Entropy as a Gate or Source for Electronic Energy Transfer in Donor-Multiple Acceptor Systems

Abstract

Charge and electronic energy transfer form the basis of many natural and artificial energy transduction systems. The energy landscapes that drive these transfer processes are often constructed from enthalpy driving force. In contrast, driving force from entropic gain, although occasionally invoked to explain some peculiar excited state dynamics, has rarely been used to actively control charge/energy flow. Here we demonstrate that entropy can either gate or power electronic energy transfer, using CsPbBr₃ colloidal nanocrystals surface-functionalized with multiple phenanthrene triplet acceptors. Triplet energy transfer from photoexcited nanocrystals to phenanthrene is followed by thermally-activated repopulation of nanocrystal excitons, leading to delayed nanocrystal emission. The lifetime of delayed emission increases with the phenanthrene/nanocrystal ratio, due to lowering of the free energy of the acceptor state by entropic gain. Moreover, entropic gain enables nominally “endothermic” sensitization of phenanthrene triplets using nanocrystals of lower exciton energy than molecular triplets. The principle of using entropy as a gate to modulate, or as a source to power, charge/energy flow is generically applicable to many other energy transduction systems containing multiple donor/acceptor units.