Ferroelectric Liquid Crystals Induced by Dopants with Axially Chiral 2,2‘-Spirobiindan-1,1‘-dione Cores

Abstract

The axially chiral dopants (R)-5,5'-, 5,6'-, and 6,6'-diheptyloxy-2,2'-spirobiindan-1,1'-dione ((R)-2, -3, and -4) were synthesized in optically pure form, and their absolute configurations were assigned by the exciton chirality method using circular dichroism spectroscopy. These new compounds were doped in four achiral liquid crystal hosts to give chiral smectic C* (SmC*) phases with spontaneous polarizations (Ps) that vary with the core structure of the host. The spontaneous polarization induced by the 5,5'-dialkoxy derivative (R)-2 is uniformly positive, whereas that induced by the 6,6'-dialkoxy derivative (R)-4 is uniformly negative and shows a different trend in host dependence. Polarization power (delta(p)) values range from +21 nC/cm2 for (R)-2 in 2',3'-difluoro-4-heptyl-4' '-nonyl-p-terphenyl (DFT) to -1037 nC/cm2 for (R)-4 in 4-(4'-heptyl[1,1'-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76). The unsymmetrical dopant (R)-3 behaves like a hybrid of the two symmetrical isomers, with lower absolute values of delta(p), on average, and varying signs of Ps. 2H NMR spectra of the doped mixtures using racemic mixtures of 2-4 with -OCD2C6H13 side-chains, in combination with phase diagrams, show that relatively minor changes in the dopant structure, that is, moving the alkoxy side-chains from the 5,5' to the 6,6' positions of the spirobiindandione core, have profound effects on dopant-host compatibility, and on the propensity of the dopant to exert chiral perturbations in the host environment. The variations in sign and magnitude of delta(p) as a function of alkoxy group positions are rationalized based on an analysis of zigzag conformations that conform to the binding site of the SmC host according to the Boulder model.

Ferroelectric liquid crystals with axially chiral 2,2′-spirobiindan-1,1′-dione cores

“…As shown in Figure 6(a), positive spontaneous polarisations were measured for all four mesogens, which is consistent with the sign of induced P S reported for the (R) enantiomer of the dialkoxy analogue 5 [19]. In all cases the spontaneous polarisation rises sharply and saturates within 3 K, which is consistent with first-order N * -SmC * or I-SmC Table 1.…”

“…In each case, resolution of the (R) and (S) enantiomers was achieved by preparative chiral phase highperformance liquid chromatography (HPLC) using a Daicel Chiralpak IC column (Chiral Technologies) and a 3:1 mixture of hexanes and EtOAc as eluent; up to 50 mg of racemate could be fully resolved (>99% ee) per injection using a 20 mL injection loop. The absolute configuration of each enantiomer was established by comparison of its CD spectrum with that of the diheptyloxy derivative (R)-5 [19]. For example, the CD spectrum of the second HPLC eluate from the racemate (RS)-QL7-10 features positive split Cotton effects that are consistent with those featured in the CD spectrum of (R)-5 (see Figure 1).…”

“…The mesogens QL7-n were synthesised by monoalkylation of racemic 5,5 -dihydroxy-2,2 -spirobiindan-1,1 -dione (6) with the appropriate alkyl bromide under basic conditions in 39-42% yields (Scheme 3) [19]. The monoalkylated intermediates 7(n) were then esterified with the corresponding 4-alkoxybenzoic acids using dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) to give the mesogens (RS)-QL7-n as racemic mixtures in 31-40% yields.…”

“…In a subsequent study, we have developed a new axially chiral dopant with a 2,2 -spirobiindan-1,1 -dione core (5) that is highly miscible in the SmC phase of various liquid crystal hosts [19,20]. Unlike 2, this dopant showed no evidence of exerting strong chiral perturbations, although its polarisation power did show a pronounced host dependence, ranging from 749 nC cm −2 in a 2-phenylpyrimidine host to 21 nC cm −2 in a difluoroterphenyl host.…”

Design and Synthesis of Chiral Nematic Liquid Crystals

“…These materials have been synthesized as racemic mixtures via multistep routes and then resolved using preparative chiral HPLC columns [239,240]. These have tended to be investigated as ferroelectric dopants, but are clearly capable of inducing chiral nematic phases in well-known classes of host liquid crystals; examples include compounds 228 and 229.…”