Fine-Tuning the Microstructure and Photophysical Characteristics of Fluorescent Conjugated Copolymers Using Photoalignment and Liquid-Crystal Ordering.

Abstract

Replicating the microstructural basis and the near 100% excitation energy transfer efficiency in naturally occurring light-harvesting complexes (LHCs) remains challenging in synthetic energy-harvesting devices. Biological photosynthesis regulates active ensembles of light-absorbing and funneling chlorophylls in proteins in response to fluctuating sunlight. Here, use of long-range liquid crystal (LC) ordering to tailor chain orientation and packing structure in liquid crystalline conjugated polymer (LCCP) layers for bio-mimicry of certain structural basis and light-harvesting properties of LHCs is reported. It is found that long-range orientational ordering in an LC phase of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) copolymer stabilizes a small fraction of randomly-oriented F8BT nanocrystals dispersed in an amorphous matrix of F8BT chains, resembling a self-doped host-guest system whereby excitation energy funneling and photoluminescence quantum efficiencies are enhanced significantly by triggering 3D donor-to-acceptor Förster resonance energy transfer (FRET) and dominant intrachain emission in the nano-crystal acceptor. Further, photoalignment of nematic F8BT layers is combined with LC orientational ordering to fabricate large-area-extended monodomains exhibiting >60% crystallinity and ≈20nm-long interchain packing order. Remarkably, these monodomains demonstrate strong linearly polarized emission, whilst also promoting a new band-edge absorption species and an extra emissive interchain excited state as compared to the non-aligned films.