Molecular Origins of Photoinduced Backward Intramolecular\nCharge Transfer

Abstract

Intramolecular\ncharge transfer (ICT) is a fundamental process in\nphotochemistry, during which charge flows from a donor (D) to an acceptor\n(A). In (probably) most dyes, ICT is enhanced upon photoexcitation\n(namely, forward ICT or FICT), endowing these compounds with positive\nsolvatochromism. Yet, many dyes display negative solvatochromism,\nsuggesting backward ICT (BICT). That is, a significant charge separation\nfrom D to A is achieved in the ground state, but charge transports\nback from A to D upon photoexcitation, leading to a reduced dipole\nmoment in the excited state. In contrast to the well-studied FICT\nsystems, molecular design guidelines to realize BICT remain elusive,\nprecluding potential applications. In this paper, we showed that BICT\ncould be achieved by incorporating double donors and double acceptors\nwith a quinoid π-bridge. The establishment of the BICT design\nrules not only deepens our understanding of charge transfer but also\ninspires future applications (i.e., nonlinear optics).