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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 π-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 ∼1300 cm-1 in accordance with the energy of ring-stretching modes for aromatic frameworks. This excited state molecular distortion is consistent with the ππ* 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 π-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 PBT model compounds exhibit biexponential luminescence decay lifetimes which were also not significantly affected by the presence of O2. Fibers of PBO and PBT 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 π-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.

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Alkali metal anions generally have narrow NMR lines that are only slightly shifted from those calculated for the gaseous anion. When, however, the anions form contact dimers, as in K+(cryptand[2.2.2])K- and Rb+(cryptand[2.2.2])Rb-, or chains, as in Rb+(18-crown-6)Rb-, the NMR peak of the alkali metal anion is significantly broadened and shifted paramagnetically. In order to obtain reliable quadrupole coupling constants, asymmetry parameters, and chemical shifts from polycrystalline samples of alkalides and electrides, spin−echo 87Rb, 85Rb, and 39K static NMR spectra were obtained for 9 alkalides, 3 electrides, and 11 model compounds. A puzzling distortion of the powder line shapes of two alkalides, an electride, and a model salt was traced to orientation-dependent transverse relaxation times (T 2) by studies of the single-crystal 87Rb NMR spectra of Rb+(cryptand[2.2.2])Cl-. To analyze the NMR powder patterns, a general computer program was developed and coupled to a nonlinear least-squares fitting program.

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