Identification of photoexcited singlet quinones and their ultrafast electron-transfer vs intersystem-crossing rates

Photoexcitation of chloranil (CA) produces initially the excited singlet state (1)CA*, as demonstrated for the first time by time-resolved spectroscopy on the femtosecond/picosecond time scale. Electron transfer 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) approximate 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-ion pairs, which are distinguished from their singlet analogues by their long (nanosecond) lifetimes. The competition between electron transfer and intersystem crossing on the early picosecond time scale also pertains to a wide variety of other photoexcited quinones related to chIoranil. Electron transfer to singlet quinone as established here adds a new dimension to the generally accepted mechanisms which proceed from the triplet state,;and the inclusion of reactions on both the triplet and the singlet manifolds provides a complete picture of photoinduced electron transfer to various quinone accepters.