Simon S. Y. Chen

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.

Three-dimensional carbazole-based dendrimers: model structures for studying charge transport in organic semiconductor films

We report the synthesis and charge transport properties of a series of three-dimensional dendrimers up to the third generation that have a 9,9′-spirobifluorene core, carbazole-based dendrons and di-n-propylfluorene surface groups. The dendrimers can all be spin-coated to form good quality amorphous films. The charge carrier mobility of the dendrimers was measured by two different methods; in an organic field-effect transistor (OFET) architecture, and by Charge Extraction by Linearly Increasing Voltage (CELIV). In the OFET configuration the first generation dendrimer had a maximum mobility of 4.1 × 10−4 cm2 V−1 s−1 and an ON/OFF ratio of 1.1 × 105. Unexpectedly, in spite of the third generation dendrimer having a volume approximately six times that of the first generation, the mobility was found to decrease by only an order of magnitude. A similar trend in mobility was seen in the CELIV results. Photoluminescence (PL) measurements in solution showed that the first generation dendrimer was comprised of non-interacting chromophores, while the second and third generation dendrimers had substantial intra-dendrimer interchromophore interactions. In the solid-state, PL measurements showed that for the first generation dendrimer there were clear inter-dendrimer interchromophore interactions with little change for the second and third generations. Comparison of the dendrimer molecular volumes in solution and the solid-state showed that in the latter, the dendrimers took up a smaller volume suggesting that there was interdigitation of the dendrons. For the first generation dendrimer the interdigitation leads to trap sites for charge transport, with the small decrease in mobility in moving from the first to the second and third generation being due to the extra intra-dendrimer interchromophore interactions. Model dendritic systems such as these can be used to gain significant insight into the subtly of charge transport phenomena in solution processable macromolecular organic semiconductors, since they offer a level of molecular control that is difficult to achieve with polymers.

Photophysics of Delocalized Excitons in Carbazole Dendrimers

The photophysical properties in solution of three generations of carbazole-based dendrons and dendrimers with fluorenyl surface groups were studied using steady-state, time-resolved femtosecond transient absorption and anisotropy, and coherent two-dimensional ultraviolet spectroscopy. It was found that increasing the generation caused a switch in the nature of the emissive state between the first-generation compounds and the second- and third-generation dendrimers. Time-resolved anisotropy measurements revealed low initial anisotropies that decreased with increasing dendrimer generation consistent with increasing intradendrimer interchromophore coupling. Two-dimensional UV spectroscopy showed that the signal from the second- and third-generation dendrimers is the product of multiple chromophores interacting. The maximum number of interacting chromophores is reached by the second generation.

Charge transport properties of carbazole dendrimers in organic field- effect transistors

We report three generations of p-type dendrimer semiconductors comprised of spirobifluorene cores, carbazole branching units and fluorene surface groups for use in organic field-effect transistors (OFETs). The group of dendrimers are defined by their generation and noted as SBF-(Gx)2, where x is the generation. Top contact-bottom gate OFETs were fabricated by spin-coating the dendrimers onto an n-octyltrichlorosilane (OTS) passivated silicon dioxide surface. The dendrimer films were found to be amorphous. The highest mobility was measured for the first generation dendrimer (SBF-(G1)2), which had an average mobility of (6.6 ± 0.2) × 10-5 cm2/V s and an ON/OFF ratio of 3.0 × 104. As the generation of the dendrimer was increased there was only a slight decrease in the measured mobility in spite of the significantly different molecular sizes of the dendrimers. The mobility of SBF-(G3)2, which had a hydrodynamic radius almost twice of SBF-(G1)2, still had an average mobility of (4.7 ± 0.6) × 10-5 cm2/V s and an ON/OFF ratio of 2.7 × 103. Density functional theory calculations showed that the highest occupied molecular orbital was distributed over the core and carbazole units meaning that both intra- and intermolecular charge transfer could occur enabling the hole mobility to remain essentially constant even though the dendrimers would pack differently in the solid-state.

The nature and role of trap states in a dendrimer-based organic field-effect transistor explosive sensor

We report the fabrication and charge transport characterization of carbazole dendrimer-based organic field-effect transistors (OFETs) for the sensing of explosive vapors. After exposure to para-nitrotoluene (pNT) vapor, the OFET channel carrier mobility decreases due to trapping induced by the absorbed pNT. The influence of trap states on transport in devices before and after exposure to pNT vapor has been determined using temperature-dependent measurements of the field-effect mobility. These data clearly show that the absorption of pNT vapor into the dendrimer active layer results in the formation of additional trap states. Such states inhibit charge transport by decreasing the density of conducting states.