Valeri Krongauz

Holographic recording and all-optical modulation in photochromic polymers

Polymers doped with photochromic spirooxazine and spiropyran dyes have been investigated for real-time holographic recording. The temporal holographic response was found to depend strongly on optical recording configurations and on the recording beam intensities. The exposure sensitivities that were required for maximal diffraction efficiency were 250 mJ/cm2 for the polymers doped with spirooxazine and 650 mJ/cm2 for those doped with spiropyran. All-optical modulation of the holographic gratings with 2000 lines/mm is possible by modulation of a separate excitation beam of incoherent UV radiation. The modulated gratings can be stored for relatively long times or erased immediately.

Searching for photochromic liquid crystals Spironaphthoxazine substituted with a mesogenic group

Abstract A number of photochromic spiropyrans and spirooxazines containing mesogenic groups were synthesized. Only one of them, spiro-indoline-naphthoxazine with mesogenic substituent, 4-(4-heptylbenzoyloxy)benzoyloxy in 5-position, revealed mesomorphic properties. Its phase behaviour, photochromism and alignment in the electric field was investigated. A structure of the mesophase is discussed.

Charge injection asymmetry: A new route to strong optical nonlinearity in poled polymers

Amorphous polymer films containing hyperpolarizable dye molecules are poled by an electric field applied parallel to the plane of the thin film. The resultant second harmonic generation is much stronger than predicted by the standard model of dipolar interactions, and furthermore indicates asymmetry both parallel and perpendicular to the poling field. These results are attributed to a new, efficient mechanism for the production of optical nonlinearity in poled films in which asymmetric charge injection into the polymer and its trapping by dye species sets up a charge gradient perpendicular to the direction of the external poling field.

The photophysics and photochemistry of 2-styryl-anthracene and related compounds: extreme variations in triplet—triplet absorption spectra with the temperature, decay anomalies and transcis photoisomerization☆

Abstract Transient triplet—triplet absorption spectra and decay curves of 2-styrylanthracene and four related compounds were determined with conventional microsecond flash spectroscopy over a wide range of temperatures between room temperature and −170 °C, in decalin and methyltetrahydrofuran as solvents. The main anomalies observed were as follows: pronounced variation in the triplet absorption spectra with the temperature and, to some extent, with the wavelength of the exciting light; “growing-in”; initial fast decay stages; non-monoexponential decay; variation in the decay rate with the wavelength of measurement. All these were observed at certain temperatures and wavelengths for each compound. Some of these anomalies may be due to the fact that 2-styrylanthracene and its derivatives exist in solution as an equilibrium mixture of two modifications, possibly rotational conformers, both in the ground and in the excited state. Some trans—cis photoisomerization, both direct and sensitized, takes place in all compounds, except possibly in 2-styrylanthracene, and is more pronounced in bromobenzene solutions.

Photo- and Thermo-Chromic Liquid Crystal Polymers with Spiropyran Groups

Abstract Side chain liquid crystal polymers containing spiropyran and mesogenic units give photo- and thermo-chromic mesophases. Structural changes of the mesomorphic systems can be induced by the reversible spiropyran-merocyanine dye conversion.

Photochromism of side-chain liquid crystal polymers containing spironaphthoxazines

Abstract Side-chain liquid crystal polyacrylates and polysiloxanes containing different photochromic spironaphthoxazine side groups were synthesized. Thermodynamic, spectral and kinetic properties of the polymers were investigated. The structure of the mesophase is discussed.

Photochemistry and multiple holographical recording in polymers with photochromic phenoxy-naphthacene-quinone side groups

Abstract The photochromism of phenoxy-naphthacene-quinones in solution was investigated and compared with that of polymers in which one of these compounds was incorporated as a side group, through flexible (CH 2 ) n spacers of various lengths. The polymers studied were polystyrene (PS), poly(methyl methacrylate) (PMMA), and polysiloxane (PSX). Up to 90% photochrome side groups could be incorporated in some of these polymers. The polymers were studied both in solution and as flexible films. The quantum yields in solutions of one of phenoxy-quinones was about 0.6 for trans → ana , and about 0.10 for ana → trans . The photoconversion trans → ana is only slightly slowed down in solutions of the polymers, but much more so in films. The rates of photoconversion observed in films, relative to a solution of the photochrome proper were as follows: PS, 12; PMMA, 25; PSX, 50; solution, 100. These rates vary slightly with the concentration of the photochrome but increase with increasing spacer length in PMMA and decrease in PSX. The glass transition temperature may play an important role. Multiple holographic recording on the polymers was investigated.

Two-dimensional poling of organic and polymer films for improved nonlinear-optical properties

Nematic films doped with molecules of high nonlinear polarizability can exhibit two-dimensional asymmetry after electric-field poling along one direction only. Consequently, these films exhibit more nonzero components of the second-order susceptibility tensor than are obtained through usual poling techniques. These films can exhibit very high nonlinear optical coefficients (close to 10(-7) esu/cm(3)) and retain most of their nonlinearity one week after removal of the poling field.

Photochromism of undercooled melts of spirooxazines

Abstract Thin amorphous films of photochromic spirooxazines were prepared by melting and undercooling these compounds. The spectra and kinetics of thermal decay of the colored merocyanine form of the photochromophores were investigated. The kinetics are substantially different above and below the glass transition temperature (Tg).

Dielectric relaxation spectroscopy and molecular dynamics of a liquid-crystalline polyacrylate containing spiropyran groups

The dielectric properties of a liquid-crystalline (LC) copolymer containing photochromic spiropyran groups have been determined over wide ranges of frequency and temperature. The observed loss curves contain a component from the dipole reorientations of the mesogenic groups and a further component from a conductivity-related process. An electrical cleaning method was used to reduce the contribution from the latter process and a subtraction method was devised to separate the overall loss curves into the two components. Using electrical/thermal treatments it was found possible to align the copolymer homeotropically but not planarly. The dielectric loss spectra for unaligned and homeotropically aligned samples were very different. The data were analysed to yield two relaxation parameters for unaligned and aligned samples and were interpreted in terms of the anisotropic motions of dipolar mesogenic groups as have been discussed previously. In addition, the temporal stability of alignment of homeotropic samples at temperatures close to the clearing temperature was determined using the dielectric method.