  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.
