posted on 1998-11-11, 00:00authored byJye-Shane Yang, Timothy M. Swager
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.