Mixing‐Induced Orientational Ordering in Liquid‐Crystalline Organic Semiconductors

Abstract

This band-like carrier conduction is beneficial in the development of high-performance organic field-effect transistors (OFETs). Among numerous layered OSCs developed thus far, unsymmetric rod-like organic molecules composed of π-electron cores linked with long alkyl chains (and/ or other substituents) have enabled the development of OSCs with excellent performance. The advantages of these materials are exemplified by the properties of substituted [1]benzothieno [3,2-b][1]benzothiophene (BTBT), [10][11][12][13][14] benzothieno [3,2b]naphtho [2,3-b]thiophene (BTNT), [15,16] and benzothieno-[6,5-b]benzothieno[3,2-b] thiophenes (BTBTTs). [17] Such molecules form unipolarly aligned monomolecular layers composed mostly of layeredherringbone (LHB) packing-an ideal form for 2D semiconducting carrier transport. In addition, each unipolar layer is stacked alternately to afford bilayer-type packing which we call bilayer-type LHB (b-LHB). [11,12,[18][19][20][21][22] A typical OSC is 2-alkyl-7-phenyl-where n represents the number of carbon atoms in the alkyl chain), which exhibits field-effect mobility greater than 10 cm 2 V −1 s −1 . [11,23] The b-LHB packing of Ph-BTBT-C n (n ≥ 5) is considerably stabilized by outer alkyl-chain layers, as demonstrated by intermolecular interaction calculations. [20,21] An intriguing feature is that the compound exhibits thermotropic liquid-crystalline (LC) transitions to smectic-E (SmE) and smectic-A (SmA) phases at elevated temperatures. [11] The large conformational degrees of freedom involved in the alkyl substituents might be associated with the emergence of LC phases. [24][25][26] Such alkylated OSCs also exhibit good solvent solubility, which is a critical feature for achieving availability in solution-based thin-film processing or printed electronics technologies. [27,28] On the basis of a molecular dynamics simulation, researchers found that the temporary LC-like layer formed at the liquid-air interface plays a critical role in the growth of a large-area and highly uniform selforganized OSC layer in solution processes. [29] However, the roles of the substituents and the process by which molecular ordering over LC states is controlled remain unclear.Recently, a substituent modification of Ph-BTBT-C n to 2-alkyl-7-phenylethynyl-BTBT (PE-BTBT-C n ), in which the phenyl group is substituted with an extended substituent of Organic semiconductors (OSCs) often exhibit thermotropic liquid-crystalline (LC) phases in which molecular orientational order is partially lost. The formation of a temporary LC state via a solution process is expected to promote printing-based device production, although a method of controlling molecular ordering over LC states has not yet been developed. Here, it is demonstrated that long-axis molecular ordering can be controlled under ambient conditions using unsymmetrically alkylated OSCs with high layered crystallinity. Introducing a small amount of OSC molecules with longer alkyl-chain lengths leads to the conversion of the long-axis disordered phase into a bila...

Boundary research between organic conductors and transistors: new trends for functional molecular crystals

“…This 2D molecular packing increases the dimensionality of the electronic structure from 1D to quasi-1D (Q1D) and consequently stabilises the metallic state, which is reminiscent of the very early days of TTF-based superconductors. 161,162 Note that this molecule shows a quinoid-like bond alternation upon oxidation especially in the central thienothiophene part, as is well-known in oligothiophenes. 175–178 These studies demonstrate that functionalised BTBT derivatives are promising electron donors in molecular conductors.…”

Alkyl-Parity Controlled Switching of Polar/Antipolar Organic Semiconductors

“…[7,11,12] Meanwhile, alkyl substitutions are effective for obtaining highly layered-crystalline organic semiconductors (OSCs) that are well suited for manufacturing organic eld-effect transistors (OFETs). [19][20][21][22][23][24][25][26][27][28][29][30][31][32] Substitutions of extended fused-ring π-electron skeletons (or π-cores) by relatively long alkyl chains enabled the development of several solution-processable OSCs with layered crystalline structures, which afforded OFETs with excellent carrier transport characteristics. [33][34][35][36][37][38][39][40][41] A notable example is phenyl-/alkylated- [1]benzothieno [3,2-b][1]benzothiophene (Ph-BTBT-C n ), in which the long-axis ends of the BTBT π-core are substituted by phenyl and alkyl groups.…”

“…[43] However, the parity of n does not affect the overall molecular packing structure, despite the clear parity dependence of terminal C-C bond orientation in the crystals. Each unidirectionally-oriented (polar) monomolecular layer is stacked alternately via head-to-head and tail-totail (where the head and tail denote phenyl and alkyl groups, respectively) alignments to form bilayer-type herringbone (b-LHB) packing [28][29][30][31][32] (which is later denoted also as antipolar-type herringbone), although a slight variation in the packing motif with an increase in n was observed in Ph-BTBT-C n . Parity-sensitive and parity-dependent variations in interlayer stacking have not been reported for any alkylated OSCs.…”

Influence of the Thiophene Ring on the Molecular Order of Structurally Simple π‐Conjugated Smectogens: 13C NMR Study

“…Organic semiconductors can be classified into (a) crystalline/amorphous solids [13] and (b) liquid crystalline materials [14] . The liquid crystalline semiconductors are those in which the core unit is constructed with π‐conjugated units that impart the required optoelectronic and photonic properties, while the terminal alkyl chains can accord flexibility and facilitate the solubility in common solvents [14–21] . The underlying feature of liquid crystalline semiconductors against crystalline organic semiconductors is the ability to form monodomain samples without defects caused by grain barriers [16] .…”

“…[14] The liquid crystalline semiconductors are those in which the core unit is constructed with π-conjugated units that impart the required optoelectronic and photonic properties, while the terminal alkyl chains can accord flexibility and facilitate the solubility in common solvents. [14][15][16][17][18][19][20][21] The underlying feature of liquid crystalline semiconductors against crystalline organic semiconductors is the ability to form monodomain samples without defects caused by grain barriers. [16] Further, the long-range orientational order in liquid crystalline semiconductors enables better charge carrier mobility.…”