PHOTOCHEMICAL GENERATION OF RADICAL CATIONS FROM ALPHA-TERTHIENYL AND RELATED THIOPHENES - KINETIC-BEHAVIOR AND MAGNETIC-FIELD EFFECTS ON RADICAL-ION PAIRS IN MICELLAR SOLUTION

The photochemistry of α-terthienyl (αT) and related compounds has been examined in homogeneous solution and in anionic micelles in the presence of electron acceptors. The absorption spectra of the radical cations from four thiophenic substrates have been characterized; for example, those derived from α-bithienyl (αB) and αT show absorption maxima at 420 and 530 nm, respectively. Triplet quenching by acceptors such as methyl viologen (MV2+) and tetracyanoethylene approaches diffusion control (k > 5 X 109 M-1 s-1). Quenching by oxygen, which is known to be dominated by singlet oxygen sensitization, involves electron transfer only to a minor extent; the highest efficiency, for αB, was only 6%. In micellar systems the behavior of the radical-ion pairs produced via electron transfer involves the competition of geminate and exit processes which occurs in the 10-7-10-6 s time domain. Both processes are slower in the larger micelles. Geminate processes are dramatically affected by magnetic fields. A model is proposed where the rate of geminate processes is suggested to depend upon the intramicellar reencounter frequency for the pair and the degree of singlet character in the triplet-derived radical-ion pair. The magnetic field effects observed are consistent with such a model. © 1990, American Chemical Society. All rights reserved.