Photoresponsive Cholesteric Liquid Crystals

Asymmetric Dimers of Chiral Azobenzene Dopants Exhibiting Unusual Helical Twisting Power upon Photoswitching in Cholesteric Liquid Crystals

“…Although our initial HTPs are lower than those reported for axial- or planar-chiral azobenzene dopant systems, ,− the maximum HTP switching ratio of 50% [(β fin – β ini )/|β ini |], obtained from N2 dissolved in JC-1041XX, is comparable with those from reported chiral azobenzene derivatives exhibiting large HTP switching. ,, Thus, we could use the photoisomerization of dopant N2 to examine the light-induced rotational motion of micro-objects on its cholesteric mixture. Feringa and co-workers − demonstrated that the collective action of molecular motors in a CLC can be translated into macroscale rotational motion.…”

“…Photoresponsive cholesteric liquid crystals comprising a nematic host and a chiral azobenzene dopant can be categorized into two types in terms of their HTP photoswitching behavior (cholesteric pitch length): the azobenzene unit’s transition from trans to cis form can induce either a decrease in HTP (lengthening the pitch length or red-shifting the reflection band) or an increase in HTP (shortening the pitch length or blue-shifting the reflection band). As mentioned above, most reported CLCs based on chiral azobenzene dopants (e.g., axially chiral, planar chiral, or para-substituted) fall into the former case. − ,,, In contrast, only two modelsspacer substitution position or molecular aspect ratio modificationhave been proposed for the latter case. Herein, we observed increases in HTP for cis-rich states for dopants that are cholesteryl azobenzene derivatives, a phenomenon rarely noted for photoisomerizable chiral dopants in nematic LC hosts.…”

“…Cholesteric liquid crystals (CLCs) have attracted great interest because the self-organization of these superstructures can be controlled under external stimuli, including temperature, − pressure, electric field, , dopants, − and light. ,− Superstructural switching of CLCs has been applied in recent years to optical switching, displays, information storage, soft actuators, and molecular motor applications. − Among established dopants, photoisomerizable azobenzene units are particularly promising because they can induce a photoresponsive actuating mechanism through isomerization between trans ( E , rodlike) and cis ( Z , bent) forms upon irradiation with UV or visible light. Such light-induced conformational changes can lead to macroscopic self-reorganization of the host system.…”

Liquid Crystals

“…These displays do not require the drive electronics with patterned electrodes or complex addressing schemes used in electroptical displays; hence, they are cost effective and can be made flexible and foldable. Thus the cholesteric structures have very far-reaching implications in technology, especially in optics and photonics (196)(197)(198)(199)(200)(201)(202)(203)(204)(205)(206).…”

Influences of Both N,N,N- and N,N,C-Coordination Modes of Tolyl-terpyridine on the Photophysical Properties of Cyclometalated Ru(II) Complexes: Combined Experimental and Theoretical Investigations on Acid/Base-Dependent Reversible Cyclometalation

“…Ru­(II) complexes derived from polypyridyl ligands are frequently used as model systems for their fruitful utilizations in diverse fields of application such as photosensitizers, functional constituents in molecular-level machines, optoelectronics, dye-sensitized solar cells (DSSCs), and sensors. − Both the ground- and excited-state behaviors of the complex arrays could be regulated upon judicious choice of the chelating ligands. − Consequently, wide varieties of ligands have been rationally designed to fine-tune the structural features and electronic aspects of the resulting Ru­(II) complexes. − [Ru­(bpy) 3 ] 2+ (bpy = 2,2′-bipyridine) and its derivatives have shown remarkable utility to this end because of their good redox stability in both the ground and excited states, a broad and tunable metal-to-ligand charge-transfer (MLCT) band in the visible domain, and enhanced excited-state lifetime (τ = 860 ns) for [Ru­(bpy) 3 ]­(PF 6 ) 2 in MeCN and quantum yields (Φ = 0.062). , This sort of emission characteristics primarily originates from lowest-lying 3 MLCT states that are energetically well-separated from the deactivating triplet metal-centered ( 3 MC) excited states. The related bis­(tridentate) complex, [Ru­(tpy) 2 ] 2+ (tpy = 2,2′:6′,2″-terpyridine), possesses many similar properties to that of [Ru­(bpy) 3 ] 2+ , but the acute bite angle of the terpyridines allows the 3 MC states to be thermally accessible, which in turn drastically affects their emission characteristics together with the lifetimes (e.g., τ = 0.25 ns for [Ru­(tpy) 2 ]­(PF 6 ) 2 ). , This observation is very frustrating as the inherent C 2 symmetry in [Ru­(tpy) 2 ] 2+ can afford isomer-free functionalization of the complexes upon substitution at the 4-position of the tpy ligand so that vectorial transport of either electron or energy could be feasible. , …”