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 ≈20 nm‐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.