Robert P. Lemieux

Molecular recognition in chiral smectic liquid crystals: The effect of core–core interactions and chirality transfer on polar order

This critical review focuses on the induction of polar order in smectic liquid crystal phases by dopants with axially chiral cores, and should be of interest to all practitioners of supramolecular chemistry. The variations in polarization power of these dopants with the core structure of the liquid crystal hosts is a manifestation of molecular recognition that reflects the nanosegregation of aromatic cores from paraffinic side-chains in smectic phases, and the collective effect of core-core interactions that enable the propagation of chiral perturbations.

Siloxane-terminated phenylpyrimidine liquid crystal hosts

We report the synthesis and characterization of trisiloxane-terminated liquid crystals with 2-phenylpyrimidine cores that form partially bilayered SmA and SmC phases. Variable temperature measurements of smectic layer spacings by powder X-ray diffraction combined with measurements of optical tilt angles and observations of birefringence changes by polarized optical microscopy reveal that the trisiloxane end-group causes the SmA–SmC transition of these compounds to be more ‘de Vries-like’ when compared to a non-siloxane analogue. One such compound, 2-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)undecyloxy)phenyl)-5-(1-chlorooctyloxy)pyrimidine (3), is characterized by a maximum layer shrinkage of only 1.6% and may be considered a bona fide de Vries material.

Tuning ‘de Vries-like’ properties in siloxane- and carbosilane-terminated smectic liquid crystals

Smectic liquid crystals with ‘de Vries-like’ properties are characterized by a maximum layer contraction of ≤1% upon transition from the non-tilted SmA phase to the tilted SmC phase. To expand the current library of ‘de Vries-like’ liquid crystals, we have developed a rational design strategy based on a concept of frustration between two structural elements, one promoting the formation of a SmA phase (chloro-terminated side-chain) and another promoting the formation of a SmC phase (siloxane-terminated side-chain). In this paper, we show that one can tune this apparent frustration—and further improve de Vries-like properties—by substituting the 2-phenylpyrimidine core in our first-generation siloxane-terminated mesogens with one of three cores known to be stronger SmC-promoting elements: 6-phenylpyridazine, 2-phenylpyridine and 2-phenylthiadiazole. We also address a fundamental design flaw of siloxane-terminated mesogens, i.e., the hydrolytic instability of siloxane oligomers, by substituting the siloxane end-group with a chemically inert carbosilane end-group. As a result of this study, we found a carbosilane-terminated 2-phenylthiadiazole mesogen that forms a SmC phase at room temperature with de Vries-like properties that are comparable to those of bona fide de Vries-like liquid crystals.

Chiral induction in thioester and oxoester liquid crystals by dispersed carbon nanotubes

Multi-walled carbon nanotubes were dispersed at low concentrations into various achiral liquid crystals having either a thioester or oxoester linkage group in the core. The presence of the carbon nanotubes resulted in chiral signatures being observed in the liquid crystals, including an electroclinic effect (a rotation of the liquid crystal director perpendicular to, and linear in, an applied electric field) in both the nematic and smectic A phases, and a macroscopic helical twist of the liquid crystal director in the nematic phase. For both experiments the chiral signatures for the thioester liquid crystals were found to be an order of magnitude larger than those of the oxoesters. We speculate that the much larger strength of the thioesters chiral properties is a result of stronger non-covalent interactions between the liquid crystal molecule and carbon nanotube.

Electroclinic effect in chiral SmA* liquid crystals induced by atropisomeric biphenyl dopants: amplification of the electroclinic coefficient using achiral additives

The atropisomeric compound (R)-2,2′,6,6′-tetramethyl-3,3′-dinitro-4,4′-bis[(4-nonyloxybenzoyl)oxy]biphenyl ((R)-1) was doped in the achiral liquid crystal hosts 2-(4-butoxyphenyl)-5-octyloxypyrimidine (2-PhP) and 4-(4′-heptyl[1,1′-biphen]-4-yl)-1-hexylcyclohexanecarbonitrile (NCB76), and electroclinic coefficients ec were measured as a function of the dopant mole fraction x1 in the chiral SmA* phase at T − TC = +5 K. The extrapolated ec values of 3.07 and 2.28 deg µm V−1 are comparable to some of the highest ec values reported for neat SmA* materials. The electroclinic coefficient of a 4 mol% mixture of (R)-1 in 2-PhP is amplified by achiral 2-phenylpyrimidine additives (5 mol%) that are longer than 2-PhP; in the best case, ec is amplified by a factor of 3.2 with 5-(tetradecyloxy)-2-(4-(tetradecyloxy)phenyl)pyrimidine (3g), which is almost twice as long as 2-PhP. However, no amplification is observed in a 4 mol% mixture of (R)-1 in NCB76 using the same series of additives. A correlation between ec values and the temperature range of the SmA* phase suggests that the amplification of ec with increasing length of the additive 3 in the (R)-1/2-PhP mixture is due primarily to a decrease in the tilt susceptibility coefficient α as the second-order SmA*–SmC* phase transition moves away from the tricritical point. Measurements of smectic layer spacing as a function of T − TC by small-angle X-ray scattering are consistent with this explanation. The results show that the variation in reduced layer spacing dA/dC with T − TC for the pure host 2-PhP fits to a square-root law, which indicates that the second-order SmA–C transition is nearly tricritical. On the other hand, the corresponding variation in dA/dC with T − TC for a 5 mol% mixture of 3g in 2-PhP fits to a linear relation, which indicates that the second-order SmA–C transition approaches typical mean-field behavior.

MODULATING THE SPONTANEOUS POLARIZATION OF A FERROELECTRIC LIQUID CRYSTAL VIA THE PHOTOISOMERIZATION OF A CHIRAL THIOINDIGO DOPANT : (R,R)-6,6'-BIS(1- METHYLHEPTYLOXY)THIOINDIGO

Abstract Doping the novel chiral photochromic dye (R,R)-6,6′-bis(1-methylheptyloxy) thioindigo into the Sc liquid crystal host MX6120 at a concentration of 7.2 mol % gives rise to a ferroelectric S∗c phase with a reduced polarization (P 0) of +14.4 nCcm−2. Results of time-resolved experiments demonstrate that the dye undergoes trans→cis photoisomerization in the Sc phase, although the efficiency of photoisomerization is significantly lower than in the nematic and isotropic liquid phases. A reversible modulation of P s on the order of 10 per cent is observed for the doped S∗c phase upon photoisomerization of the dye from a thermally relaxed trans form (+6.2 nC cm−2) to a cis-enriched photostationary state (+5.6 nC cm−2) at T – Tc = −10°C. Control experiments using mixtures of the racemic and optically active dye in a S∗c host suggest that P s photomodulation does not arise from a photomechanical effect. Consideration of the Boulder model for the molecular origins of P s provides a rationale for the observe...

Chiral induction in nematic and smectic C liquid crystal phases by dopants with axially chiral 1,11-dimethyl-5,7-dihydrodibenz[c,e]thiepin cores

Three calamitic molecules with axially chiral cores, (S)-3,9-bis[(4-dodecyloxybenzoyl)oxy]-1,11-dimethyl-5,7-dihydrodibenzo[c,e]thiepin (5a), (R)-3,9-bis[(4-butoxybenzoyl)oxy]-1,11-dimethyl-5,7-dihydrodibenzo[c,e]thiepin (5b), and (R)-3,9-bis[(4-octyloxybenzoyl)oxy]-1,11-dimethyl-5,7-dihydrodibenzo[c,e]thiepin (5c), were synthesized in optically pure form and doped in four calamitic liquid crystal hosts with I–N–SmA–SmC (INAC) phase sequences (PhP, PhB, NCB76 and DFT), and in the nematic host 5CB. Helical twisting powers (β) in the N* phase are in the range of 24–43 µm–1 with the INAC hosts, and in the range of 56–58 µm–1 with 5CB at T–TN-I = –2 K, which are comparable to those previously reported for axially chiral binaphthyls. Interestingly, these dopants were found to stabilize and broaden the temperature range of the N* phase in 5CB. With some of the dopant–INAC host mixtures, the induction of a chiral TGBA* phase was also observed. Dopant 5a induces a measurable spontaneous polarization in the SmC phase of the achiral phenylpyrimidine host PhP, giving a polarization power δp of –104 ± 20 nC cm–2. Comparisons of helical twisting powers in the N* and SmC* phases, and polarization powers in the SmC* phase, for dopants 5a–c and the dinitro analogue (+)-1,11-dinitro-3,9-bis[(4-nonyloxybenzoyl)oxy]-5,7-dihydrodibenzo[c,e]thiepin (4) suggest that the nitro substituents play a dominant role in promoting chirality transfer in the SmC* phase, whereas the helical topography of the core appears to be a more important factor in promoting chirality transfer in the N* phase.

Reversible photocyclization of achiral dithienylperfluorocyclopentene dopants in a ferroelectric liquid crystal: bistable SSFLC photoswitching

The spontaneous polarization (PS) of a ferroelectric liquid crystal is modulated reversibly by photocyclization (λ = 313 nm) of dithienyl- and dibenzothienylperfluorocyclopentene dopants with relatively high aspect ratios. Photocyclization of these dopants causes a decrease of PS due to a photomechanical effect which may originate from a loss of conformational flexibility of the dopant upon photocyclization. The degree of PS photomodulation increases with dopant concentration up to the solubility limit of 3 mol%. The maximum polarization photomodulation, ΔPS = 16.2 nC cm−2, is achieved at 6 K below the Curie point with the dopant 1,2-bis(5-(4-heptyloxyphenyl)-2-methylthien-3-yl)perfluorocyclopentene (3c, 3 mol%). The resulting PS photoswitch is fatigue resistant and photochemically bistable. Polarized spectroscopy experiments suggest that the dopants form homogeneous solutions with the FLC host and are integrated within the layer structure of the SmC* phase, with their molecular long axes approximately coincident with the director. To the best of our knowledge, this is the first example of a bistable ferroelectric liquid crystal photoswitch to be reported in the literature.

Synthesis and characterization of liquid crystals containing a non-activated 1',3',3'-trimethylspiro\[2H-1-benzopyran-2,2-indoline] group

Two series of mesogens derived from the non-activated spiropyran dyes 6-hydroxy-1,3,3-trimethylspiro\[2H -1-benzopyran-2,2-indoline] (series trimethylspiro\[2 H -1-benzopyran-2,2-indoline] (series 2) have been synthesized. Analysis by differential scanning calorimetry, polarizing microscopy and X-ray diffraction showed compounds in series 1 form monotropic nematic and SmA phases, while compounds in series 2 form only a monotropic nematic phase.

Fast switching organosiloxane ferroelectric liquid crystals

We report the synthesis and characterization of organosiloxane 2-phenylpyrimidine mesogens with chiral (R,R)-2,3-difluorooctyloxy side-chains. These compounds form chiral SmA* and SmC* phases, and show remarkable electro-optical and alignment properties as surface-stabilized ferroelectric liquid crystals, including one of the fastest optical rise times (11 µs at T − TAC = −10 K and 6 V µm−1) for an organosiloxane mesogen.