Photoinduced Optical Anisotropy Based on Axis-Selective Triplet Energy Transfer and Thermally Enhanced Reorientation in a Photo-Cross-Linkable Liquid Crystalline Polymer Film

Abstract

An axis-selective photosensitized reaction of a photo-cross-linkable liquid crystalline polymer film doped with a triplet photosensitizer (TPS) based on axis-selective triplet energy transfer achieved a thermally enhanced molecular reorientation of the film using linearly polarized (LP) 405 nm light. Four types of TPS4,4‘-(N,N ‘-bis(diethylamino))benzophenone (DBP), 4-(N-diethylamino)benzophenone (BP), 5-nitroacenaphthene (AN), and p-nitroaniline (pNA)were used to compare the efficiency of the axis selection of the energy transfer. In all cases, the photoreactivity of a film was generated when doping with less than 9 wt % of TPS. The quenching sphere played a role in the photoreactivity, while the photoinduced optical anisotropy depended on the type of TPS. The axis-selective photoexcitation of TPS and the polarization-preserved energy transfer to the photoreactive mesogenic groups dominated the photoinduced optical anisotropy as well as the thermal enhancement of the molecular reorientation.

Photoinduced Reorientation of a Liquid Crystalline Polymer with Phenyl Benzoate Mesogenic Side Groups on the Basis of an Axis-Selective Photo-Fries Rearrangement

“…In these cases, the exposure energy required for sufficient reorientation is 0.01–0.04 J/cm 2 ; small amount of photo-Fries products parallel to E (DP=3–4%) act as impurities, reducing the LC characteristics, which thermally induce in-plane reorientation perpendicular to E . A similar thermal enhancement of the photoinduced small optical anisotropy has been reported in LCPs with cinnamate, , cinnamic acids, and azobenzene side groups. − Namely, a small amount of photo-Fries products prevents the formation of the thermally induced multidomain of the mesogenic side groups. Consequently, effective in-plane self-organization is predominant at elevated temperatures.…”

“…In contrast, thermally enhanced photoinduced molecular orientation has been employed in several types of photoreactive liquid crystalline polymers (LCPs) based on the axis-selective photoreaction combined with thermally stimulated reorientation. ,− In this case, the photoalignment film can be applied to birefringent films without a polymerizable LC layer because the photoreactive film itself attains a large molecular orientation caused by the self-organization due to the LC characteristics of the material . This method uses a simple process to produce molecularly oriented polymeric films.…”

“…We have conducted systematic studies on the thermally enhanced photoinduced molecular reorientation in photoreactive LCPs comprised of cinnamate and cinnamic acid derivative side groups, which are transparent in the visible region. − The axis-selective photocross-linked groups parallel to the polarization ( E ) of the LP light control the molecular reorientation direction either parallel or perpendicular to E due to the LC characteristics of the photoreactive LCP film. Because the axis-selective photoreaction generates different distribution of the photoproducts between parallel and perpendicular directions, the self-organization abilities between these directions are different.…”

Highly sensitive photoalignment of calamitic and discotic liquid crystals assisted by axis-selective triplet energy transfer

“…41 Related studies have been carried out on the CalLC photoalignment sensitized through axis-selective triplet energy transfer for other types of sensitized photopolymers. 42,43 In this report, we elaborate on the fundamental mechanism and practical applicability of axis-selective triplet energy transfer to perform highly sensitive LC alignment. In order to elucidate the triplet energy transfer in polymer solids, we analyzed in detail phosphorescence spectra in combination with theoretical Perrin's formula.…”

Photoinduced Reorientation and Multiple Optical Data Storage in Photo-Cross-Linkable Liquid Crystalline Copolymer Films Using 405 nm Light

“…During the course of our systematic study on thermally enhanced photoinduced molecular reorientation in photo-cross-linkable materials, it was determined that polymethacrylates with 4-(4-methoxycinnamoyloxy)biphenyl (MCB) side groups exhibit efficient in-plane molecular reorientations. ,, The LC nature of the material allows the axis-selective photoreaction of the MCB side groups to generate the photoinduced optical anisotropy of the film, while a subsequent thermal treatment enhances the molecular reorientation along the photo-cross-linked anchors parallel to the polarization direction ( E ) of LP light. Furthermore, doping with a small amount of a photosensitizer such as a benzophenone derivative or p -nitroaniline improves the photoreactivity of the material while maintaining the axis-selective photoreaction when LP 405 nm light is used. , In addition, the axis-selective photoabsorption of the photosensitizers followed by a polarization-preserved excited energy transfer from the excited photosensitizers to the MCB groups generates an anisotropic reaction. However, the thermally enhanced photoinduced in-plane order parameter is smaller than that generated in a film without photosensitizers exposed to LP 365 nm light.…”