PHOTOINDUCED BACK ELECTRON-TRANSFER WITHIN GEMINATE RADICAL ION-PAIRS - OBSERVATION OF THE MARCUS INVERTED REGION IN THE SYSTEM THIONINE WITH SEVERAL AROMATIC DONORS

The Marcus "Inverted Region" is found for back electron transfer reactions within geminate radical pairs consisting of triplet-thionine and several aromatic donors. Measurements were performed in buffered methanol (pH = 8.6) at 295 K. The standard free energy change DELTA-G-et0 ranges from -54.0 to -150.5 kJ/mol by varying the donors from N,N,N',N'-tetramethyl-p-phenylendiamine, p-phenylendiamine, p-aminodiphenylamine, 1-naphthylamine, diphenylamine, 9,10-dimethylanthracene, 1,2,4-trimethoxybenzene to 9-methylanthracene. The rate constants start at 3.3.10(7) s-1, reach a maximum at 1.0.10(9) s-1 for 1-naphthylamine and decrease to 1.7.10(8) s-1 for highly exergonic reactions.