Highly conjugated D-π-A-π-D form of novel benzo[b]thiophene substituted 1,3,4‑oxadiazole derivatives; Thermal, optical properties, solvatochromism and DFT studies

Abstract

We have designed and synthesized a series of novel highly conjugated Donor-π-Acceptor-π-Donor form of unsymmetrical benzo[b]thiophene substituted 1,3,4‑oxadiazole derivatives UBTO-3(a–e) by utilizing Wittig Reaction followed by palladium catalyzed Suzuki cross coupling reaction and characterized by using standard spectroscopic techniques viz., 1H, and 13C NMR, FT-IR and GC–MS. To improve the intramolecular charge transfer property and to explore the optical properties of target compounds, Donor-Acceptor-Donor groups are linked via extending π conjugation through a phenyl spacer with para linkages. Optical/solvatochromic properties are studied in detail by UV–Vis absorption and fluorescence spectroscopy. Optical bandgap values were ranging from 2.266 to 2.704 eV. The derivatives exhibit high thermal stability that is the 5% weight loss temperature of derivatives are in the range 204–367 °C and the glass transition temperatures of the compounds UBTO-3(c, d and e) are at 146 °C, 140 °C and 133 °C respectively. Synthesized derivatives have better solubility in common organic solvents. Experimentally measured quantum yields (Φ) are in the range of 0.09 to 0.39 in toluene solvent and the molecules exhibit Stokes shifts in the range of 45–100 nm. Density functional theory computations have been performed to explore intramolecular charge transfer property and structure-property relationship of the probes. The novel unsymmetrical benzo[b]thiophene substituted 1,3,4‑oxadiazole derivatives are potential candidates and could play a significant role in organic light-emitting devices (OLED)/optoelectronic applications.