The role of C-H and C-C stretching modes in the intrinsic non-radiative decay of triplet states in a Pt-containing conjugated phenylene ethynylene

 

Anna Köhler, Amena L. T. Khan, Joanne S. Wilson, Carsten Dosche, Mohammed K. Al-Suti et al.

J. Chem. Phys. 136 (2012) 094905
[DOI][PDF]

The intrinsic non-radiative decay (internal conversion) from the triplet excited state in phosphorescent
dyes can be described by a multi-phonon emission process. Since non-radiative decay of triplet
excitons can be a significant process in organic light-emitting diodes, a detailed understanding of
this decay mechanism is important if the overall device efficiency is to be controlled. We compare
a deuterated Pt(II)-containing phenylene ethynylene with its non-deuterated counterpart in order to
investigate which phonon modes control to the non-radiative decay path.We observe that deuteration
does not decrease the non-radiative decay rate. A Franck-Condon analysis of the phosphorescence
spectra shows that the electronic excitation is coupled strongly to the breathing mode of the phenyl
ring and the C≡C carbon stretching modes, while high-energy C-H or C-D stretching modes play
an insignificant role. We, therefore, associate the internal conversion process with the carbon-carbon
stretching vibrations.

Does conjugation help exciton dissociation? A study on poly(p-phenylene)s in planar heterojunctions with C60 or TNF

 

Christian Schwarz, Heinz Bässler, Irene Bauer, Jan-Moritz Koenen, Eduard Preis, Ullrich Scherf, and Anna Köhler

Advanced Materials 24 (2012) 922–925
[DOI][PDF]

graphical abstractInternal photocurrent quantum yields near 100% can be obtained from the separation of loosely bound geminate pairs when sufficiently large electric fields are applied to organic heterojunctions. The fields needed for complete electron–hole dissociation decrease to those prevailing in organic solar cells under operating conditions when well-conjugated polymers are employed.

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Control of Aggregate Formation in Poly(3-hexylthiophene) by Solvent, Molecular Weight and Synthetic Method (Editor’s Choice Paper)

 

Christina Scharsich, Ruth H. Lohwasser, Michael Sommer, Udom Asawapirom, Ullrich Scherf, Mukundan Thelakkat, Dieter Neher, Anna Köhler

J. Poly. Sci. B: Polymer Physics 50 (2012) 442–453
[DOI][PDF]

Aggregate formation in poly(3-hexylthiophene) depends on molecular weight, solvent, and synthetic method. The interplay of these parameters thus largely controls device performance. In order to obtain a quantitative understanding on how these factors control the resulting electronic properties of P3HT, we measured absorption in solution and in thin films as well as the resulting field effect mobility in transistors. By a detailed analysis of the absorption spectra, we deduce the fraction of aggregates formed, the excitonic coupling within the aggregates, and the conjugation length within the aggregates, all as a function of solvent quality for molecular weights from 5 to 19 kDa. From this, we infer in which structure the aggregated chains pack. Although the 5 kDa samples form straight chains, the 11 and 19 kDa chains are kinked or folded, with conjugation lengths that increase as the solvent quality reduces. There is a maximum fraction of aggregated chains (about 55 ± 5%) that can be obtained, even for poor solvent quality. We show that inducing aggregation in solution leads to control of aggregate properties in thin films. As expected, the field-effect mobility correlates with the propensity to aggregation. Correspondingly, we find that a well-defined synthetic approach, tailored to give a narrow molecular weight distribution, is needed to obtain high field effect mobilities of up to 0.01 cm2/Vs for low molecular weight samples (=11 kDa), while the influence of synthetic method is negligible for samples of higher molecular weight, if low molecular weight fractions are removed by extraction.

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On the Formation Mechanism for Electrically Generated Exciplexes in a Carbazole-Pyridine Copolymer

 

Anna Hayer, Tanguy Van Regemorter, Bianca Höfer, Chris S. K. Mak, David Beljonne, Anna Köhler

J. Poly. Sci. B: Polymer 50 (2012) 361-369
[DOI][PDF]

graphical abstractAlthough carbazole-containing copolymers are frequently used as hole-transporting host materials for polymer organic light-emitting diodes (OLEDs), they often suffer from the formation of undesired exciplexes when the OLED is operated. The reason why exciplexes sometimes form for electrical excitation, yet not for optical excitation is not well understood. Here, we use luminescence measurements and quantum chemical calculations to investigate the mechanism of such exciplex formation for electrical excitation (electroplex formation) in a carbazole–pyridine copolymer. Our results suggest that the exciplex is formed via a positively charged interchain precursor complex. This complex is stabilized by interactions that involve the nitrogen lone pairs on both chain segments.

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Auf dem Weg zu neuen Solarzellen: Selbstorganisation von Polymeren fördert die Ladungstrennung

 

Christian Wißler M.A. (Hrsg.)

Mediendienst Forschung Universität Bayreuth, 2011 – Nr. 40 // 26. Oktober 2011
[PDF]

VorschaubildLichtenergie in elektrischen Strom zu verwandeln ist die Funktion von Solarzellen, die
damit eine zentrale Bedeutung für die Gewinnung und Nutzung erneuerbarer Energien
haben. Einem Forschungsteam der Universität Bayreuth, der LMU München und der
TU München ist es jetzt gelungen, in organischen Makromolekülen erstmalig den Prozess der Stromerzeugung aus Licht von Anfang an zu beobachten und zu verstehen.
Ein weltweit einzigartiger lasertechnischer Versuchsaufbau und die Verwendung von
Silizium-basierten Strukturen machten diese Forschungserkenntnisse möglich.

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Charge transport in organic semiconductors

 

Heinz Bässler, Anna Köhler

Topics in Current Chemistry Volume 218, (2011) Springer-Verlag, Berlin Heidelberg
[DOI][PDF]

Charge transport in organic semiconductors is a timely subject. Today, organic semiconductors are already widely used commercially in Xerography. For display and lighting applications they are employed as light emitting diodes (LEDs or OLEDs) or transistors, and they are making progress to enter the solar cell market.[1-6] As a result, interest in the science behind this novel class of materials has risen sharply. The optoelectronic properties of organic semiconductors differ from that of conventional inorganic crystalline semiconductors in many aspects and the knowledge of organic semiconductors is imperative to further advance with the associated semiconductor applications.[7] A central problem is the understanding of the mechanisms related to charge transport

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Role of Structural Order and Excess Energy on Ultrafast Free Charge Generation in Hybrid Polythiophene/Si Photvoltaics Probed in Real Time by Near-Infrared Broadband Transient Absorption

 

Daniel Herrmann, Sabrina Niesar, Christina Scharsich, Anna Köhler, Martin Stutzmann and Eberhard Riedle

J. Am. Chem. Soc. 133 (2011) 18220-18233
[DOI][PDF]

Despite the central role of light absorption and the subsequent generation of free charge carriers in organic and hybrid organic-inorganic photovoltaics, the precise process of this initial photoconversion is still debated. We employ a novel broadband (UV-Vis-NIR) transient absorption spectroscopy setup to probe charge generation and recombination in the thin films of the recently suggested hybrid material combination poly(3-hexylthiophene)/silicon (P3HT/Si) with 40 fs time resolution. Our approach allows for monitoring the time evolution of the relevant transient species under various excitation intensities and excitation wavelengths. Both in regioregular (RR) and regiorandom (RRa) P3HT, we observe an instant (<40 fs) creation of singlet-excitons, which subsequently dissociate to form polarons in 140 fs. The quantum yield of polaron formation through dissociation of delocalized excitons is significantly enhanced by adding Si as an electron acceptor, revealing ultrafast electron transfer from P3HT to Si. P3HT/Si films with aggregated RR-P3HT are found to provide free charge carriers in planar as well as in bulk heterojunctions and losses are due to nongeminate recombination. In contrast for RRa-P3HT/Si, geminate recombination of bound carriers is observed as the dominant loss mechanism. Site-selective excitation by variation of pump wavelength uncovers an energy transfer from P3HT coils to aggregates with a 1/e transfer time of 3 ps and reveals a factor of 2 more efficient polaron formation using aggregated RR-P3HT compared to disordered RRa-P3HT. Therefore, we find that polymer structural order rather than excess energy is the key criterion for free charge generation in hybrid P3HT/Si solar cells.

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Triplet-Triplet Annihilation in a Series of Poly(p-phenylene) Derivatives

 

S.T. Hoffmann, J.M. Koenen, U. Scherf, I. Bauer, P. Strohriegl, H. Bässler, A. Köhler

J. Phys. Chem. B 115 (2011) 8417-8423
[DOI][PDF]

We have studied the temperature dependence of phosphorescence (Ph) and delayed fluorescence (DF) in two series of poly(p-phenylene) derivatives within a temperature range from 10 to 300 K under quasi-stationary conditions. One set of materials consists of the dimer, trimer, and polymer of ethylhexyl-substituted poly(fluorene) (PF2/6) and thus allows us to assess the effects of oligomer length. The second series addresses the influence of energetic disorder and conjugation length by being composed of the polymers alkoxy-substituted poly(p-phenylene) (DOO-PPP), poly(indenofluorene) (PIF), and ladder-type poly(p-phenylene) (MeLPPP). Under low light intensities, the DF features a maximum at a certain temperature Tmax. For the dimer and trimer, the Tmax coincides with the temperature at which the phosphorescence has decayed to 1/2 of the value at 10 K, while Tmax shifts to lower temperature values along the series DOO-PPP, PIF, and MeLPPP and approaches T = 0 K for MeLPPP. By applying conventional kinetic equations we show that the occurrence of a maximum in the DF intensity is the consequence of generalized thermally activated triplet exciton transport toward quenching sites. We find the quenching rates at 0 K to be in the range of 1 s–1 for the polymers, while they are more than an order of magnitude lower for the oligomers.

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Diffusion-Limited Energy Transfer in Blends of Oligofluorenes with an Anthracene Derivative

 

R.Q. Albuquerque, C.C. Hofmann, J. Köhler, A. Köhler

J. Phys. Chem. B 115 (2011) 8063-8070
[DOI][PDF]

Organic semiconductor devices such as light-emitting diodes and solar cells frequently comprise a blend of molecular or polymeric materials. Consequently, resonant energy transfer between the components plays a major role in determining device performance. Energy transfer may take place through either single-step donor-acceptor transfer, realized for example as Forster transfer, or as a sequence of donor-donor transfers toward the acceptor site. Here we use a well-defined model system comprising an oligofluorene trimer, pentamer, or heptamer as the donor in combination with an anthracene derivative as the acceptor in order to study the rate and mechanism of energy transfer in thin films by time-resolved photoluminescence spectroscopy. We find the transfer process to be entirely dominated by sequential donor-donor transfer. In addition, we observe a strong dependence on oligomer length with an optimum energy transfer rate for the pentamer.

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A series of CBP-derivatives as host materials for blue phosphorescent organic light-emitting diodes

 

Pamela Schrögel, Ausra Tomkeviciene, Peter Strohriegl, Sebastian T. Hoffmann, Anna Köhler and Christian Lennartz

J. Mat. Chem. 21 (2011) 2266-2273
[DOI][PDF]

We report a series of CBP-derivatives with superior thermal and electronic properties for the use as host materials for blue electrophosphorescent organic light emitting diodes. We applied a systematic variation of the substitution pattern in the 2- and 2´-position of the biphenyl unit and the 3- and 6-position of the carbazole moieties. In contrast to the crystalline parent compound CBP, all methyl and trifluoromethyl substituted derivatives show amorphous behaviour. Substitution in the 2- and 2´-position of the biphenyl causes a twisting of the phenyl rings. Hence, the degree of conjugation of the molecules is limited which leads to enlarged triplet energies of approximately 2.95 eV compared to 2.58 eV for CBP. The methyl substitution at the active 3- and 6-position of the pendant carbazole units yields materials with an electrochemically stable behaviour against oxidation.

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Synthesis and characterization of platinum (II) di-ynes and poly-ynes incorporating ethylenedioxythiophene (EDOT) spacers in the backbone

 

Muhammad S. Khan, Mohammed K. Al-Suti, Hakkikulla H. Shah, Said Al-Humaimi,
Fathiya R. Al-Battashi, Jens K. Bjernemose, Louise Male, Paul R. Raithby, Ning Zhang, Anna Köhler
and John E. Warrend

Dalton Tans., 40 (2011) 10174
[DOI][PDF]

A series of trimethylsilyl-protected di-alkynes incorporating 3,4-ethylenedioxythiophene (EDOT) linker
groups Me3Si–C C–R–C C–SiMe3 (R = ethylenedioxythiophene-3,4-diyl 1a, 2,2¢-bis-3,4-
ethylenedioxythiophene-5,5¢-diyl 2a, 2,2¢,5¢,2¢¢-ter-3,4-ethylenedioxythiophene-5,5¢¢-diyl 3a) and the
corresponding terminal di-alkynes, H–C C–R–C C–H 1b–2b has been synthesized and characterized
and the single crystal X-ray structure of 1a has been determined. CuI-catalyzed dehydrohalogenation
reaction between trans-[(Ph)(Et3P)2PtCl] and the terminal di-alkynes 1b–2b in iPr2NH/CH2Cl2 (2 : 1
mole ratio) gives the Pt(II) di-ynes trans-[(Et3P)2(Ph)Pt–C C–R–C C–Pt(Ph)(Et3P)2] 1M–2M while
the dehydrohalogenation polycondensation reaction between trans-[(nBu3P)2PtCl2] and 1b–2b (1 : 1 mole
ratio) under similar reaction conditions affords the Pt(II) poly-ynes trans-[Pt(PnBu3)2–C C–R–C C-]n
1P–2P. The di-ynes and poly-ynes have been characterized spectroscopically and, for 1M and 2M, by
single-crystal X-ray which confirms the “rigid rod” di-yne backbone. The materials possess excellent
thermal stability, are soluble in common organic solvents and readily cast into thin films. Optical
absorption spectroscopic measurements reveal that the EDOT spacers create stronger donor-acceptor
interactions between the platinum(II) centres and conjugated ligands along the rigid backbone of the
organometallic polymers compared to the related non-fused and fused oligothiophene spacers.

 

Triplet Excimer Emission in a Series of 4,4′-Bis(N-carbazolyl)-2,2′-biphenyl Derivatives

 

ST Hoffmann, P Schrögel, M Rothmann, R Albuquerque, P Strohriegl and A Köhler

J. Phys. Chem. B 115 (2011) 414-421
[DOI][PDF]

Carbazole-based materials such as 4,4´-bis(N -carbazolyl)-2,2´-biphenyl (CBP) and its derivatives are frequently used as matrix materials for phosphorescent emitters in organic light emitting diodes (OLED)s. An essential requirement for such matrix materials is a high energy of their first triplet excited state. Here we present a detailed spectroscopic investigation supported by density functional theory (DFT) calculations on two series of CBP derivatives, where CH3 and CF3 substituents on the 2- and 2´-position of the biphenyl introduce strong torsion into the molecular structure. We find that the resulting poor coupling between the two halves of the molecules leads to an electronic structure similar to that of -phenyl-3,6-dimethylcarbazole, with a high triplet-state energy of 2.95 eV. However, we also observe a triplet excimer emission centered at about 2.5-2.6 eV in all compounds. We associate this triplet excimer with a sandwich geometry of neighboring carbazole moieties. For compounds with the more polar CF3 substituents, the lifetime of the intermolecular triplet excited state extends into the millisecond range for neat films at room temperature. We attribute this to an increased charge-transfer character of the intermolecular excited state for the more polar substituents.

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What controls triplet exciton transfer in organic semiconductors?

 

A Köhler, H Bässler

J. Mater. Chem. 21 (2011) 4003-4011
[DOI][PDF]

Dexter-type triplet transfer is a phenomenon that is ubiquitous in the field of molecular electronics, and that takes place at the interface of chemistry, physics and biology. It may be considered as a correlated transfer of two charges, and thus, models originally developed for charge transfer may be applied to describe triplet transfer. In dilute fluid solutions, triplet transfer from a donor to an acceptor is well-understood and it has been described in terms of Marcus theory, i.e. taking into account distortions in the molecule and its surroundings. In amorphous thin films, that are used for organic semiconductor applications, the effects of energetic disorder prevail, and they need to be considered for an appropriate description of triplet energy transfer. We present here an overview on recent experimental and theoretical work concerning a unified description of triplet energy transfer.

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What Determines Inhomogeneous Broadening of Electronic Transitions in Conjugated Polymers?

 

ST Hoffmann, H Bässler, A Köhler

J. Phys. Chem. B 114 (2010) 17037–17048
[DOI][PDF]

Energetic disorder is manifested in the inhomogeneous broadening of optical transitions in π-conjugated organic materials, and it is a key parameter that controls the dynamics of charge and energy transfer in this promising class of amorphous semiconductors. In an endeavor to understand which processes cause the inhomogeneous broadening of singlet and triplet excitations in π-conjugated polymers we analyze continuous wave absorption and photoluminescence spectra within a broad range of temperatures for (i) oligomers of the phenylenevinylene family (OPVs) and MEH-PPV in solution and (ii) bulk films of MEH-PPV and members of the poly(p-phenylene) family (PPPs). We use a Franck-Condon deconvolution technique to determine the temperature dependent S1-S0 0-0 and T1-S0 0-0 transition energies and their related variances. For planar compounds, the transition energies can be related to the oligomer length, which allows us to infer the effective conjugation length for the nonplanar compounds as a function of temperature. With this information we can distinguish between intrachain contributions to the inhomogeneous line broadening that are due to thermally induced torsional displacements of the chain elements, and other contributions that are assigned largely to dielectric interactions between the chain and its environment. We find that in solution, temperature-induced torsional displacements dominate the line broadening for the alkyl derivatives of OPVs while in the alkoxy derivatives the Van der Waals contribution prevails. In films, σ is virtually temperature independent because disorder is frozen in. We also establish a criterion regarding the ratio of inhomogeneous line broadening in singlet and triplet states. The results will be compared to a recent theory by Barford and Trembath.

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Triplet energy transfer in conjugated polymers. III. An experimental assessment regarding the influence of disorder on polaronic transport

 

ST Hoffmann, E Scheler,JM Koenen, M Forster, U Scherf, P Strohriegl,H Bässler, A Köhler

Phys.Rev. B 81 (2010) 165208
[DOI][PDF]

It is a general notion that, in organic semiconductors, the transport of electronic excitations, such as neutral excitons of singlet or triplet type and charge carriers, is controlled by both, polaron and disorder effects. For compounds with low energetic disorder triplet exciton diffusion can be described in the framework of Marcus-theory [see Sudha Devi et al.,Phys. Rev. B 78, 045210 (2008)], and a theoretical model for diffusion in more disordered compounds has been developed [see Fishchuk et al., Phys. Rev. B 78, 045211 (2008)]. Here we experimentally demonstrate that such a modified Marcus-type model is suitable to describe triplet exciton transport in commonly used poly(p-phenylene)-type polymers and oligomers. In particular, we provide aquantitative spectroscopic assessment of the polaronic and the disorder contribution to triplet exciton transport as a function of conjugation length. Franck-Condon analyses of the phosphorescence spectra and temperature-dependent triplet diffusion combined with analytic transport theory demonstrate that, in contrast to charge carriers, Marcus-type jump rates with dominantly polaronic activation energies control the motion of triplet excitons above a transition temperature.

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Hole-transporting host-polymer series consisting of triphenylamine basic structures for phosphorescent polymer light-emitting diodes

 

MW Thesen, B Höfer, M Debeaux, S Janietz, A Wedel, A Köhler, HH Johannes, H Krueger

J. Pol. Sc. A 48 (2010) 3417-3430
[DOI][PDF]

A series of novel styrene derived monomers with triphenylamine-based units, and their polymers have been synthesized and compared with the well-known structure of polymer of N,N0-bis(3-methylphenyl)-N,N0-diphenylbenzidine with respect to their hole-transporting behavior in phosphorescent polymer light-emitting diodes (PLEDs). A vinyltriphenylamine structure was selected as a basic unit, functionalized at the para positions with the following side groups: diphenylamine, 3-methylphenyl-aniline, 1- and 2-naphthylamine, carbazole, and phenothiazine. The polymers are used in PLEDs as host polymers for blend systems with the following device configuration: glass/indium–tin–oxide/PEDOT:PSS/polymer-blend/CsF/Ca/Ag. In addition to the hole-transporting host polymer, the polymer blend includes a phosphorescent dopant [Ir(Me-ppy)3] and an electron-transporting molecule (2-(4-biphenyl)-5-(4-tertbutylphenyl)-1,3,4-oxadiazole). We demonstrate that two polymers are excellent hole-transporting matrix materials for these blend systems because of their good overall electroluminescent performances and their comparatively high glass transition temperatures. For the carbazole-substituted polymer (Tg = 246°C), a luminous efficiency of 35 cd A-1 and a brightness of 6700 cd m-2 at 10 V is accessible. The phenothiazine-functionalized polymer (Tg= 220°C) shows nearly the same outstanding PLED behavior. Hence, both these polymers outperform the well-known polymer of N,N0-bis(3-methylphenyl)-N,N0- diphenylbenzidine, showing only a luminous efficiency of 7.9 cd A-1 and a brightness of 2500 cd m -2 (10 V).

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Spectral diffusion in poly(para-phenylene)-type polymers with different energetic disorder

 

ST Hoffmann, H Bässler, JM Koenen, M Forster, U Scherf, E Scheler, P Strohriegl, A Köhler

Phys. Rev. B 81 (2010) 115103
[DOI][PDF]

We have employed quasicontinuous fluorescence and phosphorescence spectroscopy within a temperature range between 10 and 500 K to monitor the spectral diffusion of singlet and triplet excitons in a series of π-conjugated polymers. We investigated (i) how spectral diffusion is controlled by the degree of energetic disorder present in the amorphous film (that is reflected by the inhomogeneous broadening of the photoluminescence spectra) and (ii) how this process depends on the range of the electronic coupling (by comparing singlet exciton diffusion via long-range Förster transfer against triplet exciton diffusion by short-range Dexter transfer). For singlets, we find that the fluorescence spectra bear out a bathochromic shift upon cooling the sample down to a critical temperature below which the shift saturates. This bathochromic shift is a signature of spectral relaxation. Random-walk theory applied to excitation transport within a Gaussian density-of-states distribution is both necessary and sufficient to rationalize the experimental results in a quantitative fashion. The same behavior is observed for triplets in weakly disordered systems, such as in a polymer containing platinum in the main chain and a ladder-type polyphenylene. In contrast we observe a hypsochromic shift of the phosphorescence spectra below a characteristic temperature for triplets in systems with at least moderate energetic disorder. The hypsochromic shift proves that triplet exciton relaxation becomes frustrated because thermally activated exciton jumps that otherwise promote spectral diffusion become progressively frozen out. The frustration effect is controlled by the jump distance and thus it is specific for triplet excitations that migrate via short-range coupling among strongly localized states as compared to singlet excitons.

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Triplet states in organic semiconductors

 

A Köhler, H Bässler

Materials Science and Engineering R, (2009) 71-109
[DOI][PDF]

Today’s technology is not possible without optoelectronic devices such as light-emitting diodes, transistors and solar cells. These basic units of modern electronic appliancesmay be made not only from traditional inorganic semiconductors, but also from organic semiconductors, i.e. hydrocarbonmolecules that combine semiconducting properties with some mechanical properties such as easy processability and flexibility. The weak van der Waals forces that bind the molecules to a solid imply a low dielectric constant, so that coulomb and exchange interactions between electrons are significant. As a result, photoexcitation or electrical excitation results in strongly bound electron–hole pairs, so-called excitons. Depending on the relative orientation of the electron and hole spin, the exciton may be of a overall singlet or triplet spin state. While the fluorescent singlet state has been investigated intensively since the first reports of organic electroluminescence, research into the properties of the phosphorescent triplet state has intensified mainly during the last decade. In this review we give an overview on the photophysical processes associated with the formation of triplet states and their decay, as well as the energy levels and energy transfer processes of triplet states. We aim to give a careful introduction for those new to this particular research area as well as to highlight some of the current research issues and intriguing questions for those familiar with the field. The main focus of this review is on molecular assemblies and polymer films, though relevant work on molecular crystals is also included where it assists in forming a larger picture.

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Synthesis and comparison of the optical properties of platinum (II) poly-ynes with fused and non-fused oligothiophenes

 

L Sudha Devi, MK Al-Suti, N Yhang, SJ Teat, L Male, HA, Sparkes, RP Rainthby, MS Khan, A Köhler

Macromolecules 42 (2009) 1131-1141
[DOI][PDF]

The synthesis and characterization of the thieno[3,2-b]thiophene and dithieno[3,2-b:2′,3′-d]thiophene containing platinum(II) poly-ynes and their molecular precursors is described and the electronic structure is established by absorption, luminescence and photoinduced absorption measurements. A comparison of the electronic structure of the fused and the nonfused oligothiophenes, thieno[3,2-b]thiophene, dithieno[3,2-b:2′,3′-d]thiophene, 2,2′-bithiophene, and 2,2′:5′,2” -terthiophene incorporated in platinum(II) poly-ynes is reported. We find the singlet S1 and triplet T1 and Tn excited states to be at higher energy in thin films made from the fused systems than from the nonfused systems. For ligands with the same number of rings, we attribute this to the decreased number of double bonds in the fused system and to the presence of an additional sulfur atom in spacers with the same number of double bonds.

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Triplet energy transfer in conjugated polymers. I. Experimental investigation of a weakly disordered compound

 

L Sudha Devi, MK Al-Suti, C. Dosche, MS Khan, RH Friend, A Köhler

Phys. Rev. B 78 (2008) 045210
[DOI][PDF]

Efficient triplet exciton emission has allowed improved operation of organic light-emitting diodes (LEDs). To enhance the device performance, it is necessary to understand what governs the motion of triplet excitons through the organic semiconductor. Here, we have investigated triplet diffusion using a model compound that has weak energetic disorder. The Dexter-type triplet energy transfer is found to be thermally activated down to a transition temperature TT, below which the transfer rate is only weakly temperature dependent. We show that above the transition temperature, Dexter energy transfer can be described within the framework of Marcus theory. We suggest that below TT, the nature of the transfer changes from phonon-assisted hopping to quantummechanical tunneling. The lower electron-phonon coupling and higher electronic coupling in the polymer compared to the monomer results in an enhanced triplet diffusion rate.

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Triplet energy transfer in conjugated polymers. II. A polaron theory description addressing the influence of disorder

 

II Fishchuk, A Kadashchuk, L Sudha Devi, P Heremans, H Bässler, A Köhler

Phys. Rev. B 78 (2008) 045211
[DOI][PDF]

Motivated by experiments monitoring motion of triplet excitations in a conjugated polymer containing Pt-atoms in the main chain (see Paper I), a theoretical formalism for electronic transport has been developed. It considers the interplay between polaronic distortion of the excited chain elements and disorder treated in terms of effective-medium theory. The essential parameters are the electronic coupling J, the polaronic binding energy λ that determines the activation energy of polaron motion Ea, and the variance σ of the density of states distribution controlling the incoherent hopping motion. It turns out that for the weak electronic coupling associated with triplet motion (J a few meV), the transfer is nonadiabatic. For a critical ratio of σ/Ea< 0.3, Marcus-type multiphonon transport prevails above a certain transition temperature. At lower temperatures, transport is disorder controlled consistent with the Miller-Abrahams formalism. Theoretical results are consistent with triplet transport in the Pt-polymer. Implications for charge and triplet motion in random organic semiconductors in general are discussed.

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Effect of solvent on the conformation of isolated MEH-PPV chains intercalated into SnS2

 

E Aharon, S Breuer, F Jaiser, A Köhler, GL Frey

Chem. Phys. Chem. 9 (2008) 1430-1436
[DOI][PDF]

Photophysical processes in conjugated polymers are influenced by two competing effects: the extent of excited state delocalization along a chain, and the electronic interaction between chains. Experimentally, it is often difficult to separate the two because both are controlled by chain conformation. Here we demonstrate that it is possible to modify intra-chain delocalization without inducing inter-chain interactions by intercalating polymer monolayers between the sheets of an inorganic layered matrix. The red-emitting conjugated polymer, MEH-PPV, is confined to the interlayer space of layered SnS2. The formation of isolated polymer monolayers between the SnS2 sheets is confirmed by X-ray diffraction measurements. Photoluminescence excitation (PLE) and photoluminescence (PL) spectra of the incorporated MEH-PPV chains reveal that the morphology of the incorporated chains can be varied through the choice of solvent used for chain intercalation. Incorporation from chloroform results in more extended conformations compared to intercalation from xylene. Even highly twisted conformations can be achieved when the incorporation occurs from a methanol:chloroform mixture. The PL spectra of the MEH-PPV incorporated SnS2 nanocomposites using the different solvents are in good agreement with the PL spectra of the same solutions, indicating that the conformation of the polymer chains in the solutions is retained upon intercalation into the inorganic host. Therefore, intercalation of conjugated polymer chains into layered hosts enables the study of intra-chain photophysical processes as a function of chain conformation.

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Exciton dynamics in blends of phosphorescent emitters

 

C Schuetz, B Höfer, F Jaiser, H Krueger, M Thesen, S Janietz, A Köhler

Physica status solidi B 245 (2008) 810-813
[DOI][PDF]

Cascading energy transfer is a usually undesired effect in organic-host guest systems that are designed for light-emitting purposes. Here we demonstrate a chemical approach to suppressing undesired energy transfer from the blue to the red emitter in multicomponent polymer blends. We have combined a red, green and blue emitting Ir-complex each with charge transporting molecules to a side-chain copolymer. The covalent attachment of the phosphorescent emitter is found to prevent cascading energy transfer through steric shielding. This approach approximately doubles the efficiency and brightness of polymer light emitting diodes.

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Dimensionality-dependent energy transfer in polymer-intercalated SnS2 nanocomposites

 

P Parkinson, E Aharon, MH Chang, C Dosche, G. Frey, A. Koehler, LM Herz

Phys. Rev. B 75 (2007) 165206
[DOI][PDF]

We have investigated the influence of dimensionality on the excitation-transfer dynamics in a conjugated polymer blend. Using time-resolved photoluminescence spectroscopy, we have measured the transfer transients for both a three-dimensional blend film and for quasi-two-dimensional monolayers formed through intercalation of the polymer blend between the crystal planes of an inorganic SnS2 matrix. We compare the experimental data with a simple, dimensionality-dependent model based on electronic coupling between electronic transition moments taken to be point dipoles. Within this approximation, the energy-transfer dynamics is found to adopt a three-dimensional character in the solid film and a two-dimensional nature in the monolayers present in the SnS2-polymer nanocomposite.

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The effect of delocalization on the exchange energy in meta- and para-linked Pt-containing carbazole polymers and monomers

 

Ning Zhang, Anna Hayer, Mohammed K. Al-Suti, Rayya A. Al-Belushi, Muhammad S. Khan, and Anna Köhler

J. Chem. Phys. 124 (2006) 244701
[DOI][PDF]

A series of novel platinum-containing carbazole monomers and polymers was synthesized and fully characterized by UV-VIS absorption, luminescence, and photoinduced absorption studies. In these compounds, a carbazole unit is incorporated into the main chain via either a para- or a meta-linkage. We discuss the effects of linkage and polymerization on the energy levels of S1, T1, and Tn. The S1-T1 splitting observed for the meta-linked monomer (0.4 eV) is only half of that in the para-linked monomer (0.8 eV). Upon polymerization, the exchange energy in the para-linked compound reduces, yet still remains larger than in the meta-linked polymer. We attribute the difference in exchange energy to the difference in wave function overlap between electron and hole in these compounds.

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Triplet Energy Back Transfer in Conjugated Polymers with Pendant Phosphorescent Iridium Complexes

 

Nicholas R. Evans, Lekshmi Sudha Devi, Chris S. K. Mak, Scott E. Watkins,

Sofia I. Pascu, Anna Köhler, Richard H. Friend, Charlotte K. Williams and

Andrew B. Holmes

J. Am. Chem. Soc. 128 (2006) 6647-6656
[DOI][PDF]

Abstract: The nature of Dexter triplet energy transfer between bonded systems of a red phosphorescent iridium complex 13 and a conjugated polymer, polyfluorene, has been investigated in electrophosphorescent organic light-emitting diodes. Red-emitting phosphorescent iridium complexes based on the [Ir(btp)2(acac)] fragment (where btp is 2-(2¢-benzo[b]thienyl)pyridinato and acac is acetylacetonate) have been attached either directly (spacerless) or through a -(CH2)8- chain (octamethylene-tethered) at the 9-position of a 9-octylfluorene host. The resulting dibromo-functionalized spacerless (8) or octamethylene-tethered (12) fluorene monomers were chain extended by Suzuki polycondensations using the bis(boronate)-terminated fluorene macromonomers 16 in the presence of end-capping chlorobenzene solvent to produce the statistical spacerless (17) and octamethylene-tethered (18) copolymers containing an even dispersion of the pendant phosphorescent fragments. The spacerless monomer 12 adopts a face-to-face conformation with a separation of only 3.6 Å between the iridium complex and fluorenyl group, as shown by X-ray analysis of a single crystal, and this facilitates intramolecular triplet energy transfer in the spacerless copolymers 17. The photo- and electroluminescence efficiencies of the octamethylene-tethered copolymers 18 are double those of the spacerless copolymers 17, and this is consistent with suppression of the back transfer of triplets from the red phosphorescent iridium complex to the polyfluorene backbone in 18. The incorporation of a -(CH2)8- chain between the polymer host and phosphorescent guest is thus an important design principle for achieving higher efficiencies in those electrophosphorescent organic light-emitting diodes for which the triplet energy levels of the host and guest are similar.

Highly Fluorescent Crystalline and Liquid Crystalline Columnar Phases of Pyrene-Based Structures

 

Anna Hayer, Véronique de Halleux, Anna Ko1hler, Abdel El-Garoughy, E. W. Meijer, Joaquín Barberá, Julien Tant, Jeremy Levin, Matthias Lehmann, Johannes Gierschner, Jérôme Cornil, and Yves Henri Geerts

J. Phys. Chem. B 110 (2006) , 7653-7659
[DOI][PDF]

A concept for highly ordered solid-state structures with bright fluorescence is proposed: liquid crystals based on tetraethynylpyrene chromophores, where the rigid core is functionalized with flexible, promesogenic alkoxy chains. The synthesis of this novel material is presented. The thermotropic properties are studied by means of differential scanning calorimetry (DSC), cross-polarized optical microscopy (POM), and X-ray diffraction. The mesogen possesses an enantiotropic Colh phase over a large temperature range before clearing. The material is highly fluorescent in solution and, most remarkably, in the condensed state, with a broad, strongly red shifted emission. Fluorescence quantum yields (¼F) have been determined to be 70% in dichloromethane solution and 62% in the solid state. Concentration- and temperature-dependent absorption and emission studies as well as quantum-chemical calculations on isolated molecules and dimers are used to clarify the type of intermolecular interactions present as well as their influence on the fluorescence quantum yield and spectral properties of the material. The high luminescence efficiency in the solid state is ascribed to rotated chromophores, leading to an optically allowed lowest optical transition.

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Morphology dependence of the triplet excited state formation and absorption in polyfluorene

A Hayer, ALT Khan, RH Friend, A Köhler

Phys. Rev. B 71 (2005) 241302
[DOI][PDF]

Poly(9,9-dioctylfluorene) (PFO) is widely used as a highly efficient blue-emitting polymer for light-emitting diode applications. Films of PFO are known to contain a planar β-phase or a disordered glassy phase that affects the emission characteristics. We have studied the morphology dependence of the photoinduced absorption in such PFO films and find that the T1-Tn absorption signal differs between the two phases in energy, linewidth, and intensity. The lower intensity we observe in the β-phase as compared to the glassy phase is evidence for a lower T1 population in the former. We attribute this to firstly, reduced intersystem crossing and secondly, increased charge generation due to both more extended conjugation along the chains and higher interchain interactions in the planar, well-ordered β-phase. The reduced energy and smaller linewidth of the triplet absorption in the β-phase as compared to the glassy phase are indicative of a more delocalized Tn state and a higher degree of order in the β-phase, respectively.

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Large magnetoresistance in nonmagnetic pi-conjugated semiconductor thin film devices

Ö Mermer, G Veeraraghavan, TL Francis, Y Sheng, DT Nguyen, M Wohlgenannt, A Köhler, MK Al-Suti, MS Khan

Phys. Rev. B. 72 (2005) 205202
[DOI][PDF]

Following the recent observation of large magnetoresistance at room temperature in polyfluorene sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different π -conjugated polymers and small molecules. The study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength. We study both hole and electron transporters at temperatures ranging from 10 K to 300 K. We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results.

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Blue-to-green electrophosphorescence of iridium-based cyclometallated materials

CSK Mak, A Hayer, SI Pascu, SE Watkins, AB Holmes, A Köhler, RH Friend

Chem. Commun., 37 (2005) 4708-4710
[DOI][PDF]

The photo- and electroluminescence properties of a series of novel, heteroleptic, mer-cyclometallated iridiumcomplexes have been fine-tuned from green to blue by changing the substituents on the pyridyl ring of the phenylpyridyl ligand. The X-ray crystal structures of two Ir-based triazolyl complexes are reported.

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Spectroscopic study of spin-dependent exciton formation rates in pi-conjugated semiconductors: Comparison with electroluminescence techniques.

C. Yang, ZV Vardeny, A Köhler, M Wohlgenannt, MK Al-Suti, MS Khan

Phys. Rev. B, 70 (2004) 241202
[DOI][PDF]

It has been found in recent measurements that the singlet-to-triplet exciton ratio in organic light-emitting diodes (OLEDs) is larger than expected from spin degeneracy, and that singlet excitons form at a larger rate than triplets. We employed the technique of optically detected magnetic resonance to measure the spindependent exciton formation rates in films of a polymer and corresponding monomer, and explore the relation between the formation rates and the actual singlet-to-triplet ratio measured previously in OLEDs. We found that the spin-dependent exciton formation rates can indeed quantitatively explain the observed exciton yields, and that singlet formation rates and yields are significantly enhanced only in polymer OLEDs, but not in OLEDs made from the corresponding monomer.

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The singlet-triplet exchange energy in conjugated polymers

A Köhler und D Beljonne

Advanced Functional Materials, (Feature article), 14 (2004),11
[DOI][PDF]

Electron–electron interactions in organic semiconductors split the lowest singlet and triplet states by the exchange energy, ?EST. Measurement of singlet and triplet emission spectra in a large number of conjugated polymers yield an almost constant ?EST value close to 0.7?eV. This is in contrast to the situation in molecules, where the exchange energy is found to depend on molecular size and to vary over a wide range. Quantum-chemical calculations are performed to address the origin of the constant exchange energy in phenylene-based conjugated polymers. The electron–hole separation in the lowest singlet and triplet excited states is found to be independent of the p-conjugated backbone, and saturates for chains longer than a few repeating units, resulting in a constant exchange energy. In shorter conjugated oligomers, confinement of the excitations destabilizes the singlet with respect to the triplet through exchange interactions and leads to a larger and size-dependent singlet–triplet energy separation.

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Morphology-dependent energy transfer within polyfluorene thin films

ALT Khan, P Sreearunothai, LM Herz, MJ Banach, A Köhler

Phys. Rev. B., 69 (2004) 085201
[DOI][PDF]

We have performed a detailed study of the photoluminescence from thin films of blue-light-emitting poly(9,9-dioctylfluorene) containing different fractions of planarized (ß-phase) chains within the glassy polymer film. By choosing solvents with a range of polarities and boiling points we were able to cast films with reliable control of the relative amounts of ß-phase chains present. We analyzed the emission spectra in terms of Franck-Condon progressions and found that, at low temperatures (8 K), the luminescence can be modeled accurately by considering two distinct contributions from the two phases present in the film. The Huang-Rhys parameter for the ß phase is shown to be approximately half the value obtained for the glassy phase, in agreement with a more delocalized exciton in the ß phase. Time-resolved photoluminescence measurements on a film containing roughly 25% of ß phase reveal a fast transfer of excitations from the glassy to the ß phase, indicating that the two phases are well intermixed. Assuming the transfer dynamics to be governed by dipole-dipole coupling, we obtain a Förster radius of 8.2±0.6nm, significantly larger than the radius typically found for excitation transfer within the glassy phase. These results are consistent with the large spectral overlap between the emission of the glassy phase and the absorption of the ß phase and explain why the latter dominates the emission even from films containing only a small fraction of ß-phase chains.

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Solution-processable conjugated electrophosphorescent polymers

AJ Sandee, CK Williams, N Evans, JR Davies, CE Boothby, A Köhler, RH Friend and AB Holmes

J. Am. Chem. Soc. 126 (2004) 7041-7048
[DOI][PDF]

We report the synthesis and photophysical study of a series of solution-processible phosphorescent iridium complexes. These comprise bis-cyclometalated iridium units [Ir(ppy)2(acac)] or [Ir(btp)2(acac)] where ppy is 2-phenylpyridinato, btp is 2-(2‘-benzo[b]thienyl)pyridinato, and acac is acetylacetonate. The iridium units are covalently attached to and in conjugation with oligo(9,9-dioctylfluorenyl-2,7-diyl) [(FO)n] to form complexes [Ir(ppy-(FO)n)2(acac)] or [Ir(btp-(FO)n)2(acac)], where the number of fluorene units, n, is 1, 2, 3, 10, 20, 30, or 40. All the complexes exhibit emission from a mixed triplet state in both photoluminescence and electroluminescence, with efficient quenching of the fluorene singlet emission. Short-chain complexes, 11-13, [Ir(ppy-(FO)n-FH)2(acac)] where n = 0, 1, or 2, show green light emission, red-shifted through the FO attachment by about 70 meV, but for longer chains there is quenching because of the lower energy triplet state associated with polyfluorene. In contrast, polymer complexes 18-21 [Ir(btp-(FO)n)2(acac)] where n is 5-40 have better triplet energy level matching and can be used to provide efficient red phosphorescent polymer light-emitting diodes, with a red shift due to the fluorene attachment of about 50 meV. We contrast this small (50-70 meV) and short-range modification of the triplet energies through extended conjugation, with the much more substantial evolution of the p-p* singlet transitions, which saturate at about n = 10. These covalently bound materials show improvements in efficiency over simple blends and will form the basis of future investigations into energy-transfer processes occurring in light-emitting diodes.

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Polymer light-emitting diodes with spin-polarised charge injection

A Hayer, A Köhler, E Arisi, I Bergenti, A Dediu, C Taliani, M Al-Suti, MS Khan

Synth. Met. 147, (2004) 155-158
[DOI][PDF]

This study is concerned with the characterisation of the injection properties of the manganite compound La0.7Sr0.3MnO3 (LSMO) and its use as a spin-polarised anode in polymer light-emitting diodes. Charge carrier injection and electroluminescence in a polymer LED with an LSMO anode is achieved. A dependence of the current–voltage characteristics of LEDs with LSMO anodes on the temperature is observed and compared to the behaviour of LEDs with standard ITO anodes. The temperature dependence is shown to be due to improved hole injection at low temperatures, possibly originating from magnetic ordering upon cooling.

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Synthesis, characterisation and optical spectroscopy of platinum(II) di-ynes and poly-ynes incorporating condensed aromatic spacers in the backbone

MS Khan, MRA Al-Mandhary, MK Al-Suti, FR Al-Battashi, S Al-Saadi, B Ahrens, JK Bjernemose, MF Mahon, PR Raithby, M Younus, N Chawdhury, A Köhler, EA Marseglia, E Tedesco, N Feeder, SJ Teat

Dalton Trans., (15): 2377-2385 2004.
[DOI][PDF]

A series of protected and terminal dialkynes with extended p-conjugation through a condensed aromatic linker unit in the backbone, 1,4-bis(trimethylsilylethynyl)naphthalene, 2a, 1,4-bis(ethynyl)naphthalene, 2b, 9,10-bis(trimethylsilylethynyl)anthracene 3a, 9,10-bis(ethynyl)anthracene 3b, have been synthesized and characterized spectroscopically. The solid-state structures of 2a and 3a have been confirmed by single crystal X-ray diffraction studies. Reaction of two equivalents of the complex trans-[Ph(Et3P)2PtCl] with an equivalent of the terminal dialkynes 1,4-bis(ethynyl)benzene 1b and 2b–3b, in iPr2NH–CH2Cl2, in the presence of CuI, at room temperature, afforded the platinum(II) di-ynes trans-[Ph(Et3P)2Pt–C ≡ C–R–C ≡ C–Pt(PEt3)2Ph] (R = benzene-1,4-diyl 1c; naphthalene-1,4-diyl 2c and anthracene-9,10-diyl 3c) while reactions between equimolar quantities of trans-[(nBu3P)2PtCl2] and 2b–3b under similar conditions readily afforded the platinum(II) poly-ynes trans-[–(nBu3P)2Pt–C ≡ C–R–C ≡ C–]n (R = naphthalene-1,4-diyl 2d and anthracene-9,10-diyl 3d). The Pt(II) diynes and poly-ynes have been characterized by analytical and spectroscopic methods, and the single crystal X-ray structures of 1c and 2c have been determined. These structures confirm the trans-square planar geometry at the platinum centres and the linear nature of the molecules. The di-ynes and poly-ynes are soluble in organic solvents and readily cast into thin films. Optical spectroscopic measurements reveal that the electron-rich naphthalene and anthracene spacers create strong donor–acceptor interactions between the Pt(II) centres and conjugated ligands along the rigid backbone of the organometallic polymers. Thermogravimetry shows that the di-ynes possess a somewhat higher thermal stability than the corresponding poly-ynes. Both the Pt(II) di-ynes and the poly-ynes exhibit increasing thermal stability along the series of spacers from phenylene through naphthalene to anthracene.

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Phosphorescence and spin-dependent exciton formation in conjugated polymers

A Köhler and JS Wilson

Organic electronics 4 (2003) 179 – special issue
[DOI][PDF]

The mechanism for the formation of singlet and triplet states in conjugated polymer-based light-emitting diodes (LEDs) is crucial in determining the overall efficiencies of these devices. If simple spin statistics apply then singlets and triplets should be formed in the ratio 25:75. However, the non-emissive nature of triplet states in these materials, as well as other loss mechanisms within the devices, mean that this ratio is not straightforward to measure. Nevertheless, recent experimental advances have made it possible to determine many of the properties of triplet states. Here we review what is now known about triplet states and their photophysics in conjugated polymers. We place particular emphasis on measurements of the singlet generation fraction in LEDs, and discuss the experimental techniques that have been used, such as direct comparison of photoluminescence and electroluminescence efficiencies, triplet absorption cross section measurements and magnetic resonance measurements. All of these techniques give values for the singlet generation fraction in polymers that are significantly larger than the 25% expected, and many of them have also shown that in shorter oligomers this value decreases to be closer to 25%. We also give a brief overview of recent theories for the processes of singlet and triplet formation in polymer devices.

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Low-energy vibrational modes in phenylene oligomers studied by THz time-domain spectroscopy

MB Johnston, LM Herz, ALK Khan, A Köhler, AG Davies, EH Linfield

Chem. Phys. Lett. 377 (2003), 256-262
[DOI][PDF]

Following the recent observation of large magnetoresistance at room temperature in polyfluorene sandwich devices, we have performed a comprehensive magnetoresistance study on a set of organic semiconductor sandwich devices made from different π -conjugated polymers and small molecules. The study includes a range of materials that show greatly different chemical structure, mobility, and spin-orbit coupling strength. We study both hole and electron transporters at temperatures ranging from 10 K to 300 K. We observe large negative or positive magnetoresistance (up to 10% at 300 K and 10 mT) depending on material and device operating conditions. We discuss our results in the framework of known magnetoresistance mechanisms and find that none of the existing models can explain our results.

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Control of alpha and beta phase formation in polyfluorene thin films via Franck-Condon Analysis

ALT Khan, MJ Banach and A Köhler

Synth. Met., 139 (2003), 905-907
[DOI][PDF]

In the blue light-emitting polymer poly(9,9-dioctylfluorene) (PFO) two distinct morphological phases have been identified, that is the glassy phase and a planar ß-phase. We show that the proportion of the formation of the two phases can be controlled by simply spinning thin films from different solvents such as chloroform and cyclopentanone. We use a Franck–Condon analysis with vibrational modes obtained from Raman spectra to analyze the emission spectra at low temperature. Films spun from cyclopentanone are dominated by from the ß-phase emission while films spun from chloroform have a high fraction of glassy phase emission. Comparison between absorption and emission spectra shows evidence for spectral diffusion from the glassy to the ß-phase. We discuss the Huang–Rhys parameters for both phases.

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Polarisation of singlet and triplet excited states in a platinum-containing conjugated polymer

JS Wilson, RJ Wilson, RH Friend, MK Al-Suti, MRA Al-Mandhary, MS Khan, A Köhler

Phys. Rev. B., 67 (2003) 125206
[DOI][PDF]

We investigate the polarization of optical transitions associated with the singlet S1 and triplet T1 and Tn excited states in a uniaxially aligned platinum-containing conjugated polymer which contains a 2-methoxy-5-(´-ethyl)-hexyloxy (MEH)-substituted phenyl ring. For the singlet S1 state, which is extended along the polymer chain, we find the corresponding absorption and emission to be polarized parallel to the chain as seen for other conjugated polymers. However, for the triplet excited states, we find that the emission from the highly localized T1 state has components both parallel and perpendicular to the polymer chain, while the absorption from T1 into the delocalized Tn state is polarized entirely parallel to the chain. We discuss this connection between the spatial extent of the excited state and the polarization of the associated optical transitions and consider how the spin-orbit coupling mechanism can influence the polarization of emission from the T1 state.

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Effect of interchain interactions on the absorption and emission of poly(3-hexylthiophene)

PJ Brown, DS Thomas, A Köhler, JS Wilson, J-S Kim, CM Ramsdale, H Sirringhaus, RH Friend

Phys. Rev. B., 67 (2003) 064203
[DOI][PDF]

The absorption spectrum of polythiophene and its derivative poly(3-hexylthiophene) (P3HT) is usually described in terms of an intrachain exciton coupled to a single phonon mode. We show that this model is too simplistic for highly ordered, regioregular P3HT and that, analogous to the case of charged polarons in this material, interchain interactions must be taken into account to correctly describe the absorption spectrum. We show that the lowest energy feature in the p-p* region of the absorption spectrum is associated with an interchain absorption, the intensity of which is correlated with the degree of order in the polymer. Correspondingly, we show that the emission from P3HT also exhibits contributions from both interchain and intrachain states, in a manner similar to that recently shown for poly(phenylenevinylene). Having reinterpreted the physical origin of the features in the absorption and emission spectra of P3HT, we then model these spectra and show how they evolve as the degree of order in the polymer is changed by varying several physical parameters including temperature and regioregularity of the polymer.

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Synthesis and characterization of new acetylide-functionalised aromatic and hetero-aromatic ligands and their dinuclear platinum complexes

MS Khan, MK Al-Suti, MRA Al-Mandhary, B Ahrens, JK Bjernemose, MF Mahon, L Male, PR Raithby, J Wilson, RH Friend, A Köhler

J. Chem. Soc., Dalton Trans., (2003) 65-73.
[DOI][PDF]

A new series of rigid rod protected and terminal dialkynes with extended p-conjugation through aromatic and hetero-aromatic linker units in the backbone, 2,5-bis(trimethylsilylethynyl)-1-(2-ethylhexyloxy)-4-methoxybenzene 1a, 2,5-bis(ethynyl)-1-(2-ethylhexyloxy)-4-methoxybenzene 1b, 5,8-bis(trimethylsilylethynyl)quinoline 2a, 5,8-bis(ethynyl)quinoline 2b, 2,3-diphenyl-5,8-bis(trimethylsilylethynyl)quinoxaline 3a, 2,3-diphenyl-5,8-bis(ethynyl)quinoxaline 3b, 4,7-bis(trimethysilylethynyl)-2,1,3-benzothiadiazole 4a and 4,7-bis(ethynyl)-2,1,3-benzothiadiazole 4b, has been synthesised. Treatment of the complex trans-[Pt(Ph)(Cl)(Et3P)2] with half an equivalent of the diterminal alkynes 1b–4b in iPr2NH–CH2Cl2, in the presence of CuI, at room temperature, afforded the platinum(II) di-yne complexes trans-[(Et3P)2(Ph)Pt–C ≡ C–R–C ≡ C–Pt(Ph)(Et3P)2] [R = 1-(2-ethylhexyloxy)-4-methoxybenzene-2,5-diyl 1c, quninoline-5,8-diyl 2c, 2,3-diphenylquinoxaline-5,8-diyl 3c, 2,1,3-benzothiadiazole-4,7-diyl 4c] in good yields. The new acetylide-functionalised ligands and the platinum(II) s-acetylide complexes have been characterised by analytical and spectroscopic methods and X-ray single crystal structure determinations for 2c–4c. The absorption spectra of the complexes 2c–4c show substantial donor–acceptor interaction between the platinum(II) centres and the conjugated ligands. The photoluminescence spectra of 1c–3c show characteristic singlet (S1) and triplet (T1) emissions. Both the singlet and triplet emissions as well as the absorption decrease in energy with increasing electronegativity of the spacer groups along the series 1c–4c.

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Synthesis and optical characterisation of platinum (II) poly-yne polymers incorporating substituted 1,4-diethylbenzene derivatives and and investigation of the intermolecular interactions in the diethylbenzene molecular precursors

MS Khan, MRA Al-Mandhary, MK Al-Suti, TC Corcoran, Y Al-Mahrooqi, JP Attfield, N Feeder, WIF David, K Shankland, RH Friend, A Köhler, EA Marseglia, E Tedesco, CCTang, PR Raithby, JC Collings, KP Roscoe, AS Batsanov, LM Stimson, TB Marder

N. J. Chem. 27 (2003) 140-149
[DOI][PDF]

A series of 1,4-diethynylbenzene (1) derivatives, H–C ≡ C–R–C ≡ C–H with R = C6H3NH2 (2), C6H3F (3), C6H2F2-2,5 (4), C6F4 (5), C6H2(OCH3)2-2,5 (6) and C6H2(OnC8H17)2-2,5 (7) has been synthesised and their crystal structures determined by single crystal (2–5) or powder (6, 7) X-ray diffraction. The C ≡ CH · · · pC ≡ C hydrogen bonds dominating structure 1 are gradually replaced by C ≡ C–H · · · F ones with the increase of fluorination (3 → 5), or completely replaced by C ≡ CH · · · N and NH · · · pC ≡ C bonds in 2, and C ≡ CH · · · O in 6 and 7. The related platinum-based polymers, trans-[–Pt(PnBu3)2–C ≡ C–R–C ≡ C–]n (R = as above and C6H4,) have been prepared and characterised by spectroscopic methods and thermogravimetry, which show that the amino- and methoxy-derivatives have lowest thermal stability while the fluorinated ones exhibit increasing thermal stability with increasing fluorination. Optical spectroscopic measurements reveal that substituents on the aromatic spacer group do not create strong donor–acceptor interactions along the rigid backbone of the organometallic polymers.

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Synthesis, characterisation and optical spectroscopy of diynes and poly-ynes containing derivatised fluorenes in the backbon

MS Khan, MRA Al-Mandhary, MK Al-Suti, B Ahrens, MF Mahon, L Male, PR Raithby, CE Boothby, A Köhler

J. Chem. Soc., Dalton Trans., (2003) 74-84.
[DOI][PDF]

A series of trimethylsilyl-protected and terminal mono- and bis-alkynes based on 9,9-dioctylfluorene, 2-(trimethylsilylethynyl)-9,9-dioctylfluorene 1a, 2-ethynyl-9,9-dioctylfluorene 1b, 2,7-bis(trimethylsilylethynyl)-9,9-dioctylfluorene 2a, 2,7-bis(ethynyl)-9,9-dioctylfluorene 2b, have been synthesised. Reaction of trans-[(PnBu3)2PtCl2] with 2 equivalents of the terminal ethyne 1b yields the mononuclear platinum(II) diyne 3, reaction of trans-[(Ph)(Et3P)2PtCl] with 0.5 equivalents of the diterminal ethyne 2b gives the dinuclear platinum(II) diyne 4 while 1 : 1 reaction between trans-[(PnBu3)2PtCl2] and 2b gives the platinum(II) poly-yne 5. Treatment of 2,5-dioctyloxy-1,4-diiodobenzene with 1b in 1 : 2 stoichiometry produces the organic di-yne 6 while 1 : 1 reaction between 2,5-dioctyloxy-1,4-diiodobenzene and 2b, 2,7-bis(ethynyl)fluorene or 2,7-bis(ethynyl)fluoren-9-one produces the organic co-poly-ynes 7–9. All the new materials have been characterised by analytical and spectroscopic methods and the single crystal X-ray structures of 2a and 3 have been determined. The diynes and poly-ynes are soluble in organic solvents and are readily cast into thin films. Optical spectroscopic measurements reveal that the attachment of octyl side-chains on the fluorenyl spacer reduces inter-chain interaction in the poly-ynes while a fluorenonyl spacer creates a donor–acceptor interaction along the rigid backbone of the organometallic poly-ynes and organic co-poly-ynes.

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Fluorescence and phosphorescence in organic materials – (Review article)

A Köhler, JS Wilson, RH Friend

Adv. Mater. 14 (2002) 701-707 and Advanced Engineering Materials 4 (2002) 453-459.
[DOI][PDF]

Progress reports are a new type of article in Advanced Engineering Materials, dealing with the hottest current topics, and providing readers with a critically selected overview of important progress in these fields. It is not intended that the articles be comprehensive, but rather insightful, selective, critical, opinionated, and even visionary. We have approached scientists we believe are at the very forefront of these fields to contribute the articles, which will appear on an annual basis. The article below describes the latest advances in fluorescence and phosphorescence in organic materials.

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The singlet-triplet energy gap in organic and Pt-containing phenylene ethynylene polymers and monomers

A Köhler, JS Wilson, RH Friend, MK Al-Suti, MS Khan, A Gerhard, H Bässler

J. Chem. Phys. 116 (2002) 9457-9463.
[DOI][PDF]

We have studied the evolution of the T1 triplet excited state in an extensive series of phenylene ethynylene polymers and monomers with platinum atoms in the polymer backbone and in an analogous series of all-organic polymers with the platinum(II) tributylphosphonium complex replaced by phenylene. The inclusion of platinum increases spin–orbit coupling so T1 state emission (phosphorescence) is easier to detect. For both, the platinum-containing polymer series and for the all-organic polymers, we find the T1 state to be at a constant separation of 0.7±0.1 eV below the singlet S1 state. It is not possible to change this singlet–triplet splitting by altering the size or the charge-transfer character of the polymer repeat unit or by changing the electron delocalization along the polymer backbone. The S1–T1 gap can be increased by confining the S1 state in oligomers and monomers.

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Synthesis, characterisation and electronic properties of a series of platinum(II) poly-ynes containing novel thienyl-pyridine linker groups

MS Khan, MRA Al-Mandhary, M Al-Suti, N Feeder, S Nahar, A Köhler, RH Friend, PJ Wilson, PR Raithby

J. Chem. Soc., Dalton Trans. (2002) 2441-2448.
[DOI][PDF]

The synthesis and characterisation of a series of novel poly-yne polymers, trans-[–(PnBu3)2Pt(C ≡ C–L–C ≡ C)-]n (L = 2-(2′-thienyl)-pyridine (L1), 2,5-bis(2′-thienyl)pyridine (L2), 6,6′-bis(2?-thienyl)-3,3′-bipyridine (L3)), and the corresponding platinum dimers trans-[(PEt3)2(Ph)Pt(C ≡ CLC ≡ C)Pt(Ph)(PEt3)2] are described. The dimers were prepared by the reaction of two equivalents of trans-[Pt(PEt3)2(Ph)Cl] with one equivalent of the diterminal diyne HC ≡ C–L–C ≡ CH, in CuI catalysed dehydrohalogenation reactions in CH2Cl2–Pri2NH, while the polymers are obtained similarly from the diyne and one equivalent of trans-[Pt(PnBu3)2Cl2]. The optical absorption and photoluminescence spectra of the polymers with L1 and L2 linker groups are reported. The optical gaps are 465 nm (2.67 eV) and 486 nm (2.55 eV), respectively. The photoluminescence spectrum for each polymer exhibits one band at room temperature. At 10 K, low-energy emission bands emerge which are attributed to emission from the triplet excited state. The results of these studies are compared with those obtained for other platinum-containing poly-yne and related organometallic polymers.

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Structural characterisation of a series of acetylide-functionalised oligopyridines and the synthesis, characterisation and optical spectroscopy of platinum di-yne and poly-ynes containing oligopyridyl linker groups in the backbone

MS Khan, MRA Al-Mandhary, MK Al-Suti, AK Hisahm, PR Raithby, B Ahrens, MF Mahon, L Male, EA Marseglia, E Tedesco, RH Friend, A Köhler, N Feeder, SJ Teat

J. Chem. Soc., Dalton Trans. (2002) 1358-1368.
[DOI][PDF]

A series of trimethylsilyl-protected bis(ethynyl)oligopyridine derivatives Me3SiC ≡ C–R–C ≡ C–SiMe3 (R = 2,2′-bipyridine-5,5′-diyl (1a), 2,2′-bipyridine-6,6′-diyl (2a), 2,2′:6′,2?-terpyridine-6,6?-diyl (3a), 4′-phenyl-2,2′:6′,2?-terpyridine-6,6?-diyl (4a)) has been synthesised and 2a–4a have been characterised by single crystal X-ray crystallography. The corresponding terminal di-ynes H–C ≡ C–R–C ≡ CH (1b–4b) and their dinuclear platinum(II) complexes trans-[(Et3P)2(Ph)Pt–C ≡ C–R–C ≡ C–Pt(Ph)(Et3P)2] (1M–4M) have been characterised spectroscopically and by single-crystal X-ray crystallography for 2M. Novel platinum(II) poly-yne polymers trans-[Pt(PBun3)2–C ≡ C–R–C ≡ C–]n (1P–4P) containing the oligopyridyl linker groups in the backbone have been synthesised by the CuI-catalysed dehydrohalogenation polycondensation reaction of 1b–4b and trans-[(Bu3P)2PtCl2] in Pri2NH–CH2Cl2. The polymeric materials exhibit decreasing thermal stability with increasing number of pyridine units in the linker group. In the absorption and phosphorescence spectra, the platinum(II) poly-yne and di-yne complexes 1P, 1M show red shifts whereas the complexes 2P–3P, 2M–3M show blue shifts of the S1 and T1 states. At room temperature, the phosphoresence spectra indicate some excimer formation whereas at 10 K, only intra-chain emission occurs. The results of the photophysical studies are compared with those obtained for other platinum(II)-containing poly-ynes and related organometallic polymers.

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Spin-dependent exciton formation in pi-conjugated compounds

JS Wilson, AS Dhoot, AJAB Seeley, MS Khan, A Köhler, RH Friend

Nature 413 (2001) 828-831.
[DOI][PDF]

The efficiency of light-emitting diodes (LEDs) made from organic semiconductors is determined by the fraction of injected electrons and holes that recombine to form emissive spin-singlet states rather than non-emissive spin-triplet states. If the process by which these states form is spin-independent, the maximum efficiency of organic LEDs will be limited to 25 per cent1. But recent reports have indicated fractions of emissive singlet states ranging from 22 to 63 per cent2±5, and the reason for this variation remains unclear. Here we determine the absolute fraction of singlet states generated in a platinum-containing conjugated polymer and its corresponding monomer. The spin-orbit coupling introduced by the platinum atom allows triplet-state emission, so optically and electrically generated luminescence from both singlet and triplet states can be compared directly.We find an average singlet generation fraction of 22 6 1 per cent for the monomer, but 57 6 4 per cent for the polymer. This suggests that recombination is spin-independent for the monomer, but that a spin-dependent process, favouring singlet formation, is effective in the polymer. We suggest that this process is a consequence of the exchange interaction, which will operate on overlapping electron and hole wavefunctions on the same polymer chain at their capture radius.

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The energy gap law for triplet states in Pt-containing conjugated polymers and monomers

JS Wilson, N Chawdhury, MRA Al-Mandhary, M Younus, MS Khan, PR Raithby, A Köhler, RH Friend

J. Am. Chem. Soc. 123 (2001) 9412-9417.
[DOI][PDF]

The energy gap law established for aromatic hydrocarbons and rare earth ions relates the nonradiative decay rate to the energy gap of a transition through a multiphonon emission process. We show that this energy gap law can be applied to the phosphoresce of a series of conjugated polymers and monomers for which the radiative decay rate has been enhanced through incorporation of a heavy metal. We find that the nonradiative decay rate from the triplet state T1increases exponentially with decreasing T1-S0 gap for the polymers and monomers at 300 and 20 K. Comparison of the nonradiative decay of polymers with that of their corresponding monomers highlights the role of electron-lattice coupling.

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Triplet states in a series of Pt containing ethynylenes

JS Wilson, A Köhler, RH Friend, MK Al-Suti, MRA Al-Mandhary, MS Khan, PR Raithby

J. Chem. Phys. 113 (2000) 7627-7634.
[DOI][PDF]

By use of optical steady state and time resolved spectroscopy, we studied the evolution of the triplet excited state in a series of six ethynylenic polymers of the structure [-Pt(PBu3n)2 -C ≡ C-R-C ≡ C-]n where the spacer unit R is systematically varied to give optical gaps from 1.7–3.0 eV. The inclusion of platinum in the polymer backbone induces a strong spin-orbit coupling such that triplet state emission (phosphorescence) associated with the conjugated system can be detected. Throughout the series we find the S1-T1 (singlet-triplet) energy splitting to be independent of the spacer R, such that the T1 state is always 0.7±0.1?eV below the S1 state. With decreasing optical gap, the intensity and lifetime of the triplet state emission were seen to reduce in accordance with the energy gap law.

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Evolution of lowest singlet and triplet excited states with number of thienyl rings in platinum poly-ynes.

N Chawdhury, A Köhler, RH Friend, W-Y Wong, M Younus, PR Raithby, J Lewis, MRA Al-Mandhury, YC Corcoran, MS Khan

J. Chem. Phys. 110 (1999) 4963-4970.
[DOI][PDF]

Soluble, rigid-rod organometallic polymers trans-[-Pt(PBu3n)2–C ≡ C–R–C ≡ C–]8 (R ≡ bithienyl 2, terthienyl 3) have been synthesized in good yields by the CuI-catalyzed dehydrohalogenation reaction of trans-[-Pt(PBu3n)2Cl2] with one equivalent of the diterminal alkynyl oligothiophenes H–C ≡ C–R–C ≡ C–H in CH2Cl2/iPr2NH at room temperature. We report the thermal properties, and the optical absorption, photoluminescence, and photocurrent action spectra of 1 (trans-[–Pt(PBu3n)2–C ≡ C–R–C ≡ C–]8, R ≡ thienyl), 2 and 3 as a function of the number of thiophene rings within the bridging ligand. With increasing thiophene content, the optical gap is reduced and the vibronic structure of the singlet emission changes toward that typical for oligothiophenes. We also find the intersystem crossing from the singlet excited state to the triplet excited state to become reduced, while the singlet–triplet energy gap remains unaltered. The latter implies that, in these systems, the T1 triplet excited state is extended over several thiophene rings. The photoconducting properties do not depend on the size of the thiophene fragment. We discuss and compare our results with studies on oligothiophenes and related organometallic polymers.

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The effects of H2O and O2 on the photocurrent spectra of MEH-PPV

N Chauwdhury, A Köhler, MG Harrison, DH Hwang, AB Holmes, RH Friend

Synth. Met. 102 (1999) 871-872.
[DOI][PDF]

Photocurrent (PC) and current-voltage measurements were taken on MEH-PPV [poly(2-methoxy-5-(2′-ethyl-hexyloxy)-paraphenylene vinylene] in an as-made photocell, ITO / MEH-PPV (265 nm) / Al, in the photocell under ambient air, under a mixture of oxygen + nitrogen, and in vacuum after annealing. We discuss the different spectral responses and current-voltage characteristics obtained for the different atmospheres.

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Donor-acceptor interactions in organometallic and organic poly-ynes

A Köhler, M Younus, MRA Al-Mandhary, PR Raithby, MS Khan, RH Friend

Synth. Met. 101 (1999) 246-247.
[DOI][PDF]

We compare the spectroscopic properties of donor-acceptor type poly-ynes of the type [D-C=C-A-C=C-]n with a fluorine-substituted phenylene or a thieno-pyrazine unit as acceptor, and platinum or alkoxy-substituted phenylene as donor.

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Charge separation in localised and delocalised electronic states in polymeric semiconductor

A Köhler, DAd Santos, D Beljonne, Z Shuai, J-L Brédas, RH Friend, AB Holmes, A Kraus, K Müllen

Nature 392 (1998) 903-906.
[DOI][PDF]

Conjugated polymers such as poly(p-phenylene vinylene)s (PPVs) allow low-cost fabrication of thin semiconducting films by solution processing onto substrates. Several polymeric optoelectronic devices have been developed in recent years, including field-effect transistors, light-emitting diodes, photocells and lasers. It is still not clear, however, whether the description of electronic excitations in these materials is most appropriately formulated within a molecular or semiconductor (band-theory) picture. In the former case, excited states are localized and are described as excitons; in the latter they are delocalized and described as free electron–hole pairs. Here we report studies of the electronic states associated with optical excitations in the visible and ultraviolet range for the conjugated polymer poly(2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene) (MEH-PPV), by means of photocurrent measurements and quantum-chemical calculations. We find several photocurrent spectral features between 3 and 5 eV which are coupled with bands in the absorption spectrum. On modelling the excited states in this energy range, we have discovered an important feature that is likely to be general for materials composed of coupled molecular units: that mixing of delocalized conduction- and valence-band states with states localized on the molecular units produces a sequence of excited states in which positive and negative charges can be separated further at higher energies. In other words, these excited states facilitate charge separation, and provide a conceptual bridge between the molecular (localized) and semiconductor (delocalized) pictures.

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Synthesis, electrochemistry and spectroscopy of blue platinum(II) poly-ynes and di-ynes

M Younus, A Köhler, S Cron, N Chawdhury, MRA Al-Mandhary, MS Khan, J Lewis, NJ Long, RH Friend, PR Raithby

Angew. Chemie, Internat. Ed. 37 (1998) 3036-3039.
[DOI][PDF]

The smallest band gap observed so far (1.77 eV) for an organometallic polymer is exhibited by the blue, rigid-rod polymer 2, which is prepared by the reaction of trans-[PtCl2(PnBu3)2] with one equivalent of 1.

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Synthesis and electronic structure of platinum-containing poly-ynes with aromatic and heteroaromatic rings

N Chauwdhury, A Köhler, RH Friend, M Younus, NJ Long, PR Raithby, J Lewis

Macromolecules 31 (1998) 722-727.
[DOI][PDF]

We have studied the dependence of intersystem crossing and the spatial extent of singlet and triplet excitons in platinum-containing poly-yne polymers with the general formula [Pt(PR3)2C ≡ CLC ≡ C]n (R = Et, nBu; L = pyridine, phenylene, or thiophene) as a function of electron delocalization in the spacer group L. We also report the synthesis route of those compounds. The optical absorption, photoluminescence, and photoinduced absorption of the corresponding polymers and monomers have been measured. We find that conjugation is increased but intersystem crossing is reduced by the electron-rich thiophene unit, while the opposite occurs for the electron-deficient pyridine unit as compared to the phenylene unit. For all investigated systems, we find that the singlet excited state and a higher lying Tn triplet excited state extend over more than a repeat unit while the T1 triplet state remains localized to less then one repeat unit.

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Electronic excitations in luminescent conjugated polymers

RH Friend, GJ Denton, JJM Halls, NT Harrisson, AB Holmes, A Köhler, A Lux, SC Moratti, K Pichler, N Tessler, K Towns, HF Wittmann

Solid State Commun. 102 (1997) 249-258.
[DOI][PDF]

We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved and single-layer EL diodes achieve luminous efficiencies above 2 Lumen W-1 and peak brightnesses up to 90 000 cd m-2. We discuss measurements of photoconductivity, photovoltaic response, photoluminescence excitation spectra and stimulated emission in films of PPV. We consider that the photoexcited state in these films of PPV is the intrachain singlet exciton. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments and can be used as the active lasing medium when incorporated in suitable microcavity structures.

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UV photocurrent spectroscopy in poly(p-phenylene vinylene) and derivatives

A Köhler, DAd Santos, D Beljonne, Z Shuai, J-L Brédas, RH Friend, SC Moratti, AB Holmes

Synth. Met. 84 (1997) 675-676.
[DOI][PDF]

We present photocurrent measurements of poly(p-phenylene vinylene) (PPV) and derivatives for photon energies up to 5.0 eV. We observe photocurrent peaks related to the first, second and third absorption peaks, which we associate with the dissociation of a succession of increasingly energetic excitons. We have performed INDO/SCI calculations for the five-ring PPV oligomer, which show different distributions of the electronic charge of the exciton wavefunction over the chain at different excitation energies. We discuss the significance of these states for the photoconduction in PPV and question the applicability of a band-based model.

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Electronic processes of conjugated polymers in semicondutor device structures

RH Friend, GJ Denton, JJM Halls, NT Harrisson, AB Holmes, A Köhler, A Lux, SC Moratti, K Pichler, N Tessler, K Towns, HF Wittmann

Synth. Met. 84 (1997) 463-470.
[DOI][PDF]

We report progress in the processing and application of poly(phenylene vinylene), PPV, as the emissive layer in electroluminescent diodes, LEDs. Photoluminescence efficiencies above 60% for solid films of PPV are now achieved, and single-layer EL diodes achieve luminous efficiencies above 2 Lumens/W and peak brightnesses up to 90,000 cd/m2. We demonstrate that PPV of this type can show stimulated emission in sub-picosecond pump-probe experiments, and can be used as the active lasing medium when incorporated in suitable microcavity structures.

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Spatial extent of the singlet and triplet excitons in transition metal-containing poly-ynes

D Beljonne, HF Wittmann, A Köhler, S Graham, M Younus, J Lewis, PR Raithby, MS Khan, RH Friend, JL Brédas

J. Chem. Phys. 105 (1996) 3868-3877.
[DOI][PDF]

We present a joint experimental and theoretical investigation of the electronic excitations in transition metal-containing phenylene ethynylenes. The influence of the metal on the nature of the lowest singlet and triplet excited states is characterized. We find that p conjugation occurs through the metal sites, which deeply modifies the optical properties of the conjugated chains. We also analyze the chain-length dependence of the singlet–singlet, S0?S1, singlet–triplet, S0?T1, and triplet–triplet, T1?Tn, transition energies; both experimental data and theoretical results indicate that the lowest triplet exciton, T1, is strongly localized on a single phenylene ring while the S1 and Tn states extend over a few repeating units. Finally, we estimate the geometric relaxation phenomena occurring in the lowest excited states and perform a Huang–Rhys analysis of the triplet emission spectrum in model systems.

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Enhanced photocurrent response in photocells made with platinum-poly-yne/C60 blends by photoinduced electron-transfer

A Köhler, HF Wittmann, RH Friend, MS Khan, J Lewis

Synth. Met. 77 (1996) 147-150.
[DOI][PDF]

We report photocurrent and photoluminescence measurements on blends of the organometallic polymer Pt-poly-yne and C60. Due to efficient intersystem crossing, triplet excitons with a binding energy of at least 0.8 eV are created after photoexcitation of Pt-poly-yne. We find that electrons are transferred from Pt-poly-yne onto C60 by efficient ionization of the triplet exciton. This process gives rise to a photocurrent quantum yield of up to 1–2% in photocells fabricated in a layer structure of indium-tin oxide (ITO)/Pt-poly-yne and 7 wt.% C60/aluminium. Evidence for the ionization of the strongly bound triplet excitons is provided by studies of luminescence in the blends. There is only partial quenching of the singlet emission band, but complete quenching of the triplet emission band, and the lifetime of the triplet emission at room temperature is reduced from 2 to 0.1 µs upon addition of 7 wt.% C60.

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Synthesis of new conjugated thiophene polymers

R Cervini, AB Holmes, SC Moratti, A Köhler, RH Friend

Synth. Met. 76 (1996) 169-171.
[DOI][PDF]

Results are presented which explore the synthesis, characterization and photoluminescence of new types of conjugated copolymers which incorporate fused thiophene units. Such materials possess intense purple-red colours and some absorb in the near-infrared. The redox processes showed that the polymers were both p- and n-dopable.

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Photocurrent measurements on aggregates in ladder-type poly(p- phenylene)

A Köhler, J Gruener, RH Friend, K Müllen, U Scherf

Chem. Phys. Lett. 243 (1995) 456-461.
[DOI][PDF]

We report measurements of the photovoltaic effect on single layer devices made with a ladder-type poly-(paraphenylene) (LPPP). Besides the photocurrent in the blue spectral range associated with p to p* intra-chain absorption we observe an additional response in the yellow part of the spectrum centered at 2.25 eV. We also observe a weak absorption feature at the same energy, and we consider that both arise from absorption into p electron states formed over several aggregated polymer chains. The photocurrent due to excitation of these states is enhanced by two orders of magnitude compared to blue excitation and vanishes under reverse bias (i.e. ITO negative). We find that the photocurrent response due to the aggregate states is more sensitive to oxygen exposure than the response due to intra-chain absorption.

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The photovoltaic effect in a platinum poly-yne

A Köhler, HF Wittmann, RH Friend, MS Khan, J Lewis

Synth. Met. 67 (1994) 245-249
[DOI]

We have carried out measurements of the photovoltaic effect in a platinum-containing poly-yne. A photocell was fabricated with the platinum poly-yne sandwiched between indium-tin oxide (ITO) and aluminium electrodes. A strong photoresponse was seen at the threshold of the singlet exciton absorption near 3.2 eV, with a quantum yield for carrier generation of up to 0.6%. This is comparable to that found in similar devices made with poly(phenylenevinylene) (PPV). It is known that the presence of the platinum atom gives very efficient crossover from singlet to triplet exciton, and we discuss here the effect of triplet excitons on the photogeneration of charge carriers. We also analyse the spectral response of the photocurrent in terms of charge carrier ranges.

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