FLUORESCENT POROUS POLYMER-FILMS AS TNT CHEMOSENSORS - ELECTRONIC ANDSTRUCTURAL EFFECTS

The synthesis, spectroscopy, and fluorescence quenching behavior of pe ntiptycene-derived phenyleneethynylene polymers, 1-3, are reported. Th e incorporation of rigid three-dimensional pentiptycene moieties into conjugated polymer backbones offers several design advantages for soli d-state (thin film) fluorescent sensory materials. First, they prevent ,pi-stacking of the polymer backbones and thereby maintain high fluore scence quantum yields and spectroscopic stability in thin films. Secon d, reduced interpolymer interactions dramatically enhance the solubili ty of polymers 1-3 relative to other poly(phenyleneethynylenes). Third , the cavities generated between adjacent polymers are sufficiently la rge to allow diffusion of small organic molecules into the films. Thes e advantages are apparent from comparisons of the spectroscopic and fl uorescence 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 ex ergonicity 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 polyme r 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-aro matic compounds from other species, such as quinones. In short, thinne r films show a larger response to nitro-aromatic compounds, but show a lower response to quinones. Such differences are explained in terms o f polymer-analyte interactions, which appear to be electrostatic in na ture. The rapid fluorescence response (quenching) of the spin-cast fil ms of 1-3 to nitro-containing compounds qualifies these materials as p romising TNT chemosensory materials.