Photochemically Induced Changes of Optical Anisotropy and Surface of LB- Multilayers Built up by an Amphiphilic and Liquid Crystalline Copolymer Containing Azobenzene Moieties

Geue, Thomas; Stumpe, Joachim; Pietsch, U.; Haak, Michael; Kaupp, Gerd

doi:10.1080/10587259508033522

Cited by 22 publications

(28 citation statements)

References 20 publications

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“…The preferred formation of H-aggregated clusters and their efficient photoinduced destruction stimulated by the photochemical variation of a small number of stacking chromophores seems to be a very common phenomenon in the photochemistry of ordered assemblies and solids. Recently, we have found that such photoinduced deaggregation takes place in liquid crystalline guest−host systems, , in the glassy and viscoelastic state of liquid crystalline polymers, and in LB multilayers of amphotropic polyacrylates. − , Several photochromic moieties like azobenzene, stilbene, or cinnamic acid derivatives show this effect. ,− Similar results are reported by Hoyle 61 and Whitten …”

Section: Discussionsupporting

confidence: 69%

Langmuir−Blodgett Films of Photochromic Polyglutamates. 9. Relation between Photochemical Modification and Thermotropic Properties

1996

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Thermotropic polyglutamates with photochromic azobenzene side groups are polymers of the “hairy rod” type. They form LB multilayer assemblies, in which the rod-like macromolecules are oriented in the dipping direction. The initial LB films have a well-defined bilayer structure in which the rod-like azobenzene moieties are tilted toward the α-helical backbones and form H-aggregates. The change of aggregation and orientational order of the azobenzene chromophores is studied for polyglutamates with different spacer lengths upon irradiation and annealing. The lamellar order and in-plane anisotropy of the chromophores is irreversibly lost on UV irradiation. New ordered structures with a more symmetric distribution of the side chains around the main chain helix, a modified spacing, changed aggregation, and a different in-plane anisotropy are established after subsequent vis irradiation or annealing. The established supramolecular order strongly depends on the spacer length. A shorter spacer (C2) between the backbone and the photochromic moiety causes an isotropic in-plane distribution of the chromophores, whereas in the case of longer spacers (C4 or C6) an arrangement of the chromophores parallel to the main chain helices is restored. The change in the molecular arrangement of the side groups is controlled by the ordering tendencies and the dynamic properties of the combined liquid crystalline main and side chain polymers.

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Section: Discussionsupporting

confidence: 69%

Langmuir−Blodgett Films of Photochromic Polyglutamates. 9. Relation between Photochemical Modification and Thermotropic Properties

1996

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“…The short wavelength of the maximum and the decrease of absorbance indicate the aggregation and a decreasing tilt angle of the azobenzene side groups. This strictly corresponds to the structure model proposed from the X-ray reflection and grazing incidence diffraction measurements. − …”

Section: Resultsmentioning

confidence: 56%

“…The manipulation of LB order by external forces or photoreactions is of great interest. In the case of thermotropic polymers with photochromic azobenzene groups, this can be achieved by annealing or irradiating. − …”

Section: Introductionmentioning

confidence: 99%

Light-Induced Orientation Phenomena in Langmuir−Blodgett Multilayers

1997

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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.

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“…In contrast, tiny roughness (Δ d < 0.5 nm) in addition to the initial bumpy structure appears at the in-plane reorientation area of AN (NA)-coated films (Figures a–c). Changes in the surface topology upon the photoinduced reorientation are observed in an azobenzene-containing Langmuir–Blodgett thin film . Similarly, molecular reorientation at the free surface of the PNBAM film slightly alters the surface topology due to the molecular motion, but direction selectivity of the surface roughness is hardly observed.…”

mentioning

confidence: 94%

“…Changes in the surface topology upon the photoinduced reorientation are observed in an azobenzene-containing Langmuir−Blodgett thin film. 14 Similarly, molecular reorientation at the free surface of the PNBAM film slightly alters the surface topology due to the molecular motion, but direction selectivity of the surface roughness is hardly observed.…”

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confidence: 99%

Orientation Direction Control in Liquid Crystalline Photoalignable Polymeric Films by Adjusting the Free-Surface Condition

et al. 2016

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Adjusting the free-surface condition facilely controls the in-plane and out-of-plane orientations in liquid crystalline polymer (LCP) films. Top coating with aromatic molecules onto LC polymethacrylate films with N-benzylideneaniline (NBA) or 4-methoxybiphenyl (MB) side groups (PNBAM or PMBM) and subsequent annealing generate a random planar orientation while simultaneously removing the coated aromatic molecules, whereas annealing noncoated films induces a homeotropic orientation of the mesogenic side groups. Additionally, irradiating a top-coated PNBAM film with linearly polarized (LP) 365 nm light induces an in-plane molecular reorientation of the NBA side groups without changing the orientation in the homeotropically oriented region. Changes in the surface topology of the LCP films due to the reorientation processes are investigated in detail. Inkjet coating with aromatic molecules and LP 365 nm light exposure precisely controls the in-plane and out-of-plane alignment pattern in a PNBAM film.

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Photochemically Induced Changes of Optical Anisotropy and Surface of LB- Multilayers Built up by an Amphiphilic and Liquid Crystalline Copolymer Containing Azobenzene Moieties

Cited by 22 publications

References 20 publications

Langmuir−Blodgett Films of Photochromic Polyglutamates. 9. Relation between Photochemical Modification and Thermotropic Properties

Langmuir−Blodgett Films of Photochromic Polyglutamates. 9. Relation between Photochemical Modification and Thermotropic Properties

Light-Induced Orientation Phenomena in Langmuir−Blodgett Multilayers

Orientation Direction Control in Liquid Crystalline Photoalignable Polymeric Films by Adjusting the Free-Surface Condition

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