Molecular engineering of organic semiconductors: design of self-assembly properties in conjugated thiophene oligomers

Abstract

In order to analyze the correlation between charge transport and structural properties in conjugated oligomers, sexithiophene, 6T, was substituted by hexyl groups, both on the terminal a positions (a,cvDH6T) and as pendant groups in the ß position (ß,ß' 6 ). Structural characterizations by X-ray diffraction show that vacuum-evaporated thin films of 6T and , consist of layered structures in a monoclinic arrangement, with all-trans planar molecules standing on the substrate. When compared to 6T, , 6 is mainly characterized by a very large increase of molecular organization at the mesoscopic level, evidenced by a much longer range ordering. Electrical characterizations indicate that the conductivity of , 6 is largely anisotropic, with a ratio of 120 in favor of the conductivity parallel to the substrate plane, i.e. along the stacking axis. The charge carrier mobility, determined on field-effect transistors fabricated from these conjugated oligomers, also shows an increase by a factor of 25 when passing from 6T to , 6 , reaching a value of 5 X 10-2 cm* 12 3V-1 s-1. In contrast, ß,ß' 6 presents very low conductivity and mobility, the latter being below detection limit. These results are attributed to the self-assembly properties brought by alkyl groups in the a,w position.

Influence of alkyl chain substitution on sexithienyl-metal interface morphology and energetics

“…This angle is larger than the one reported for DH6T on silicon oxide ͑21.3°͒. 9 However, rodlike molecules are prone to exhibit a number of different crystal structure polymorphs ͑with dif- 17 whose existence critically depends on the nature of the substrate and deposition conditions. We therefore assume that multilayer DH6T exists in a different polymorph on ML-DH6T/Ag͑111͒ than it does on SiO 2 ; in addition, some alkyl chains may not be in the all-trans conformation.…”

“…7 However, the growth of molecules may be substantially changed by alkyl chain addition. 8,9 Consequently, the electronic structure of a molecular film and also its interface electronic properties may depend on different molecular organization. 10 For the case of the 6T derivative ␣ , -dihexylsexithienyl ͓DH6T; chemical structure shown in Fig.…”

Liquid crystalline organic semiconductors for organic transistor applications

“…Garnier et al 18 studied OFETs of dialkyl-oligothiophene derivatives, in which the alkyl chains attached to an oligothiophene affected the solubility and the molecular assembly of the molecular layers, and OFETs of dihexyl-quaterthiophene, which showed a liquid crystal phase, that were fabricated by spin-coating at a high temperature. 19,20 The Phillips research group then studied thienylethynyl-terthiophene derivatives as liquid crystalline organic semiconductors and succeeded in fabricating single-domain crystalline OFETs by a phase transition from a nematic phase on an alignment layer, 21 whereas Funahashi et al 22 reported the FET performance in the liquid crystalline phase of a phenyl-terthiophene derivative (3-TTP-Ph-5).…”

Synthesis of Two Novels-Shaped Dibenzo[c,l] Chrysene Derivatives, Crystal Structure, and the Evaluation of their Photophysical Properties

“…Over the past decade significant research has been done and the search for the new π-conjugated systems has been ongoing because of the important, rapidly growing number of applications in electronic devices such as semiconducting materials [2][3][4][5], organic solar cells [6], sensors [7], organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and flexible displays. π-conjugated materials have been extensively studied for their optoelectronic properties due to the offering of low-cost, large-area, and flexible electronic devices [8][9][10]. S-shaped dibenzo[c,l]chrysene derivatives are regarded as a hexacyclic polyaromatic hydrocarbon compound containing two cove-regions, and they have potential applications in biological systems and organic semiconductor materials [11,12].…”