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NMR of guest‐host systems: 8CB in chiral nematic porous glasses
Author(s) -
Manning Alan P.,
Giese Michael,
Terpstra Andrea S.,
MacLachlan Mark J.,
Hamad Wadood Y.,
Dong Ronald Y.,
Michal Carl A.
Publication year - 2014
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.4101
Subject(s) - liquid crystal , chemistry , mesoporous material , chemical physics , mesoporous silica , isotropy , pulsed field gradient , proton nmr , crystallography , materials science , organic chemistry , molecule , optoelectronics , optics , physics , catalysis
Liquid crystals confined to porous materials often have different critical phenomena and ordering than in the bulk. Through the selection of pore size, structure and guest liquid crystal, these systems could enable a variety of functional materials for applications such as sensors and displays. A recent example of such a system is chiral nematic mesoporous films infiltrated with liquid crystal 4‐cyano‐4'‐ n ‐octylbiphenyl (8CB), which has reversible thermal switching of its optical bandgap. The optical bandgap is lost when the ordered 8CB guests are heated above ∼50 °C, where the 8CB becomes isotropic. In this study, we have used NMR cryoporometry and pulsed‐field gradient diffusion measurements to determine the pore sizes and structures of various chiral nematic mesoporous silica and organosilica films. Temperature and orientation‐dependent wideline 15 N NMR spectra of films infiltrated with 15 N‐labelled 8CB guests show that the ordering of the 8CB mesogens is consistent with an average orientation parallel to the chiral nematic pore axes. Inclusion of a large, orientation‐dependent shift was necessary to fit the spectra, probably due to susceptibility differences between the 8CB guests and the organosilica host. Copyright © 2014 John Wiley & Sons, Ltd.