Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

The spectral diffusion of singlet and triplet excitons in 9,9-dioctylfluorene-based conjugated copolymers is investigated using photoluminescence spectroscopy at both low (5 K) and room temperature (300 K). Inclusion of a N,N-diphenyl-4-(pyridin-2-yl)aniline moiety into the polymer backbone allows subsequent cyclometalation with platinum acetylacetonate to increase the spin-orbit coupling and yield radiative decay from the triplet state. For suitably low fractions (<= 5%) of bulky ligand inclusion, cyclometalated or not, the resulting longer sequences of fluorene units are able to adopt the chain-extended beta-phase conformation. Comparison between the phosphorescence spectral diffusion in glassy- and beta-phase Pt-copolymer samples provide insight into the triplet exciton transfer in more- or less-disordered conjugated polymer films. It is found in the glassy-phase samples with shorter conjugation lengths that the triplet exciton relaxation becomes frustrated at low temperature due to a freezing out of the thermally activated hops required to move from one conjugated segment to another. In contrast, for films containing beta-phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through infra-segment motion. This work demonstrates controlled use of changes in molecular conformation to optimize triplet diffusion properties in a member of the widely deployed fluorene-based conjugated copolymers.

Automated summary

This study investigates the spectral diffusion of singlet and triplet excitons in 9,9-dioctylfluorene-based conjugated copolymers. It demonstrates that changes in molecular conformation can optimize the triplet diffusion properties.