OBLATE HEXAALKOXYTRIPHENYLENE SOLUTES IN A PROLATE NEMATIC SOLVENT - A DEUTERIUM NMR-STUDY OF ALKYL CHAIN ORDERING

We present deuterium NMR spectra of a series of hexaalkoxytriphenylene solutes (THEn, where n = 5, 6, 7, and 8 is the number of carbons in t he labeled alkoxy chains) dissolved in the calamitic solvent phase V. The observed methylene quadrupolar splitting patterns (segmental order profiles) are markedly different from those obtained in the neat colu mnar discotic phases of the THEn mesogens; they also differ significan tly from order profiles of alkoxy chains attached to calamitic mesogen s. Theoretical simulations of the spectra give the assignment of the s plittings and, moreover, enable studies of the accommodation of the ob late THEn solute in the calamitic solvent's orientationally biased mea n field. The chord model with the rotational isomeric state descriptio n of the THEn's six alkoxy chains quantitatively describes this unusua l mixture of oblate and prolate molecules. The model shows that the ac commodation of the THEn mesogenic core in the calamitic solvent's mean field does not favor maximum extension of the chains: The oblate THEn disk is tangent to the solvent director, and consequently, the radial disposition of extended chain contours on the periphery of the THEn d isk is frustrated by the excluded volume interactions that constitute the solvent's mean field.