VIBRONIC SPECTRA OF OXAZINE 750 DYE IN AQUEOUS MEDIA: A COMPUTATIONAL STUDY

The MN12SX/6-31++G(d,p)/SMD theory level exactly reproduced both positions of the main maximum and short-wavelength shoulder of OX750 absorption in an aqueous solution. The optimal functional for calculating the vibronic absorption spectra of different oxazine dyes in an aqueous solution was discussed based on the author’s present and previous studies. The absorption spectrum shoulder is caused by the vibronic transition. The vibrations involved in vibronic transitions correspond to large-scale molecular movements, are low-frequency, and very weak compared to the others. However, excitation significantly influences the vibrations including the most intensive ones. Photoinduced charge redistribution is local and there is no charge transfer over the dye molecule as a whole. Aliphatic hydrogen atoms prevent water molecules from accessing the N24 nitrogen atom. Considering H-bonded "solute-solvent" interactions by three water molecules led to a redshift of the entire spectrum by ≈15 nm. A strengthening of H-bonds with water molecules upon OX750 excitation was found, which explains this bathochromic effect. The intensity of low-frequency vibrations (including those involved in vibronic transitions) increases with the addition of bound water molecules, especially in an excited state. The vibration of the N-H bond of the imino group is strengthened (especially in an excited state) due to water molecule binding. Noticeable polarization of one water molecule bounded was revealed upon dye excitation. The vibronic model was also applied to calculate the emission spectrum of OX750 in the aqueous media.