Diffusion-FacilitatedDirect Determination of IntrinsicParameters for Rapid Photoinduced Bimolecular Electron-Transfer Reactionsin Nonpolar Solvents.

Bimolecular fluorescence-quenchingreactions involving electron-transferbetween electronically excited 5,10,15,20-tetraphenyl-21H,23H-porphine (TPP*) and 1,4-benzoquinone (BQ) or1,4-naphthoquinone (NQ) were investigated using a set of alkane solventsthat enabled the rapid reaction kinetics to be probed over a wideviscosity range, while minimizing changes in other relevant solventparameters. Relative diffusion coefficients and reaction distanceswere recovered directly from analysis of fluorescence decay curvesmeasured on a nanosecond time scale. The electron transfer from TPP*to BQ requires reactant contact, consistent with tightly associatedexciplex formation in these nonpolar solvents. In contrast, electrontransfer from TPP* to NQ displays a clear distance dependence, indicativeof reaction via a much looser noncontact exciplex. This differenceis attributed to the greater steric hindrance associated with contactbetween the TPP*/NQ pair. The diffusion coefficients recovered fromfluorescence decay curve analysis are markedly smaller than the correspondingmeasured bulk relative diffusion coefficients. Classical hydrodynamicstheory was found to provide a satisfactory resolution of this apparentdiscrepancy. The calculated hydrodynamic radii of TPP and NQ correlatevery well with the van der Waals values. The hydrodynamic radius obtainedfor 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 demonstratesthe utility of a straightforward methodology, based on widely availableinstrumentation and data analysis, that is broadly applicable fordirect determination of kinetic parameter values for a wide varietyof rapid bimolecular fluorescence quenching reactions in fluid solution. [ABSTRACT FROM AUTHOR]