The development of methods for non-contact detection of high explosives is a central theme in the global security agenda. In particular, the race is on to create portable, sensitive and selective techniques for deployment in a wide range of theatres from the battlefield to airports. Methods based upon fluorescence-quenching in polymeric organic semiconductors have received significant attention. Conjugated dendrimers are a class of organic semiconductors that have delivered success in the field of organic light emitting diodes due to their tunable properties via structural modification, high luminescence efficiency, respectable electrical conductivity and amenable solution processability. These features make conjugated dendrimers promising materials for use in chemosensors, and in particular in the sensing of trace amounts of explosive analytes. This thesis describes research conducted on the detection of nitroaromatic explosives with a family of three generations of carbazole dendrimers using both optical and electronic means.
The fluorescence response of the dendrimers in question to five analytes (three nitroaromatic analogues of TNT, one taggant for explosives and one control) relevant to the detection of high explosives were first characterised in the solution phase. Stern-Volmer constant measurements were used to quantify the fluorescence quenching efficiencies of the analytes with respect to the three generations of dendrimers. The Stern-Volmer constants of the first generation (G1) were smaller than those of the higher generation second (G2) and third (G3) for all the analytes, although there was little difference between the values for G2 and G3. Furthermore, time-resolved measurements indicated that the quenching was the result of both static and collisional interactions between dendrimer and analyte. These trends were attributed to the fact that both G2 and G3 are multi-chromophoric with strong inter-chromophore interactions, which result in the formation of long-lived delocalised excitons on the dendrons that are more susceptible to quenching. Nitroaromatic analytes (i.e. 2,4-dinitrotoluene, DNT) were also found to cause more efficient fluorescence quenching than the non-nitrated control benzophenone (BP) and the nitroaliphatic 2,3-dimethyl-2,3-dinitrobutane (DMNB), which is an extensively used taggant in commercial plastic explosives. This behaviour is consistent with the differences in the electron affinities of the analytes and confirms that the fluorescence quenching occurs via an oxidative process.
The fluorescence quenching of the dendrimers in the solid state upon exposure to analyte vapours was then investigated and compared to the sensing behaviours in solution. Unlike the results in solution, the fluorescence quenching of all three dendrimers in the solid state were very similar. This indicated that there exists little correlation between solution and solid-state fluorescence quenching. In contrast to quenching in solution, factors such as the vapour pressure of the analyte and film morphology, play a major role in the solid state quenching. In addition, heating of the sensor films can break the analyte-dendrimer binding and lead to fluorescence recovery.
The dendrimers in question also feature respectable charge transport (mobility properties in particular) and can be incorporated into electronic devices such as an organic field-effect transistor (OFET). G1 possessed the highest carrier mobility in this dendrimer family, so it was selected to be the active layer in a bottom-gate, top-contact OFET to conceptually investigate the sensing performance of dendrimers from an electrical perspective. It was found that the permeation of 4-nitrotoluene (pNT) vapour into the G1 active layer resulted in quick and significant decreases in the OFET channel current and carrier mobility. The reason for this was investigated with transfer characteristic measurements and temperature-dependent carrier mobility measurements. The transfer characteristic measurements of a G1 OFET prior to and after pNT exposure and after heating at 80 °C for 5 min revealed that the decrease in carrier mobility was only dictated by the presence of pNT. The temperature-dependent carrier mobility measurements on a G1 OFET showed “Arrhenius” behaviour and the results demonstrated that there were only intrinsic trap states (~0.3 eV) before pNT exposure and extra pNT-induced trap states (~0.7 eV) after exposure. The Gaussian disorder model (GDM) was used for modelling the charge transport in the dendrimer. The Gaussian width σ describes the width of state distribution in an organic semiconductor with charges hopping between these states. The G1 OFET displayed a single temperature dependence in the measured temperature range before pNT exposure, suggesting a single charge conduction pathway. After pNT exposure, there were two distinct regions with different temperature dependencies, suggesting two conduction pathways: one disturbed by pNT trap states with broadening of the Gaussian width, and the other unaffected with an unchanged Gaussian width. The broadening of state distribution decreases the density of transport states, and thus the carrier mobility decreases.
This work primarily investigates the interactions between dendrimers and explosive analytes and their roles in fluorescence-based sensing and electrical sensing. Understanding of the interactions and sensing mechanisms can potentially guide the design of novel sensing materials and innovation in new sensing devices.