High intrachain order promotes triplet formation from recombination of long-lived polarons in poly(3-hexylthiophene) J-aggregate nanofibers

Alan K Thomas, José A Garcia, Jordan Ulibarri-Sanchez, Jian Gao, John K Grey
ACS Nano 2014 October 28, 8 (10): 10559-68
Photoluminescence (PL) of single poly(3-hexylthiophene) (P3HT) J-aggregate nanofibers (NFs) exhibits strong quenching under intensity-modulated pulsed excitation. Initial PL intensities (I(0)) decay to steady-state levels (ISS) typically within ∼ 1-10 μs, and large quenching depths (I(0)/I(SS) >2) are observed for ∼ 70% of these NFs. Similar studies of polymorphic, H-aggregate type P3HT NFs show much smaller PL quenching depths (I(0)/I(SS) ≤ 1.2). P3HT chains in J-type NF π-stacks possess high intrachain order, which has been shown previously to promote the formation of long-lived, delocalized polarons. We propose that these species recombine nongeminately to triplets on time scales of >1 ns. The identity of triplets as the dominant PL quenchers was confirmed by subjecting NFs to oxygen, resulting in an instantaneous loss of triplet PL quenching (I(0)/I(SS) ∼ 1). The lower intrachain order in H-type NFs, similar to P3HT thin-film aggregates, localizes excitons and polarons, leading to efficient geminate recombination that suppresses triplet formation at longer time scales. Our results demonstrate the promise of self-assembly strategies to control intrachain ordering within multichromophoric polymeric aggregate assemblies to tune exciton coupling and interconversion processes between different spin states.


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