SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF SOME BENZOXAZOLE AND BENZOTHIAZOLE COMPOUNDS

Several benzoxazole and benzothiazole compounds have been prepared and their extended configurations characterized by optical absorption and emission spectroscopy. In general, solutions of these compounds fluor esce strongly and exhibit emission spectral profiles which mirror thei r respective excitation spectra. One exception to this correlation res ults from a chromophore with a nonplanar ground state configuration wh ich disrupts the extended pi-network, promoting a strong hypsochromic shift of the absorption spectrum. The absorption, excitation, and emis sion spectra of these compounds also show a strong vibronic progressio n of similar to 1300 cm(-1) in accordance with the energy of ring-stre tching modes for aromatic frameworks. This excited state molecular dis tortion is consistent with the pi pi nature of the optical excitation . Also, the energy gap between excitation and emission 0-0 bands of th ese benzoxazole and benzothiazole compounds and their polymeric forms are strongly influenced by the minimum allowed intermolecular space. I n dilute solutions or for structures with bulky substituents, only sma ll energy differences are observed between excitation and fluorescence 0-0 bands. In contrast, solid state samples devoid of side groups exh ibit significantly larger energetic displacements accompanied by a pro nounced broadening of both excitation and emission spectral profiles. These results suggest that strong intermolecular pi-stacking interacti ons occur for the planar benzoxazoles and benzothiazoles in the solid state. Excited state lifetime decay measurements for PBO model compoun ds in toluene are monoexponential with essentially identical lifetimes under evacuated and standard pressure conditions. In the solid state, PBO and PET model compounds exhibit biexponential luminescence decay lifetimes which were also not significantly affected by the presence o f O-2. Fibers of PBO and PET revealed three oxygen independent, but wa velength dependent emitting species. The presence of only one emitting species for these benzoxazole compounds in solution, compared with th eir multiexponential lifetime behavior in the solid state, further sup ports strong pi-interactions between these molecules in the solid stat e. This molecular configuration permits benzoxazole and benzothiazole compounds to undergo photoinduced electron transfer in the solid state , which in the presence of oxygen leads to the generation of superoxid e.