Crystalline Complexes
5.1. Introduction
Liquid crystalline (LC) materials bearing banana-shaped mesogens become interesting topics duo to their special electro-optical properties, such as spontaneous polarized capabilities and nonlinear optics.[42,61] Based on various intermolecular arrangements, several kinds of special mesophases in accordance with banana-shaped (or bent-core) molecular designs with particular mesophases, including columnar stacking, tilted smectic, and three dimensional structures, named as B1 to B7 phases were developed and identified.[4] Traditionally, electro-optical switching behaviors were observed in the smectic B1, B2, B5, and B7 phases, where the B2 (SmCP) phase had been prevailingly investigated. Depending on the polar direction and molecular tilted direction in neighboring layers of the SmCP phase, ferroelectric (F) and antiferroelectric (A) states possessed identical/inverse polarizations and synclinic (S)/anticlinic (A) arrangements with alike/opposite molecular tilted aspects between layer to layer, respectively. Hence, four kinds of different supramolecular architectures denoted SmCAPA, SmCSPA, SmCAPF, and SmCSPF were recognized as homochiral (SmCAPA and SmCSPF) and racemic (SmCSPA and SmCAPF) conditions separately.[8] With respect to the bent-core molecular architectures, the traditional banana-shaped liquid crystals were generally formed by two bent-substituted rigid arms connected to a central cyclic ring (through polar or non-polar functional groups)
with a suitable bent angle and linking, where appropriate lengths of flexible chains were attached.[5] Among these bent-shaped modeling frameworks, the structural variations of achiral molecular designs, such as the central parts,[6] lateral substituents,[25a] linking groups,[62] terminal chains,[63] and the number of rings,[7]
would affect their physical properties to different extents in small molecular systems.
Recently, poly-molecular systems, i.e., dimeric,[31] main-chain polymeric,[32d]
side-chain polymeric,[33a] and dendritic structures,[34b] were also developed to investigate the influence of molecular configurations on mesomorphic and electro-optical properties. Moreover, some novel supramolecular bent-core interactions or their nanocomposite architectures have been integrated into organic or inorganic parts to display special electro-optical characteristics, for instance, bent-core derivatives embedded with nanoparticles,[37] bent-core H-bonded supramolecules,[38]
and bent-core structures with silyl and siloxyl linkages.[21]
To retain the electro-optical switching behavior in the bent-core structures is constantly the important assignment in the field of banana-shaped LC research.
However, even if many kinds of bent-core small molecular systems displayed particular polar switching current behaviors, duo to the higher viscosities and larger inter-/intra-molecular interactions in polymers, such switching current behaviors were not easy to be obtained (or detected) in analogous bent-core polymer derivatives.[33b-c,64] Interestingly, the switching current behavior of the SmCP phase in the monomeric units was sustained and the ferro-electricity could be modified by the polymer structural design of dimethylsiloxane diluted polysiloxane side-chain copolymer frameworks.[33a] According the previous works, bent-core H-bonded small molecules could yield lower mesophasic transition temperatures, enthalpies, and threshold voltages than those of fully covalent-bonded five-ring banana-shaped molecular analogues and H-bonded side-chain polymeric derivatives, which
suggested that softer bent-core intermolecular arrangements were present in both H-bonded and small molecular designs.[2,38a,39]
Regarding the small molecular systems containing two different fully covalent-bonded components, unusual molecular arrangements and mesomorphic behaviors were developed by blending rod-like liquid crystals with bent-core LC dopants.[65] Meanwhile, the blended polymeric systems (mainly side-chain polymers) were expanded by doping,[60] copolymerization,[66] and H-bonded complexation[67] with chiral/achiral liquid crystalline components to induce the particular mesomorphic and electro-optical properties.
As shown in Figure 5.1, in order to investigate the polymeric dopant effects of bent-core covalent- and H-bonded structures on the mesomorphic and electro-optical properties of banana-shaped LC complexes, two series of H-bonded complexes HPm/CBn (i.e., bent-core H-bonded side-chain homopolymer HP mixed with bent-core covalent-bonded small molecule CB) and CPm/HBn (i.e., bent-core covalent-bonded side-chain homopolymer CP mixed with bent-core H-bonded small molecular complex HB) with various m/n molar ratios (m/n = 15/1, 10/1, 15/1, 1/1, 1/5, 1/10, and 1/15) were developed. The mesomorphic and electro-optical properties of the H-bonded complexes were investigated and characterized by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), powder X-ray diffraction (XRD), and electro-optical (EO) switching current experiments. Herein, the polar smectic (SmCAPA) phase with switching current behavior (or spontaneous polarization) was introduced and stabilized in some of the banana-shaped LC H-bonded complexes HPm/CBn and CPm/HBn by blending various m/n molar ratios of soft H-bonded bent-core moieties with rigid covalent-bonded bent-core moieties, where one of the moieties was homopolymerized as side-chain H-bonded
their mesophasic ranges and spontaneous polarization (Ps) values could be adjusted by the m/n molar ratios (polymeric moieties vs. small molecular moieties) in the banana-shaped LC H-bonded complexes HPm/CBn and CPm/HBn.
Figure 5.1. Chemical structures of banana-shaped LC H-bonded complexes HPm/CBn and CPm/HBn (where m/n = 15/1, 10/1, 15/1, 1/1, 1/5, 1/10, and 1/15) and their composing H-bonded and covalent-bonded bent-core side-chain homopolymers (HP and CP, respectively) as well as H-bonded and covalent-bonded bent-core small molecules (HB and CB, respectively).
CPCPmm//HHBBnn
5.2. Eeperimental 5.2.1. Methods
1H NMR spectra were recorded on a Varian Unity 300 MHz spectrometer using d6-dioxane and CDCl3 as solvents and mass spectra were determined on a Micromass TRIO-2000 GC-MS. Elemental analyses (EA) were performed on a Heraeus CHN-OS RAPID elemental analyzer. Gel permeation chromatography (GPC) analyses were conducted on a Waters 1515 separation module using polystyrene as a standard and THF as an eluant. Mesophasic textures were characterized by polarizing optical microscopy (POM) using a Leica DMLP equipped with a hot stage. Infrared (IR) spectra were investigated by Perk-Elmer Spectrum 100 instrument. Temperatures and enthalpies of phase transitions were determined by differential scanning calorimetry (DSC, model: Perkin Elmer Pyris 7) under N2 at a heating and cooling rate of 10
°Cmin-1. Synchrotron powder X-ray diffraction (XRD) measurements were performed at beamline BL17A of the National Synchrotron Radiation Research Center (NSRRC), Taiwan, where the wavelength of X-ray was 1.33366 Å. The powder samples were packed into a capillary tube and heated by a heat gun, whose temperature controller was programmable by a PC with a PID feedback system. The scattering angle theta was calibrated by a mixture of silver behenate and silicon. The electro-optical properties were determined in commercially available ITO cells (from Mesostate Corp., thickness = 4.25 μm, active area = 1 cm2) with rubbed polyimide alignment coatings (parallel rubbing direction). A digital oscilloscope (Tektronix TDS-3012B) was used in these measurements, and a high power amplifier connected to a function generator (GW Model GFG-813) with a d.c. power supply (Keithley 2400) was utilized in the d.c. field experiments. During electro-optical measurements, the modulations of textures by applying electric fields were observed by POM.