Picosecond Dynamics of Proton Transfer of a 7-Hydroxyflavylium Salt in Aqueous–Organic Solvent Mixtures

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 115; no. 40; pp. 10988 - 10995
• Main Authors: Freitas, Adilson A, Quina, Frank H, Maçanita, António A. L
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 13.10.2011
• Subjects: A: Kinetics, Spectroscopy; Chlorides; Decay; Diffusion; Diffusion rate; Dioxanes - chemistry; Excitation spectra; Flavonoids - chemical synthesis; Flavonoids - chemistry; Fluorescence; Glycols; Propylene Glycols - chemistry; Protons; Salts - chemistry; Solvents; Spectrometry, Fluorescence; Thermodynamics; Time Factors; Water - chemistry
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp2069754

The intermediacy of the geminate base–proton pair (A*···H+) in excited-state proton-transfer (ESPT) reactions (two-step mechanism) has been investigated employing the synthetic flavylium salt 7-hydroxy-4-methyl-flavylium chloride (HMF). In aqueous solution, the ESPT mechanism involves solely the excited acid AH+* and base A* forms of HMF as indicated by the fluorescence spectra and double-exponential fluorescence decays (two species, two decay times). However, upon addition of either 1,4-dioxane or 1,2-propylene glycol, the decays become triple-exponential with a term consistent with the presence of the geminate base–proton pair A*···H+. The geminate pair becomes detectable because of the increase in the recombination rate constant, k rec, of (A*···H+) with increasing the mole fraction of added organic cosolvent. Because the two-step ESPT mechanism splits the intrinsic prototropic reaction rates (deprotonation of AH+*, k d, and recombination, k rec, of A*···H+) from the diffusion controlled rates (dissociation, k diss, and formation, k diff[H+], of A*···H+), the experimental detection of the geminate pair provides a wealth of information on the proton-transfer reaction (k d and k rec) as well as on proton diffusion/migration (k diss and k diff).