Spin-multiplicity of a moderately coupled triplet–doublet spin pair in a biphenylene-linked porphyrin dimer

Abstract

Electron spin transient nutation spectroscopy has been applied to a moderately coupled excited triplet and doublet spin-pair. In a biphenylene-linked copper(II)-free base porphyrin dimer, laser irradiation of either the free base half or the copper half, leads to excitation to the excited states and relaxation to the lowest excited state of the dimer. In this lowest excited state, the free base moiety in the lowest triplet state is coupled to the copper porphyrin ground doublet state, and thus forming a triplet–doublet spin pair. Time-resolved electron paramagnetic resonance (TREPR) spectra of the dimer in frozen solution at 40 K show much wider spectral width and a prominent band in the centre of the spectra than either of those for the free base monomer triplet or that of the copper(II) monomer porphyrin. These features are quite different from the strongly-coupled and weakly-coupled triplet–doublet spin pairs. Transient nutation method by pulsed EPR is used to examine the spin-multiplicity of the excited state in the dimer at three magnetic field positions. It was found that spin-multiplicity depends largely on the magnetic field. This is rationalized in terms of an intermediate size of coupling. In such intermediate coupling cases, the degree of mixing of the triplet and doublet basis wavefunctions depends greatly on the relative orientation between magnetic field and molecular axis because the ratio of the coupling size to an energy difference between the unperturbed species, triplet and doublet, varies with magnetic field direction. A splitting of TREPR spectra in partially oriented samples suggests that exchange coupling |J| is around 10 mT, which reasonably matches the results of the pulsed experiments.