Time resolved ESR study on energy difference of quartet and doublet states in radical-triplet encounter pairs

The energy difference J of quartet and doublet states in a radical-triplet pair (RTP) has been studied in pairs of triplet naphthalene (Np) derivative/galvinoxyl(Galv) and triplet biphenyl (Biph) derivative/Galv systems by time resolved ESR spectroscopy. Most of the systems studied show an unusual positive sign for J that corresponds to the relative location of the quartet state being below the doublet state. In order to explain this unusual sign of J in RTPs, the effects of intermolecular charge transfer (CT) states were examined. The observation was explained by introducing configuration interaction between zero-order RTP (RTPo) and CT (CT0) states in the encounter pair, where the RTP0 state is located above the CTo state. Among the RTP systems examined, triplet 1-chloro-Np/Galv, triplet 1-bromo-Np/Galv and triplet 4-cyano- Biph/Galv systems show a normal negative value of J. On the basis of the estimated energy gap between the RTP0 and CT0 states, it was concluded that configuration interaction is not significant in these three RTP systems. The value of J in these RTP systems is dominated by exchange interaction, giving a negative sign. The solvent effect on the sign of J in a triplet Np/Galv system is also reported, as evidence that the sign of J is controlled by the CT0 state.