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...
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