Azo polymers for reversible optical storage. 1. Poly[4'-[[2-(acryloyloxy)ethyl]ethylamino]-4-nitroazobenzene]

Photodegradation of azobenzene-based self-assembled monolayers characterized by in-plane birefringence

Azobenzene-based self-assembled monolayers (azo-SAMs) are photoactive and become orientationally ordered when illuminated with linearly polarized light (LPL), making them attractive as dynamic alignment layers in liquid crystal cells. Azo-SAMs, however, are chemically unstable when exposed to both air and light. We have characterized the photodegradation of a methyl red-based SAM by measuring with a high-sensitivity polarimeter the optical anisotropy induced by illumination with linearly polarized actinic light after the sample is irradiated with circularly polarized light (CPL) in air. The number of unbleached, photoactive molecules in the SAM decays exponentially with CPL exposure time, lowering the reorientation rate during photowriting with LPL. Azo-SAMs in an argon atmosphere, in contrast, are chemically stable and remain photoactive even after exposure to CPL.

4-[2-(2,2-Dimethyl-4,6-dioxo-1,3-dioxan-5-ylidene)hydrazin-1-yl]benzonitrile

In the title compound, C₁₃H₁₁N₃O₄, the dioxane ring adopts an envelope conformation with the C atom bonded to the dimethyl group in the flap position [deviation = 0.613 (1) Å]. The nitrile group and the attached benzene ring are roughly coplanar [maximum deviation = 0.087 (1) Å]. An intra­molecular N—H⋯O hydrogen bond involving the hydrazinyl group generates an S(6) ring. The N—N and C—N bond lengths indicate that the compound may be a mixture of the azo and hydrazone tautomeric forms but the presence of the N-bound H atom supports the hydrazone form. The crystal structure is stabilized by weak inter­molecular C—H⋯O, C—H⋯N and C—H⋯π inter­actions.

Adsorption of azothiophene dye having an N-bridging bidentate tail group on gold

The novel azothiophene derivatives [AT-di(CnSAc) (n=6 and 12)], an azo linkage (N=N) that bridges one phenyl ring with an N-bridging dialkylthioacetate tail and one thiophene ring, are synthesized and characterized. The azothiophene derivatives substituted with electron-withdrawing groups are blue in color, exhibit a bathochromic shift of a longer wavelength, and are electrochemically active. The formation and characterization of AT-di(CnSAc) SAMs with a bidentate tail group of the N-bridging dialkylthioacetate have been studied by surface sensitive techniques such as grazing angle Fourier transform infrared (FT-IR), a quartz crystal microbalance (QCM), spectroscopic ellipsometry, X-ray photoelectron spectroscopy (XPS), and cyclic voltammetry (CV). It has been shown that the adsorption reaction of the thioacetate group is almost spontaneous and the N-bridging dialkylthioacetate SAMs with longer methylene length have a packing density higher than that with a shorter methylene length. However, the sulfur tethers of the N-bridging dialkylthioacetate show incomplete binding of sulfur atoms in AT-di(CnSAc) SAMs.

Efficient surface-relief gratings in hydrogen-bonded polymer-azobenzene complexes

We show that efficient photoinduced surface-relief gratings can be inscribed in polymer-azobenzene complexes which are bonded by phenol-pyridine hydrogen bonding. The grating inscription was studied as a function of chromophore concentration and the molecular weight of the host polymer, both of which can be easily tuned without demanding organic synthesis. Stable gratings with modulation depth as high as 440 nm and with diffraction efficiency exceeding 40% were inscribed in the equimolar complexes. Our results demonstrate that phenol-pyridine hydrogen bonding not only allows one to increase the chromophore content until each polymer unit is occupied but is also sufficiently strong to induce mass migration of the polymer chains in a manner comparable to covalently functionalized polymers.

All-optical subdiffraction multilevel data encoding onto azo-polymeric thin films

By exploiting photoinduced reorientation in azo-polymer thin films, we demonstrate all-optical polarization-encoded information storage with a scanning near-field optical microscope. In the writing routine, five-level bits are created by associating different bit values to different birefringence directions, induced in the polymer after illumination with linearly polarized light. The reading routine is then performed by implementing polarization-modulation techniques on the same near-field microscope in order to measure the encoded birefringence direction.

Time-resolved studies of photoinduced birefringence in azobenzene dye-doped polymer films

We measured transient photoinduced birefringence (delta n) in various azobenzene dye films by pumping with a nanosecond pulse at 532 nm and probing at 633 nm. The switch-on times for the photoinduced birefringence range from nanoseconds to milliseconds and are systematically related with the lowest optical transition energies for those films. Moreover, our results suggest that the transient photoinduced birefringence measurement is a convenient way to determine the relative energies of pi-pi(*) and n-pi(*) states in azo-based materials.

Photo-mechanical effects in azobenzene-containing soft materials

The change in shape inducible in some photo-reversible molecules using light can effect powerful changes to a variety of properties of a host material. The most ubiquitous natural molecule for reversible shape change is the rhodopsin-retinal protein system that enables vision, and this is perhaps the quintessential reversible photo-switch. Perhaps the best artificial mimic of this strong photo-switching effect however, for reversibility, speed, and simplicity of incorporation, is azobenzene. This review focuses on the study and application of reversible changes in shape that can be achieved with various systems incorporating azobenzene. This photo-mechanical effect can be defined as the reversible change in shape inducible in some molecules by the adsorption of light, which results in a significant macroscopic mechanical deformation of the host material. Thus, it does not include simple thermal expansion effects, nor does it include reversible but non-mechanical photo-switching or photo-chemistry, nor any of the wide range of optical and electro-optical switching effects for which good reviews exist elsewhere.

Cholesteric polymer guest-host mixture with circularly polarized fluorescence: two ways for phototuning of polarization and its intensity

A photosensitive fluorescent cholesteric guest-host mixture consisting of a nematic polyacrylate, a chiral, photochromic dopant sensitive to UV light, and a fluorescent dopant was prepared. The nematic polyacrylate contains 4-phenyl-4'-methoxybenzoate nematogenic side groups and photochromic 4-cyanoazobenzene side groups. The chiral-photochromic dopant formed by isosorbide and cinnamic acid is capable of E-Z photoisomerization and [2 + 2] photo-cycloaddition under light irradiation. The planarly oriented films possess a selective light reflection in the visible spectral region coinciding with the emission peak of the fluorescent dopant. The fluorescence emitted by the planarly oriented films of the mixture is strongly circularly polarized and characterized by a large value of the dis-symmetry factor. At temperatures below glass transition (T(g)) the polarized light action of an Ar(+) laser (488 nm) leads to the photo-orientation of the azobenzene fragments resulting in a strong and reversible disruption of the selective reflection and a decrease of the dis-symmetry factor of fluorescence. UV irradiation leads to E-Z isomerization and/or [2 + 2] cycloaddition of the chiral-photochromic dopant, causing an irreversible shift of the maximum of the dis-symmetry factor to a long-wavelength spectral region under subsequent annealing at temperatures higher than T(g). Such multifunctional glass-forming guest-host mixtures combining photosensitive and fluorescent properties with the unique optical properties of cholesteric liquid crystals can be considered as promising material for optical data processing technologies and photonic applications.