Donor-Acceptor Fluorophores for Energy-Transfer-Mediated Photocatalysis.

Triplet-triplet energy transfer (EnT) is a fundamental activation pathway in photocatalysis. In this work, we report the mechanistic origins of the triplet excited state of carbazole-cyanobenzene donor-acceptor (D-A) fluorophores in EnT-based photocatalytic reactions and demonstrate the key factors that control the accessibility of the LE (locally excited triplet state) and CT (charge-transfer triplet state) via a combined photochemical and transient absorption spectroscopic study. We found that the energy order between CT (charge transfer singlet state) and LE dictates the accessibility of LE/CT for EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate LE and facilitate EnT from the chemically more tunable CT state for photosensitization. Following the above design principle, a new D-A fluorophore with strong D-A character and weak redox potential is identified, which exhibits high efficiency for Ni(II)-catalyzed cross-coupling of carboxylic acids and aryl halides with a wide substrate scope and high selectivity. Our results not only provide key fundamental insight on the EnT mechanism of D-A fluorophores but also establish its wide utility in EnT-mediated photocatalytic reactions.