Electron Spin Polarization of Photo-Excited Copper Coproporphyrin I: From Monomers to Dimers

The results of the electron paramagnetic resonance (EPR) and transient EPR (TREPR) of copper complexes of coproporphyrin I in different solvents before and after the laser pulse photo-excitation have been presented. Continuous-wave EPR spectra of the CuCPP-1 complex in o-terphenyl indicate the presence of only monomer fragments, while in the solution of the chloroform and isopropanol mixture, the complexes dimerize and the amount of dimers is five times larger than that of monomer complexes. Parameters describing EPR spectra of monomer and dimer CuCPP-1 complexes have been determined. It was established that the fine structure tensor of the dimer complex is rotated with respect to the g-tensor, which coincides with the tensor of monomer complexes. TREPR spectra of CuCPP-1 complexes in o-terphenyl and in the chloroform and isopropanol mixture after the laser photo-excitation are mainly due to spin-polarized ground states of monomer and dimer complexes, respectively. The TREPR spectra of the monomer CuCPP-1 show the emissive spin-polarized signal of the ground state. For dimer fragments, the net polarization is observed in the form of absorption and there is a small contribution from the multiplet polarization, which decays fast in time. The time dependence of TREPR of CuCPP-1 complexes in the chloroform and isopropanol mixture is described with allowance for these contributions from the ground state of the dimer and the contribution from the ground state of the monomer, which is manifested at larger times. Differences in the spin polarization of ground states and their possible origin are discussed.