Widely Tunable, Photoinvertible Cholesteric Liquid Crystals

Abstract

The helicoidal structure of cholesteric liquid crystals (CLCs) selectively refl ects light according to the Bragg condition, which at normal incidence simplifi es to:where n is the average index of the mixture and P is the pitch, defi ned as the length of one complete rotation of the helix. [ 1 ] Researchers have examined the utility of this mesophase in a range of topical areas including optics (switchable or tunable fi lters), photonics (selective shutters), and lasing (mirrorless feedback cavity). [ 1 , 2 ] Like other liquid-crystal phases, CLCs are responsive to a range of stimuli including temperature, electric fi elds, and light. The color switching or tuning response to each of these stimuli has been recently reviewed. [ 3 ] In particular, photoresponsive CLCs offer the capability of dynamic selective refl ection cued by light itself. Photoresponsive CLCs have been examined for more than forty years. Early examinations looked at photodegradation of halide-containing cholestryl mixtures [ 4 , 5 ] as well as guest-host mixtures with photochromics such as azobenzene. [ 6 ] Vinogradov et al. were the fi rst to demonstrate comparably larger optical responses by employing photoresponsive chiral materials to formulate the CLC mixture. [ 7 ] In this case, a menthone chiral dopant yielded a approximately 400-nm tuning of the refl ection bandgap with exposure to a 442-nm helium-cadmium laser. Convergent to this effort, Feringa, [ 8 , 9 ] Schuster, [10][11][12] and others pursued the synthesis of novel photochiral materials, which at the time were focused towards the development of an optical switch. In these examinations, as well as others that have followed, a variety of photochromic chiral materials have been examined including menthone, [ 7 , 13 , 14 ] chiral olefi ns, fulgides, [ 15 , 16 ] azobenzene, [17][18][19][20][21][22][23][24] and overcrowded alkenes. [25][26][27] Phototuning of the CLC refl ection notch has been presented in a number of recent reports. These works have primarily utilized azobenzene nematic liquid-crystal hosts [ 20 , 28-32 ] or chiral dopants. [17][18][19][20][21][22][23][24] The limited tuning range of guest-host azobenzene mixtures ( ≈ 50 nm) is expanded through the utilization of azobenzene nematic liquid-crystal hosts ( ≈ 250 nm) or further expanded by the use of azobenzene chiral dopants ( > 2000 nm). Unfortunately, a limitation of azo-based CLCs is the slow dark relaxation. While the at times days-long dark relaxation has been dramatically reduced through polymer stabilization, [ 32 ] materials with inherently faster dark relaxation are desirable.Towards this end, this work uses an overcrowded alkene (OA1; 9-(2-phenyl-2,3-dihydro-cyclopenta[a]naphthalen-1-ylidene)-9 H -fl uorene) as a chiral dopant. Overcrowded alkenes are known to have comparably rapid relaxation. Feringa and co-workers have championed these materials and showed their utility as optical switches, molecular motors, and as phototunable CLCs. [8,9] The structure and photoinversion of OA1 is shown in Figure 1 . ...

Beam coupling in hybrid photorefractive inorganic-cholesteric liquid crystal cells: Impact of optical rotation

“…We note first the existence of chiral dopants which are widely tunable with UV light. 34 Using such a dopant, in the same cell, one could in principle tune the cholesteric gap through the wavelength of the ambient laser light. In this system, the beam coupling would be observed to change as a function of the intensity of the imposed UV irradiation.…”

Infrared Regulating Smart Window Based on Organic Materials

“…In the past decade, a number of light responsive chiral dopants which can change their HTP either upon isomerization or helical inversion have been used to tune the position of a cholesteric reflection notch over a wide range. [116][117][118] A light responsive system which can change its reflection bandwidth based on the intensities of light would be very attractive. First steps for such a responsive system in the visible and infrared regions have been taken by White et al using a chiral azobenzene photoisomer doped in a Ch-LC cell.…”

Stimuli‐Directed Dynamic Reconfiguration in Self‐Organized Helical Superstructures Enabled by Chemical Kinetics of Chiral Molecular Motors

“…The resulting tunability of geometrical charity has been significantly investigated to create photoactiving chiral catalysts in solvents51, 52 and polar switchovers on solid surfaces 53, 54, 55. To further overcome the obstacle of Brownian motion, the guest‐host mesogenic hybrid systems make it possible to achieve long‐range ordered function in photoresponsive 1D photonic crystals with standing helixes56, 57, 58 and rotating microobjects on lying helixes 59, 60. Therefore, the kinetic analysis of chiral motor in liquid‐crystalline system is necessary to gain more bottom‐up information for the complex reconfiguration of artificial mesogenic construction, not only including the pitch variation of standing helixes and the in‐plane rotation of helical axes in lying helixes, but also taking account of the angular motion of helical orientation in spatial dimensions for emerging applications in areas of tunable photonics, intelligent energy‐saving, and controllable bioengineering.…”