Light-Induced Orientation Phenomena in Langmuir−Blodgett Multilayers

Abstract

The double-layer structure of Langmuir−Blodgett (LB) multilayer assemblies formed by a thermotropic polyacrylate with 2-hydroxyethyl and [[[[(2-methylbutyl)oxy]phenyl]azo]phenoxy]hexyl side groups is modified by annealing and on ultraviolet (UV) irradiation. Both procedures cause order−disorder transitions. The interaction with linearly polarized visible light results in a reversible reorientation of the azobenzene side groups maintaining the layered structure. In this way the optical anisotropy of the LB films is modified reversibly and continuously. In the original multilayer the photoreorientation process is restricted by the dense packing of the side groups. But the process takes place after weakening the intermolecular interactions. This has been achieved by a suitable preparation of the films, by annealing of the thermotropic polymer, or by a photochemically induced decrease of the supramolecular order. The photogeneration of anisotropy is governed by the molecular aggregation, the orientational order, the thermotropic ordering tendency of the polymer to establish a homeotropic alignment, and the irradiation conditions (wavelength, power density, polarization direction). Continued irradiation or annealing above the glass transition temperature (T g) results in a decrease of absorbance indicating smaller tilt angles. The photoinduced dichroism is long-term stable but can be erased by irradiation and heating.

Synthesis and Characterization of Copolymers of Methyl Methacrylate and 11-(4-Ethoxyazobenzene-4′-oxy)undecyl Methacrylate

“…The lack of shift in the absorption maxima for our polymers confirms that there is no interaction between the azobenzene groups. Such interaction is common for polymers with polar azobenzene side chains, [25,38] or in Langmuir-Blodgett films, [39] but generally does not occur in non-polar side chain polymers. [27,40] The broken lines in Figure 3 represent absorption spectra of the copolymer sample after irradiation with 366 nm light.…”

High in-plane alignment of liquid crystalline methacrylate copolymers bearing photoreactive 2-styrylpyridine and mesogenic 4-methoxybiphenyl groups

“…[1][2][3][4][5][6][7][8][9][10][11] For the LC polymers, LC polymers with azobenzene group as a photoisomerizable group 8,[12][13][14][15][16] or cinnamoyl group as a photocyclodimerizable group [17][18][19][20][21] have been mainly studied on photoalignment after linearly polarized ultraviolet (LPUV) light irradiation or on thermal alignment after the anisotropic photoreaction. In these reports, spacer lengths of alkylene groups, 5,15,19,20 compositions of non-photosensitive comonomers, 6,17,19,21 and comonomer units 8,19,22 as structural factors of the photosensitive LC polymers were focused on the orientational order and directions, i.e.…”

Polarized light spectroscopy of azobenzene or disperse red 1 with Mn12 single-molecule magnets in PMMA film hybrid materials

“…Although photofunctional polymers and liquid crystalline materials containing azobenzene groups have been widely studied [19][20][21][22], for most of these systems photomodulation of the inorganic species' electronic properties is not understood.…”