Correspondence concerning the interpretation of the fine structure in the electronic absorption spectra of symmetrical cyanine dyes

In the two papers, (i) Pontremoli C, Chinig`o G et al. Photosensitizers for photodynamic therapy: structure-activity analysis of cyanine dyes through design of experiments. Dyes Pigments 2023; 210:111047 and (ii) Yoo N, Lim JY et al. A study on red cyanine dyes employing bis(fluorosulfonyl)imide anions for color filters of virtual and augmented reality displays. Dyes Pigments 2023; 220:111734, it was suggested that the short wavelength subband in the electronic absorption spectra of symmetrical cyanine dyes corresponds to the HOMO-LUMO+1 electronic transition. However, all experimental observations lead to the explanation that the fine structure in the spectra is a vibronic fine structure and a sub-band is a vibronic transition from v = 0 to v', where v is the vibrational quantum number of the totally symmetric carbon-carbon valence vibration of the polymethine chain in the ground electronic state S0 and v' is that in the first excited electronic state S1. The long wavelength subband corresponds to the vibronic 0-0 transition and the first short wavelength sub-band to the vibronic 0-1 transition.