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

Abstract Radical enhanced intersystem crossing (EISC) of S 1 –T 1 and T 1 –S 0 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 S 1 and T 1 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 S 1 - and T 1 -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 S 1 - and T 1 -quenching processes ( | P n QP | and | P n DP | , respectively) were determined to be 7 P eq and 0.8 P eq , respectively, where P eq is the equilibrium Boltzmann polarization at room temperature. According to the CIDEP theory, the large | P n QP / P n DP | ratio of ca. 9 indicates that the S 1 –T 1 EISC occurs through a long distance interaction which is mediated by intermolecular charge transfer state, while | P n QP | of 7 P eq indicates T 1 –S 0 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.