Six novel star-shaped compounds with a triphenylamine core and carbazole or fluorene sidearms have been synthesized by Suzuki cross-coupling. The star-shaped molecules are able to form molecular glasses. They were characterized regarding their thermal, optical, and electrochemical properties. The new compounds were tested as organic semiconductors in solution processed organic field-effect transistors (OFETs). Mobilities of 3 × 10 -4 cm 2 /(V s), high on/off ratios of up to 10 5 , and low threshold voltages were obtained. The new materials show very small hysteresis and an exceptionally high stability under ambient conditions.
The precise quantification of the pyroelectric coefficient p is indispensable for the characterization of pyroelectric materials and the development of pyroelectric-based devices, such as radiation sensors or energy harvesters. A summary of the variety of techniques to measure p is given in the present review. It provides a classification after the thermal excitation and an outline of capabilities and drawbacks of the individual techniques. The main selection criteria are: the possibility to separate different contributions to the pyroelectric coefficient, to exclude thermally stimulated currents, the capability to measure p locally, and the requirement for metallic electrodes. This overview should enable the reader to choose the technique best suited for specific samples.
In this paper we present a new synthesis for N-alkylated 2,7-dibromocarbazoles, from which 2,7-linked carbazole trimers have been synthesized by Suzuki cross coupling. The compounds were characterized regarding their thermal, optical, and electrochemical properties and compared to analogous fluorene trimers, which also have been prepared by Suzuki reactions. The carbazole and fluorene trimers exhibit thermal stabilities of more than 300 °C and are able to form molecular glasses. Only one of the carbazole trimers is crystalline. Both carbazole and fluorene trimers exhibit a bright blue fluorescence with a maximum at 393 nm and show similar absorption characteristics. Cyclovoltammetric experiments revealed the electrochemical stability of the fluorene trimers. Their HOMO and LUMO levels are at -5.6 and -2.5 eV, respectively. In contrast to the fluorenes, the 2,7-linked carbazole trimers showed nonreversible oxidations in the CV experiments.
We time resolve the event of singlet separation into free carriers in an oligofluorene using femtosecond field assisted pump-probe technique. Steady state photoinduced absorption ͑PA͒ measurements have also been employed to identify triplet absorption ͑T 1 − T n ͒ in the spectral region of 1.8 eV. As a result of an applied electric field of 2.2 MV/ cm singlets are observed to undergo a dissociation process consisting of first singlet splitting into bound polaron pairs and second further dissociation of polaron pairs into free carriers. Subsequent free carrier recombination yields a new population of singlets and triplets. Our results suggest that nongeminate recombination leads to the formation of 35% singlets and 65% triplets.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.