Ultrafast Intersystem Crossing in 9,10-Anthraquinones and Intramolecular Charge Separation in an Anthraquinone-Based Dyad

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Abstract
Femtosecond transient absorption spectroscopy was employed to determine quantitatively the ultrafast S1−T1 intersystem crossing in a 2-substituted 9,10-anthraquinone derivative (3), kisc = 2.5 × 1012 s-1. Notwithstanding this rapid process, photoexcitation of dyad 1 is followed by competition between intersystem crossing and intramolecular charge separation, the latter leading to a short-lived (2 ps) singlet charge-transfer (CT) state. The local triplet state itself undergoes slower charge separation to populate a relatively long-lived (130 ns) triplet CT state. An earlier report about the formation of an extremely long-lived CT state (>900 μs) in 1 was found to be erroneous and was related to the sacrificial photo-oxidation of the dimethylsulfoxide solvent used in that study. Finally, some important criteria have been formulated for future experimental validation of “unusually long-lived” CT states.