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.


Liquid crystalline techniques and experimental details
The optical textures of the mesophases were studied with a Nikon polarizing microscope Eclipse LV 100N POL equipped with a Linkam hot-stage and Linkam LINKSYS32 central processor and microphotographs were taken with a Nikon DP12 digital camera.
The transition temperatures and enthalpies were measured by differential scanning calorimetry with a DISCOVERY DSC Q100 calorimeter operated at a scanning rate of 10 ºC min -1 on both heating and cooling and under atmosphere of N 2 .Thermogravimetric analysis (TGA) was performed to determine the thermal stability of the compound and was carried out on a TA TGA Q500 analyzer under a N 2 atmosphere.The sample was S-5 heated at 10 ºC min -1 from room temperature to 850 ºC.The XRD patterns at the mesophase were obtained with a pinhole camera (Anton-Paar) operating with a pointfocused Ni-filtered Cu-K beam.The sample was held in Lindemann glass capillaries (1 mm diameter) and heated, when necessary, with a variable-temperature oven.The capillary axis is perpendicular to the X-ray beam and the pattern is collected on flat photographic film perpendicular to the X-ray beam.Spacing was obtained via Bragg's law.The XRD pattern of the cooled mesophase was obtained with a Bruker D8 Advance diffractometer with a Sol-X energy dispersive detector, working at 40 kV and 30 mA and employing CuK α (λ = 1.5418Å) filtered radiation.The diffractograms were registered with a step size of 0.02 º and exposure time of 0.5 s per step and a 2θ range of 2.5-30 º. and a platinum wire auxiliary electrode.Ferrocene was used as an internal standard, an all potentials were referenced to the ferrocene/ferrocenium redox couple.
The HOMO energy values of TRISMe was estimated as -5.08 eV from the first oxidation potential with respect ferrocene/ferrocenium redox couple 1 and considering a value of -4.8 eV for Fc with respect to zero vacuum level.This value is obtained from the calculated value of -4.6 eV for the standard electrode potential (E) using a normal hydrogen electrode (NHE) on the zero vacuum level and the value of 0.2 V for Fc vs. NHE. 2,3e LUMO energy values was estimated from the difference between the HOMO level and the optical gap (3.07 eV), which render a LUMO level of -2.01 eV.S-9

7-. Absorption spectra
UV-vis studies were carried out on a PerkinElmer Lamba XLS+ spectrometer.As it can be seen, the annealed samples used for SCLC measurements were not uniformly aligned.However, as it was possible to observe by rotating the samples between crossed polarizers, several areas (orientational domains) were of a gray or almost black shade changing little with rotation, while other areas were much brighter and showed a much higher brightness variation with rotation.This indicates that in each measurement areas there were several homeotropic or almost homeotropic domains.As the anisotropy of charge mobility in columnar mesophases is estimated to be around 3-4 orders of magnitude, the contribution of nonhomeotropic domains to the measured current is minor and one can assume that most of the SCLC signal derives from intracolumnar mobility.

Figure
Figure S6.a) Polarizing optical photomicrograph of TRISMe at room temperature upon cooling from the isotropic liquid, following the thermal treatment shown in b).

Figure S7 .
Figure S7.Comparison of the experimental powder X-ray diffractograms of the films of TRISMe prepared by drop-casting and annealed 5 min at 150ºC (gray) and the diffractogram simulated from the single crystal X-ray data (red).

5 Figure S8 .
Figure S8.Transfer curves of TRISMe after annealing the device for 5 min.at 150 ºC measured at a source-drain voltage of -80V