Self-assembled discotics as molecular semiconductors

Terminal halogen-containing rod-like liquid crystals: Synthesis, self-assembly, photophysical and mechanochromism properties

Five series of cyanostilbene-based rod-like liquid crystals containing one different terminal atom (H, F, Cl, Br and I) at one end and one terminal aliphatic chain with different numbers of carbon atoms at the other end were reported by Suzuki coupling and Knoevenagel reactions. The influence of terminal halogen atoms and terminal chain length on the self-assembly, AIE behavior, temperature-dependent emission and mechanochromism behavior was explored by POM, DSC, XRD, SEM, absorption spectra and emission spectra. All the compounds are enantiotropic liquid crystals. The lowest non-halogen substituted homologue exhibited solo N phase, but the higher non-halogen substituted homologues exhibited mesogenic transition from SmA phase to N phase upon rising temperature. All the lowest halogen substituted homologues exhibited mesogenic transition from SmA phase to N phase upon rising temperature and all the higher homologues only exhibited SmA. The distinct mesogenic phase transition could be attributed to the intermolecular interaction produced by terminal halogen and the rigidity of the terminal aliphatic chain. All the non-halogen substituted compounds and halogen substituted compounds with smaller terminal halogen atom (F, Cl and Br atom) exhibited AIE behaviors, whereas the iodinated compounds exhibited extremely weak emission in solution and aggregated states due to the heavy atoms effect. These compounds also exhibited distinct solid-state emission with blue or cyan fluorescence, which could be quenched by increasing temperature. The reversible mechanochromism behavior was also achieved in all the compounds. The mechanical force induced quench in emission in non-halogen substituted compounds and halogen substituted compounds with smaller terminal halogen atom (F, Cl and Br atom), whereas enhancement in iodinated compounds. The reversible mechanochromism behavior endowed these compounds with potential applications in rewritable paper and anti-counterfeiting. The interesting properties in these liquid crystals would be attributed to the balance of the halogen-halogen interactions, heavy atom effect, steric-hindrance effect and chain length. These investigations would be helpful to understand the relationship between chemical structures and properties.

Point and line defects in checkerboard patterned hybrid nematic films: A computer simulation investigation

We consider a nematic liquid crystal film confined to a flat cell with homeotropic and planar patterned hybrid anchoring and show, using Monte Carlo simulations, the possibility of the system to stabilize line and point defects. The planar anchoring surface is patterned with a chessboardlike grid of squares with alternating random or parallel homogeneous planar anchoring. The simulations show only line defects when the individual domains are small enough, but also point defects when the domain size is significantly larger than the sample thickness. In the latter case, defect lines are not observed in domains with random surface anchoring, although lines and points are connected by a thick line which separates two regions with different director tilts. Increasing the anchoring strength, the defect lines appear a few layers above the surface, with the two ends just above the randomly oriented domains.

Columnar Mesomorphism in a Methylthio-Decorated Triindole for Enhanced Charge Transport

We report a semiconducting triindole-based discotic liquid crystal (TRISMe) functionalized with six p-methylthiophenyl groups at its periphery. While initially a crystalline solid at room temperature, TRISMe transitions to a columnar hexagonal mesophase upon heating and retains this supramolecular organization upon subsequent cooling, despite having only three flexible alkyl chains attached to the core's nitrogens. The incorporation of methylthio groups effectively hinders tight molecular packing, stabilizing the columnar arrangement of this disk-shaped molecule. Single crystal analysis confirmed the high tendency of this compound to organize into a columnar architecture and the role played by the methylthio groups in reinforcing such structure. The mesomorphic behavior of TRISMe provides an opportunity for processing from its molten state. Notably, our research reveals significant differences in charge transport depending on the processing method, whether solution drop-casting or melt-based. TRISMe shows hole mobility values averaging 3 × 10-1 cm2 V-1 s-1 when incorporated in diode-type devices from the isotropic melt and annealed at the mesophase temperature, estimated by SCLC (space-charge-limited current) measurements. However, when integrated into solution-processed organic field-effect transistors (OFETs), crystalline TRISMe exhibits a hole mobility of 3 × 10-4 cm2 V-1 s-1. The observed differences can be attributed to a beneficial supramolecular assembly achieved in the mesophase in spite of its lower order. These results emphasize the material's potential for applications in easy-to-process electronic devices and highlight the potential of methylthio moieties in promoting columnar mesophases.

Useful synthetic artifacts? The impact of ubiquitous linker-adjacent groups on the self-assembly of discotic dimers

Although discotic dimers commonly feature bulky ether substituents adjacent to the linking group, the impact of these chains on self-assembly remains unclear. A series of dibenzo[a,c]phenazine dimers with alkoxy groups ortho to the linker were prepared and their solution conformational dynamics and liquid crystalline properties examined. The presence of a methoxy substitutent adjacent to the bridging group increased the phase stability, whereas longer chains dramatically decreased clearing temperatures. NMR solution studies indicated that adjacent groups increased the preference of dimers to adopt unfolded conformers. DFT models indicated that the unfolded structures were nonplanar and hence less compatible with columnar ordering, leading to a destabilization of the mesophases.

High Electrical Conductivity and Hole Transport in an Insightfully Engineered Columnar Liquid Crystal for Solution-Processable Nanoelectronics

Discotic liquid crystals (DLCs) are widely acknowledged as a class of organic semiconductors that can harmonize charge carrier mobility and device processability through supramolecular self-assembly. In spite of circumventing such a major challenge in fabricating low-cost charge transport layers, DLC-based hole transport layers (HTLs) have remained elusive in modern organo-electronics. In this work, a minimalistic design strategy is envisioned to effectuate a cyanovinylene-integrated pyrene-based discotic liquid crystal (PY-DLC) with a room-temperature columnar hexagonal mesophase and narrow bandgap for efficient semiconducting behavior. Adequately combined photophysical, electrochemical, and theoretical studies investigate the structure-property relations, logically correlating them with efficient hole transport. With a low reorganization energy of 0.2 eV, PY-DLC exhibits superior charge extraction ability from the contact electrodes at low values of applied voltage, achieving an electrical conductivity of 3.22 × 10-4 S m-1, the highest reported value for any pristine DLC film in a vertical charge transport device. The columnar self-assembly, in conjunction with solution-processable self-healed films, results in commendably elevated values of hole mobility (≈10-3 cm2 V-1s-1). This study provides an unprecedented constructive outlook toward the development of DLC semiconductors as practical HTLs in organic electronics.

A novel B,O,N-doped mesogen with narrowband MR-TADF emission

Modification of an unsymmetric B,O,N-doped aromatic core with peripheral mesogenic units triggers self-assembly into a columnar hexagonal mesophase, which is stable between 22 and 144 °C. The columnar assembly is preserved in a glassy state below 22 °C. The B,O,N-doped mesogen displays narrowband sky-blue multiresonance thermally activated delayed fluorescence (MR-TADF) under diluted conditions and bright excimer emission in condensed phase. Our combined experimental and theoretical approach provides insight into the development of strongly aggregating liquid crystalline MR-TADF emitters.

Temperature-dependent hole mobility in pyrene-thiophene-based room-temperature discotic liquid crystals

π-Conjugated pyrene-thiophene-based room-temperature discotic liquid crystals armed with four peripheral aliphatic chains are reported to study their potential use in a hole-transporting organic semiconductor. The charge carrier mobility studies using the ToF method revealed room temperature hole mobility in the order of 10-4 cm2 V-1 s-1 for both mesogens. However, the mobility values for compound 1a were observed in the order of 10-3 cm2 V-1 s-1 at high temperatures. Such molecular systems can potentially be used in nonlinear organic electronic applications.

Mesomorphism Modulation of Perfluorinated Janus Triphenylenes by Inhomogeneous Chain Substitution Patterns

Two isomeric series of compounds with "inverted" chains' substitution patterns, 7,10-dialkoxy-1,2,3,4-tetrafluoro-6,11-dimethoxytriphenylene and 6,11-dialkoxy-1,2,3,4-tetrafluoro-7,10-dimethoxytriphenylene, labelled respectively p-TPFn and m-TPFn, and two non-fluorinated homologous isomers, 3,6-dibutoxy-2,7-dimethoxytriphenylene and 2,7-dibutoxy-3,6-dimethoxytriphenylene, p-TP4 and m-TP4, respectively, were synthesized in three steps and obtained in good yields by the efficient transition-metal-free, fluoroarene nucleophilic substitution via the reaction of appropriate 2,2'-dilithium biphenylenes with either perfluorobenzene, C6 F6 , to yield p-TPFn and m-TPFn, or o-difluorobenzene, C6 H4 F2 , for p-TP4 and m-TP4, respectively. The single-crystal structures of p-TPF4, m-TPF4 and p-TP4, unequivocally confirmed that the cyclization reactions occurred at the expected positions, and that the fluorinated molecules stack up into columns with short separation, a propitious situation for the emergence of columnar mesophases. The mesomorphous properties were found to be greatly affected by both chains' length and positional isomerism: a Colhex phase is found for p-TPF4 and m-TPF4, but mesomorphism vanishes in p-TPF6, and changes for the isomeric homologs m-TPFn, with the induction for n≥6 of a lamello-columnar phase, LamColrec . As expected, both non-fluorinated compounds are deprived of mesomorphism. These compounds emit blue-violet colour in solution, independently of the chains' substitution pattern, and the absolute fluorescence quantum yields can reach up to 46 %. In thin films, fluorescence is slightly redshifted.

First examples of room-temperature discotic nematic liquid crystals exhibiting ambipolar charge carrier mobilities

Four new room-temperature nematic liquid-crystalline (LC) dimers consisting of a wedge-shaped 3,4,5-tridecyloxy gallic ester molecule linked to either cyanobiphenyl, cholesteryl, pentaalkynylbenzene or triphenylene based moieties are reported. Dimers with pentaalkynylbenzene and triphenylene moieties in their room-temperature discotic nematic (ND) mesophase show ambipolar charge carrier mobilities of the order of 10-5 cm2 V-1 s-1 and 10-3 cm2 V-1 s-1, respectively, as measured using a time-of-flight (ToF) technique.

Triphenylamine-Containing Benzoic Acids: Synthesis, Liquid Crystalline and Redox Properties

The synthesis, characterization and liquid crystalline and electrochemical properties of novel triarylamines, in which the triphenylamine platform is non-symmetrically modified with a 4-(6-oxyhexyloxy)benzoic acid group, are reported. Compounds show columnar liquid crystalline behavior, as confirmed through the use of polarized optical microscopy, differential scanning calorimetry and X-ray diffraction. Electrochemical properties were measured using cyclic voltammperometry, obtaining low oxidation potentials and HOMO values that were optimum for consideration as organic semiconductors in hole transport layers. In addition, the photoredox activity of one of these derivatives in dichloromethane was studied under light irradiation. A photooxidation/assembly process under white light irradiation occurs without the assistance of hydrogen bonding amide functional groups.