Synthesis and Linear and Nonlinear Optical Properties of Three Push–Pull Oxazol-5(4<i>H</i>)‑one Compounds

Three uncharged push–pull oxazol-5­(4<i>H</i>)-ones were synthesized and thoroughly characterized. The examined molecules contained electron-donor and electron-acceptor groups interacting via a π-conjugated bridge. Spectral properties of the oxazol-5­(4<i>H</i>)-ones were studied in detail in three solvents of different polarities. The results indicate a solvatochromic shift toward lower energy for the charge-transfer state. The compounds are weakly fluorescent in polar solvents, but they have high fluorescence quantum yields in nonpolar solvents. Their two-photon absorption (2PA) properties were characterized by the open- and closed-aperture Z-scan technique, by the pump–probe technique, and by the two-photon excited fluorescence method. The dyes exhibit relatively high effective two-photon absorption cross sections ranging from 490 to 2600 GM at ∼100 GW/cm², according to the Z-scan results, which are found, however, to contain significant contribution from higher-order absorption processes. In addition, these compounds display good photostability