Structures and conformations of acridinium mono- and polymethine cyanine dyes and their UV–Vis absorption spectra in protic solvents: A PCM/TD-DFT study

Structures of the acridinium mono- and polymethine cyanine dyes with different lengths of polymethine chain of four compounds were obtained using DFT calculations. The differences of the energies and free energies of their corresponding conformers in methanol and in water were determined using the DFT/B3LYP and PCM(UAKS)-DFT/B3LYP approaches. The absorption maxima computed using the PCM(UAKS)-TDDFT/B3LYP/6-311+G(d,p) calculations for the four acridine-containing cyanine dyes and their conformers are in agreement with the experiments. It was found that the absorption maxima of all the acridinium cyanine dyes are mainly caused by the electronic transition from HOMO to LUMO.

► Conformers of novel mono- and polymethine cyanine dyes were theoretically studied.
► Conformers in either methanol or water were more preferable than in gas phase.
► Absorption maxima were due to the HOMO–LUMO transitions for all dye conformers.
► The longer polymethine chain of the dyes caused the larger red-shift.