Synthesis and photophysical studies on 2‑styryl phenanthro[9,10‑d]oxazole derivatives

A new series of 2‑styryl phenanthro[9,10‑d]oxazoles was readily accessible from the condensation reaction of 9,10‑phenanthroquinone with cinnamaldehydes in the presence of lactic acid. All these styryl dyes were isolated in good yields and characterized by various analytical and spectroscopic techniques. One of the dyes containing NO₂ group (3d) was structurally characterized by single crystal X-ray analysis. These dyes displayed emission in blue to green region with larger Stokes shift values characteristic to the nature of substituents. In addition, positive solvatochromic trend was observed by increasing the solvent polarity suggestive of a more stabilized polar excited state. Moreover, the addition of trifluoroacetic acid leads to a prominent blue-shift in visible and emission color changes owing to the protonation of the nitrogen atom of oxazole moiety. Among the all, the oxazole derivative having NMe₂ group (3b) exhibits good response to acidic pH in the range of 3.0 to 5.6 with a good linearity upon decreasing the pH from 8.0 to 2.16. The absorption studies were further supported by density functional theory calculations.