Energy Separation between Quartet and Doublet Spin States of Radical−Triplet Encounter Pairs; Unusual Ferromagnetic Interaction in a 1,1-Diphenyl-2-picrylhydrazyl and Triplet Coronene Pair

Abstract

Time-resolved ESR (TR-ESR) measurements were performed for chemically induced dynamic electron polarization (CIDEP) created in various systems of the 1-diphenyl-2-picrylhydrazyl (DPPH) radical and triplet organic molecules. TR-ESR spectra of DPPH show a net emission CIDEP signal in most of the DPPH−triplet systems. The only exception was observed for a DPPH−coronene system in which the TR-ESR spectrum of DPPH presents a net absorption CIDEP signal. The origin of these net emission and absorption CIDEP signals is interpreted in terms of the radical−triplet pair mechanism (RTPM) for a triplet quenching process. To confirm that the CIDEP is created by RTPM, we simulated the time profiles of the TR-ESR signal with modified Bloch and kinetic equations. Stern−Volmer type analyses of the CIDEP intensities also support the assignment for the proposed CIDEP mechanism. According to the signal sign rule in RTPM, we examined the energy difference, J, of the quartet and doublet spin states of the radical−triplet encounter pair (RTP). J is defined by the equation J = E(2RTP) − E(4RTP) where E(2RTP) and E(4RTP) represent the energies of doublet and quartet RTP states, respectively. According to the CIDEP signal, most of the RTPs in DPPH−triplet molecule systems show a normal negative J-value, while a DPPH−triplet coronene pair shows an unusual positive J-value. The mechanism for this unusual antiferromagnetic coupling in the DPPH−triplet coronene pair is explained by introducing an intermolecular charge-transfer interaction.