title: Diffusion-facilitated direct determination of intrinsic parameters for rapid photoinduced bimolecular electron-transfer reactions in nonpolar solvents.
abstract: Bimolecular fluorescence-quenching reactions involving electron-transfer between electronically excited 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP*) and 1,4-benzoquinone (BQ) or 1,4-naphthoquinone (NQ) were investigated using a set of alkane solvents that enabled the rapid reaction kinetics to be probed over a wide viscosity range, while minimizing changes in other relevant solvent parameters. Relative diffusion coefficients and reaction distances were recovered directly from analysis of fluorescence decay curves measured on a nanosecond time scale. The electron transfer from TPP* to BQ requires reactant contact, consistent with tightly associated exciplex formation in these nonpolar solvents. In contrast, electron transfer from TPP* to NQ displays a clear distance dependence, indicative of reaction via a much looser noncontact exciplex. This difference is attributed to the greater steric hindrance associated with contact between the TPP*/NQ pair. The diffusion coefficients recovered from fluorescence decay curve analysis are markedly smaller than the corresponding measured bulk relative diffusion coefficients. Classical hydrodynamics theory was found to provide a satisfactory resolution of this apparent discrepancy. The calculated hydrodynamic radii of TPP and NQ correlate very well with the van der Waals values. The hydrodynamic radius obtained for BQ is a factor of 6 times smaller than the van der Waals value, indicative of a possible tight cofacial geometry in the (TPP(+)/BQ(-))* exciplex. The present work demonstrates the utility of a straightforward methodology, based on widely available instrumentation and data analysis, that is broadly applicable for direct determination of kinetic parameter values for a wide variety of rapid bimolecular fluorescence quenching reactions in fluid solution.
authors.affiliation: †CSIRO Manufacturing Flagship, Bayview Avenue, Clayton, Victoria 3168, Australia.
authors.forename: Andrew D
authors.initials: AD
authors.lastname: Scully
authors.affiliation: ‡Department of Applied Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan.|§Department of Pharmacy, Chiba Institute of Science, 3 Shiomi-cho, Choshi, Chiba 288-0025, Japan.
authors.forename: Hiroyasu
authors.initials: H
authors.lastname: Ohtaka
authors.affiliation: ‡Department of Applied Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Takezaki
authors.affiliation: ‡Department of Applied Chemistry, Okayama University of Science, 1-1 Ridai-cho, Okayama 700-0005, Japan.
authors.forename: Toshihiro
authors.initials: T
authors.lastname: Tominaga
mesh_terms: D000473:Alkanes / Q000737:chemistry*; D016227:Benzoquinones / Q000737:chemistry*; D004058:Diffusion; D004579:Electron Transport; D057446:Hydrodynamics; D009285:Naphthoquinones / Q000737:chemistry*; D055668:Photochemical Processes; D011166:Porphyrins / Q000737:chemistry*; D011789:Quantum Theory; D012997:Solvents / Q000737:chemistry*
chemical_list: C000599518:5,10,15,20-tetraphenyl-21H,23H-porphine; D000473:Alkanes; D016227:Benzoquinones; D009285:Naphthoquinones; D011166:Porphyrins; D012997:Solvents; C004532:quinone; C035342:1,4-naphthoquinone