Theoretical simulation of the Qx-band absorption and fluorescence spectra of cis-isobacteriochlorin: Including the Duschinsky and Herzberg–Teller effects

Highlights: • Low-lying excited states of cis-isobacteriochlorin were calculated and analyzed in detail. • Highly resolved absorption and fluorescence spectra of cis-isobacteriochlorin were simulated. • Theoretical results allowed to identify the experimental bands and to assign them to the specific vibrational transitions. • The HT contribution and Duschinsky effects were investigated separately. The characteristics of the low-lying electronic states of cis-isobacteriochlorin were investigated by Density Functional Theory and its time-dependent extension. The high-resolved absorption and fluorescence spectra were simulated in the framework of the Herzberg-Teller approximation with the inclusion of the normal mode mixing effect. The simulated spectra were in accordance with the experimental findings. Our theoretical simulations allowed to identify the experimental bands and to assign them to the specific vibrational transitions. The individual Herzberg-Teller and Duschinsky effects were investigated separately and their respective contribution on the vibrational intensity changes of different normal modes were clarified.