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7497810 
Journal Article 
Donor-Acceptor Fluorophores for Energy-Transfer-Mediated Photocatalysis 
Lu, J; Pattengale, B; Liu, Q; Yang, S; Shi, W; Li, S; Huang, J; Zhang, J 
2018 
Yes 
Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 
140 
42 
13719-13725 
English 
30277771 
WOS:000448755200035 
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 3LE (locally excited triplet state) and 3CT (charge-transfer triplet state) via a combined photochemical and transient absorption spectroscopic study. We found that the energy order between 1CT (charge transfer singlet state) and 3LE dictates the accessibility of 3LE/3CT for EnT, which can be effectively engineered by varying solvent polarity and D-A character to depopulate 3LE and facilitate EnT from the chemically more tunable 3CT 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.