Fluorescent Porous Polymer Films as TNT Chemosensors:  Electronic and Structural Effects

Abstract

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.

Design of fluorescent materials for chemical sensing

“…139,140 The most successful use of these semiconductor materials as fluorescent probes has been the design of sensors for the detection of vapors of nitroaromatic explosives such as trinitrotoluene (TNT) and dinitrotoluene (DNT). 5,141 These semiconductive materials have been incorporated into sensors used for ultra-trace explosive detection in the search for hidden landmines. Devices with femtograms detection limits of TNT have been fabricated.…”

“…With the war on terrorism, the need for accurate, reliable, real-time biological and chemical sensing is in the spotlight. 4,5 Finally, chemical sensing allows for the study and control of chemical processes from the laboratory to the industrial scale, and plays an important role in the food industry for the control of food quality and safety. 6 The list of interesting analytes to be detected is lengthy 2 and there is a need for rapid and low-cost testing methods for a wide range of clinical bioprocesses and in areas of chemical and environmental applications.…”

Structure—Property relationships for exciton transfer in conjugated polymers

“…It is for this reason that thin films of CPs can serve as unparalled, highly sensitive chemosensors (exemplarily for 2,4,6-trinitrotoluene (TNT) 12 ). In general, increases in the diffusion length of the exciton within the CP thin film will enhance the sensitivity of the chemosensor.…”

Assessing the sensing limits of fluorescent dendrimer thin films for the detection of explosive vapors

“…For this reason we chose a standard film preparation method for all three dendrimers that would give films approximately 35-40 nm thick, which is relatively thick when compared to some of the thin films (<10 nm) that have been reported in the literature. [6,[17][18][19][20][21] The analytes used in this study were DNT, pNT and DMNB; both DNT and pNT are chemically similar to TNT with DNT an impurity found in TNT, while DMNB is a taggant added to plastic explosives to facilitate their detection by trained canines. Crucially they all have sufficiently high electron affinities to quench the PL although they differ significantly in terms of their vapor pressures.…”

“…This process is often described in terms of the relative energies of the lowest unoccupied molecular orbital (LUMO) of the sensing material and analyte. [6,7] The sensing of analytes using PL quenching is a straightforward process. In the absence of an analyte, photo-excitation of the fluorescent sensor leads to an exciton that can decay radiatively.…”