Effect of Cell Thickness on the Electro-optic Response of Polymer Stabilized Cholesteric Liquid Crystals with Negative Dielectric Anisotropy

It has previously been shown that for polymer-stabilized cholesteric liquid crystals (PSCLCs) with negative dielectric anisotropy, the position and bandwidth of the selective reflection notch can be controlled by a direct-current (DC) electric field. The field-induced deformation of the polymer network that stabilizes the devices is mediated by ionic charges trapped in or near the polymer. A unique and reversible electro-optic response is reported here for relatively thin films (&le;5 &mu;m). Increasing the DC field strength redshifts the reflection notch to longer wavelength until the reflection disappears at high DC fields. The extent of the tuning range is dependent on the cell thickness. The transition from the reflective to the clear state is due to the electrically controlled, chirped pitch across the small cell gap and not to the field-induced reorientation of the liquid crystal molecules themselves. The transition is reversible. By adjusting the DC field strength, various reflection wavelengths can be addressed from either a different reflective (colored) state at 0 V or a transparent state at a high DC field. Relatively fast responses (~50 ms rise times and ~200 ms fall times) are observed for these thin PSCLCs.</p

One- and Two-Photon Spectroscopy of Donor−Acceptor−Donor Distyrylbenzene Derivatives: Effect of Cyano Substitution and Distortion from Planarity

The one- and two-photon spectroscopic properties of four symmetrically substituted donor−acceptor−donor distyrylbenzenes with either di-n-butyl- or diphenylamino donor groups and cyano acceptor groups are reported. It has been found that the position of the substitution of the electron-withdrawing cyano groups on the central phenylene ring as compared to the vinylene bond strongly affects the observed properties. In particular, the molecules with cyano substitution on the α-carbon of the vinylene linkage are characterized by weak fluorescence, short fluorescence lifetimes, and two-photon cross sections (δ) that are comparable to analogous molecules with no acceptor groups. In contrast, the molecules with acceptor substitution on the central phenylene ring are strongly fluorescent and have δ values roughly twice those of the vinyl-substituted molecules. These results are discussed in terms of the larger deviation of the conjugated backbone from planarity and the smaller distance between the donors and acceptors when the cyano groups are substituted on the vinylene carbon rather than the central phenylene ring

One- and Two-Photon Spectroscopy of Donor−Acceptor−Donor Distyrylbenzene Derivatives: Effect of Cyano Substitution and Distortion from Planarity

The one- and two-photon spectroscopic properties of four symmetrically substituted donor−acceptor−donor distyrylbenzenes with either di-n-butyl- or diphenylamino donor groups and cyano acceptor groups are reported. It has been found that the position of the substitution of the electron-withdrawing cyano groups on the central phenylene ring as compared to the vinylene bond strongly affects the observed properties. In particular, the molecules with cyano substitution on the α-carbon of the vinylene linkage are characterized by weak fluorescence, short fluorescence lifetimes, and two-photon cross sections (δ) that are comparable to analogous molecules with no acceptor groups. In contrast, the molecules with acceptor substitution on the central phenylene ring are strongly fluorescent and have δ values roughly twice those of the vinyl-substituted molecules. These results are discussed in terms of the larger deviation of the conjugated backbone from planarity and the smaller distance between the donors and acceptors when the cyano groups are substituted on the vinylene carbon rather than the central phenylene ring

Structure−Property Relationships for Two-Photon Absorbing Chromophores: Bis-Donor Diphenylpolyene and Bis(styryl)benzene Derivatives

The two-photon absorption properties of a series of bis dialkylamino- or diarylamino-substituted diphenylpolyenes and bis(styryl)benzenes have been investigated. Two-photon absorption cross sections, δ, as large as 1420 × 10^(-50) cm^4 s/photon-molecule have been observed for molecules with this general bis-donor structure. The effect of the type and length of the conjugated chain and of dialkylamino or diarylamino substitution on the position and magnitude of the peak two-photon absorptivity is reported. The transition dipole moments for the transitions between the ground state and the first excited singlet state (M_(ge)) and between the first and second excited singlet states (M_(ee‘)) have been estimated using experimental data from the one- and two-photon spectra. It was found that increases in chain length result mainly in an increase in M_(ge), whereas the addition of donor end groups or going from diphenylpolyene- to phenylene-vinylene-type bridges leads primarily to an increase in Mee‘. The trends in the energy of the lowest excited singlet states and in the transition moments for the diphenylpolyene series as a function of chain length are in agreement with those calculated by quantum mechanical methods. These results furnish a link between structural features in these classes of molecules and the electronic dipole couplings and state energies that control the strength of the two-photon absorption. In bis(aminophenyl)polyenes containing up to four double bonds (m) the lowest excited singlet state is a B_u state, as opposed to the case of simple polyenes and diphenylpolyenes, for which it is an A_g state for m > 2. The relationship of the state ordering in these systems with the observed values of the radiative and nonradiative decay rates is also discussed

Phototropic Guest–Host Liquid Crystal Systems: Environmental Effects on Naphthopyran Kinetics

Photoinduced isothermal and order-increasing phase transitions can be observed in guest–host liquid crystal mixtures containing naphthopyran derivatives. The changes are attributed to the different geometry and a less energetically favorable interaction with the liquid crystal molecules for the ground state naphthopyran species with respect to the photoproduct. In this study, we have investigated the thermal relaxation of the naphthopyran photoproduct back to the initial species as a function of temperature and excitation conditions. It was found that the thermal relaxation is typically first-order and its characteristic rate has a temperature dependence that is different in the anisotropic and isotropic states of the mesogenic solvents. In particular, the activation energy for thermal relaxation is larger in the ordered phases of mesogenic solvents relative to their isotropic phase. The findings suggests that the naphthopyran relaxation is hindered by the organization of the host environment. These results will be useful to build a more complete understanding of the mutual guest–host interactions in naphthopyran-containing guest–host LC systems

Supplementary document for Prediction of Metallo-dielectric Transmission Filter Performance Based on Underlying Dispersion Relations - 6835132.pdf

Derivation of dispersion relatio

Supplementary document for Prediction of Metallo-dielectric Transmission Filter Performance Based on Underlying Dispersion Relations - 6835132.pdf

Derivation of dispersion relatio

Thermoplasmonic controlled optical absorber based on a liquid crystal metasurface

Metasurfaces can be realized by organizing subwavelength elements (e.g., plasmonic nanoparticles) on a reflective surface covered with a dielectric layer. Such an array of resonators, acting collectively, can completely absorb the resulting resonant wavelength. Unfortunately, despite the excellent optical properties of metasurfaces, they lack the tunability to perform as adaptive optical components. To boost the utilization of metasurfaces and realize a new generation of dynamically controlled optical components, we report our recent finding based on the powerful combination of an innovative metasurface-optical absorber and nematic liquid crystals (NLCs). The metasurface consists of self-assembled silver nanocubes (AgNCs) immobilized on a 50 nm thick gold layer by using a polyelectrolyte multilayer as a dielectric spacer. The resulting optical absorbers show a well-defined reflection band centered in the near-infrared of the electromagnetic spectrum (750–770 nm), a very high absorption efficiency (∼60%) at the resonant wavelength, and an elevated photothermal efficiency estimated from the time constant value (34 s). Such a metasurface-based optical absorber, combined with an NLC layer, planarly aligned via a photoaligned top cover glass substrate, shows homogeneous NLC alignment and an absorption band photothermally tunable over approximately 46 nm. Detailed thermographic studies and spectroscopic investigations highlight the extraordinary capability of the active metasurface to be used as a light-controllable optical absorber