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 fluoresce strongly and exhibit emission spectral profiles which mirror their respective excitation spectra. One exception to this correlation results from a chromophore with a nonplanar ground state configuration which disrupts the extended pi-network, promoting a strong hypsochromic shift of the absorption spectrum. The absorption, excitation, and emission spectra of these compounds also show a strong vibronic progression of similar to 1300 cm(-1) in accordance with the energy of ring-stretching modes for aromatic frameworks. This excited state molecular distortion is consistent with the pi pi* nature of the optical excitation. Also, the energy gap between excitation and emission 0-0 bands of these benzoxazole and benzothiazole compounds and their polymeric forms are strongly influenced by the minimum allowed intermolecular space. In dilute solutions or for structures with bulky substituents, only small energy differences are observed between excitation and fluorescence 0-0 bands. In contrast, solid state samples devoid of side groups exhibit significantly larger energetic displacements accompanied by a pronounced broadening of both excitation and emission spectral profiles. These results suggest that strong intermolecular pi-stacking interactions occur for the planar benzoxazoles and benzothiazoles in the solid state. Excited state lifetime decay measurements for PBO model compounds 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 of O-2. Fibers of PBO and PET revealed three oxygen independent, but wavelength dependent emitting species. The presence of only one emitting species for these benzoxazole compounds in solution, compared with their multiexponential lifetime behavior in the solid state, further supports strong pi-interactions between these molecules in the solid state. 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 superoxide.
