Solvatochrony: solvatochronic photocatalysts and their applications in intermolecular [2+2] cycloaddition reactions of unactivated styrenes

Abstract

Solvent effects in chemical reactions are typically associated with energetic control. In photocatalysis, however, the ability of solvents to influence excited-state lifetimes offers a promising avenue for improving reaction efficiency. Herein, we report a solvatochronic effect whereby the excited-state lifetime of a photocatalyst can be extended by use of an appropriate solvent. Specifically, a linear, heteroleptic Au(i) complex bearing carbazolide and N-heterocyclocarbene ligands exhibits an excited-state lifetime of 12 mu s in the Lewis basic solvent DMSO, but only 0.30 mu s in the non-Lewis basic solvent CH2Cl2. This lifetime prolongation arises from a combined effect of accelerated intersystem crossing to the ligand-localized triplet state and suppressed nonradiative decay, driven by solvent basicity and viscosity, respectively. The long-lived Au excited state facilitates Dexter-type energy transfer to unactivated styrenes, initiating intermolecular [2 + 2] cycloaddition reactions. These findings reveal a new mode of solvent control-temporal modulation-and present a general, catalyst-structure-independent strategy for enhancing photocatalytic reactivity.