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.

Untangling the Mechanochromic Properties of Benzothiadiazole-Based Luminescent Polymorphs through Supramolecular Organic Framework Topology.

Two new luminophore polymorphs of 4-bromo-7-(4-nonylphenyl)benzo[c][1,2,5]thiadiazole (1α and 1ß) exhibiting different color emissions, which switch into each other in response to shear force and solvent vapors, are presented and their X-ray structure is determined. Supramolecular organic framework topology (SOFT) studies on the two polymorphic structures led us to conclude that the mechanochromic phase transformation can be explained on the basis of modifications in their respective topological nets: mab and pcu for 1α and 1ß, respectively, as a result of the breaking and restoration of a number of weak supramolecular interactions. The color changes accompanying this transformation have been rationalized with the help of time-dependent density functional theory. We firmly believe that our findings will inspire future research on the design of novel stimuli-responsive organic materials with switchable properties based on their supramolecular interactions by establishing clear SOFT-property relationships.

Stimuli-Responsive Benzothiadiazole Derivative as a Dopant for Rewritable Polymer Blends.

A new rod-shaped benzothiadiazole fluorophore, namely, 4,7-di-(4-nonylphenyl)benzo[c][1,2,5]thiadiazole, which strongly emits fluorescence both in solution and in solid state has been synthesized, and its photophysical properties were rationalized with the help of density functional theory calculations. This molecule crystallizes in two distinct light-emitting crystalline phases, which can be interconverted in response to pressure, temperature, and solvent vapors. Powder X-ray diffraction indicates that in both polymorph, molecules adopt a lamellar packing, the different interlayer spacing being the main difference between the two structures. Single-crystal analysis of one of the polymorphs allows us to identify weak interaction planes, which presumably facilitates the polymorphic transformation through mechanically or thermally induced sliding processes. The polymorphic transformation and the origin of the switchable fluorescence have been rationalized through a spectroscopic and theoretical study. This study suggests that the different colors observed are due to different intermolecular aromatic interactions owing to the displacement of the molecules with respect to the layer normal. Interestingly, blending this molecule with a biodegradable polymer such as poly(vinyl alcohol) gives rise to a thermally activated reversible switchable fluorescent system, which entitles this material as an attractive candidate for technological applications, such as thermal sensors, security inks, or rewritable paper.

Fluorescent and Electroactive Monoalkyl BTD-Based Liquid Crystals with Tunable Self-Assembling and Electronic Properties.

We report here on a series of redox active benzothiadiazole-based luminophores functionalized on one edge with a phenyl-nonyl substituent, which confers these molecules a rodlike shape and a tendency to self-assemble into layered superstructures. On the other edge, the molecules are endowed with different p-substituted phenyl rings, which allows the modulation of their redox and optical properties on the basis of the electronic nature of the terminal substituents. We have found that just one lateral alkyl chain is sufficient to induce mesomorphism in these molecules, which present nematic or smectic mesophases upon thermal treatment. Single-crystal analysis allows us to get an insight into the nature of the forces responsible for different supramolecular assemblies in these derivatives, and point to a strong contribution of the terminal groups in the different arrangements observed. The interesting redox and optical properties together with their self-assembling tendencies render these new materials interesting candidates for optoelectronics.

Mobility versus Alignment of a Semiconducting π-Extended Discotic Liquid-Crystalline Triindole.

The p-type semiconducting properties of a triphenylene-fused triindole mesogen, have been studied by applying two complementary methods which have different alignment requirements. The attachment of only three flexible alkyl chains to the nitrogen atoms of this π-extended core is sufficient to induce columnar mesomorphism. High hole mobility values (0.65 cm2 V-1 s-1) have been estimated by space-charge limited current (SCLC) measurements in a diode-like structure which are easily prepared from the melt, rendering this material a good candidate for OPVs and OLEDs devices. The mobility predicted theoretically via a hole-hopping mechanism is in very good agreement with the experimental values determined at the SCLC regime. On the other hand the hole mobility determined on solution processed thin film transistors (OFETs) is significantly lower, which can be rationalized by the high tendency of these large molecules to align on surfaces with their extended π-conjugated core parallel to the substrate as demonstrated by SERS. Despite the differences obtained with the two methods, the acceptable performance found on OFETs fabricated by simple drop-casting processing of such an enlarged aromatic core is remarkable and suggests facile hopping between neighboring molecular columns owing to the large conducting/isolating ratio found in this discotic compound.

Saddle-Shaped Cyclic Indole Tetramers: 3D Electroactive Molecules.

We present a joint theoretical and experimental study of a series of cyclic indole tetramers aimed at understanding the fundamental electronic properties of this 3D platform and evaluating its potential in the construction of new semiconductors. To this end, we combined absorption and Raman spectroscopy, cyclic voltammetry, and spectroelectrochemistry with DFT calculations. Our results suggest that this platform can be easily and reversibly oxidized. Additionally, it has a HOMO that matches very well with the workfunction of gold, therefore charge injection from a gold electrode is expected to occur without significant barriers. Interestingly, the cyclic tetraindoles allow for good electron delocalization in spite of their saddle-shaped structures. The steric constraints introduced by N-substitution significantly inhibits ring inversion of the central cyclooctatetraene unit, whereas it only barely affects the optical and electrochemical properties (a slightly higher oxidation potential and a blueshifted absorption upon alkylation are observed).

Triindole-bridge-triindole dimers as models for two dimensional microporous polymers.

New dimers with two triindole subunits bound together through different linkers (p-phenylene or diacetylene groups) have been synthesized and studied as model systems to determine the differences in the electron transfer ability of the two bridging units. Our results show that whereas a p-phenylene bridge nearly isolates the two subunits of the dimers a diacetylene bridge allows a high level of electronic connection between them.

Organic Semiconductors toward Electronic Devices: High Mobility and Easy Processability.

Chemists worldwide are making strong efforts toward finding an organic semiconductor with optimum charge-transport stability and processability properties, approaching the problem from several angles. The number and variety of organic semiconductors have expanded exponentially during the past few years, allowing their classification and identification of the advantages and drawbacks of the different candidates, and leading to an increasingly better understanding of factors affecting charge carrier mobility. In this Perspective, we present different types of materials available, classified according to their supramolecular order. As it will be shown, high intermolecular order, although beneficial for charge transport, may compromise processability. The required balance between processing and high mobility can be achieved only through an adequate design at both the molecular and supramolecular levels.