Photophysical properties of some new fluorescent 1,2,4-oxadiazoles

In this thesis, we synthesized some new 1,2,4-oxadiazoles having aryl and styryl moieties functionalized with different electron-donating and -withdrawing groups, such as OCH3, SCH3, CN, and NO2. The photophysical properties of the compounds were also investigated by utilizing UV-Vis absorption, steady-state fluorescence, and time-resolved fluorescence spectroscopy techniques, in detail. It was found that just the compounds with the structure comprising an electron-donating group on the aryl-moiety and an electron-withdrawing group on the styryl-moiety exhibit fluorescence emission in the solution. The absorption and fluorescence measurements have revealed that the spectral position of the fluorescence emission exhibits strong redshifted behavior with the increase in solvent polarity, while the absorption curve remains almost unchanged. In addition, the fluorescence quantum yield of both compounds was found to be in a decreasing trend with increasing solvent polarity, which suggests the dipole-dipole interactions between the fluorophore molecule having strong intramolecular charge transfer (ICT) characteristics and solvent. The quantum chemical calculations comprising density functional theory also elucidated the suggested strong ICT characteristic for both compounds. Furthermore, it was observed solid-state fluorescence emission in all compounds. The compounds with the structure comprising an electron-donating group on the aryl-moiety and an electron-withdrawing group on the styryl-moiety were found to exhibit relatively higher quantum yields among them. Also, it was found that the fluorescence quantum yield tends to decrease or increase depending on the type of substituent on the aryl and styryl moieties, and the results are well consistent with time-resolved fluorescence measurements. The findings revealed that the non-radiative decay rate in the emission mechanism of the compounds dominantly governs their fluorescence emission in solid-state, depending on the type of substituent on the aryl and styryl moieties.