Inclusion Complexes Oriented in Thermotropic Liquid-Crystalline Solvents Studied with Carbon-13 NMR

Abstract

The inclusion complex of cryptophane-A and chloroform dissolved in two non-chiral liquid crystalline environments was investigated via 13 C NMR. Stable solutions of oriented complexes were prepared using aromatic (ZLI 1132) and aliphatic (ZLI 1695) thermotropic nematic liquid crystals as solvents; ordering of the complexes was manifested by the 1 H-13 C dipolar splitting of the 13 C resonance of labeled chloroform. In both solutions, the dipolar splitting for the bound ligands was substantially larger than that obtained These results demonstrate the potential for using restored dipolar couplings to investigate structural and dynamical aspects of inclusion complexes in solution.2

NMR studies of chloroform@cryptophane-A and chloroform@bis-cryptophane inclusion complexes oriented in thermotropic liquid crystals

“…Utilizing previously obtained information concerning the relative orientation of chloroform in cryptophane cages [32], the results are consistent with overall alignment of both complexes such that the cage principal axis lies roughly perpendicular to the LC director-in agreement with previous results and predictions [27]. These results demonstrate the potential for utilizing NMR measurement of anisotropic interactions restored by LC matrices for probing the structure, dynamics, and ordering behavior of host-guest inclusion complexes.…”

“…Our motivation has been to investigate the use of LC alignment to perform novel NMR studies of structural and dynamical aspects of host-guest interactions. In our previous experiments involving aligned complexes of chloroform and cryptophane-A [27], we observed enhanced ordering for trapped ligands in aligned complexes as manifested by the observed 1 H-13 C intramolecular dipolar couplings and performed variable-temperature NMR studies of the guest 13 C signals to probe the molecular alignment in an aliphatic LC (ZLI 1695). Additionally, observations of linewidth reductions for the trapped ligand resonance upon high-power 1 H heteronuclear decoupling indirectly suggested that intermolecular dipolar couplings between host and guest had been partially restored as well.…”

“…It was also recently demonstrated that aligned cryptophane-A complexes could be prepared and studied using thermotropic liquid crystalline matrices [27]. Dissolving molecules in liquid crystals (LCs) generally results in greatly increased anisotropy in the molecular rotational diffusion, thereby partially ''restoring'' contributions to the NMR Hamiltonian that would otherwise be averaged away in isotropic solutions.…”

“…[35][36][37][38][39][40] for selected reviews). Similarly, recent efforts to align inclusion complexes with LCs [27,41,42] have been motivated by the desire to enhance the NMR signatures of host-guest interactions, including the recent demonstration of selective chiral recognition of enaniomeric guests by an aligned chiral host (a-cyclodextrin, manifested by the differential enhancement of restored 2 H quadrupolar couplings) [42]. Our motivation has been to investigate the use of LC alignment to perform novel NMR studies of structural and dynamical aspects of host-guest interactions.…”

“…Here we report on our studies of chloroform@crypto-phane-A and chloroform@bis-cryptophane host-guest inclusion complexes aligned in an aromatic liquid crystalline matrix (ZLI 1132), and provide a more complete description of our approach to characterize the ordering in such systems via the analysis of CSA shifts and restored dipolar couplings of the guests [27]. Large 1 H- 13 with free ligands.…”