Photophysical Properties of Zinc Coproporphyrin I Tetraethyl Ester in Different Solvents Probed by TR EPR Spectroscopy

The results of time-resolved electron paramagnetic resonance (TR EPR) of zinc complexes of coproporphyrin I tetraethyl ester (ZnCPP-1) in solvents: o-terphenyl and chloroform/isopropanol mixture on the time after the laser pulse photoexcitation have been presented. The TR EPR spectra of the ZnCPP-1 complex in o-terphenyl indicate the presence of only one type of the photoexcited triplet state, while in the solution of the chloroform/isopropanol, the TR EPR spectrum is a sum of two different triplet spectra. The triplet spectrum detected in o-terphenyl is described by the zero-field splitting (ZFS) parameters and the aaa/eee electron spin polarization (ESP) pattern characteristic of zinc porphyrin derivative triplets. This spectrum is assigned to the monomeric ZnCPP-1 complex. A new spectrum detected together with the monomer spectrum in the chloroform/isopropanol has a reverse ESP pattern. The new spectrum is especially clearly observed in the time interval of 1.4–3 μs after the laser pulse when the intensity of the spectrum from the monomer triplet is substantially reduced. In addition to the reverse ESP pattern, an increase in the ZFS parameters of the new spectrum is observed. The density functional theory (DFT) calculation of the monomer and dimer structures and their energies shows that the dimer formation is energetically favorable. On the basis of the TR EPR data and DFT calculation, we suppose that the ZnCPP-1 complex is dimerized in the solution of the chloroform and isopropanol mixture and the new spectrum is most likely assigned to the dimer.