Diversity of excited state deactivation paths in heteroazaaromatics with multiple intermolecular hydrogen bonds

Abstract

Transient absorption and luminescence studies were performed for several heteroazaaromatic molecules which possess both hydrogen bonding donor and acceptor groups, and which are capable of forming cyclic, double intermolecular hydrogen bonds. In protic solvents, these systems exhibit complex photophysics, manifested by a strong quenching of the fluorescence. In dipyrido[2,3‐a:3′,2′‐i]carbazole and 7,8,9,10‐tetrahydropyri‐do[2,3‐a]carbazole, photoinduced double proton transfer is observed in the first excited singlet state, leading to tautomeric species which emit strongly Stokes‐shifted fluorescence. The large and temperature‐independent rate of this reaction is observed for the case in which a cyclic complex with an alcohol molecule is formed already in the ground state. Another excited state depopulation channel, internal conversion caused by proton movement in the cyclic complex, is observed in alcohol solutions of 2‐(2′‐pyridyl)‐indoles and pyrido[2,3‐a]carbazole. In these cases, formation of cyclic, doubly hydrogen‐bonded structures occurs in the excited state and involves solvent rearrangement. The rate of this process changes with solvent viscosity and becomes negligible at low temperatures, when the viscosity is high. Fast internal conversion has also been observed in hydrogen‐bonded complexes with azaaromatic bases. Its mechanism can be explained by intermolecular charge transfer interactions. In alcohol solutions of 2‐(2′‐pyridyl)indoles, transient absorption results point to yet another channel of excited state deactivation – photoionization, occurring faster than S1 decay.

Fluorescence Quenching by Pyridine and Derivatives Induced by Intermolecular Hydrogen Bonding to Pyrrole-Containing Heteroaromatics

“…However, our transient picosecond studies for pyridine solutions 42 have not detected the presence of any transient band in the visible region that could be assigned to a CT state. Also, no spectral changes were observed that would indicate a proton transfer along the hydrogen bond.…”

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

“…In view of the above, it seemed instructive to investigate the photophysical characteristics of other bifunctional molecules related to 7AI . The properties of alcohol solvates of 1-azacarbazole ( 1AC ) appear to be similar to those of 7AI . − On the other hand, large differences have been observed for a series of bifunctional azaheterocyclic compounds composed of indole, carbazole, or pyrrole hydrogen-bonding donors and of pyridine acceptor units. − We have shown that the room temperature fluorescence of several methylene-bridged 2-(2‘-pyridyl)indoles is strongly quenched in alcohol solutions. , The quantum yields of fluorescence in alcohols are nearly 2 orders of magnitude lower than in aprotic polar and nonpolar solvents. This difference is observed only for molecules which possess both, the hydrogen bond donor, the NH group, and the hydrogen bond acceptor, the pyridine type nitrogen atom.…”

“…[32][33][34][35] On the other hand, large differences have been observed for a series of bifunctional azaheterocyclic compounds composed of indole, carbazole, or pyrrole hydrogen-bonding donors and of pyridine acceptor units. [36][37][38][39][40][41][42] We have shown that the room temperature fluorescence of several methylene-bridged 2-(2′-pyridyl)indoles is strongly quenched in alcohol solutions. 36,37 The quantum yields of fluorescence in alcohols are nearly 2 orders of magnitude lower than in aprotic polar and nonpolar solvents.…”

Effect of hydrogen bonding and complexation with metal ions on the fluorescence of luotonin A

“…To gain insight into the details of the photophysical processes initiated by hydrogen bonding, the effect of HFIP was examined in CH 2 The equilibrium constants of 1:1 (K 1 ) and 1:2 (K 2 ) complexes were calculated by global analysis of the experimental results in the 330-390 nm range. Since HFIP was added in a large excess compared to LuA, the absorbance change was described by the function: Addition of Zn(NO 3 ) 2 caused a similar spectral change, but the driving force of Zn 2+ complexation was significantly smaller than that of Cd 2+ .…”

“…[1][2][3][4][5][6] Light absorption of these species altered the charge density on the heteroatoms inducing considerable change in the acid-base strength. Consequently, photoinduced proton transfer or excited-state tautomerization could occur.…”