Electronic Conduction in the Chiral Nematic Phase of an Oligothiophene Derivative

Abstract

[a] Cholesteric liquid crystals (CLCs) are promising optical materials for reflective displays [1] and imaging devices [2] because the periodicities of their helical structures are of the order of wavelengths in visible light. Circularly polarized photoluminescence [3] and laser emission from cholesteric liquid crystals have been investigated recently, [4][5][6][7] but only optical pumping was possible in these studies in which fluorescent dyes were doped in the cholesteric matrixes. The realization of electrical pumping systems, which are more practical, requires the construction of semiconductive cholesteric systems.Electronic charge-carrier transport has been confirmed in smectic systems [8][9][10][11][12] and discotic columnar phases [13][14][15][16] in which intermolecular packing structures promote intermolecular electronic charge transfer. In contrast, only ionic conduction caused by contamination with ionic impurities has been observed in nematic or cholesteric phases, [17][18][19][20][21][22][23] which exhibit more liquidlike fluidity than smectic and discotic mesophases. Electronic conduction has also been observed in the glassy nematic phases of some low-and high-molecular-weight materials. [24,25] In particular, the glassy chiral nematic phase of a fluorene derivative exhibited good hole-transporting properties, with mobility on the order of 10 À4 cm 2 V À1 s À1 , in addition to luminescence of circularly polarized light. [25] No reports exist, however, on electronic charge-carrier transport in fluidic nematic or chiral nematic phases, which are more tunable with respect to the temperature and electric field than their glassy counterparts.As well as the effect of ionic impurity, the transfer integral in achiral and chiral nematic phases displaying only uniaxial alignment of molecules, is small. Therefore fast intermolecular charge transfer is difficult compared to smectic phases, in which, due to the large degree of intermolecular orbital overlap within layers, sufficiently large intermolecular transfer integrals occur, causing fast intermolecular electronic charge transfer. In addition, competition between electronic and ionic charge transport remains another problem in these phases. Carrier transport characteristics have been investigated in the smectic phases of liquid-crystalline semiconductors doped with impurities that can trap holes. [26] In the SmB phase, two-dimensional trap-controlled electronic charge-carrier transport was dominant, while competition between ionic and electronic charge-carrier transport was observed in the SmA phase. [26] Based on Walden's rule, the rate of ionic charge transport is determined by the viscosity of the media and the ionic ratio. Therefore, the mechanism of carrier transport in the mesophase is determined by a balance between the viscosity and the intermolecular transfer integral. Ionic processes are dominant in achiral and chiral nematic phases that possess lower viscosities and have intermolecular transfer integrals smaller than those of smectic phases. ...

Chiral Liquid Crystalline Electronic Systems

“…In the LC phases with a nematic order, ionic conduction had been predominant. However, electronic charge carrier transport has been confirmed in the nematic phases consisting of the rod-like and disk-like molecules comprising extended π-conjugated cores [31][32][33][34]. In chiral LC phases, electronic charge carrier transport should be possible if π-conjugated moieties aggregate closely, resulting in a large overlap between the π-conjugated units.…”

Self-assembled 1D Semiconductors: Liquid Crystalline Columnar Phase

“…A large number of columnar and smectic liquid crystals had been prepared to study their charge transport and semiconducting applications. This chapter primarily focuses on semiconducting columnar phases of DLCs and we refer readers to some recent articles for information on advances in the field of calamitic and polymeric LC semiconductors [10,[20][21][22][23]. …”

“…However, hole transport was recently confirmed in d columnar lamellar (Col L ) phase and e helical (H) phase. Also shown are top views of the twodimensional lattices (ellipses denote disks that are tilted with respect to Col axis): f hexagonal; g and j rectangular; h rectangular face-centred and i oblique the nematic and cholesteric phases of calamitic liquid crystals bearing large π-conjugated cores [21,22]. Sustained efforts led to the development of LC semiconductors that combine high charge-carrier mobility with several other advantages including long-range self-assembling, self-healing, ease of processing and solubility in organic solvents all of which are highly desirable for device applications.…”

High Ambipolar Mobility in a Highly Ordered Smectic Phase of a Dialkylphenylterthiophene Derivative That Can Be Applied to Solution‐Processed Organic Field‐Effect Transistors

“…The bulk carriertransport properties have been investigated in discotic columnar, [10,11] smectic, [12][13][14] nematic, [15,16] and cholesteric [17] phases by using the time-of-flight (TOF) technique and pulse-radiolysis time-resolved microwave conductivity. Recently, high hole drift mobilities exceeding 0.1 cm 2 V -1 s -1 have been reported in the highly ordered smectic phases of alkynylquaterthiophene and dithienylbenzene derivatives; [18,19] furthermore a high microscopic band mobility exceeding 1 cm 2 V -1 s -1 has also been observed in the columnar phase of hexabenzocoronene derivatives.…”