A time-resolved EPR study of weakly coupled triplet-doublet pairs of copper(II)-free base porphyrin dimers

Abstract

In triplet-doublet pairs in which the electron spins are coupled weakly through a long distance, the photodynamics of the triplet are changed as a result of the interaction with the doublet. The electron spin polarization patterns of such pairs and their time developments are expected to reveal how the interaction controls the coupled system and its dynamics. While extensive efforts have been devoted to studying strongly coupled triplet-doublet systems, relatively few examples have been observed for the weakly coupled systems. Time-resolved electron paramagnetic resonance (EPR) spectra of copper(n)-porphyrin-free base porphyrin dimers, in which the two halves are linked via an alkyl-chain or a p-biphenyl group, are presented together with a framework of spin-polarized spectra in weakly coupled triplet-doublet systems. In these complexes, two pathways, i.e., intramolecular energy transfer ( EnT ) and intersystem crossing (ISC ), are available to generate a weakly coupled triplet-doublet state, where the free base half is in the excited triplet and the copper half is the ground state doublet. The excitation wavelength dependence of the spectra reveals mechanisms, which can account for characteristic spin polarization patterns of the weakly coupled triplet-doublet pairs and their time developments.