Molecular Origins of Photoinduced Backward Intramolecular Charge Transfer

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