Role of ground state structure in photoinduced tautomerization in bifunctional proton donor-acceptor molecules: 1H-pyrrolo[3,2-h]quinoline and related compounds

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 1 H-pyrrolo[3,2-h]quinoline, 7,8,9,10-tetrahydropyrido[2,3-a]carbazole, pyrid o[2,3-a]carbazole, dipyrido[2,3-a:3',2'-i]carbazole, and 2-(2'pyridyl)indol es. In alcohol solutions, all these compounds reveal the phenomenon of exci ted state intermolecular double proton transfer, occurring in complexes of the excited chromophore with an alcohol molecule. This process was identifi ed by comparing the fluorescence of the phototautomeric products with the e mission of molecules synthesized to serve as chemical models of the tautome ric structures. Detailed investigations demonstrate that the excited state reaction occurs in solvates that, already in the ground state, have an appr opriate stoichiometry and structure. These species correspond to 1:1 cyclic , doubly hydrogen bonded complexes with alcohol. Other types of complexes w ith alcohol were also found, which, upon excitation, undergo deactivation t o the ground state via internal conversion. The relative populations of the two forms of alcohol solvates, characterized by different photophysics, va ry strongly across the series. The properties of the presently investigated compounds differ from those of a structurally related 7-azaindole and 1-az acarbazole, in which the phototautomerization involves solvent relaxation a round the excited chromophore. Molecular dynamics calculations, performed t o predict and compare the ground-state structure of 7-azaindole and 1H-pyrr olo[3,2-h]quinoline alcohol complexes, allow one to rationalize the observe d differences in the excited-state reaction mechanisms for the two kinds of systems.