Comparison of fluorescent, phosphorescent and TADF luminophores for explosive detection

The detection of nitroaromatic compounds is crucial for protecting people from explosive hazards, as well as for preserving ecosystems from their toxic effects. Photoluminescence-based sensors offer one approach to detect trace levels of explosives. The detection mechanism is commonly based on a bimolecular interaction between the luminophore and explosive molecule, in which a photoinduced electron transfer (PET) from the luminophore to the analyte causes a quenching of the photoluminescence. The sensitivity and quenching behaviour of the sensor is closely related to the interaction strength between these two compounds, and the emission mechanism of the luminophore. In this thesis, the influence of the emission mechanisms (fluorescence, phosphorescence, and thermally activated delayed fluorescence - TADF) on the photoluminescent response towards the detection of nitroaromatic explosives was investigated. To evaluate the luminescence quenching kinetics of representative emitters, steady-state and time-resolved photoluminescence measurements are applied in solution and vapour sensing experiments. The quenching behaviour exhibited by the TADF compounds in solution, was found to deviate from a linear Stern-Volmer relationship, and to be strongly dependent on the intrinsic parameters of the TADF emitter: the product of the initial ISC and RISC quantum yields; the sum of all singlet rate constants; and the sum of all triplet rate constants; with the singlet and triplet populations being quenched with different efficiencies. To model the quenching dynamics of this class of emitters, modified Stern–Volmer equations were derived and applied to different TADF molecules when used as luminophores for sensing a range of PET quenchers. Vapour sensing was investigated through a combination of steady-state and time-resolved luminescence measurements, showing for the first time that TADF molecules can be used as a sensor for nitroaromatic vapours. These results indicate that in the solid state, the singlet and triplet populations are each quenched by the analyte, and that the temporal response may be modified by blending the luminophore in different polymer hosts. In summary, the results presented in this thesis demonstrate for the first time the advantages of applying TADF materials as photoluminescent sensors of nitroaromatic compounds and, more broadly, PET quenchers, in both solution and vapour sensing. Besides offering a promising route to significantly increase the sensitivity of PET quencher, the careful monitoring of the singlet and triplet population of TADF quenching systems can be a valuable tool to achieve selectivity in photoluminescence sensors.