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Organic thin film transistors (TFTs) are of interest for lowcost, large-area electronic applications, such as active-matrix displays, electronic paper, flexible microelectronics, and chemical sensors.[1±8] The performance of organic TFTs is determined primarily by the field effect mobility of the charge carriers in the organic semiconductor layer and by the efficiency of injecting and extracting carriers at the source and drain contacts. For virtually all classes of organic semiconductors, the intrinsic carrier mobility depends critically on the degree of molecular ordering and on the extent of the p±p stacking in the material.[9±11] Consequently, optimizing the chemical structure of the organic semiconductor with regard to optimum molecular ordering and maximum orbital overlap continues to be of great importance for the further advancement of organic TFT technology. Despite the extraordinary number of organic semiconductors that have been synthesized and evaluated for use in organic TFTs, [12] the relationships between molecular structure and electrical TFT performance remain sketchy at best. The fused hydrocarbon pentaceneÐby all accounts a rather unspectacular moleculeÐcontinues to deliver the highest electrical performance, regardless of the method of film deposition (thermal evaporation, vapor phase epitaxy, conversion of a solution-processed precursor) and regardless of the contact configuration (top or bottom contacts).[13±15]A useful and practical strategy for investigating the relationships between molecular structure and electrical performance is the systematic variation of the number of repeat units in a particular type of molecular semiconductor. This can be done either by varying the number of units in the conjugated backbone of the molecule, or by varying the length of alkyl substituents. One particularly useful class of model compounds for this type of investigation are the oligothiophenes, due to their relatively straightforward synthesis and because of the wide range of possible modifications in their chemical structure. Organic TFTs based on oligothiophenes were first reported by Horowitz and Garnier [16±18] and later by Dodabalapur, Katz, and others. [19±21] Carrier mobilities reported for a-sexithiophene (a-6T) TFTs have improved from 10 ±4 cm 2 / V s to greater than 0.01 cm 2 /V s.[18±21] Substituting alkyl chains at the a-and a¢-positions of the a-6T molecule led to an increase in carrier mobility to 0.13 cm 2 /V s. [22,23] Carrier mobilities near 0.2 cm 2 /V s have been reported for a-octithiophene (a-8T) TFTs with active layers deposited at 150 C and higher.[24] For many years, oligothiophenes and their alkyl-substituted derivatives have been among the most intensely investigated organic semiconductors and have even led to the demonstration of fast integrated circuits.[3]We have synthesized and evaluated a series of alkyl-substituted oligothiophenes with chromophore length ranging from four to six thiophene units (a,a¢-didecylquaterthiophene, a,a¢-didecylquinquethiophene, and a,a¢-didecy...

A systematic study of the influence of generation number on the phase behavior of LC dendrimers is presented. For this purpose, phase behaviors and structures of first to fifth generations of liquid crystalline (LC) carbosilane dendrimers with 8, 16, 32, 64, and 128 terminal cyanobiphenyl groups were investigated. Investigation of thermal behavior of the LC dendrimers by means of polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction experiments reveals smectic-type mesophases over a wide temperature region. It is shown that with increasing generation number the isotropization temperature increases whereas the enthalpy of this phase transition decreases. The strongest influence of spherical molecular architecture on the phase behavior of the LC dendrimers appears at high generations. In the case of LC dendrimer of the fifth generation, it leads to the formation of two levels of a structural organization. This dendrimer forms different supramolecular nanostructures of columnar type in addition to smectic-like arrangement of mesogenic groups. Possible structures of all mesophases formed are discussed.