Experimental and Theoretical Studies of the Proton-Hopping Reaction of 7-Hydroxyquinoline in Viscous Hydroxylic Media

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 1998-11, Vol.102 (45), p.8871-8880
• Main Authors: García-Ochoa, I, Bisht, P. B, Sánchez, F, Martinez-Atáz, E, Santos, L, Tripathi, H. B, Douhal, A
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp981489+

Experimental and theoretical (PM3) studies of 7-hydroxyquinoline in glycerol and in ethylene glycol show the occurrence of a proton-transfer reaction in the ground as well as in the first singlet electronically excited states. Both studies indicate that the H-bond bridge formed in the 1:1 complex provides a stabilization of the keto form in the S0 state (λabs= 420 nm). In S1, a photoinduced proton-transfer reaction solely occurs in the bridged or well-prepared H-bonded enol form, producing a fraction of the keto tautomer that emits a largely Stokes shifted band (λemis = 530 nm). The time-resolved fluorescence measurements show that the dynamics of this proton-hopping reaction is viscosity-dependent (0.5 ns in ethylene glycol and 0.8 ns in glycerol). Theoretical calculations indicate the coexistence of cis and trans rotamers of the dye in the gas phase, in agreement with the observation in a jet-cooled molecular beam. The optimized geometries of the 1:1 complexes of both cis-enol and keto tautomers with both solvents indicate that proton-transfer dynamics involves a global nuclear motion of the H-bond bridge. In both associated tautomers, the 2-OH group of the glycerol molecule does not participate in the H-bond bridge involved in the tautomerization. Analysis of the HOMO and LUMO shows that the driving force of the proton-hopping reaction originates in a partial intramolecular charge transfer from the proton-donating site to the accepting group within the dye molecule.