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Synthesis of NIR-II Fluorophores by a C(sp2)-C(sp2/sp) Coupling Reaction Driven by Charge-Transfer Interaction
Jie Li1, Xin-Xin Liu2, Ling-Ya Peng2
1Key Laboratory of Radiopharmaceuticals, Ministry of Education, College of Chemistry, Beijing Normal University, Beijing, 100875, P. R. China.
Abstract:
Here, we report a C(sp2)-C(sp2/sp) coupling reaction driven by charge transfer interaction. Due to the strong intermolecular noncovalent interaction, an essential electron donor-acceptor (EDA) complex is formed, confirmed solidly by absorption spectra, 1H NMR and single crystal X-ray diffraction. The EDA complex lowers the activation barrier (Ea=9.17 kcal/mol) and facilitates the formation of the C-C bond, which is the rate-limiting step revealed by H/D and 12C/13C KIE studies. Kinetic investigations reveal that the reaction is a second-order reaction. Furthermore, high-level theoretical calculations support the proposed mechanism. The mono- and di-substituted products were obtained by varying the reaction conditions. A series of structurally diverse and novel second near-infrared (NIR-II) fluorophores based on benzo[1,2-c : 4,5-c']bis([1,2,5]thiadiazole) (BBTD) were synthesized by utilizing this coupling reaction.
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