tetraphenylporphine and quinone

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.

Topics: Alkanes; Benzoquinones; Diffusion; Electron Transport; Hydrodynamics; Naphthoquinones; Photochemical Processes; Porphyrins; Quantum Theory; Solvents

Photoinduced electron transfer with the electron donor-acceptor linked system of zinc-5,10,15,20-tetraphenylporphyrin (ZnTPP) as donor and benzoquinone (BQ) as acceptor was designed to mimic the electron transfer process in photosynthetic reaction center. Within the framework of the bimolecular electron transfer mechanisms of scanning electrochemical microscopy (SECM), the fundamental features of photoinduced electron transfer were demonstrated by recording the feedback approach curves under different light wavelength, intensity, and concentration of BQ. Meanwhile, relevant theoretical study was employed to support our proposal. It demonstrated that SECM as a new platform could also provide crucial information for simple simulation of the electron transfer process in photosynthetic reaction center.

Topics: Benzoquinones; Chlorophyll; Coloring Agents; Electrodes; Electron Transport; Light; Photosynthesis; Porphyrins; Spectrophotometry, Ultraviolet; Thermodynamics; Tin Compounds; Zinc