On the spectral profile change in the Q band absorption spectra of metalloporphyrins (Mg, Zn, and Pd): A first-principles study.

We performed a systematic study of the vibrationally resolved absorption spectra in the Q band of three metalloporphyrins (Mg, Zn, and Pd) to understand the spectral changes in this series, including both the Franck-Condon (FC) and Herzberg-Teller (HT) contributions. The ground (S0) and the lowest singlet excited (S1) states were, respectively, simulated by the static and time-dependent density functional theory, with which the Duschinsky rotation effect was considered. Different functionals and basis sets were tested and compared with experiment. Results show that the long-range corrected functional CAM-B3LYP can nicely describe the spectral fingerprints of these metalloporphyrins, while the B3LYP functional significantly underestimates the FC contributions. We found that the absorption fine structures of these molecules are mainly caused by the HT vibronic couplings. The experimentally observed enhancements to the on-site 0-0 absorption peak in the series of Mg, Zn, and Pd are nicely reproduced. Enhanced absorption intensity is caused by larger FC contributions of molecules with heavier metal ions. The structure-spectroscopy relationship was analyzed, and it was found that the smaller cavity size of the porphyrin ring can significantly enhance the oscillator strength of the S0 → S1 transition.