Photoinduced motions in azo-containing polymers.

Photoalignment of Liquid-Crystal Systems.

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Chiroptical Molecular Switches

All optical fabrication of Surface Relief Grating (SRG) on azobenzene functionalized polymer films is reviewed in this article. The uniqueness of the single step erasable photofabrication of large amplitude surface relief structures on azo polymer films is mainly due to the photoisomerization and photoanisotropic behavior of the azobenzene group. The design and synthesis of several classes of suitable azobenzene functionalized polymers for the SRG formation is presented. Experimental studies are carried out in a systematic manner to understand the chemical and physical intricacies involved in the SRG formation process. Structural requirements and their limitations for the efficient fabrication of SRGs on azo functionalized polymer films are also discussed. Understanding the fundamental mechanism of SRG formation on the azo functionalized polymer films enables us to make use of these structures in a variety of applications.

Surface relief structures on azo polymer films

Two types of photomodulation of orientations of liquid crystals (LCs) are reviewed: 1) order–disorder phase transitions of LCs induced by photochemical reactions of photochromic molecules and 2) order–order alignment change of LCs (change in LC directors) induced by photochemical reactions or without photochemical events. Both processes produce a large refractive-index modulation, which forms the basis of a range of photonic applications. Various modes of photomodulation of orientations of LCs with plausible mechanisms and their possible applications in photonics are described.

Photomodulation of liquid crystal orientations for photonic applications

New side-chain liquid crystalline polyesters have been prepared by melt transesterification of diphenyl tetradecanedioate and a series of mesogenic 2-[ω-[4-[(4-cyanophenyl)azo]phenoxy]alkyl]-1,3-propanediols, where the alkyl spacer is hexa-, octa-, and decamethylene in turn. The polyesters have molecular masses in the range 5000-89 000. Solution 13 C NMR spectroscopy has been employed to identify carbons of polyester repeat units and of both types of end groups. Polyester phases and phase transitions have been investigated in detail by polarizing optical microscopy and differential scanning calorimetry for the hexamethylene spacer architecture with different molecular masses. Using FTIR polarization spectroscopy, the segmental orientation in unoriented polyester films induced by argon ion laser irradiation has been followed and an irradiation-dependent order parameter for the cyanoazobenzene mesogens calculated. FTIR is also utilized to follow the temperature-dependent erasure of the induced orientation. Optical storage properties of thin unoriented polyester films are examined through measurements of polarization anisotropy and holography. A resolution of over 5000 lines/mm and diffraction efficiencies of about 40% have been achieved. Lifetimes greater than 30 months for information stored have been obtained, even though the glass transition temperatures are about 20 °C. Complete erasure of the information can be obtained by heating the films to about 80 °C, and the films can be reused many times without fatigue.

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

The laser-induced anisotropy in thin films of an extensive number of cyanoazobenzene side-chain liquid crystalline polytetradecanedioates, -dodecanedioates, and -adipates selectively deuterated at different positions have been investigated with polarized FTIR spectroscopy. The analysis of the segmental orientation based on dichroic ratios of characteristic absorption bands shows that, in polyesters with long main-chain spacing (tetradecanedioates and dodecanedioates), not only the light sensitive azo chromophore but also the main-chain methylene segment and to a smaller extent the flexible spacer are preferentially oriented perpendicular to the laser light polarization. The extent of orientation increases with increasing spacer length. On the other hand, in the shorter adipates only the chromophore and the spacer are likewise oriented. Rapid-scan FTIR analysis performed on-line with the laser irradiation reveals that the alignment of the aliphatic segments arises simultaneously with the chromophore orient...

Selectively deuterated liquid crystalline cyanoazobenzene side-chain polyesters. 3. Investigations of laser-induced segmental mobility by Fourier transform infrared spectroscopy

A series of new mesogenic azobenzene propanediols with nitro, methoxy, cyano and no substituent have been synthesized and their mesomorphic behaviour mapped. The diols are employed in a melt transesterification with diphenyl tetradecanedioate to produce side-chain polyesters with high molar masses. Diols and polyesters have similar UV-visible spectra reflecting the acceptor character of the substituent. Polarization FTIR spectroscopy has been used to determine the mesogenic order parameter for the polyesters with different substituents at four laser irradiation wavelengths (413, 458, 488 and 515 nm). Diffraction gratings in thin polyester films have been recorded through polarization holography. Cyano and unsubstituted azobenzene polyesters are shown to exhibit the highest diffraction efficiencies on the order of 13%.

The influence of substituents on the orientational behaviour of novel azobenzene side-chain polyesters

Two sets of specifically deuterated cyanoazobenzene side-chain polyadipates and polytetrade-canedioates have been prepared by transesterification in the melt. Combinations of three different, selectively deuterium labeled 2-[6-[4-[(4-cyanophenyl)azo]phenoxy]hexyl]-1,3-propanediols or the non-deuterated analog and diphenyl adipate or tetradecanedioate in perdeuterated or non-deuterated versions were employed. The polyesters have weight average molecular masses in the range 21 000–183 000 as determined by SEC. The phase behaviour of the polyesters have been investigated in detail by differential scanning calorimetry. Whereas the polytetradecanedioates show a complex thermal behaviour with a number of different phases, the polyadipates are less complex and both nematic and smectic A phases have been identified by polarizing optical microscopy. Solution 1H, 13C and 2H NMR spectroscopy have been utilized for both polyester structural assessment and determination of deuterium distribution and content. ...

Selectively Deuterated Liquid Crystalline Cyanoazobenzene Side-Chain Polyesters. 2. Preparation and Characterization of Polyesters

Diphenyl perdeuterated adipate and tetradecanedioate, and three different, selectively deuterium labeled 2-[6-[4-[(4-cyanophenyl)azo]phenoxy]hexyl]-1,3-propanediols have been prepared by exchange and reduction techniques. 1H and 2H NMR spectroscopic investigations have revealed the deuterium exchange in the esters to be higher than 93% and evenly distributed. Similarly, a varying but high deuterium exchange was detected in the selectively labeled diols. All deuterated samples were additionally characterized in detail by 13C NMR spectroscopy; the deuterium isotope effect conclusively assisted some of the shift assignments. No isotope effect could be detected by visible spectroscopy in any of the deuterated diols. On the other hand, FTIR in all deuterated compounds reveals a substantial frequency shift of all carbon-deuteron vibrations as compared to the corresponding carbon-hydrogen vibrations. Polarizing optical microscopic and differential scanning calorimetric investigations have identified all...

Christian Kulinna

Papers

Novel Side-Chain Liquid Crystalline Polyester Architecture for Reversible Optical Storage

Selectively deuterated liquid crystalline cyanoazobenzene side-chain polyesters. 3. Investigations of laser-induced segmental mobility by Fourier transform infrared spectroscopy

The influence of substituents on the orientational behaviour of novel azobenzene side-chain polyesters

Selectively Deuterated Liquid Crystalline Cyanoazobenzene Side-Chain Polyesters. 2. Preparation and Characterization of Polyesters

Selectively Deuterated Liquid Crystalline Cyanoazobenzene Side-Chain Polyesters. 1. Preparation and Characterization of Precursors and Diols