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 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.

The red‐edge effects: 30 years of exploration

“…The possibility of photoselection between ground-state conformers of the tryptophan zwitterion has been also demonstrated (201). For some ESPT reactions, the structures of ground-state solutesolvent complexes are important (202), while for others they are not. We can even observe that for some excited-state isomerizations there is no excited-state equilibrium, despite the fact that all the species originate from the same ground-state precursor (203).…”

Excited‐State Double Hydrogen Bonding Induced by Charge Transfer in Isomeric Bifunctional Azaaromatic Compounds

“…The change from an aprotic to a protic solvent causes a decrease in the fluorescence intensities of this band, F 1 , and the appearance of a new red-shifted fluorescence band, labelled as F 2 (Figure 30.1). As can be seen in Figure 30.2, the spectral locations and shapes of these [73]. These findings leave no doubt that low-energy fluorescence originates in the tautomeric species, obtained as a result of a double-proton transfer reaction occurring in complexes of the photoexcited chromophore with alcohol.…”

“…From the plot of ln [(OD À OD 0 )/[(OD ¥ À OD)] versus ln [A], the number of alcohol molecules in a complex can be obtained. The values extracted from this plot point to the 1:1 stoichiometry at a low concentration of alcohols [72,73] (see Figure 30.1).…”

“…Moreover, the F 1 emission becomes structured and very similar to the emission observed at 293 K in non-polar solvents. However, in spite of the recovery of the fluorescence intensities, the sum of F 1 and F 2 quantum yields in alcohols at low [73]. Copyright 1999 American Chemical Society temperature is still much lower than the fluorescence quantum yield at 293 K in non-polar or polar aprotic solvents [72].…”

“…These findings leave no doubt that low-energy fluorescence originates in the tautomeric species, obtained as a result of a double-proton transfer reaction occurring in complexes of the photoexcited chromophore with alcohol. However, the fact that the decrease in the intensity of the F 1 fluorescence was much greater than the concomitant increase in the tautomeric emission led the model that assumes the presence of only two species, and thus only one form of the alcohol solvate, to be considered as probably excessively simple [73]. This finding has been explained by the presence of at least two different forms of alcohol complexes.…”