Enhanced intersystem crossings of S1–T1 and T1–S0 in coronene– and pyrene–galvinoxyl systems as studied by a pulsed ESR method

Radical enhanced intersystem crossing (EISC) of S1–T1 and T1–S0 in excited molecule–radical systems was investigated by observing chemically induced dynamic electron polarization of radicals with a time-resolved (TR) ESR method. Time profiles of CIDEP were observed in excited coronene–radical systems, where the radical was 2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO), 2-phenyl-4,4,5,5-tetramethylimidazoline-3-oxide-1-oxyl (PTIO), 1,1-diphenyl-2-picrylhydrazyl (DPPH), 1,3,5-triphenylverdazyl (TPV) or galvinoxyl (Galv). In two systems containing TEMPO and PTIO radicals, signals due to the quenching of coronene in both S1 and T1 states were observed, while in other three systems containing DPPH, TPV, and Galv radicals only the signals induced by triplet coronene–radical pair interaction could be observed. To make the quantitative analysis on CIDEP created in S1- and T1-quenching processes, a pulsed ESR method was applied for a coronene–Galv system chosen as a model system. The absolute magnitude of CIDEP of Galv radical created by S1- and T1-quenching processes (|PnQP| and |PnDP|, respectively) were determined to be 7Peq and 0.8Peq, respectively, where Peq is the equilibrium Boltzmann polarization at room temperature. According to the CIDEP theory, the large |PnQP/PnDP| ratio of ca. 9 indicates that the S1–T1 EISC occurs through a long distance interaction which is mediated by intermolecular charge transfer state, while |PnQP| of 7Peq indicates T1–S0 enhanced ISC is promoted by exchange interaction at the nearly contact coronene–Galv pair. Another system of pyrene–Galv pair was also discussed to confirm the proposed interaction model for the enhanced ISC.