Investigation of the competition between electron and energy transfer in the quenching of aromatic ketones in the triplet state using picosecond transient grating spectroscopy

The competition between electron transfer (ET) and triplet energy transfer (TT) in the quenching of benzophenone, xanthone, and anthraquinone in the t riplet state by molecules with both a sufficiently small oxidation potentia l and low triplet state was investigated in the picosecond to microsecond t ime scales. In the longer time scale, the product distribution depends stro ngly on the relative exergonicity of ET and TT processes, the yield of the lower energy product being at least four times larger than that of the othe r product. Picosecond transient grating measurements reveal that if TT is m ore exergonic than ET, the TT product is predominantly formed by two sequen tial ET reactions, i.e., by spin-allowed back ET within the triplet geminat e ion pair formed upon ET quenching. However, if ET is more exergonic than TI, no conversion from the TT product to the ET product could be detected. In this case, the product distribution in the microsecond time scale seems to reflect the competition between the two processes. When both processes a re exergonic, ET appeared to be always faster than TT. This is in agreement with the severe orbital overlap requirement for TT via the Dexter exchange mechanism.