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Diffusion-Facilitated Direct Determination of Intrinsic Parameters for Rapid Photoinduced Bimolecular Electron-Transfer Reactions in Nonpolar Solvents
journal contribution
posted on 2015-03-26, 00:00 authored by Andrew
D. Scully, Hiroyasu Ohtaka, Makoto Takezaki, Toshihiro TominagaBimolecular 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.