Thorsten Schwalm

The Gilch Synthesis of Poly(p-phenylene vinylenes): Mechanistic Knowledge in the Service of Advanced Materials

A consistent picture is presented of the mechanistic details and intermediates of the Gilch polymerization leading to poly(p-phenylene vinylenes) (PPVs). In-situ generated p-quinodimethanes are shown to be the real monomers, and spontaneous formation of the initiating radicals is effected by dimerization of some of these monomers to dimer diradicals, the latter also being the reason why significant amounts of [2.2]paracyclophanes are formed as side-products. Chain propagation predominantly proceeds by radical chain growth, occasionally interrupted by polyrecombination events between the growing α,ω-macro-diradicals. Based on this knowledge, oxygen is identified as a very efficient molar-mass regulating agent, and the temporary gelation of the reaction mixtures is interpreted to be the consequence of a very high entanglement of the polymers immediately after their formation. Last but not least, it is rationalized why the usually considered constitutional defects in Gilch PPVs might not be the only and most relevant ones with respect to the efficiency and durability of the organic light emitting devices produced thereof, and why cis-configurated halide-bearing vinylene moieties should be perceived as being among the most critical candidates. These considerations result in the recommendation of straightforward measures that should lead to clearly improved PPVs.

The impact of contact formation on the light emission from ambipolar transistors

In this letter the effect of the charge carrier injection on the performance of ambipolar light-emitting organic field-effect transistors will be investigated. For the analysis, the light output and spatial information of the recombination zone from different devices will be compared. The three investigated devices provide either Ohmic contacts for one or both charge carrier types or hindered injection for both. It will be demonstrated that the light emission in the different operation regimes of the transistor can be used to characterize the contact properties at source and drain.

Rodlike Tetracene Derivatives

Efficient and versatile synthetic access to rodlike tetracene derivatives was developed by means of Diels-Alder cycloaddition, halogenation, halogen-metal exchange, and transition metal mediated coupling reactions. Herein, the synthesis and structural, electrical, and charge-transport properties of three of the resulting materials, namely, 2-(tetracen-2-yl)tetracene, 1,4-bis(2-tetracenyl)benzene, and 2,5-bis(2-tetracenyl)thiophene, are presented. Good crystallization behavior on SiO2 substrates, narrowing of the bandgap by 0.2 eV, and a decrease of the ionization potential of more than 0.5 eV compared to tetracene were observed. Charge-carrier field-effect mobilities on the order of 10-1  cm2  V-1  s-1 , on/off ratios of 105 , and threshold voltages Vth <15 V were found in thin-film organic field-effect transistors prepared by standard high-vacuum deposition techniques.