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

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.