Fine-tuning the Microstructure and Photophysical Characteristics of Fluorescent Conjugated Copolymers Using Photoalignment and Liquid-crystalline Ordering

Abstract

Replicating the microstructure and near-unity excitation energy transfer efficiency in natural light-harvesting complexes (LHCs) remains a major challenge for synthetic energy-harvesting devices. Biological photosynthesis can spontaneously regulate the active ensembles of involved energy absorbing and funnelling chlorophyll-containing proteins in response to fluctuating sunlight. Here we utilize liquid crystalline (LC) ordering to fine-tune the polymer packing and photophysical properties in liquid crystalline conjugated polymer (LCCP) films for LHC biomimicry and optimizing photoluminescence quantum efficiency (PLQE). We show that the long-range orientational ordering present in a LC phase of poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) stabilizes a small fraction of randomly-oriented F8BT nanocrystals dispersed in an amorphous matrix of disordered F8BT chains, hence resembling a self-doped host-guest system whereby excitation energy funnelling and PLQE are reinforced significantly by three-dimensional donor-to-acceptor Forster resonance energy transfer (FRET) and dominant intrachain emission in the nano-crystalline acceptor. Furthermore, the photoalignment of nematic F8BT layers is combined to fabricate long-sought large-area-extended monodomains which exhibit >60% crystallinity and ~20 nm-long interchain packing order, whilst also promoting linearly polarized emission, a new band-edge absorption species, and an extra emissive interchain excited state. Our micro-PL spectral results support the feasibility of making use of self-doped F8BT nematic films for bio-mimicry of certain structural basis and light-harvesting properties of naturally occurring LHCs.