Deuterium Studies of Guest Solubilization in Liquid Crystals/Photochemistry of Cyclopropenes
Abstract
The solubilization of eight aromatic ketones and C6D6 in the liquid crystalline phases of trans,trans-4'-butylbicyclohexyl-4-carbonitrile(CCH-4), trans,trans-4'-ethylbicyclohexyl-4-carbonitril (CCH-2) and a binary mixture (1:2) of CCH-4 and CCH-2 (EB) has been investigated by deuterium NMR spectroscopy using quadrupolar splitting and T1 measurements. All of the solutes were found to be homogeneously soluble in the nematic phase of these solvents. However, solubilization of these solutes in the smectic phase of these solvents was found to be dependent on temperature, solute size, and solute structure.
All of the solutes investigated were found to have very low solubilities (ca. 0.2-2.0 mol%) in the smectic phase of these solvents. In general, the solubility of a given solute in the smectic phase increases with decreasing temperature. When the bulk concentration is above the solubility limit in the smectic phase, the solute order parameters (via ^2H NMR) appear to decrease with decreasing temperature. This results from formation of a biphasic system which consists of a solute-depleted bulk smectic phase and a solute rich phase separated nematic phase. The solubility limit in the smectic phase also appears to be dependent on the size of the solute, with a solubility increasing with decreasing solute size. The nature of substituents on the solute seems to affect the solubilization in the smectic phase as well.
At lower temperatures (at or near the K→S phase transition) we have observed, as have others, a solute-induced phase (characterized by isotropic like ^2H NMR spectra). We have shown that this solute-induced phase is a viscous isotropic liquid.
The direct solution photochemistry of 1,3,3-trimethylcyclopropene and 1-t-butyl-3,3-dimethylcyclopropene has been investigated. Primary photoproducts consisting of alkynes, allenes, and 1,3-dienes are formed from vinylcarbene intermediates, a result of cleavage of the C1-C3 or C2-C3 bond. Cleavage of the more subsituted (C1-C3) bond to yield a vinylcarbenes can account for >80% observed products. Most of the products are formed via 1,2- and 1,4-hydrogen migrations from the intermediate vinylcarbenes.
Photolysis of 1,3,3-trimethylcyclopropene 1-^13C supports the intermediacy of vinylcarbenes in product formation from single excited cyclopropenes. In addition, evidence which strongly suggests the importance of a second pathway for formation of alkyne products from alkylcyclopropenes has been obtained. This pathway involves the intermediacy of vinylidene, formed by 1,2-hydrogen migration/ring opening (sequential or concerted) from the singlet excited-state of cyclopropene.