The synthesis, spectroscopy, and fluorescence quenching behavior of pentiptycene-derived phenyleneethynylene polymers, 1−3, are reported. The incorporation of rigid three-dimensional pentiptycene moieties into conjugated polymer backbones offers several design advantages for solid-state (thin film) fluorescent sensory materials. First, they prevent π-stacking of the polymer backbones and thereby maintain high fluorescence quantum yields and spectroscopic stability in thin films. Second, reduced interpolymer interactions dramatically enhance the solubility of polymers 1−3 relative to other poly(phenyleneethynylenes). Third, the cavities generated between adjacent polymers are sufficiently large to allow diffusion of small organic molecules into the films. These advantages are apparent from comparisons of the spectroscopic and fluorescence quenching behavior of 1−3 to a related planar electron-rich polymer 4. The fluorescence attenuation (quenching) of polymer films upon exposure to analytes depends on several factors, including the exergonicity of electron transfer from excited polymer to analytes, the binding strength (polymer-analyte interactions), the vapor pressure of the analyte, and the rates of diffusion of the analytes in the polymer films. Films of 1−3 are particularly selective toward nitro-aromatic compounds. The dependence of fluorescence quenching on film thickness provides an additional criterion for the differentiation of nitro-aromatic compounds from other species, such as quinones. In short, thinner films show a larger response to nitro-aromatic compounds, but show a lower response to quinones. Such differences are explained in terms of polymer−analyte interactions, which appear to be electrostatic in nature. The rapid fluorescence response (quenching) of the spin-cast films of 1−3 to nitro-containing compounds qualifies these materials as promising TNT chemosensory materials.

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The photochemical behavior of the three positional isomers of trans-aminostilbene is reported and compared to that for unsubstituted trans-stilbene. The absorption spectrum of the para isomer displays a single intense long-wavelength band; however the meta and ortho isomers display two less intense bands as a consequence of configuration interaction. All three isomers are fluorescent and display similar solvent-induced shifts of their fluorescence maxima. The fluorescence rate constant of the para isomer is larger than that of the ortho or meta isomer. The para isomer has a short singlet lifetime and high photoisomerization quantum yield, similar to those for trans-stilbene. In contrast, the ortho and meta isomers have long singlet lifetimes and low photoisomerization quantum yields. Isomerization of the para isomer is a singlet-state process, whereas isomerization of both the ortho and meta isomers is a triplet-state process. The lower bound for the barrier for singlet-state torsion of the ortho and meta isomers is 7 kcal/mol. The barrier height is found to be determined by the effects of the amino substituent upon the relative energies of the fluorescent singlet and twisted singlet states.

Mice chimeric for the expression of alpha4 integrins were used to dissect the roles of these receptors in development and traffic of lymphoid and myeloid cells. During fetal life, T cell development is alpha4 independent, but after birth further production of T cells becomes alpha4 dependent. Precursors for both T and B cells require alpha4 integrins for normal development within the bone marrow. In contrast, monocytes and natural killer cells can develop normally without alpha4 integrins. Thus, there are lymphocyte-specific, developmentally regulated requirements for alpha4 integrins in hematopoiesis in the bone marrow. We also show that alpha4 integrins are essential for T cell homing to Peyer's patches, but not to other secondary lymphoid organs, including spleen, lymph nodes, and intestinal epithelium.