Hamish Cavaye

The binding and fluorescence quenching efficiency of nitroaromatic (explosive) vapors in fluorescent carbazole dendrimer thin films

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.

Photophysical properties of 9,10-disubstituted anthracene derivatives in solution and films.

We have carried out absorption, time-resolved fluorescence, and fluorescence quantum yield measurements of four new soluble anthracene derivatives. They show natural radiative lifetimes in the range of 2.5-4.4 ns, which is 5-10 times shorter than those reported for unsubstituted anthracene. The 9,10-bis(phenylethynyl)anthracene (BPEA) derivatives show the largest fluorescence transition dipoles, which is attributed to extended π-conjugation between anthracene and phenyls through acetylene linkages. Spin-cast films of the BPEA derivatives show strong fluorescence quenching by weakly emitting low energy excitations, which is attributed to excimer-like traps. Quenching is significantly reduced when bulky dendrons are attached so that they give maximum coverage of the emitting chromophore and prevent their aggregation. The results show that anthracene derivatives can be developed into efficient solution-processable fluorescent emitters for the blue and green spectral regions.

Molecular versus exciton diffusion in fluorescence-based explosive vapour sensors.

The diffusion of p-nitrotoluene vapours into polymer or dendrimer sensing films follows Super Case II dynamics in which the quenching efficiency is strongly correlated to an accelerating analyte front propagating through the neat film rather than being reliant on exciton diffusion.

Vertical morphology in solution-processed organic solar cells

Motivated by a report of an all-solution processed P3HT/PCBM ‘bilayer’ organic solar cell, we have investigated the vertical morphology by using a combination of techniques, including neutron reflectometry. By correlating the device performance with the active layer morphology, we establish that sequential solution processing is an elegant way to make high efficiency bulk heterojunction organic solar cells.