The Control of the Cholesteric Pitch by Some Azo Photochemical Chiral Switches

Abstract

A few chiral azo compounds, which undergo reversible photochemical switching, are presented. Of these, the most interesting contain the binaphthyl moiety and belong to the C2 (derivatives 1 and 2) or C1 symmetry group (derivatives 3 and 4). These binaphthyl compounds display intense CD and high beta values. Photochemical switching has profound effects on both the CD and beta values of these compounds; in the case of compound 3, the sign of beta changes upon isomerisation. Compound 2 has, to our knowledge, the highest beta of the switches reported in the literature and also seems the most interesting owing to its fast response to photochemical stimuli. Nematic phases can be transformed into cholesteric phases with reflection bands in the visible region by doping with reasonable amounts of 1 and 2. The reflection colours can be changed reversibly by photoisomerisation of the switches. Thermal reversion of the colourless UV photostationary state to the green isomeric EE state or to intermediate coloured states is temperature dependent. This can allow the thermal history of a sample to be traced.

Rotational Reorganization of Doped Cholesteric Liquid Crystalline Films

“…Azobenzene switch 5 was synthesized according to a literature procedure 5 , with a modification of the synthetic procedure of the first step. Apart from the first step, yields and spectral data were in accordance with literature values.…”

“…Apart from the first step, yields and spectral data were in accordance with literature values. 5 In the first step, the mono-acetylation of (S)-2,2'-diamino-1,1'-binaphthyl was accomplished using the following procedure. Acetic anhydride (0.31 ml, 3.3 mmol) in acetonitrile (10 ml) was added dropwise to a solution of (S)-2,2'-diamino-1,1'-binaphthyl (720 mg, 2.53 mmol) and Et 3 N (0.46 ml, 3.3 mmol) in acetonitrile (30 ml) at 0 °C.…”

Synthesis and Properties of Photoisomerizable Derivatives of Isosorbide and Their Use in Cholesteric Filters

“…This degradation can be assigned primarily to the thermal instability of the menthone-derived chiral component, but also to the fact that this chiral component is a monoacrylate and, therefore, gives rise to polymer networks with relatively low crosslinking density upon photopolymerization. [22] Alternatively, chiral isomerizable derivatives of stilbene [23,24] or azobenzene [25] that are reversibly switchable could be used. However, these types of compounds absorb in the visible region of the electromagnetic spectrum.…”

Phototunable Azobenzene Cholesteric Liquid Crystals with 2000 nm Range

“…[46][47][48][49] These azobenzene chiral dopants have potential advantages in that 1) low concentrations of the dye are required (therefore reducing coloration) and 2) the potential exists for large-range tuning. In materials in which the change in refractive index is small, the tuning range (Dl b ) is equal to the product of the average index and the change in pitch (DP) (Eq.…”

“…[26,27,43] Convergent to these efforts is the use of photosensitive chiral molecules as molecular switches. While the overall goal of these research areas is somewhat dissimilar, the molecules synthesized by Ferringa and co-workers; [32,44,45] Spada, Gottarelli, and coworkers; [46][47][48] and Li et al [49] offer high HTP materials that allow direct accessibility to the pitch of the CLC helix. Most notably, Spada/Gottarelli and later Li developed axially chiral bis(azo) molecules with an HTP of nearly 60 mm À1 in the trans/trans state.…”