Dj. Photinos et al., OBLATE HEXAALKOXYTRIPHENYLENE SOLUTES IN A PROLATE NEMATIC SOLVENT - A DEUTERIUM NMR-STUDY OF ALKYL CHAIN ORDERING, Journal of the American Chemical Society, 115(23), 1993, pp. 10895-10900
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.