Structural and Solvent Effects on the Photophysical Behavior of Pyridyl-Substituted 1,3,4-Oxadiazole Derivatives

The photophysical properties of two pyridyl isomers of 1,3,4-oxadiazole, namely 2-(4-(dodecyloxy)phenyl)-5-(pyridin-3-yl)-1,3,4-oxadiazole (Oxa3py) and 2-(4-(dodecyloxy)phenyl)-5-(pyridin-4-yl)-1,3,4-oxadiazole (Oxa4py), were systematically investigated to elucidate the impact of molecular structure and different solvent environments on the ground and excited states. The effect of the pyridinic nitrogen position was evaluated in terms of solvatochromism, charge transfer properties, and aggregation behavior in methanol-water mixtures. Both compounds exhibit positive solvatochromism, showing a considerable bathochromic shift of 45 nm for Oxa3py and 66 nm for Oxa4py in their emission spectra obtained in carbon tetrachloride and dimethyl sulfoxide. The McRae-Bayliss model and the empirical Reichardt solvent polarity scale, E_T(30), indicate that dipolar effects govern this behavior. The compounds formed aggregates in methanol-water mixtures, displaying a pronounced hypsochromic shift and a distinct influence on emission intensities. Time-dependent density functional theory (TD-DFT) calculations combined with natural transition orbitals (NTOs) analysis confirm the occurrence of intramolecular charge transfer from the dodecyloxyphenyl group to the pyridyl moiety. These findings provide fundamental insight into structure-property relationships and demonstrate how environmental factors and molecular architecture jointly modulate photophysical behavior. This knowledge contributes to the rational design of new optoelectronic materials based on simple heteroaromatic scaffolds.