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Optical polarimetry measurements of the orientational order of a discoticliquid crystal based on a pyrene derivative and confined in parallel-alignednanochannels of monolithic mesoporous alumina, silica, and silicon as afunction of temperature, channel radius (3 - 22 nm) and surface chemistryreveal a competition of radial and axial columnar order. The evolution of theorientational order parameter of the confined systems is continuous, incontrast to the discontinuous transition in the bulk. For channel radii largerthan 10 nm we suggest several, alternative defect structures, which arecompatible both with the optical experiments on the collective molecularorientation presented here and with a translational, radial columnar orderreported in previous diffraction studies. For smaller channel radii ourobservations can semi-quantitatively be described by a Landau-de Gennes modelwith a nematic shell of radially ordered columns (affected by elastic splaydeformations) that coexists with an orientationally disordered, isotropic core.For these structures, the cylindrical phase boundaries are predicted to movefrom the channel walls to the channel centres upon cooling, and vice-versa uponheating, in accord with the pronounced cooling/heating hystereses observed andthe scaling behavior of the transition temperatures with channel diameter. Theabsence of experimental hints of a paranematic state is consistent with abiquadratic coupling of the splay deformations to the order parameter.

Thermotropic Orientational Order of Discotic Liquid Crystals in Nanochannels: An Optical Polarimetry Study and a Landau-de Gennes Analysis