Excited-State Amine−Imine Double Proton Transfer in 7-Azaindoline

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Abstract

Ground-state thermodynamics and excited-state amine/imine tautomerism in 7-azaindoline (7AZD) mediated by hydrogen bond formation have been studied by means of absorption and emission spectroscopies. The association constants in cyclohexane (298 K) were determined to be 80, 2.5 × 102, and 7.8 × 102 M-1, for the formation of 7AZD dimer, 7AZD/azacyclohexanone, and 7AZD/acetic acid dual hydrogen-bonded complexes, respectively. The 7AZD/acetic acid complex undergoes a fast (≫ 3 × 109 s-1) excited-state double proton transfer (ESDPT) reaction, resulting in a prominent imine-like tautomer emission. Proton-transfer isomers of 7AZD have been identified through syntheses and spectral characterization of various 7AZD methyl derivatives. In contrast, ESDPT is prohibited in cases of 7AZD dimer and 7AZD/azacyclohexanone hydrogen-bonded complex. The results, in combination with a comparative study on 7-azaindole, generalize the amine/imine tautomerism, which can be fine-tuned by the length of π electron conjugation coupled with types of associated guest molecules, further supporting the proposed catalytic-versus-noncatalytic model for the ESDPT reaction.

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