Role of Ground State Structure in Photoinduced Tautomerization in Bifunctional Proton Donor−Acceptor Molecules: 1H-Pyrrolo[3,2-h]quinoline and Related Compounds

Abstract

Spectral, synthetic, and theoretical studies were performed for a family of bifunctional compounds possessing both a hydrogen bond donor (aromatic NH group) and an acceptor (pyridine-type nitrogen atom). The series included 1H-pyrrolo[3,2-h]quinoline, 7,8,9,10-tetrahydropyrido[2,3-a]carbazole, pyrido[2,3-a]carbazole, dipyrido[2,3-a:3‘,2‘-i]carbazole, and 2-(2‘pyridyl)indoles. In alcohol solutions, all these compounds reveal the phenomenon of excited state intermolecular double proton transfer, occurring in complexes of the excited chromophore with an alcohol molecule. This process was identified by comparing the fluorescence of the phototautomeric products with the emission of molecules synthesized to serve as chemical models of the tautomeric structures. Detailed investigations demonstrate that the excited state reaction occurs in solvates that, already in the ground state, have an appropriate stoichiometry and structure. These species correspond to 1:1 cyclic, doubly hydrogen bonded complexes with alcoh...