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.

中文翻译：

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用于能量转移介导的光催化的供体-受体荧光团

三重态-三重态能量转移 (EnT) 是光催化中的基本活化途径。在这项工作中，我们报告了基于 EnT 的光催化反应中咔唑 - 氰基苯供体 - 受体（DA）荧光团的三重激发态的机械起源，并证明了控制 3LE（局部激发三重态）可及性的关键因素和通过结合光化学和瞬态吸收光谱研究的 3CT（电荷转移三重态）。我们发现 1CT（电荷转移单线态）和 3LE 之间的能量顺序决定了 EnT 3LE/3CT 的可及性，这可以通过改变溶剂极性和 DA 特征来有效地设计，以减少 3LE 的数量并促进 EnT 从化学更可调的 3CT光敏状态。遵循上述设计原则，鉴定了一种具有强 DA 特性和弱氧化还原电位的新型 DA 荧光团，该荧光团对 Ni(II) 催化的羧酸和芳基卤化物的交叉偶联表现出高效率，具有广泛的底物范围和高选择性。我们的结果不仅为 DA 荧光团的 EnT 机制提供了关键的基本见解，而且还确立了其在 EnT 介导的光催化反应中的广泛用途。