Columnar Liquid Crystalline Glasses by Combining Configurational Flexibility with Moderate Deviation from Planarity: Extended Triaryltriazines

Abstract

Triply phenanthryl‐ and tetrahelicenyl‐substituted triazine‐hexaalkyl esters with short alkyl chains show glass transitions conveniently above room temperature within the hexagonal columnar liquid crystalline state, resulting in a solid columnar order at room temperature. As the hexagonal columnar mesophase is easily aligned with the director perpendicular to a solid substrate, such glassy columnar liquid matrices are aimed at for the orientation of guest emitters, to obtain anisotropic emission. A condition for face‐on alignment on substrates are attainable melting and clearing temperatures, which is achieved with the moderately nonplanar tetrahelicenyl derivatives in spite of their short alkyl periphery. An unusual phase transition between two columnar mesophases of same hexagonal symmetry, but very different long‐distance regularity of the column lattice, is found in one phenanthryl homolog.

Dissymmetric Triaryltriazines: Small Mass Columnar Glasses

“…We have earlier reported that methyl 3‐cyanobenzoate is trimerized in triflic acid at room temperature without substantial hydrolysis and degradation to non‐mesogenic 4 , whereas 3 itself and its longer‐chain homologs are better obtained by saponification of the methyl homolog 4 followed by appropriate re‐esterification than by the hydrolysis‐plagued trimerization of the appropriate alkyl 3‐cyanobenzoate [18] . We also noted previously that degradation upon attempted trimerization in triflic acid is a problem with 3‐cyanobenzoic acid, but not with 3‐cyanophenylacetic acid [17] . Accordingly, we now found that the co‐trimerization of the latter with methyl 3‐cyanobenzoate affords a mixture of products 4 – 7 from which 5 and 6 can be isolated conveniently by column chromatography in about 30 % of the statistically expected yield.…”

“…[18] We also noted previously that degradation upon attempted trimerization in triflic acid is a problem with 3-cyanobenzoic acid, but not with 3-cyanophenylacetic acid. [17] Accordingly, we now found that the co-trimerization of the latter with methyl 3-cyanobenzoate affords a mixture of products 4-7 from which 5 and 6 can be isolated conveniently by column chromatography in about 30 % of the statistically expected yield.…”

“…Data for 1 and 2 are taken from the literature. [17] Cr-Cr Cr-Col hex Col hex -Iso Cr-Iso T g (Col) d col d disk m…”

“…Compared with monotropic 2 , the only symmetrical homolog that exhibits an attainable isotropic phase (T cl =149 °C, T m =205 °C, T g =39 °C) [17] and a columnar glass at room temperature (at the expense of a much larger molecular weight), 12 – 15 all show an improved T g , and it is striking that 15 with two large, slightly non‐planar tetrahelicenyl fragments exhibits a thermodynamically stable mesophase with an enantiotropic temperature range, whilst its more disk‐shaped, but symmetrical, homolog 2 with three [4[helicenyl arms does not. In contrast, in the more planar phenanthrene homologs, the enantiotropic mesophase range is greatly reduced when going from symmetrical 1 to the bis‐phenanthryl homolog 14 : whilst T cl becomes attainable (>375 °C→291 °C), T m increases (150 °C→205 °C), and T g decreases (81 °C→50 °C).…”

“…But the smaller the molecular weight, the more difficult it is to obtain a glass transition temperature T g conveniently above room temperature. Molecular flexibility and configurational multiplicity combined with viscosity‐augmenting polar substituents are ingredients that have allowed us recently to obtain columnar LC glasses with above‐room‐temperature T g from extended triaryltriazine hexaesters 1 and 2 bearing three phenanthyl or three tetrahelicenyl groups on the s ‐triazine core [17] (Scheme 1). Given the presence of six propyl or butyl ester groups and three extended aryl arms, the molecular weights of these columnar glass formers are considerable (1126 and 1361 Da), and whilst 1 shows no attainable LC‐to‐isotropic transition (i. e. clearing point) up to 375 °C that would also allow its uniform alignment by cooling from the isotropic liquid, 2 shows only a monotropic mesophase (observable only on cooling, the melting point being higher than the clearing point).…”