IDENTIFICATION OF PHOTOEXCITED SINGLET QUINONES AND THEIR ULTRAFAST ELECTRON-TRANSFER VS INTERSYSTEM-CROSSING RATES

Photoexcitation of chloranil (CA) produces initially the excited singl et state (1)CA, as demonstrated for the first time by time-resolved s pectroscopy on the femtosecond/picosecond time scale. Electron transfe r from aromatic donors (D) to singlet chloranil leads to short-lived ( ca. 5 ps) singlet radical-ion pairs, (1)[D.+, CA(.-)]. This ultrafast quenching process competes with intersystem crossing (k(ISC) approxima te to 10(11) s(-1)) to generate the triplet excited state, (3)CA. The follow-up electron transfer from D to (3)CA yields triplet radical-i on pairs, which are distinguished from their singlet analogues by thei r long (nanosecond) lifetimes. The competition between electron transf er and intersystem crossing on the early picosecond time scale also pe rtains to a wide variety of other photoexcited quinones related to chI oranil. Electron transfer to singlet quinone as established here adds a new dimension to the generally accepted mechanisms which proceed fro m the triplet state,;and the inclusion of reactions on both the triple t and the singlet manifolds provides a complete picture of photoinduce d electron transfer to various quinone accepters.