Xiang Li, Matthew Bird, Gina Mauro, Sadayuki Asaoka, Andrew R. Cook, Hung-Cheng Chen, and John R. Miller
J. Phys. Chem. B 119, ASAP (2015).
[Find paper at ACS Publications]
Abstract:
Triplet excitons created in poly-2,7-(9,9-dihexyl)fluorene (pF) chains with end trap groups in solution are efficiently transported to and captured by the end groups. The triplets explore the entire lengths of the chains, even for ~100 nm long chains, enabling determination of the completeness of end-capping. The results show that the chains are continuous: they may contain transient barriers or traps, such as those from fluctuations of dihedral angles, but they are free of major defects that stop motion of the triplets. Quantitative determinations are aided by the addition of a strong electron donor, TMPD, which removes absorption bands of the end-trapped triplets. For chains having at least one end trap, triplet capture is quantitative on the 1 µs time scale imposed by the use of the donor. Fractions of chains having no end traps were 0.15 for pF samples with anthraquinone (AQ) end traps and 0.063 with naphthylimide (NI) end traps. These determinations agreed with measurements by NMR for short (<40 polymer repeat units (pru)) chains, where nmr determinations are accurate. the results find no evidence for traps or barriers to the transport of triplets, and places limits on the possible presence of defects as impenetrable barriers to less than one per 300 pru. the present results present a paradigm different from the current consensus, derived from observations of singlet excitons, that conjugated chains are divided into "segments," perhaps by some kind of defects. for the present pf chains, the segmentation either does not apply to triplet excitons or is transient so that the defects are healed or surmounted in times much shorter than 1 µs. triplets on chains without end trap groups transfer to chains with end traps on a slower time scale. rate constants for these bimolecular triplet transfer reactions were found to increase with the length of the accepting chain, as did rate constants for triplet transfer to the chains from small molecules like biphenyl. A second set of polyfluorenes with 2-butyloctyl side chains was found to have a much lower completeness of end-capping.