Effect of triplet spin dynamics memory in electron-transfer reaction. Fourier transform-electron paramagnetic resonance spectroscopy

Abstract

Electron-transfer reactions, in ethanol solutions at approx. 243 K, between the photoexcited triplets of two metallotetraphenylporphyrins, ZnTPP and MgTPP (donors), and neutral duroquinone, DQ (acceptor), were studied by selective laser excitation-Fourier transform-EPR spectroscopy. The authors demonstrate that this highly time-resolved EPR differentiates between photochemical processes, occurring early in time, in similar chemical systems. The early spectra (10 ns < /tau//sub d/ < 100 ns) of the DQ/sup /center dot/-/ generated by /sup 3/Zn TPP are totally absorptive, reflecting the out-of-plane active triplet spin state of /sup 3/ZnTPP. On the other hand, the early spectra of the DQ/sup /center dot/-/ radical generated by /sup 3/MgTPP (10 ns < /tau//sub d/ < 400 ns) are totally emissive, reflecting the in-plane active triplet spin state(s) of /sup 3/MgTPP. These results verify that the CIDEP effects due to the triplet mechanism strongly depend on the triplet spin memory of the particular precursor. Qualitative analysis of the temporal changes of the signal intensities results in the following: (i) apparent spin-lattice relaxation (SLR) times of the triplet precursors, i.e., T/sub 1/(/sup 3/ZnTPP) approx. 30 ns, T/sub 1/(/sup 3/MgTPP) approx. 200 ns; (ii) SLR times of the doublet radicals, i.e., T/sub 1/(DQ/sup /center dot/-/) approx. 7-8 ..mu..s,more » in the presence of ZnTPP; T/sub 1/(DQ/sup /center dot/-/) approx. 2-5 ..mu..s, in the presence of MgTPP; (iii) electron-transfer rates, i.e., k/sub et/(/sup 3/ZnTPP+DQ) approx. 10/sup 9/ M/sup -1/ s/sup -1/, k/sub et/-(/sup 3/MgTPP+DQ) approx. 2 /times/ 10/sup 8/ M/sup -1/ s/sup -1/. They suggest that the different results, ZnTPP vs MgTPP systems, are due to triplet exciplex formation between /sup 3/MgTPP and DQ.« less