title: Picosecond dynamics of proton transfer of a 7-hydroxyflavylium salt in aqueous-organic solvent mixtures.
abstract: 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)).
authors.affiliation: Centro de Química Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisbon, Portugal. adilsondefreitas@ist.utl.pt
authors.forename: Adilson A
authors.initials: AA
authors.lastname: Freitas
authors.forename: Frank H
authors.initials: FH
authors.lastname: Quina
authors.forename: António A L
authors.initials: AA
authors.lastname: Maçanita
mesh_terms: D004146:Dioxanes / Q000737:chemistry*; D005419:Flavonoids / Q000138:chemical synthesis / Q000737:chemistry*; D011409:Propylene Glycols / Q000737:chemistry*; D011522:Protons; D012492:Salts / Q000737:chemistry; D013050:Spectrometry, Fluorescence; D013816:Thermodynamics; D013997:Time Factors; D014867:Water / Q000737:chemistry*
chemical_list: C569376:7-hydroxy-4-methylflavylium chloride; D004146:Dioxanes; D005419:Flavonoids; D011409:Propylene Glycols; D011522:Protons; D012492:Salts; D014867:Water; C025223:1,4-dioxane