Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers |
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Authors: | Nikol T Lambeva Claudius C Mullen Xuyu Gao Qingjing Wu Robert A Taylor Youtian Tao Donal D C Bradley |
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Institution: | 1. Department of Physics, University of Oxford, Oxford, UK;2. Key Lab for Flexible Electronics and Institute of Advanced Materials, Nanjing Tech University, Nanjing, P. R. China |
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Abstract: | 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 β-phase conformation. Comparison between the phosphorescence spectral diffusion in glassy- and β-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 β-phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through intra-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. |
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Keywords: | chain conformations polyfluorenes spectral diffusion triplet state β-phase |
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