Excited-state amine-imine double proton transfer in 7-azaindoline

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 x 10(2), and 7.8 x 10(2) 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 (much greater than 3 x 10(9) 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 pi electron conjugation coupled with types of associated guest molecules, further supporting the proposed catalytic-versus-noncatalytic model for the ESDPT reaction.