Strategies to Reduce Rate of Charge Recombination
Modulating excitons generated as a result of photoinduced electron transfer in crowded environments is vital for the development of photo-functional materials.1 The hetero-junctions (HJs) in organic photovoltaics are termed as “transport highways” for the charge carriers to the respective electrodes.2Careful design and organization of molecular architectures at the HJs in organic solar cells dictates the fate of excitons generated. Molecular organization relies on interplay between various inter/intra molecular interactions such as multi-pole electrostatic interactions, dispersion and inductive effects, p-p interactions, hydrogen bonding etc. which determines electronic and optical properties associated with these materials. Myriads of models have been proposed in enhancing the survival times of the excitons generated at the HJs. Mullen and co-workers3 substantiated that compromise and dominance of various inter and intra molecular interactions operating in donor (D) - acceptor (A) self-assembled systems could generate segregated D-D/A-A stacks, D-A interdigitating alternate stacks etc. Aida and co-workers4 demonstrated the photochemical generation of spatially separated charge carriers through co-axial nanotubular arrangement of D and A. Wasielewski et al.5extended the survival time of charge separated states through self-assembled D-A tetramers, trefoils, dimers and hydrogen bonded foldamers.Recent report from our group6 demonstrated the importance of supramolecular vesicular scaffold in reducing the rate of charge recombination of the charge separated states.7Recently we are successful in synthesizing near-orthogonal D-A helical and columnar stacks wherein the latter undergo self-assembly in CHCl3 to form spherical aggregates which couldhelp in sustaining the charge transfer intermediates for longer timescales through D-A stacks. The following scheme represents the different models of D-A self-assembled systems reported and under investigation.
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