Novel low-molecular weight styryl dyes based on 2-chloroimidazo[1,2-a]pyridine-3-carbaldehyde – Synthesis and fluorescent molecular rotor studies

Abstract

A library of fluorescent molecular rotors (FMRs) were synthesized by condensation of 2-chloroimidazo[1,2-a]pyridine-3-carbaldehyde with various active methylene moieties and their viscosity sensing properties were studied. The substituted 2-chloroimidazo[1,2-a]pyridine-3-carbaldehydes were prepared by a multistep reaction starting from 2-aminopyridine or 5-methyl-2-aminopyridine. Substitution of styryl group at the third position of imidazo[1,2-a]pyridine ring, the lone pair of bridging nitrogen as well as hyperconjugated methyl group are in proper resonance within the molecule and sustain aromaticity in ring leading to the red shifted absorption. Large red shifted absorptions were observed after substituting electron withdrawing group at the third position as compared to the known 7-[(E)-2-(phenyl)vinyl]-2-phenylimidazo[1,2-a]pyridine analogues where the styryl groups are at the second position of imidazo[1,2-a]pyridines ring. These donor- π-acceptor extended styryl FMRs were investigated for their viscosity sensitivity and found to show values between 0.2135 and 0.5356 in glycerol-methanol system using fluorescence emission spectra. These low molecular weight FMRs are found to show up to 31 fold enhancement in emission intensity in viscous environment. Oscillator strengths (f) and transition dipole moments (µeg) have been studied to understand charge transfer within the molecules. The structures are confirmed by 1H NMR, 13C NMR, Mass and elemental analysis.