Coumarin 337 in Aqueous Solution: Interplay Between Excitation and Hydration

Abstract

Excitation of the C337, highly fluorescent dye in an aqueous solution was studied by the TD-DFT. Through the B3PW91/6-311++G(d, p)/IEFPCM theory level, we obtained the C337 vibronic absorption spectrum which exactly reproduces the experimental maximum. Significant differences are shown between the results of calculating the transition energies using the models of vertical and vibronic transitions. Upon excitation, the main shift of the electron density occurs from C10 to C4 neighboring atom. Thus, large-scale ICT not occur. The effect of excitation on the vibrational spectra of C337 was analyzed. A single C337 molecule and its five different hydration complexes with strongly bound water molecules have been analyzed. Various hydrate complexes of the dye have been studied, and the effect of hydration on its excitation vibrational properties has been elucidated. It was taken into account that a water molecule can attach to the nitrogen atom N1 ("A" type) and the C=O carbonyl ("B" type), and cyano C≡N ("C" type) groups of the dye.