Ao Zhang

Competition and promotion of different mesophases in a series of novel unsymmetrical discotic dimers via subtle modification of spacers and peripheral side chains

The highly ordered columnar plastic (Colhp) phase with charge carrier mobility up to 10−2 cm2 v−1 s−1 which is comparable to that of organic single crystals is considered as a potential charge transport material applied in electronic devices. The Colhp phase formed by monomers hexapropoxytriphenylene (HAT3) and hexabutoxytriphenylene (HAT4) as well as their corresponding symmetrical dimers can be supercooled into glassy states but with no monodomains obtained. It is expected that longer-lasting glassy states with monodomains can be engineered by making them subtly unsymmetrical. Unsymmetrical dimers with a Colhp phase can be constructed by the combination of any two of the three different monomers (HAT3, HAT4 and HAT5). Herein, three series of triphenylene-based dimers T3,4Dm, T4,5Dm, and T3,5Dm were synthesized by changing the spacer length (m = 6–12) and their mesophase was investigated by differential scanning calorimetry, polarizing optical microscopy, and wide-angle X-ray diffraction. These dimers can not only retain the order of the corresponding monomers but also obtain highly ordered single domains much more easily. To our surprise, the superlattice structure was firstly observed among these unsymmetrical dimers which made them not only be tractable models for discotic polymers with charge carrier mobility up to 10−2 cm2 v−1 s−1 but also provide a practical way for cultivating single crystals from discotic liquid crystals. The molecular stacking suggested that two different subunits tend to self-recognize and stack together, which was firstly reported by us and is very meaningful for supramolecular self-organization.

A systematic study on the influences of linkage length on phase behaviours and charge carrier mobilities of discotic dimers

ABSTRACT Two series of discotic dimers T3Dn and T5Dn based on hexapropoxytriphenylene (HAT3) and hexapentyloxytriphenylene(HAT5), respectively, with polymethylene linkage O(CH2)nO (n = 3–12) have been synthesised. Their mesomorphism was investigated by differential scanning calorimetry, polarising optical microscopy and X-ray diffraction. The results showed that side chains induced a phase transition from colhp phase to colh phase, namely dimers T3Dn (n = 6–12) based on HAT3 exhibiting a single colhp phase, dimers T4Dn (n = 6, 7, 11, 12) based on HAT4 showing a highly ordered colhp phase in low-temperature region and a colh phase in high-temperature region, dimers T4Dn (n = 8–10) based on HAT4 displaying a single colhp phase and dimers T5Dn (n = 6–11) based on HAT5 indicating a single colh phase. Dimers T4Dn showed a phase competition between colh phase and colhp phase induced by linkages. Surprisingly, a unique phenomenon was found by us, that is, those compounds in which linkage lengths were twice those of side chains showed the largest enthalpies, the smallest intracolumnar spacings and the highest charge-carrier mobilities among their homologues, respectively, which implied that they formed the most highly ordered phase among their homologues. Graphical Abstract

Systematic studies on structure–properties relationship of main chain discotic liquid crystalline polyethers: effects of the spacer lengths and substitution positions

The elucidation of structure–properties relationship influenced by spacer lengths and substitution positions is of crucial importance for the application of discotic liquid crystalline polymers (DLCPs), but few systematic investigations have been reported. Herein, an exhaustive study was carried out in order to gain a deeper insight into the structure–properties relationship of main chain DLC polyethers Pa,b-n; these were regiospecifically synthesized via polycondensation by changing spacer length n from 8 to 12 and varying substitution positions a,b- indicated 2,3-, 2,6-, 2,7-, and 3,6-substitution positions. To our surprise, when the spacer length was twice that of the side chain, the polymers Pa,b-10 displayed the highest order in each series. This was attributed to the perturbations from steric hindrance and space filling effects being minimized. It was disclosed that angular rotation of a triphenylene unit prevailed in the main chain DLCPs and increasing of the rotation freedom in the angular range was desirable for achieving higher order; this was directly confirmed by investigating the charge carrier mobilities of P2,3-10, P2,6-10, P2,7-10 and P3,6-10. Results showed that P3,6-10 displayed the highest order in variant substitution positions. More inspiring, the charge carrier mobility of P3,6-10 could reach the value 1.9 × 10−3 cm2 V−1 s−1, which was comparable to that of the corresponding monomer hexapentyloxytriphenylene (HAT5). This could provide a practical way for conductive polymers design by incorporating a liquid crystal unit into the main chain to establish charge transportation properties via self-assembling into ordered structures.

2D Organic Superlattice Promoted via Combined Action of π–π Stacking and Dipole–Dipole Interaction in Discotic Liquid Crystals

A series of discotic liquid crystals based on hexapentyloxytriphenylene (HAT5) have been investigated where one out of the six ether side chains of a triphenylene core was replaced by an ester side chain and named for 5a-5h. During the process of studying these compounds, the characteristic straight line defect of ordered columnar structure was identified by polarizing optical microscopy (POM) and liquid crystal state over a wide temperature range was obtained by differential scanning calorimetry (DSC). Basic phase structure and molecular arrangement were assigned by one-dimensional wide-angle X-ray diffraction (1D WAXD), small-angle X-ray scattering (SAXS), two-dimensional wide-angle X-ray diffraction (2D WAXD), and transmission electron microscope (TEM). Combined with sharp and regular dots in 2D WAXD patterns and characteristic peaks at small angle in SAXS pattern which indicated the existence of superlattice, we proved that 2D superlattice formed from self-assembly of discotic molecules with a polar group via π-π stacking and dipole-dipole interaction. In order to verify the effect of orientation on charge carrier mobility, their electron and hole mobilities were measured by time-of-flight (TOF) device, among which the charge carrier mobility could achieve almost twice as that of HAT5. The formation of superlattice no doubt improved their electronic properties and made them more attractive in organic electronics.

2D superlattice nanostructures formed via self-assembly of discotic liquid crystals dominated by long range dipole–dipole interaction

2D organic superlattice nanostructures of high charge carrier mobility of up to 8.1 × 10−2 cm2 V−1 s−1 were formed via hierarchical self-assembly of discotic liquid crystals as demonstrated by X-ray diffraction pattern and TEM. The supperlattice nanostructures, first reported by us, are expected to be used in optoelectronic devices such as OLEDs, OPVs and OFETs.

Synthesis and Mesophase of a Triphenylene-Based Discotic Liquid Crystal with Plastic Columnar Phase

A novel triphenylene-based discotic liquid crystal 2, 3, 6, 7, 10, 11-hexabutylcarboxyltriphenylene was synthesized. The chemical structure was determined by 1HNMR and FT-IR spectrum, and its mesophase was investigated by DSC and POM methods. The compound showed classical texture of columnar plastic phase under POM at the mesophase temperature from175°C-185°C during cooling.

Synthesis and Mesophase of a Chiral Triphenylene-Based Discotic Liquid Crystal

A chiral triphenylene-based compound 2-(2-octyloxy)-3,6,7,10,11-penta-pentyloxy triphenylene was synthesized by Mitsunobu reaction as shown in Fig.1. The structure of compound was determined by 1HNMR and FT-IR spectrum and its mesomorphism was clarified by DSC and POM methods.

Mesophase Studies and Charge Transportation Properties of 3,6-dicarbethoxy-2,7,10,11-tetra-pentyloxytriphenylene

In this study, a triphenylene-based discotic liquid crystal 3,6-dicarbethoxy-2,7,10,11-tetra-pentyloxytriphenylene was synthesized. The chemical structure was characterized by FTIR, 1HNMR.The mesophases of the material synthesized here was characterized by using polarizing optical microscopy (POM), differential scanning calorimetry (DSC). It was found that the compound displayed columnar phase during cooling at 164.7 °C and the columnar phase can be preserved even at room temperature. The charge transportation properties of the compound were measured by using time-of-flight method.