Influence of the meso-substituents on the spectral features of free-base porphyrin

The role of meso-substituents on the spectral features of free-base porphyrins is explored. Meso-tetra(4-pyridyl) porphyrin is compared with meso-tetra(2-thienyl)porphyrin and meso-tetra(pentafluorophenyl)porphyrin. Our results indicate that some of the asymmetric Q-bands in the free-base porphyrin tend to become symmetric relative to the adopted meso-substituent. The results show that the outlying perturbations lead the free-base quasi-degenerated Q(x1), Q(x2), Q(y1), and Q(y2) bands to be closer in energy. Combined, absorption, fluorescence and Raman spectroscopies endorse our conclusions showing that both the frequencies and the Huang-Rhys factors associated with every vibronic progression are noticeably affected by the investigated meso-substituents. Our results confirm that the B-band is also multi-featured in agreement with what is found for the Q-bands. (C) 2020 Elsevier B.V. All rights reserved.