Experimental and theoretical absorption spectra of A3V-Type porphyrine derivatives

Abstract

In order to interpret the experimental absorption spectra of meso-substituted porphyrin derivatives of A3Btype (A = PhOCnH2n+1, n = 8, 16; V = Ph-NH2, Ph-OH, Py), a quantum-chemical simulation of the structure and electronic absorption spectra (EAS) of these compounds were performed. The optimization of the geometric structure corresponding to the ground electronic state was performed by the DFT/B3LYP/6-311++G method. To analyze the effect of the alkyloxy-substituent length (-OCnH2n+1) on the electronic properties of these compounds, the calculations were performed for the molecules with n = 1-4. The calculated absorption spectra (obtained by TD-DFT method) included six the lowest singlet-singlet excitations of the ground electronic state of the molecules. This computational procedure allowed to simulate absorption spectra in the wave length range of 300-800 nm, which corresponds to the range of the experimental spectra. An interpretation of the spectra is given and it is noted that the Soret band of the studied compounds includes two electronic transitions: 31A and 41A from HOMO-1 to LUMO and LUMO+1. The short-wavelength shoulder of the Soret band corresponds to the π-π transition with charge transfer from the meso-substituent to the porphyrin macrocycle. It was established that the variation of substituents in the fragment B, which significantly effects the physicochemical properties of these compounds (phase transition temperatures, dipole moment, and molecular packing in condensed phases), leads only to a slight change in the position of the bands in visible region of electronic absorption spectra, which was confirmed by quantum chemical calculations.