Conjugated dual hydrogen-bond mediating proton-transfer reaction in 3-hydroxyisoquinoline

Proton-transfer tautomerism mediated by the conjugated dual hydrogen-bonding (CDHB) effect in the ground state as well as in the excited state has been studied in 3-hydroxyisoquinoline (3HIQ). In cyclohexane, upon increasing concentration or adding guest molecules possessing the bifunctional hydrogen-bonding property, spectral and dynamic analyses indicate the existence of equilibria between various proton-transfer tautomers, including enol monomers, enol dimers (or 1:1 enol/guest complex), and keto/enol complexes (or 1:1 keto/guest complexes). The equilibrium constants among each species have been determined, which can be qualitatively rationalized through an ab inito molecular orbital calculation. The results conclude that the CDHB formation and its strength play a key role to fine-tune the ground-state equilibria toward the keto form. Upon excitation the enol CDHB complexes undergo a rapid proton-transfer reaction (k(pt) >> 5 x 10(9) s(-1)), resulting in a unique keto emission. Surprisingly, however, drastically different excited-state relaxation dynamics between the keto dimer and keto/enol complex were observed. The result was tentatively rationalized by a deactivation mechanism induced by vibrations of low-frequency hydrogen-bonding modes associated with the CDHB strength.