Effect of acetylene linkage on solvatochromism of betaine consisting of 1-methylpyridinium and phenolate units

Abstract

Solvatochromic behavior of 4-[2-(1-methylpyridinium-4-yl)ethynyl]phenolate (5) in various solvents was investigated to elucidate the effect of C≡C linkage. 5 was found to exhibit negative solvatochromism, where the observed maximum absorption wavelengths were short in polar solvents. Plots of the absorption energies of 4-(1-methylpyridinium-4-yl)phenolate (3) and 4-[2-(1-methylpyridinium-4-yl)ethenyl]phenolate (2) against those of 5 showed liner correlation. Their slopes were 0.80 for 3 and 0.91 for 2, indicating the C≡C linkage increase sensitivity of solvent polarity more effectively than the CC linkage. The excitation energies in various solvents were estimated by TD-DFT calculations using the polarizable continuum model (PCM) with SMD universal solvation model with the four different functional (LC-ωPBE, ωB97XD, CAM-B3LYP, and B3LYP). The LC-ωPBE method was found to have the closet agreement with the experimental solvatochromic sensitivity toward solvent polarity.

Keywords

• Solvatochromism
• Betaine
• TD-DFT
• Reichardt’s dye
• Kamlet-Taft solvent parameter