Fine-structure spectra of metal complexes of porphin in tetrahydrofuran at low temperatures: Manifestation of nonplanarity effects

The fine-structure fluorescence spectra of Mg and Zn porphins in solid tetrahydrofuran matrices at the liquid helium temperature are recorded for the first time. The fluorescence spectra of Mg porphin molecules deposited from the gas phase on a sapphire substrate simultaneously with tetrahydrofuran molecules are measured. Based on the data obtained from the fluorescence and fluorescence excitation spectra, it is ascertained that there are two noninteracting spectrally different long-and short-wavelength forms of metal complexes of porphins in the ground state. In the case of Mg porphin, the spectral gap for these two forms at 4.2 K amounts to 330 cm^?1. The short-wavelength form is attributed to the nonplanar saddle conformation of a porphin molecule, while the long-wavelength form is associated with the nonplanar domed conformation, in which the central metal ion is displaced out of the plane of the porphyrin ligand. The fine-structure fluorescence spectra of both forms of Mg and Zn porphins are measured in tetrahydrofuran at 4.2 K upon selective laser excitation. The frequencies of the normal vibrations in the ground electronic state are determined and the reasons for the differences in the vibrational frequencies of the forms are discussed.