Synthesis, characterization, crystal structure, and fluorescence behavior of new 4-aryl-6-(2,5-dichlorothiophen-3-yl)-2-methylpyridine-3-carbonitrile derivatives

Abstract

A series of new 4-aryl-6-(2,5-dichlorothiophen-3-yl)-2-methylpyridine-3-carbonitrile derivatives 4(a-u) were synthesized starting with Friedel-Crafts acylation of 2,5-dichlorothiophene to form compound (1), which undergoes Claisen-Schmidt condensation with the corresponding aryl aldehyde 2(a-u) to produce chalcones 3(a-u) as intermediate compounds. The reaction of chalcones 3(a-u) and 3-aminocrotononitrile furnished the new title compounds 4(a-u). The pyridines 4(a-u) were characterized by elemental analysis, ATR-FTIR, 1H NMR, 13C NMR, DEPT 135, ESIMS, and HRESIMS. The proposed structures agree with the expected physical as well as spectral data. Single crystal x-ray structure for some derivatives were determined and confirmed unambiguously the proposed structures. Compounds 4(a-u) exhibit distinct fluorescent properties in solution. The steady state fluorescence spectra of the compounds studied showed that all the compounds are blue emitters. Quantum chemical calculations have been performed to study the geometry of all compounds based on DFT/TDDFT calculations at B3LYP/6-311G (d,p) level of theory. The calculated geometries are in good agreement with determined single crystal structures. In order to get more shed into the red shifted emission of compounds 4(d-i) with respect to relevant absorption behavior, TD-DFT study was performed. The red shifted emission is found to be controlled by the conformational relaxation in the excited state on the emission spectra.