| Summary: | 博士 === 國立中正大學 === 化學研究所 === 87 === Proton-transfer tautomerism mediated by the conjugated dual hydrogen-bonding (CDHB) effect in the ground 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 monomer, enol dimer (or 1:1 enol/guest complex) and keto/enol complex (or 1:1 keto/guest complex). 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 (kpt >> 5 x 109 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.
The enol-keto proton-transfer tautomerism of 7-hydroxyquinoline (7HQ) mediated by hydrogen-bonding formation has been studied in the ground as well as in the excited state. Thermodynamics of self-association and hydrogen-bonded complexes of 7HQ in various nonpolar solvents were obtained by means of absorption, emission and theoretical approaches. Specific hydrogen-bonding sites in the complex were determined by applying various derivatives of 7HQ incorporated with guest molecules possessing only either a proton donating or accepting site. The result can be qualitatively rationalized by a correlation of the hydrogen-bonding strength with respect to the donor’s acidity and/or acceptor’s basicity. In benzene, the 7HQ cyclic dimer undergoes a fast excited-state double proton transfer reaction, resulting in a unique keto-tautomer emission. Surprisingly, however, the 1:1 7HQ(enol)/acetic acid complex possessing only a single hydrogen bond undergoes an excited-state double proton-transfer reaction, forming a keto/acid complex. As a result, a proton-transfer mechanism incorporating the rotational diffusion dynamics of guest molecules, i.e. acetic acid, is proposed.
The proton-transfer tautomerism of 4-Azabenzaimidazole (4ABI) mediated by hydrogen bonding formation has been studied in the ground as well as in the excited state by means of absorption and emission spectroscopies. Thermodynamics of self-association and hydrogen-bonded complexes in nonpolar solvents were obtained. Proton-transfer isomers of 4ABI have been determined by syntheses and spectral characterization of various 4ABI methyl derivatives. The 4ABI dimer and 1:1 4ABI/acetic acid complex possessing cyclic dual hydrogen bonds undergoes a fast excited-state double proton transfer reaction, resulting in a unique proton-transfer tautomer emission. Surprisingly, however, the ESDPT is prohibited in the 4ABI/2-azacyclohexanone cyclic hydrogen-bonded complex. The results in combination with an ab initio approach lead us to propose a proton-transfer tautomerism mediated by the relative excited-state thermodynamics.
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