Polar Order and Symmetry Breaking at the Boundary between Bent‐Core and Rodlike Molecular Forms: When 4‐Cyanoresorcinol Meets the Carbosilane End Group

Abstract

Two isomeric achiral bent-core liquid crystals involving a 4-cyanoresorcinol core and containing a carbosilane unit as nanosegregating segment were synthesized and were shown to form ferroelectric liquid-crystalline phases. Inversion of the direction of one of the COO groups in these molecules leads to a distinct distribution of the electrostatic potential along the surface of the molecule and to a strong change of the molecular dipole moments. Thus, a distinct degree of segregation of the carbosilane units and consequent modification of the phase structure and coherence length of polar order result. For the compound with larger dipole moment (CN1) segregation of the carbosilane units is suppressed, and this compound forms paraelectric SmA and SmC phases; polar order is only achieved after transition to a new LC phase, namely, the ferroelectric leaning phase (SmCLs PS ) with the unique feature that tilt direction and polar direction coincide. The isomeric compound CN2 with a smaller dipole moment forms separate layers of the carbosilane groups and shows a randomized polar SmA phase (SmAPAR ) and ferroelectric polydomain SmCs PS phases with orthogonal combination of tilt and polar direction and much higher polarizations. Thus, surprisingly, the compound with the smaller molecular dipole moment shows increased polar order in the LC phases. Besides ferroelectricity, mirror-symmetry breaking with formation of a conglomerate of macroscopic chiral domains was observed in one of the SmC phases of CN1. These investigations contribute to the general understanding of the development of polar order and chirality in soft matter.

Stereochemical Rules Govern the Soft Self‐Assembly of Achiral Compounds: Understanding the Heliconical Liquid‐Crystalline Phases of Bent‐Core Mesogens

“…The investigated compounds 1/ n (Scheme ) are based on the 140° bent 4‐cyanoresorcinol core ( α =140°) with a reduced molecular bent compared to the 120° angle of ordinary bent‐core molecules . In these molecules the 4‐cyanoresorcinol core is connected with two electron deficit terephthalate wings, providing a dense packing . A summary of previous work on selected individual members of the 4‐cyanoresorcinol based LCs and the development of the distinct models of their phase structures is given in reference as well as in Section S4 and Table S2 in the Supporting Information.…”

“…[45][46][47][48][49][50][51] In these moleculest he 4-cyanoresorcinol core is connectedw ith two electron deficit terephthalate wings, providing ad ense packing. [52][53][54][55][56] As ummary of previous work on selected individual members of the 4-cyanoresorcinol based LCs and the development of the distinct modelso ft heir phase structures is given in reference [57] as well as in Section S4 and Ta ble S2 in the Supporting Information. Compounds 1/n with n = 14 and 16 are of special interest, because for 1/16 the existence of ah eliconicals mectic LC phase was first observed by opticali nvestigations [19,[58][59][60][61] and AFM, [19] and an incommensurate 3-layers tructure was corroborated for 1/14 by soft resonant X-ray scattering at the carbon K-edge (RSoXR).…”

Polar Order, Mirror Symmetry Breaking, and Photoswitching of Chirality and Polarity in Functional Bent‐Core Mesogens

“…4‐Cyanoresorcinol is a very useful bent‐core unit with reduced bending angle, which has led to a number of new and unique mesophases, for example, the nontilted antiferroelectric SmAP A phase and a short‐pitch heliconical smectic phase designated as SmC s P F hel . In previous work, the 4‐cyanoresorcinol core was combined with two identical wings, either azobenzenes or phenyl benzoates …”

“…[52][53][54][55][56][57][58][59][60][61] Compoundsw ith molecular shapes at the cross-over between linear (rod-like) and bent shapes( BCLCs), such as mesogenic dimers with odd spacers, [62][63][64][65][66][67][68][69] hockeys tick molecules, [70][71][72] and molecules with reduced length of the bent core [73] or with ar educed bending angle [74] have received significant attention, because they providen ew LC phase structures. 4-Cyanoresorcinol is av ery useful bent-core unit with reduced bending angle, which has led to an umber of new and unique mesophases, [37,38,[75][76][77][78][79][80][81][82] for example, the nontilted antiferroelectric SmAP A phase [83] and as hort-pitch heliconical smectic phase designatedasS mC s P F hel . [84][85][86] In previous work, the 4-cyanoresorcinol core was combined with two identical wings, either azobenzenes or phenyl benzoates.…”

“…[84][85][86] In previous work, the 4-cyanoresorcinol core was combined with two identical wings, either azobenzenes or phenyl benzoates. [37,38,[76][77][78][79][80][81][82][83][84][85][86] Herein, we report new BCLCs derived from a4 -cyanoresorcinol central core connected to two different rod-like wings, in which one is azobenzene-based (A)a nd the other is either a benzoylated 4-hydroxybenzoatew ing (B)i nc ompounds ABn or ap henyl terephthalate wing (T)i nc ompounds ATn;t hese are different in the direction of the COO group betweent he two outer benzene rings (see Scheme 1). The terminal alkoxy chain at the ester-based wing in both types of compounds was fixed to m = 14, whereas that connected to the azobenzene containing sidea rm has chain lengths n = 8, 12, 16, and 20.…”