Mesomorphism, Isomerization, and Dynamics in a New Series of Pyramidic Liquid Crystals

Abstract

Nona-alkanoyloxy tribenzocyclononene (CTV-n, where n is the number of carbons in the side chains) were prepared for n = 2 to 14. The homologues of this series appear in two stable isomeric forms, rigid crown and flexible saddle. We report on their isomerization equilibria and dynamics in solution and on their mesomorphic properties in the neat state. The crown-saddle equilibrium and interconversion kinetics of the CTV-8 isomers were studied in dimethyl formamide solutions using high-resolution (1)H NMR in the temperature range from 50 to 130 degrees C. At lower temperatures, the isomerization is too slow to measure. In this range the equilibrium saddle fraction increases from approximately 0.40 to approximately 0.65, whereas the isomerization rate increases from approximately 10(-)(4) to approximately 1 s(-)(1). The saddle isomer undergoes fast pseudorotation at room temperature, but below about -50 degrees C, it becomes slow enough to affect the NMR line width. The rate parameters for this process were estimated from the carbon-13 spectra in methylene chloride solutions to be, k(p)(-100 degrees C) approximately 1.7 x 10(3) s(-)(1) and E(a) approximately 9.6 kJ/mol. The slow crown-saddle isomerization at room temperature (half-life of about one year) allows quantitative separation (by chromatography) of the two isomers and their separate investigation. When the alkanoyloxy side chains are sufficiently long both isomers are mesogenic (n >or= 4 for the saddle and n >or= 5 for the crown), exhibiting hexagonal columnar mesophases. The structure, dynamics, and mesomorphic properties of these mesophase were investigated by X-ray diffraction, optical polarizing microscopy, differential scanning calorimetry, and NMR. The lattice parameters of the crown and saddle mesophases of corresponding homologues are almost identical and increase monotonically with increasing length of the side chains. The clearing temperatures of the saddle isomers are consistently lower than those of the corresponding crowns. Within each series, the clearing temperatures are almost independent of the length of the side chains (156 to 170 degrees C for the crown and 115 to 148 degrees C for the saddle). The thermal and kinetic properties of the neat compounds lead to peculiar phase sequences, as observed in the polarizing microscope and in the DSC thermogram, involving repeated, back and forth, interconversion between the two isomers. Carbon-13 MAS NMR measurements of the crown and saddle mesophases of several homologues were carried out. The spectra of the crown mesophase exhibit dynamic features consistent with planar 3-fold molecular jumps about the column axes. A quantitative analysis for the CTV-8 crown homologue yielded the following Arrhenius parameters, A = 3.1 x 10(22)s(-)(1) and E(a) = 130.1kJ/mol. These unusually high values suggest that the barrier to the jump process is temperature dependent, decreasing with increasing temperature. The rate of this 3-fold jump process is slower for the lower homologues and faster for the higher ones. I...

Isolation of the saddle and crown conformers of cyclotriveratrylene (CTV) oxime

“…31 The intermediacy of the more flexible saddle conformation had been proposed by as an intermediate in CTV crown umbrella inversion by Collet, who elegantly demonstrated the barrier to crown-to-crown inversion via racemization of chiral d9-CTV to be 26.5 kcal/mol 32 which most likely proceeds via the saddle conformation. 12 Zimmerman also studied the mesomorphic properties of nonasubstituted tribenzocyclononene derivatives 27 wherein the crown conformation is destabilized relative to the saddle form due to steric interactions with neighboring aryl rings. 12 For CTV, the saddle conformation is thermodynamically disfavored due to the negative steric crowding of proton H a ( Figure 1) with the face of the central aromatic ring at the back of the saddle.…”

“…24,25 CTV has been employed as a supramolecular building block to construct more complex cage-like cryptophanes. 26 CTV derivatives have also been studied for their mesomorphic properties 27 and are capable of forming liquid crystals by themselves 28 and as complexes with C60. 28 CTV has also been studied as a crystal engineering tecton giving network structures of unusual topology.…”

Diskotische Flüssigkristalle: Von der maßgeschneiderten Synthese zur Kunststoffelektronik

“…[179] Luz und Zimmermann et al studierten das dynamische Verhalten des nonasubstituierten Tribenzocyclononatriens 74 d (Schema 31). [180] Diese Verbindung zeigte eine spezielle Form von Mesomorphie, in der das Kronen-und Sattelkon- 13 C-MAS-NMR-Spektren belegen, dass sich die Moleküle in der Mesophase des Kronenkonformers um die Säulenachse reorientieren, während sie in der Mesophase des Sattelkonformers statisch sind. Die Arbeitsgruppe um Nierengarten synthetisierte den Hexagallussäureester 74 e, der eine nematische Mesophase bildet (Schema 31).…”

Recent NMR Studies of Thermotropic Liquid Crystals

“…Some discotic mesogens can exist with their molecular core not flat and even not rigid such as derivatives of tribenzocyclononene [307] and octanonyloxy tetrabenzo-orthocyclophane, TBC-9 [308]. Octa-alkyloxy tetrabenzo-orthocyclophanes (TBC-n where n is the number of carbons in the side chains) exhibit columnar mesophases of various symmetries, depending on the length of the side chains.…”