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 alcohol. Other types of complexes with alcohol were also found, which, upon excitation, undergo deactivation to the ground state via internal conversion. The relative populations of the two forms of alcohol solvates, characterized by different photophysics, vary strongly across the series. The properties of the presently investigated compounds differ from those of a structurally related 7-azaindole and 1-azacarbazole, in which the phototautomerization involves solvent relaxation around the excited chromophore. Molecular dynamics calculations, performed to predict and compare the ground-state structure of 7-azaindole and 1H-pyrrolo[3,2-h]quinoline alcohol complexes, allow one to rationalize the observed differences in the excited-state reaction mechanisms for the two kinds of systems.

Photoinduced two‐proton tautomerization has been studied in two types of chromophores: (i) alcohol complexes of azaaromatic molecules possessing both proton donor and acceptor groups; (ii) constitutional isomers of porphyrin. The reaction path for the intermolecular process may involve solvent reorientation around the excited chromophore. In this case, rapid internal conversion is activated, efficiently competing with proton transfer. Another possibility arises if cyclic, doubly‐hydrogen‐bonded complexes exist already in the ground state. Excitation of such species leads to a fast tautomerization, which is not stopped even at low temperatures. Excited‐state double proton transfer also has been observed in cyclic dimers in the crystal phase. In porphyrin isomers, the rate of the process is a function of the strength of the intramolecular hydrogen bond. Dependence of the phototautomerization rate on temperature and on the nature of the surrounding matrix has also been studied.