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.
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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