Reentrant Nematic Phenomenon in a New Class of Low Molar Mass, Single-Component Liquid Crystals

Prabhu, Rashmi; Yelamaggad, C. V.

doi:10.1021/jp304003c

Cited by 5 publications

(13 citation statements)

References 63 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…The qualitative miscibility study further supported the N−SmA−N re sequence with the nematic phase exhibiting re-entrance. 81 An optical transmission profile obtained clearly supports the phase transitions deduced from both calorimetric and optical studies. 74 To reveal the molecular arrangement, the SmA phase occurring between two nematic phases was probed with the help of an XRD technique.…”

Section: Articlesupporting

confidence: 69%

Structure–Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens

Prabhu

Yelamaggad

2015

J. Phys. Chem. B

Self Cite

View full text Add to dashboard Cite

In this contribution, we present the results of our extensive investigations on the synthesis and phase behavior of five series of dimer-like compounds formed by covalently linking a promesogenic, cyanobiphenyl core, with a nonmesogenic N-(n-alkyl)-salicylaldimine segment via an oxy(oligomethylene)oxy spacer of varying length and parity. This work is a continuation of our previous short study where the occurrence of re-entrant nematic (Nre) phase seemed to be vitally dependent on the parity of the spacer and the length of the terminal chain. Thus, working along these lines the effects on the thermal (Nre phase) behavior of the various spacers and the terminal tails have been investigated comprehensively. In particular, with the aim of exploring the fundamental relationships between molecular structural features and thermal properties, five spacers, namely, oxy(hexyl)oxy, oxy(heptyl)oxy, oxy(octyl)oxy, oxy(nonyl)oxy, and oxy(decyl)oxy spacers, have been used. Each series consists of six mesogens, as the terminal tail attached to the salicylaldimine core has been varied from n-pentyl to n-decyl. For the sake of comparison and completeness, previously reported materials have also been studied in detail, and data are included here. The characterization results derived from the optical and calorimetric studies clearly illustrate a pronounced odd-even effect; however, the effect attenuates as the length of the spacer is elongated. The even-members show higher clearing temperatures and enthalpies of transition than those of adjoining odd-members. Three series of mesogens possessing an even-parity spacer show thermodynamically stable nematic (N) and/or smectic A (SmA) phase(s); the lower homologues display only the N phase, and the middle ones exhibit both N and SmA phases while the higher members show the SmA phase solely. In contrast, the vast majority of the odd-spaced compounds of two series show a monotropic N phase. X-ray diffraction study confirms the partial bilayer order of the SmA (SmAd) phase with L < d < 2L where d is the layer spacing and L is the molecular length. Contrary to what was previously noted, none of the new compounds synthesized favor the stabilization of the Nre phase. At the present time, it is extremely difficult to point out the exact cause for the origin of re-entrant behavior in some compounds exclusively, as they hardly differ in their structure. It was also found that the phase transitional properties of even-members are comparable to that observed for 4'-n-alkoxy-4-cyanobiphenyls (nOCBs) and 4-n-alkyl-4-cyanobiphenyls (nCBs). In essence, our study validates the point that dimer-like compounds resemble liquid crystal (LC) dimers, as well as monomers to some degree, in the context of their thermal behavior; thus, they can be regarded as the bridging structures between LC monomers and dimers.

show abstract

Section: Articlesupporting

confidence: 69%

Structure–Property Correlations in Cyanobiphenyl-Based Dimer-Like Mesogens

Prabhu

Yelamaggad

2015

J. Phys. Chem. B

Self Cite

View full text Add to dashboard Cite

show abstract

“…Scheme shows the synthetic steps followed to prepare the eight pairs of enantiomers. To begin with, 4′‐hydroxy‐4‐cyanobiphenyl was alkylated with α,ω‐dibromoalkanes in the presence of anhydrous K 2 CO 3 in butanone to obtain 4′‐((ω‐bromoalkyl)oxy)‐4‐cyanobiphenyls ( 1 a – h ) in good yields . The reaction of 2,4‐dihydroxybenzaldehyde with cyanobiphenyls ( 1 a – h) in the presence of a mild base in butanone yielded the respective O ‐alkylated products, namely, 4′‐((ω‐(4‐formyl‐3‐hydroxyphenoxy)alkyl)oxy)‐4‐cyanobiphenyls ( 2 a – h ) .…”

Section: Resultsmentioning

confidence: 99%

“…Powder XRD studies have shown that the SmA phase formed by polar mesogens (i.e., with a terminal cyano group) generally consist of antiparallel pairs . The pairing of polar mesogens results from the strong intermolecular longitudinal association so as to minimize the dipolar energy contribution effectively.…”

Section: Resultsmentioning

confidence: 99%

“…To begin with, 4'-hydroxy-4-cyanobiphenyl was alkylated with a,w-dibromoalkanes in the presence of anhydrous K 2 CO 3 in butanone to obtain 4'-((w-bromoalkyl)oxy)-4-cyanobiphenyls (1a-h)i ng ood yields. [59][60][61] The reactiono f2 ,4-dihydroxybenzaldehyde with cyanobiphenyls (1a-h) in the presenceo famild base in butanone yielded the respective O-alkylated products, namely,4 '-((w-(4-formyl-3-hydroxyphenoxy)alkyl)oxy)-4-cyanobiphenyls (2a-h). [58][59][60][61][62] Under Mitsunobu reaction conditions, [63] 4-nitrophenol was O-alkylated with the (S)-and (R)-2-octanol, to obtain (R)-4-(octan-2-yloxy)nitrobenzene (3)a nd (S)-4-(octan-2-yloxy)nitrobenzenes (5); [64][65][66][67][68] these nitro materials were catalytically hydrogenated using H 2 (1 atm, balloon) and Pd/C (10 %) in THF,t oo btain the respective (R)-4-(octan-2-yloxy)aniline (4)a nd (S)-4-(octan-2yloxy)aniline (6)i nn otable yields.…”

Section: Synthesis and Molecular Structural Characterizationmentioning

confidence: 99%

“…[59][60][61] The reactiono f2 ,4-dihydroxybenzaldehyde with cyanobiphenyls (1a-h) in the presenceo famild base in butanone yielded the respective O-alkylated products, namely,4 '-((w-(4-formyl-3-hydroxyphenoxy)alkyl)oxy)-4-cyanobiphenyls (2a-h). [58][59][60][61][62] Under Mitsunobu reaction conditions, [63] 4-nitrophenol was O-alkylated with the (S)-and (R)-2-octanol, to obtain (R)-4-(octan-2-yloxy)nitrobenzene (3)a nd (S)-4-(octan-2-yloxy)nitrobenzenes (5); [64][65][66][67][68] these nitro materials were catalytically hydrogenated using H 2 (1 atm, balloon) and Pd/C (10 %) in THF,t oo btain the respective (R)-4-(octan-2-yloxy)aniline (4)a nd (S)-4-(octan-2yloxy)aniline (6)i nn otable yields. [64][65][66][67][68] In the final step, anilines 4 and 6 were condensed with aldehydes 2a-h in the presence of aw eak acidi ne thanol to obtain the targeted optically active, non-symmetric dimers in excellent yields.…”

Section: Synthesis and Molecular Structural Characterizationmentioning

confidence: 99%

See 2 more Smart Citations

Optically Biaxial, Re‐entrant and Frustrated Mesophases in Chiral, Non‐symmetric Liquid Crystal Dimers and Binary Mixtures

Padmini

Babu

Nair

et al. 2016

Chemistry An Asian Journal

View full text Add to dashboard Cite

Sixteen optically active, non-symmetric dimers, in which cyanobiphenyl and salicylaldimine mesogens are interlinked by a flexible spacer, were synthesized and characterized. While the terminal chiral tail, in the form of either (R)-2-octyloxy or (S)-2-octyloxy chain attached to salicylaldimine core, was held constant, the number of methylene units in the spacer was varied from 3 to 10 affording eight pairs of (R & S) enantiomers. They were probed for their thermal properties with the aid of orthoscopy, conoscopy, differential scanning calorimetry and X-ray powder diffraction. In addition, the binary mixture study was carried out using chiral and achiral dimers with the intensions of stabilizing optically biaxial phase/s, re-entrant phases and important phase sequences. Notably, one of the chiral dimers as well as some mixtures exhibited a biaxial smectic A (SmA ) phase appearing between a uniaxial SmA and a re-entrant uniaxial SmA phases. The mesophases such as chiral nematic (N*) and frustrated phases viz., blue phases (BPs) and twist grain boundary (TGB) phases, were also found to occur in most of the dimers and mixtures. X-ray diffraction studies revealed that the dimers possessing oxybutoxy and oxypentoxy spacers show interdigitated (SmA ) phase where smectic periodicity is over 1.4 times the molecular length; whereas in the intercalated SmA (SmA ) phase formed by a dimer having oxydecoxy spacer the periodicity was found to be approximately half the molecular length. The handedness of the helical structure of the N* phases formed by two enantiomers was examined with the aid of CD measurements; as expected, these enantiomers showed optical activities of equal magnitudes but with opposite signs. Overall, it appears that the chiral dimers and mixtures presented herein may serve as model systems in design and developing novel materials exhibiting the apolar SmA phase possessing D symmetry and nematic-type biaxiality.

show abstract