Vibronic fine-structure in the S0 → S1 absorption spectrum of zinc porphyrin: A Franck-Condon simulation incorporating Herzberg-Teller theory and the Duschinsky effect

Although zinc porphyrin is a suitable specimen for studying the electronic structures and fine optical spectra of large molecules, few theoretical investigations have been performed to simulate and assign the vibrational bands in highly resolved spectra. On the basis of density functional theory and its TD extension, the S-0 -> S-1 absorption spectrum (that is, the Q band) of zinc porphyrin was simulated using the Franck-Condon (FC) approximation and incorporating the Herzberg-Teller (HT) and Duschinsky contributions to electronic transition dipole moments. Our theoretical results fit well with the optical spectrum obtained by experimental observations. The spectral profile of the Q band of zinc porphyrin is primarily described by the transition of the vibrational normal modes v(8), v(18), v(49), v(56), v(57), v(63), v(64) and v(87). Comparison of the FC and FCHT spectra with experimental results indicate that the S-0 -> S-1 absorption spectrum of zinc porphyrin is primarily due to the HT mechanism. In this regard, for the vibronically allowed or very weak transitions like those of the Q band in zinc porphyrin, the results indicate that the HT effect is more dominant than the FC contribution. In addition, we have tentatively assigned the vibrational lines in the simulated absorption spectrum. When compared with the available experimental spectra, our simulated results accurately reproduce most of the dominant transitions. (C) 2012 Elsevier Inc. All rights reserved.