Second harmonic generation (SHG) investigations of different phases of banana shaped molecules

Kentischer, F.; Macdonald, Rainer; Warnick, Peter; Heppke, G.

doi:10.1080/026782998206137

Cited by 65 publications

(47 citation statements)

References 10 publications

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“…[22,[24][25][26] However, comparatively few works have addressed structureproperty relations of molecular ferroelectrics in a systematic manner. [8,19,27] One of the few studies focusing on proper organic ferroelectric materials is by Kishikawa et al, who probed ferroelectric columnar hexagonal liquid crystalline urea with C 4 H 9 to C 16 H 33 long alkyl chains. [28] The observed inverse proportionality between alkyl chain length and remnant polarization value was explained to rely on the geometrical filling alone.…”

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confidence: 99%

Tuning the Ferroelectric Properties of Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

Urbanavičiūtė

Meng

Cornelissen

et al. 2017

Adv Elect Materials

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This study demonstrates how simple structural modification of a prototypical organic ferroelectric molecule can be used to tune its key ferroelectric properties. In particular, it is found that shortening the alkyl chain length of trialkylbenzene-1,3,5-tricarboxamide (BTA) from C18H37 to C6H13 causes an increase in depolarization activation energy (approximate to 1.1-1.55 eV), coercive field (approximate to 25-40 V mu m(-1)), and remnant polarization (approximate to 20-70 mC m(-2)). As the polarization enhancement far exceeds the geometrically expected factor, these observations are attributed to an increase in the intercolumnar interaction. The combination of the mentioned characteristics results in a record polarization retention time of close to three months at room temperature for capacitor devices of the material having the shortest alkyl chain. The long retention and the remnant polarization that is as high as that of P(VDF:TrFE) distinguish the BTA-C6 material from other small molecular organic ferroelectrics and make it a perspective choice for applications that require cheap, flexible, and lightweight ferroelectrics.

Funding Agencies|NWO Nano program; Vetenskapsradet; Swedish Government Strategic Research Area in Materials Science on Functional Materials at the Linkping University (Faculty Grant SFO Mat LiU) [2009 00971]

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confidence: 99%

Tuning the Ferroelectric Properties of Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

Urbanavičiūtė

Meng

Cornelissen

et al. 2017

Adv Elect Materials

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“…The resulting phases exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering within the layer plane, and molecular tilt, which together produce chiral layers with a handedness that depends on the direction of the tilt relative to the polar axis. Very large second order nonlinear optical (NLO) coefficients have been measured in the ferroelectric state of such materials, bearing some promising applications in NLO devices [8,9].From a theoretical point of view, the relationship of phase behavior to the specific bent-core molecular shape is of fundamental interest. In this paper, we investigate a minimal excluded volume model of bent-core mesogens, focusing on the molecular origin of polar and/or chiral symmetry breaking.…”

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Phase behavior of bent-core molecules

et al. 2003

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Recently, a new class of smectic liquid crystal phases (SmCP phases) characterized by the spontaneous formation of macroscopic chiral domains from achiral bent-core molecules has been discovered. We have carried out Monte Carlo simulations of a minimal hard spherocylinder dimer model to investigate the role of excluded volume interations in determining the phase behavior of bent-core materials and to probe the molecular origins of polar and chiral symmetry breaking. We present the phase diagram as a function of pressure or density and dimer opening angle ψ. With decreasing ψ, a transition from a nonpolar to a polar smectic phase is observed near ψ = 167• , and the nematic phase becomes thermodynamically unstable for ψ < 135• . No chiral smectic or biaxial nematic phases were found. [5]. In all of these examples, chirality is an intrinsic property built into the chemical structure of the LC molecules. Recently, a new class of smectic LC phases (SmCP phases) characterized by the spontaneous formation of macroscopic chiral layers from achiral molecules has been discovered [6,7]. The molecules comprising these phases have 'bow' or 'banana' shaped cores. The resulting phases exhibit two spontaneous symmetry-breaking instabilities: polar molecular orientational ordering within the layer plane, and molecular tilt, which together produce chiral layers with a handedness that depends on the direction of the tilt relative to the polar axis. Very large second order nonlinear optical (NLO) coefficients have been measured in the ferroelectric state of such materials, bearing some promising applications in NLO devices [8,9].From a theoretical point of view, the relationship of phase behavior to the specific bent-core molecular shape is of fundamental interest. In this paper, we investigate a minimal excluded volume model of bent-core mesogens, focusing on the molecular origin of polar and/or chiral symmetry breaking. Of particular interest is the coupling between polar and chiral symmetry breaking. In all bentcore materials studied to date, polar symmetry breaking is accompanied by chiral symmetry breaking induced by molecular tilt. This empirical fact raises the question whether there is a fundamental connection between polarity and chirality in molecular fluids. Another empirical observation is that bent-core materials exhibiting SmCP phases generally do not exhibit nematic phases, although two exceptions have recently been reported [10,11]. One objective of this study is to establish the molecular shape requirements for the occurence of the nematic phase in bent-core materials. Finally, we explore the possibility of biaxial nematic ordering in bent-core materials, motivated by recent experimental indications [11,12].Hard core models are particularly appealing due to their simplicity and relative ease of computation, both in simulation and theory. In particular, hard spherocylinders have been widely studied as simple models for conventional LCs [13,14]. This model exhibits rich phase behavior including isotropic, nematic, s...

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“…First, bentcore molecules have an electron donor-acceptor system along each rod-like segment of the molecule giving them a higher hyperpolarisability than calamitic molecules. [11,12] Second, they have reportedly greater flexoelectric coefficients [5,[13][14][15] which could be expected to lead to a greater flexoelectric polarisation and, consequently, SH signal in the nematic phase. Flexoelectric effects are predicted to be strongest for molecules asymmetric in shape and with a large permanent dipole.…”

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confidence: 99%

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

et al. 2016

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Second-harmonic generation (SHG) in the nematic phase of bent-core oxadiazole-based liquid crystals (LCs) was studied and compared to that for the rod-like compound 4-cyano-4ʹ-n-octylbiphenyl (8CB). Weak, isotropically scattered second-harmonic (SH) light was observed for all materials, consistent with SHG by nematic director fluctuations. The SH intensity produced by the bent-core materials was found to be up to~3.4 times that of 8CB. We discuss this result in terms of the dependence of SH intensity on temperature, elastic constants and flexoelectric coefficients. We have calculated the latter by using a molecular field approach with atomistic modelling, thus demonstrating how molecular parameters contribute to the flexoelectric coefficients and illustrating the potential of this method for predicting the flexoelectric behaviour of bent-core LCs. We show that the increased SH signal in the bent-core compounds is partly due to their nematic phases being at a much higher temperature, and also potentially due to them having greater flexoelectric coefficients, up to~1.5 times those of 8CB. These estimates are consistent with reports of increased flexoelectric coefficients in bent-core compounds in comparison to rod-like compounds.ARTICLE HISTORY

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Second harmonic generation (SHG) investigations of different phases of banana shaped molecules

Cited by 65 publications

References 10 publications

Tuning the Ferroelectric Properties of Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

Tuning the Ferroelectric Properties of Trialkylbenzene‐1,3,5‐tricarboxamide (BTA)

Phase behavior of bent-core molecules

Second-harmonic generation and the influence of flexoelectricity in the nematic phases of bent-core oxadiazoles

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