Ultrasonic assisted synthesis of styrylpyridinium dyes: Optical properties and DFT calculations

The ultrasonic technique has received considerable attention in several fields; in particular, it gained rapid momentum in organic synthesis due to the larger reaction rates, milder reaction conditions, and better yields. We report herein a facile synthesis of a series of styrylpyridinium based dyes under ultrasonic irradiation. Within short reaction time (15 min) under ultrasonic irradiation, compared to normal laboratory conditions, (4-16 h), we can achieve good to excellent yields. The reaction time is shortened because ultrasound can accelerate the generation of the nucleophile of the pyridinium salt and subsequently a nucleophilic addition of an aldehyde followed by dehydration affords the styrylpyridinium dye, (Knoevenagel condensation). The photophysical properties of all compounds are comprehensively investigated in different solvents. All the compounds exhibit negative solvatochromism both in absorption and fluorescence emission spectra. Such behavior is due to the higher dipole moment of these molecules at the ground state. DFT calculations were performed to understand the electronic structure of the molecules. Our results show the high efficacy of sonochemistry over other methods for preparation of styrylpyridinium dyes.