Reversible Isothermal Twist–Bend Nematic–Nematic Phase Transition Driven by the Photoisomerization of an Azobenzene-Based Nonsymmetric Liquid Crystal Dimer

Abstract

The liquid crystal nonsymmetric dimer, 1-(4-butoxyazobenzene-4'-yloxy)-6-(4-cyanobiphenyl-4'-yl) hexane (CB6OABOBu), shows enantiotropic twist-bend nematic, NTB, and nematic, N, phases. The NTB phase has been confirmed using polarized light microscopy, freeze fracture transmission electron microscopy, and X-ray diffraction. The helicoidal pitch in the NTB phase is 18 nm. The NTB-N (TNTBN) and N-I (TNI) transition temperatures are reduced upon UV light irradiation, with the reduction in TNTBN being much larger than that in TNI. An isothermal, reversible NTB-N transition may be driven photochemically. These observations are attributed to a trans-cis photoisomerization of the azobenzene fragment on UV irradiation, with the cis isomers stabilizing the standard nematic phase and the trans isomers stabilizing the NTB phase. The dramatic changes in TNTBN provide evidence that the transition between the normal nematic and twist-bend nematic with spontaneous breaking of chiral symmetry is crucially dependent on the shape of molecular dimers, which changes greatly during the trans-cis isomerization.

The Dependency of Nematic and Twist-bend Mesophase Formation on Bend Angle

“…al. have very recently demonstrated a N TB phase transition driven by the photoisomerisation of an azo group; this is an elegant demonstration of the fact that the ability to change the bend angle can give rise to (or eliminate) the N TB phase45. Such results confirm to us the fact that the incidence of the N TB phase is ultimately a product of the gross topology of the molecule, and thus the minimisation of free volume that leads to condensed phases3846.…”

Conformational landscapes of bimesogenic compounds and their implications for the formation of modulated nematic phases

“…The discovery of the twist-bend phase (N TB ) has given fresh impetus to the study of dimeric liquid crystals [1][2][3][4][5][6][7][8][9][10][11][12][13]. First predicted by Dozov [13], this liquid-crystalline state of matter has a locally helical structure with a pitch measured to be on the order of 10 nm [4,14], with this phase displaying a number of unusual and unique optical textures [15].…”

“…A number of reviews concerning the twist-bend phase have been published recently [18][19][20]. A relatively large number of dimeric liquid crystals are known to exhibit this state of matter [1,3,8,9,11,12,[21][22][23][24][25][26][27][28][29][30], and there is a growing number of liquid-crystalline oligomers known to exhibit the twist-bend phase [18,[31][32][33][34]. Additionally, this phase has been observed in a phenylpiperazine derived bent-core liquid crystal [4].…”

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

“…Compounds with molecular shapes at the cross‐over between linear (rod‐like) and bent shapes (BCLCs), such as mesogenic dimers with odd spacers, hockey stick molecules, and molecules with reduced length of the bent core or with a reduced bending angle have received significant attention, because they provide new LC phase structures. 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 .…”