Tuning helical twisting power and photoisomerisation kinetics of axially chiral cyclic azobenzene dopants in cholesteric liquid crystals

ABSTRACT

Significant attention has been paid to improve the helical twisting power (β) and Δβ between the two different isomers of axially chiral azobenzene dopants in cholesteric liquid crystals (CLCs); however, the correlations between the vales (β and Δβ) with the molecular structures as well as photoisomerisation kinetics are far from clear. In this study, a series of binaphthyl-azobenzene cyclic dopants R1–R3 with different lengths of alkoxy chain was synthesised, which exhibited photochemically reversible trans–cis isomerisation in both organic solvents and liquid crystal hosts. When doping into a nematic liquid crystal, dopant R2 with one linking alkoxy group showed the highest values of β and Δβ. The results revealed that the β value was related to the dihedral angle between two naphthyl planes and the miscibility between the dopants and the host molecule. Moreover, Δβ was also depended on the photoisomerisation quantum yields. With increasing length of alkoxyl chain, the photoisomerisation rate constant of dopants increased upon ultraviolet irradiation and decreased for the reverse process upon visible light irradiation either in isotropic acetonitrile or in CLCs. These results enable the precise tuning of the pitch and selective reflection wavelength of CLCs.

Graphical abstract

KEYWORDS:

• Cyclic azobenzenes
• chiral dopants
• cholesteric liquid crystals
• photoisomerisation
• helical twisting power

Acknowledgments

The authors acknowledge the financial support of Natural Science Foundation of Anhui Province, China [Grant Number 708085MF150], Fundamental Research Funds for the Central Universities [Grant Number JZ2018HGPB0276], National Natural Science Foundation of China (NSFC) [Grant Numbers 61805066 and 61107014] and Distinguished Youth Foundation of Anhui Province [Grant Number 1808085J03].

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

Supplementary material data can be accessed here.

Additional information

Funding

This work was supported by the National Natural Science Foundation of China: [Grant Numbers 61805066 and 61107014], Natural Science Foundation of Anhui Province: [Grant Number 1708085MF150], Fundamental Research Funds for the Central Universities: [Grant Number JZ2018HGPB0276] and Distinguished Youth Foundation of Anhui Province: [Grant Number 1808085J03].