Photocontrollable Photonic Crystals Based on Porous Silicon Filled with Photochromic Liquid Crystalline Mixture

Abstract

Novel photocontrollable photonic crystals based on porous silicon prepared by electrochemical etching are elaborated and their great potential for creation of photoswitchable materials is demonstrated. For this purpose, silicon‐based photonic porous wafers are filled with photochromic liquid crystalline (LC) nematic mixture formulated by 4‐pentyl‐4′‐cyanobiphenyl and a specific azobenzene derivative. Results of electron paramagnetic resonance spectroscopy reveal that molecules of LC mixture are aligned along the etched channels in silicon plate, i.e., in normal direction. UV irradiation (375 nm) of the resulting composite causes a shift of photonic band gap to longer wavelengths by ≈10 nm. This phenomenon is explained by isothermal photoinduced nematic–isotropic phase transition associated with the bent‐shaped Z isomer formation of the azobenzene compound. During this transition, effective refractive index slightly increases as the initial homeotropic alignment of LC molecules transforms to the isotropic nonaligned state. This process is completely reversible under visible light action (428 nm) and can be repeated many times. Obtained photonic crystal photochromic LC composites can be considered as new promising material for photonic applications.

Chiral Photoresponsive Liquid Crystalline Materials Derived from Cyanoazobenzene Central Core: Effect of UV Light Illumination on Mesomorphic Behavior

“…Similarly, the SmA* phase at higher temperatures transforms into two N* phases, while at lower temperatures modulation of the size of its domains has been observed. The obtained results will contribute to better understanding of the molecular structure-mesomorphic property relationship for the liquid crystalline materials possessing the photosensitive moiety as such materials attract considerable attention of the scientific community due to high application potential of those materials for various photonic devices [5,8,56].…”

Light-induced switchable adsorption in azobenzene- and stilbene-based porous materials

“…It is known that azobenzene-based molecules encapsulated in porous materials can undergo controlled stimulus-responsive behavior regarding their photophysical properties or solventmediated permeability properties (Figure 4) [60][61][62]88]. In flexible systems, the trans-cis isomerization of azobenzene is known to induce host structural transformation on the influence of PCN-250 (Fe MOF) Coupling of LISA with magnetic fields triggered record-high gas desorption of adsorbed CO 2 [81] Trends in Chemistry external stimuli [89,90]. These structural changes can result in gas adsorption property changes in the host-guest composite.…”

Design and Self-Assembling Behaviour of Calamitic Reactive Mesogens with Lateral Methyl and Methoxy Substituents and Vinyl Terminal Group

“…The resultant material may combine the anisotropy and large susceptibility of low molar-mass LCs with mechanical elasticity and relatively simple processability of macromolecular materials. During recent decades, an increased interest in advanced macromolecular materials possessing certain properties [ 4 , 10 , 11 , 12 , 13 ], specifically in LC side-chain polymers and elastomers possessing the self-assembling behaviour, has occurred due to their high potential for various smart applications, such as artificial muscles, micro-valves, mechanical actuators, smart surfaces and propulsion systems, etc. [ 5 , 14 ].…”

“…However, it is very difficult to theoretically predict, describe and keep under control the macroscopic parameters of the self-assembling materials [ 18 , 19 , 20 ]. Many experimental efforts in reactive mesogen design [ 4 ] have been carried out while building-up new photosensitive and photo-controllable macromolecular materials [ 10 ], incorporating a photo-responsive azo group into the molecular core. Its presence allows the effective driving of the resulting optical, structural, mesomorphic and dielectric properties by non-contact stimuli, i.e., irradiation by UV or even by visible light [ 16 , 21 , 22 , 23 ].…”

“…However, each part of the reactive mesogen molecule, namely the molecular core, type and position of the lateral substitution, linkage groups, length and type of the flexible chains, etc., can significantly change the resulting self-assembling behaviour of the designed material. The methyl [ 10 , 22 , 38 , 39 ] and methoxy [ 39 , 40 , 41 ] groups and halogen atoms [ 38 , 42 ], namely fluorine [ 42 , 43 , 44 , 45 , 46 , 47 ], chlorine [ 38 , 41 , 42 , 46 , 48 , 49 ], bromine [ 38 , 43 ] and iodine [ 50 ], are the most widely used and successful types of lateral substituents utilised for the effective tuning of the self-assembling behaviour, especially for the non-chiral and chiral calamitic LC materials. The type and position of the lateral substituents on the molecular core always drastically change the liquid crystalline behaviour of the resulting LC material when compared with its non-substituted analogue [ 38 , 39 ].…”