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Broadening the temperature range of blue phases using $azo$ compounds of various molecular geometries assembled from modular "LEGO" molecular units

Igor A. Gvozdovskyy, Vitalii O. Chornous, Halyna V. Bogatyryova, Oleksandr M. Samoilov, Longin N. Lisetski, Serhiy V. Ryabukhin, Yurii V. Dmytriv, Mykhaylo V. Vovk

TL;DR

This work investigates broadening the blue phase (BP) temperature range in highly chiral cholesteric mixtures formed by cholesteryl oleyl carbonate (COC) and the nematic host E7, using azo dopants assembled from modular LEGO units. It shows that UV-induced cis–trans photoisomerization increases the BP stability window, with cis isomers more effective than trans; the magnitude of broadening depends on the molecular geometry of the azo dopants and their concentration. A quantitative cis-isomer efficiency $ξ$ and a concentration-dependent “cinema hall” model are introduced to interpret how dopant structure and loading influence $ΔT_{BP}$. The results demonstrate a controllable, structure-driven approach to tuning BP thermal ranges, enabling potential photonic applications that exploit BRL within visible light. The approach integrates molecular engineering with reversible photo-switching to achieve precise BP stabilization metrics, such as $ΔT_{BP}$ and $ξ$, across a library of LEGO-like azo architectures.

Abstract

The temperature range of the blue phases (BPs) formed in highly chiral mixtures based on cholesteryl oleyl carbonate (COC) and the nematic liquid crystal E7 was studied in the presence of various chemical structures. The $azo$ compounds used were of both chiral and achiral nature, and their molecular geometry was modified by substitution of modular "LEGO" molecular units of varying alkyl chain lengths and types of bridging groups, which could substantially affect the mesomorphic properties of the matrix mixture. It was shown that in many cases these dopants effectively broadened the BP temperature range. This effect depends on both the variation in the molecular geometry of the $azo$ compounds and the increase in the $cis$-isomer concentration under UV irradiation. The presence of the $cis$-isomers formed have a stronger impact on broadening the BP temperature range than the initial $trans$-isomers. These results demonstrate that the temperature range of BPs can be precisely controlled via a combination of molecular engineering and $trans$-$cis$ photo-isomerization.

Broadening the temperature range of blue phases using $azo$ compounds of various molecular geometries assembled from modular "LEGO" molecular units

TL;DR

This work investigates broadening the blue phase (BP) temperature range in highly chiral cholesteric mixtures formed by cholesteryl oleyl carbonate (COC) and the nematic host E7, using azo dopants assembled from modular LEGO units. It shows that UV-induced cis–trans photoisomerization increases the BP stability window, with cis isomers more effective than trans; the magnitude of broadening depends on the molecular geometry of the azo dopants and their concentration. A quantitative cis-isomer efficiency and a concentration-dependent “cinema hall” model are introduced to interpret how dopant structure and loading influence . The results demonstrate a controllable, structure-driven approach to tuning BP thermal ranges, enabling potential photonic applications that exploit BRL within visible light. The approach integrates molecular engineering with reversible photo-switching to achieve precise BP stabilization metrics, such as and , across a library of LEGO-like azo architectures.

Abstract

The temperature range of the blue phases (BPs) formed in highly chiral mixtures based on cholesteryl oleyl carbonate (COC) and the nematic liquid crystal E7 was studied in the presence of various chemical structures. The compounds used were of both chiral and achiral nature, and their molecular geometry was modified by substitution of modular "LEGO" molecular units of varying alkyl chain lengths and types of bridging groups, which could substantially affect the mesomorphic properties of the matrix mixture. It was shown that in many cases these dopants effectively broadened the BP temperature range. This effect depends on both the variation in the molecular geometry of the compounds and the increase in the -isomer concentration under UV irradiation. The presence of the -isomers formed have a stronger impact on broadening the BP temperature range than the initial -isomers. These results demonstrate that the temperature range of BPs can be precisely controlled via a combination of molecular engineering and - photo-isomerization.
Paper Structure (17 sections, 7 equations, 26 figures, 11 tables)

This paper contains 17 sections, 7 equations, 26 figures, 11 tables.

Table of Contents

  1. Introduction
  2. Experimental
  3. Materials
  4. Methods
  5. Results and discussions
  6. The highly chiral mixture doped with azo compounds: impact of the cis-isomer on broadening the blue phase temperature range
  7. Influence of the structure of azo compounds assembled from "LEGO" molecular units on the blue phase temperature range
  8. Broadening of blue phase temperature range by the base azo molecule as a key element of "LEGO" molecular unit.
  9. Influence of the chiral fragment of azo compounds on the photoinduced broadening of the blue phase temperature range.
  10. Influence of various linking groups in azo compounds on the photoinduced broadening of the blue phase temperature range.
  11. Effect of alkyl chain length on the broadening of the blue phase temperature range in homologous azo compounds.
  12. Synthesis and chemical characterization of azo compounds
  13. Characterization of the base highly chiral mixture: phase transitions and spectral properties of the blue phases
  14. Determination of the helical twisting power (HTP) of chiral azo compounds
  15. Phase transitions during heating and cooling of the base highly chiral mixture (HChM) doped with chiral and achiral azo compounds
  16. ...and 2 more sections

Figures (26)

  • Figure 1: Schematic presentation of blue phases consisting of double-twist cylinders (a). 3D crystal unit cell formed by a network of disclination lines for: (b) simple cubic-–-BP-II and (c) body-centered cubic–--BP-I lattices. (d) An amorphous structure of high temperature BP-III ("fog" phase).
  • Figure 2: Chemical structure of the chiral azo compounds characterized by: (a) azo moiety, (b) alkyl chains of various lengths (i.e., different number of carbon atoms) and (c) chiral fragment (l-menthol). A schematical build-up of azo compounds by means of "LEGO" molecular units: (d) azo moiety, (e) alkyl chain and (f) chiral fragment.
  • Figure 3: Chemical structures of compounds and their weight ratio in the base highly chiral mixture formed by: (a) 65 wt% cholesteryl oleyl carbonate (COC) as chiral component and (b) 35 wt% nematic liquid crystal E7, which is a mixture with 51 wt% 4-cyano-4'-n-pentyl-biphenyl (5CB), 25 wt% 4-cyano-4'-n-heptyl-biphenyl (7CB), 16 wt% 4-cyano-4'-n-oxyoctyl-biphenyl (8OCB) and 8 wt% 4-pentyl-4"-n-pentyl-p-terphenyl (5CT).
  • Figure 4: Schematic illustration of the photo-induced transformation of achiral (a, b) and chiral (c, d) dopants, using aChD-3490 and ChD-4212 as examples, respectively. The trans-isomers are represented by: (a) a "baseball bat-shaped" molecule for achiral compounds and (b) a "golf club-shaped" molecule for chiral compounds. The cis-isomers are represented by: (c) a "boomerang-shaped" molecule for achiral compounds and (d) a "hockey stick-shaped" molecule for chiral compounds. The reversible photo- and thermal isomerization of azo molecule is indicated by rainbow arrows.
  • Figure 5: BP temperature range of the base HChM (solid black squares), doped with 5 wt% of (a) chiral and (b) achiral azo compounds as a function of irradiation time $\textit{t}_{\text{irr}}$.
  • ...and 21 more figures