Fluorescence Properties of Pyridine and Pyridine-Carbonitrile Derivatives: Photophysical and Structural Analysis

This study investigates a series of novel 2-methoxypyridine derivatives, specifically 6-(2,5-dichlorothiophen-3-yl)-4-(Ar)-2-methoxypyridine-3-carbonitrile (1?6), 6-(2,5-dichlorothiophen-3-yl)-4-(Ar)-2-methoxypyridine (7?9), 6-(5-chlorothiophen-2-yl)-4-(Ar)-2-methoxypyridine-3-carbonitrile (10?14), and 6-(5-chlorothiophen-2-yl)-4-(Ar)-2-methoxypyridine (15 and 16), where Ar represents various heteroaryl groups. A single crystal X-ray diffraction analysis of selected derivatives (2, 3, 12, and 13) revealed stabilization through C?H???O/N/S hydrogen bonding, ????? stacking, and C?H???? interactions. The photophysical properties of all derivatives were characterized via UV-Vis absorption and fluorescence emission spectroscopy in solvents of varying polarity. The derivatives exhibited blue emissions ranging from 379 to 535 nm, with quantum yields from 1.55% to 29.9% and significant Stokes shifts (30 to 200 nm). Notably, mono-chlorinated derivatives demonstrated enhanced fluorescence efficiency compared to their di-chlorinated counterparts. Furthermore, the incorporation of a cyano (CN) group induced pronounced bathochromic shifts in both absorption and emission spectra, while derivatives 6 and 14, featuring nitrogen atoms in their heteroaryl substituents, exhibited exceptionally large Stokes shifts. Density functional theory (DFT) calculations at the B3LYP/6-31g(d,p) level confirmed that key photophysical parameters are predominantly solvent-independent. This research underscores the potential of these 2-methoxypyridine derivatives as versatile fluorophores for applications in chemical sensing, material science, and optoelectronics, providing valuable insights into structure-property relationships for the development of efficient blue-emitting materials.