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We report three generations of dendrimers incorporating either a fluorene or spirobifluorene core with carbazole dendrons and fluorene surface groups that are effective sensing materials for the detection of nitrated explosives by fluorescence quenching. The photophysical properties of the dendrimers were investigated with a combination of steady-state absorption and photoluminescence and time-resolved photoluminescence. We show that the first-generation dendrimers behave as single chromophores while the higher-generation dendrimers contain multiple chromophores that interact to give excimer-like emissive states. Stern−Volmer measurements with nitrated analytes show that the quenching efficiency decreases with generation for the planar fluorene-cored dendrimers and increases with generation for the more three-dimensional spirobifluorene-cored dendrimers. These contrasting trends are shown to be caused primarily by changes in the quenching efficiency of static interactions with the nitrated analytes, which is a consequence of the choice of core. Our results highlight the potential for exploiting such excimer-like states for chemical sensing, particularly in the case of nitrated explosives.

We present a study on three generations of fluorescent carbazole dendrimers that exhibit strong binding with nitroaromatic compounds accompanied by photoluminescence (PL) quenching, making them attractive sensing materials for the detection of explosives such as 2,4,6-trinitrotoluene (TNT). The absorption and release of vapors of the (deuterated) TNT analogue 4-nitrotoluene (pNT) from thin films of the dendrimers were studied with a combination of time-correlated neutron reflectometry and PL spectroscopy. When saturated with pNT the PL of the films was fully quenched and could not be recovered with flowing nitrogen at room temperature but only upon heating to 40-80 °C. Although the majority of the absorbed pNT could be removed with this method the recovered films were found to still contain a residual pNT concentration of ~0.1 molecules per cubic nanometer. However, the proportion of the PL recovered increased with generation with the third generation dendrimer exhibiting close to full recovery despite the presence of residual pNT. This result is attributed to a combination of two effects. First, the dendrimer films present a range of binding sites for nitroaromatic molecules with the stronger binding sites surviving the thermal recovery process. Second, there is a large decrease of the exciton diffusion coefficient with dendrimer generation, preventing migration of the excitation to the remaining bound pNT.

Abstract:Fluorescence-based sensing with organic semiconductors is a powerful method for the detection of a broad range of analytes including explosives, chemical weapons and drugs.Diffusion of an analyte into an organic semiconductor thin film, and its subsequent interaction with the chromophore are key factors that govern the sensing performance of a chemosensor.In this study the diffusion behaviour of an explosive analyte analogue into a sensing film of a conjugated dendrimer was investigated using a quartz crystal microbalance (QCM) and correlated with neutron reflectivity measurements. The mechanistic insights of paranitrotoluene (pNT) sorption in the films of different thicknesses of a first generation dendrimer with fluorenyl surface groups, carbazole dendrons and a spirobifluorene core were studied and interpreted in terms of the underlying kinetics and thermodynamics. Sorption measurements suggest that the process of diffusion of pNT vapour into the dendrimer films is Super Case II, which involves swelling of the film. Swelling of the film was confirmed by neutron reflectometry measurements, which also showed uniform distribution of the pNT molecules throughout the entire film thickness. The activation energy barrier and change in Gibbs free energy in the sorption process were calculated from the QCM responses. The sorption process was found to be thermodynamically (not kinetically) controlled and independent of film thickness. This work sheds insight into the structure-property relationships that govern the performance of organic semiconductor fluorescence-based chemosensors.

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