Probing Columnar Discotic Liquid Crystals by EPR Spectroscopy with a Rigid-Core Nitroxide Spin Probe**

Within the last decade columnar discotic liquid crystals (DLCs) have attracted considerable interest, not least for their potential technological applications as one-dimensional conductors, in sensors, field-effect transistors, and photovoltaic solar cells. The coupling within stacked polyaromatic cores provides an efficient structure for charge transport along the columns thus providing one-dimensional pathways for charge and energy transfer with an efficiency that depends on the extent and stability of the overlap of the p-extended cores. Understanding the dynamics and microscopic phase behavior of columnar discotics at the molecular scale is particularly challenging but fundamental for the link between new systems and devices with desired functionalities. The most extensively studied columnar LCs are based on hexasubstituted triphenylenes, of which hexakis(n-hexyloxy)triphenylene (HAT6) is a representative example. HAT6 and other members of the HATn series have been studied by many methods including broad-band dielectric spectroscopy, DSC, X-ray diffraction, Muon spectroscopy, deuteron NMR spectroscopy, and quasi-electric neutron scattering. EPR spectroscopy of nitroxide spin probes (SPs) is a valuable method for the study of both structure and dynamics of complex partially disordered systems such as proteins and their complexes, DNA/RNA, biological membranes, nanoparticles, and soft matter. EPR spectroscopy has the ability to resolve directly molecular re-orientational dynamics over time scales of 10 –10 7 s through the variation in spectral line shapes. Because of the high sensitivity of the EPR technique only very low concentrations of the probe, about 100 mm, are required experimentally so that the host system is essentially unperturbed. Continuous-wave (CW) EPR spectra provide three types of important information about the partially ordered fluid state, namely, molecular dynamics, local order of molecules averaged over a small volume, and global or long-range order in a multi-domain system. The spectra are very sensitive to changes in both dynamics (correlation times) and the order (order parameter) of the SP within the LC system. For instance, rod-shaped EPR SPs such as cholestane derivatives have been successfully applied to study calamitic 4-n-alkyl-4’-cyanobiphenyl (nCB) nematic liquid crystals. However, currently available spin probe molecules are incompatible with discotic systems. Here we report the first application of EPR spectroscopy with a purpose-designed paramagnetic SP compatible with columnar discotic LCs. The SP design requires two important features. The probe itself should resemble the host matrix molecules as closely as possible to favour intercalation and cause minimal disruption to the phase, and the SP fragment itself must be orientationally rigid with respect to the discotic core. Discotic SP 1 was designed to meet these criteria and is shown in Scheme 1 together with HAT6. Its synthesis is shown in Scheme 2 and described in the Supporting Information. We demonstrate herein that the novel probe, when combined with variable-temperature CW EPR, is a sensitive