Fluorescent carbazole dendrimers for the detection of nitroaliphatic taggants and accelerants

The detection of explosive taggant 2,3-dimethyl-2,3-dinitrobutane (DMNB) and accelerant nitromethane (NM) by oxidative quenching of dendrimer fluorescence is examined. Two fluorescent dendrimers incorporating a 3,6-disubstituted-9-n-hexylcarbazole-based core and first-generation biphenyl-based dendrons linked directly or with acetylene bridges are reported. The dendrimers display good photoluminescence (PL) quantum yields in both solution and thin films and appropriate excited state energies for oxidation by the nitroaliphatic analytes. The dendrimer natural excited state lifetimes in solution of 6.8 and 7.4 ns were found to be significantly longer than previously reported fluorescent conjugated polymers and dendrimers for sensing applications. Steady-state PL quenching measurements in solution revealed the highest quenching efficiencies for the detection of nitroaliphatics reported to date of 59 ± 1 M ^-1 for DMNB and 78 ± 1 M ^-1 for NM. Furthermore, PL lifetime quenching measurements confirmed that the dendrimers were quenched by a predominantly collisional quenching mechanism. As such, the unprecedented quenching efficiencies with nitroaliphatics in solution are due to the combination of the long excited state lifetimes of the dendrimers and efficient collisional quenching. The fluorescence of dendrimer thin films was also reversibly quenched by exposure to pulses of sub-saturation concentrations of analyte vapours. However, in the thin film case the sensitivity towards DMNB was found to be greater than NM, highlighting the disparity between solution and thin film fluorescence quenching measurements.