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

Abstract

In the two papers, (i) Pontremoli C, Chinigò 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 sub-band 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 sub-band corresponds to the vibronic 0–0 transition and the first short wavelength sub-band to the vibronic 0–1 transition.