Cross-Linked Functionalized Poly(3-hexylthiophene) Nanofibers with Tunable Excitonic Coupling

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Abstract
We show that mechanically and chemically robust functionalized poly(3-hexylthiophene) (P3HT) nanofibers can be made via chemical cross-linking. Dramatically different photophysical properties are observed depending on the choice of functionalizing moiety and cross-linking strategy. Starting with two different nanofiber families formed from (a) P3HT-b-P3MT or (b) P3HT-b-P3ST diblock copolymers, cross-linking to form robust nanowire structures was readily achieved by either a third-party cross-linking agent (hexamethylene diisocyanate, HDI) which links methoxy side chains on the P3MT system, or direct disulfide cross-link for the P3ST system. Although the nanofiber families have similar gross structure (and almost identical pre-cross-linked absorption spectra), they have completely different photophysics as signaled by ensemble and single nanofiber wavelength- and time-resolved photoluminescence as well as transient absorption (visible and near-IR) probes. For the P3ST diblock nanofibers, excitonic coupling appears to be essentially unchanged before and after cross-linking. In contrast, cross-linked P3MT nanofibers show photoluminescence similar in electronic origin, vibronic structure, and lifetime to unaggregated P3HT molecules, e.g., dissolved in an inert polymer matrix, suggesting almost complete extinction of excitonic coupling. We hypothesize that the different photophysical properties can be understood from structural perturbations resulting from the cross-linking: For the P3MT system, the DIC linker induces a high degree of strain on the P3HT aggregate block, thus disrupting both intra- and interchain coupling. For the P3ST system, the spatial extent of the cross-linking is approximately commensurate with the interlamellar spacing, resulting in a minimally perturbed aggregate structure.

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