Glassy Liquid Crystal Films as Broadband Polarizers and Reflectors via Spatially Modulated Photoracemization

Abstract

Liquid crystals are a class of organic compounds that are fluid with a uniaxial, lamellar, helical or columnar arrangement at the molecular level. They have found extensive use as electro-optic devices, and many other potential applications are being explored. If these molecular arrangements could be frozen in the solid state, the resultant glassy liquid crystals (GLCs) would be uniquely suited for various optical and optoelectronic applications. However, most existing liquid crystals fail to vitrify upon cooling; in fact, they tend to form polycrystalline films that scatter light. In the last two decades, numerous attempts have been made to create GLCs with limited success. Previous approaches were largely empirical, and few GLCs were found to be stable against spontaneous crystallization from the non-equilibrium glassy state. We have implemented a general approach in which liquid crystalline moieties are chemically bonded to selected volume-excluding cores.[1] These two structural elements are crystalline as separate entities, but the hybrid system readily forms glass on cooling while retaining the order characteristic of liquid crystalline mesomorphism. In addition, this material class is characterized by excellent film-and fiber-forming abilities as well as superior morphological stability, emulating polymeric materials and yet readily processable because of low melt viscosity and favorable dynamic mechanical properties.[2]Of all the GLCs that have been reported to date, chiralnematics appear to be the most interesting because of their unique supramolecular structure and the associated optical properties. A chiral-nematic film can be represented as a stack of quasinematic layers, each characterized by its own director and all sharing a common surface normal, often referred to as the helical axis. From one layer to the next, the director makes an incremental rotation about the axis, resulting in a right-or left-handed helix. A left-handed film will selectively reflect the left-handed circularly polarized component of unpolarized incident light without attenuating the right-handed circularly polarized component. An attractive feature of this material approach is the nearly 100 % energy efficiency accompanying the conversion of unpolarized light to circularly polarized light.[3] Nevertheless, the GLCs produced to date give constant-pitch films, thus limiting the bandwidth to less than 100 nm in the visible region. [4] According to the Hajdo±Eringen theory, [5] the bandwidth of a chiral-nematic film can be significantly broadened by introducing a pitch gradient. Broer et al. [6] implemented an approach based on counter-diffusion of chiral and nematic monomers in an increasingly viscous environment as photopolymerization with crosslinking proceeds to preserve the pitch gradient. Here we demonstrate band broadening by spatially modulated photoracemization of (R)-dinaphtho[2,1-d:1¢,2¢-f][1,3]dioxepin, III, in GLC films comprising I and II, see Figure 1. In this approach, counter-diffusion of the two enantiomers of I...

Nature‐Inspired Emerging Chiral Liquid Crystal Nanostructures: From Molecular Self‐Assembly to DNA Mesophase and Nanocolloids

“…Using this methodology, Kent Optronics developed a product named as e-TransFlector, such an electrically switchable transreflective LC device can be rapidly and reversibly switched between full-color reflection, half-reflection, and fully transparent states through applying a low electric field as shown in Figure 5d-f. Several other research groups have also put extensive efforts to develop the cholesteric films with switchable reflection bandwidth. [99][100][101][102][103][104][105][106][107][108][109][110][111][112][113][114][115] For example, Schenning and co-workers reported electrically switchable broadband photonic reflection in infrared regions by developing polymer network-stabilized cholesteric thin films. [99,100] White and co-workers found that broadband photonic reflection could be dynamically switched in polymer stabilized cholesteric LCs with negative dielectric anisotropy under DC electric fields, Figure 3.…”

Cholesteric Liquid Crystals with a Broad Light Reflection Band

“…The bandgap can be broadened as a direct consequence of a spatially modulated photoracemization reaction 176, 177. Glass‐forming CLCs are blended with a chiral dopant with a high HTP and that is susceptible to photoracemization (i.e., CLC to NLC phase conversion).…”

“…The solid (dashed) curve corresponds to the response of the pristine (photoracemized) film. Reproduced with permission 176. Copyright 1999, Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim.…”

Surface anchoring effects on spectral broadening of cholesteric liquid crystal films

“…11͒ or from spatially modulated photoracemization. 12 A CLC film with Grandjean ͑planar͒ texture can be modeled as a stack of quasinematic LC layers, each characterized by its own director, the average orientation of elongated LC molecules in the layer. The LC director rotated a small angle from one layer to the next about a common surface normal, referred to as the helical axis.…”