Enhanced solid-state fluorescence in the oxadiazole-based excited-state intramolecular proton-transfer (ESIPT) material: Synthesis, optical property, and crystal structure

We report highly fluorescent oxadiazole-based excited-state intramolecular proton-transfer (ESIPT) material, 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-phenol (SOX) in solid state film (Φ_f = 0.47) as well as in solution (Φ_f = 0.40). From the single crystal X-ray crystallography, a molecular geometry of SOX was found to be nearly planar due to the strong intramolecular hydrogen-bond between the hydroxyl and oxadiazole groups to give rise to the virtually single keto fluorescence. In view of the molecular arrangement, a specific dimer interaction caused by a Coulomb attraction in the SOX crystal was most likely associated with a sliding-away stacking, which contributed to the intense solid-state fluorescence. On the other hand, 2,5-bis-[5-(4-tert-butyl-phenyl)-[1,3,4]oxadiazol-2-yl]-benzene-1,4-diol (DOX) with dual ESIPT sites but otherwise the same as SOX showed a significantly red-shifted orange emission (λ_em = 573 nm) of keto tautomer relative to the bluish-green emission (λ_em = 486 nm) of SOX in chloroform. Similarly, the fluorescence emission of DOX in solid-state film (Φ_f = 0.13) was highly enhanced from that in solution (Φ_f = 0.02). Interestingly, SOX and DOX showed well-defined room-temperature phosphorescence. Kinetic studies on the ESIPT keto fluorescence as well as the phosphorescence were investigated using picosecond laser experiments.