Multicolor fluorescent nanomaterials that exhibit multiple distinguishable emission signals are especially attractive due to their potential applications in flexible full-color displays, in next-generation lighting sources, and in probes to decipher multiple biological events simultaneously. Recently, we found that a photoswitchable fluorescent NP composed of a photochromic diarylethene (DAE) and a fluorescent benzothiadiazole (BTD) unit exhibits a remarkable nonlinear fluorescence photoswitching due to the efficient intermolecular FRET process in the densely packed NP state, in which only a small amount of the non-fluorescent closed-ring isomer (quencher) was enough to quench the whole fluorescence signal. This unique property allows us the demonstration of high-contrast multicolor fluorescence photoswitching. In this study, we tried to prepare several photoswitchable NPs, which have a different emission maximum in the fluorescence unit and a different absorption maximum in the closed-ring isomer of the photochromic DAE unit. All compounds showed the giant amplified fluorescence quenching in the NP state. Based on this property, we tried to demonstrate the sequential red-green-blue (RGB) fluorescence color photoswitching in a multicomponent photochromic fluorescent NP containing three different fluorescence-colored molecules and the wavelength-selective multicolor fluorescence photoswitching in a mixture of two emission colored photochromic NPs composed of different pairs of a photoswitching unit and a fluorescence unit upon irradiation with appropriate wavelength of lights. Such multicolor fluorescence photoswitchable systems have great potential for various applications.
We have developed several fluorescence photoswitchable molecules based on a photochromic energy transfer or electron transfer process and successfully demonstrated reversible fluorescence photoswitching with non-destructive readout capability even at the single-molecule level. In this study, as a novel method to achieve reversible fluorescence photoswitching and non-destructive fluorescence readout, we focused on the stimuli-responsive orientation behavior of a liquid crystalline polymer (LCP) containing a photochromic azobenzene unit. We attempted to cooperatively control the molecular orientation of a fluorescent dye by incorporating into an azobenzene LCP (PMAz6Ac) film and reversibly switch the fluorescence intensity along with the orientation change of PMAz6Ac induced by the polarized-light irradiation or thermal annealing process. We successfully observed cooperative orientation behavior of a doped fluorescent dye along with the orientation change of PMAz6Ac by choosing an appropriate fluorescent dye. This cooperative orientation allows us to demonstrate the reversible modulation of fluorescence intensity with non-destructive readout under irradiation with the polarized excitation light.
Light-driven micro-particles (MPs) were prepared by modification of photo-responsive azobenzene(Azo) groups as molecular motors. In a nematic liquid crystal, 5CB, we have achieved remote control of the Azo-modified MPs (Azo/MPs), which constructed with glass spheres (GSs) or rod-like glass particles (GRs) as MPs, by light stimuli. As results of photo-induced isomerization from UV-visible extinction spectroscopy and from phase evaluation under microscope observation, the motion behavior suggest to be correlated with the generation of micro-sized isotropic phase (I-phase) regions, which could be caused by disorganization of the 5CB molecules in the nematic phase (N-phase) due to photo-induced continuous isomerization of Azo groups.
A few research group reports formation of ordered structures of colloidal particles due to topological defects in a nematic liquid crystal. In this paper, we describe the photochemical phase transition of a nematic liquid crystal by the photoisomerization of azobenzene dyes, focusing especially on the dynamical effect of the trans–cis–trans photoisomerization cycle of a push-pull azobenzene dye. Then, we discuss the effects of light irradiation on the motion of small objects dispersed in the nematic and smectic liquid crystals contatining a push-pull azobenzene dye, and successfully manipulate those objects by pushing, trapping and dragging them.
In this study, faster response speed of ON/OFF switching of multi-bilayered films containing azo-functionalized polymer liquid crystal and polyvinyl alcohol, PVA, is investigated. The multi-bilayered films were found to reflect a light of specific wavelength, and showed reversible change in the reflection intensity by irradiation with non-polarized visible light and UV light. The multi-bilayered film having high absorbance around 300-500 nm due to stacking several decades of azobenzene containing layer. So, the change in reflection intensity of multi-bilayered film takes long time by light irradiation. We synthesized polyacrylates copolymers with azobenzene and biphenyl side chains where biphenyl groups is having no absorbance at UV and visible region. Multi-bilayered films were fabricated by spin-coating method and the switching time of reflection was investigated comparing to the multi-bilayered film containing azobenzene homopolymer, the ON switching times of reflection for the copolymers were faster than homopolymer needing 900 s. It is attributed to that inducing UV light (λ = 365 nm) easily entered into the film by introducing of biphenyl groups.
Photo-responsive multi-bilayered film consisting of azobenzene polymer liquid crystals (PAzo) and polyvinylalcohol
(PVA) was prepared on a glass substrate by spin coating of the polymer solutions alternately. The reflectivity of the
multi-bilayered film disappeared by annealing at 80 °C. The disappearance of the reflection by the annealing was related
to the thermal out-of-plane molecular orientation of PAzo even in the multi-bilayered film, leading to a very small
difference in refractive indices between PAzo and PVA. The reflectance of the multi-bilayered film was increased again
by UV irradiation because of the transformation from the
out-of-plane orientation to the in-plane random orientation,
resulting in the restoration of difference in the refractive indices. In this way, the on-off switching of the reflection was
achieved by combination of the thermally spontaneous out-of-plane molecular orientation and following
photoisomerization of PAzo comprising the multi-bilayered film.
Photochemically tunable photonic band gap materials were prepared by infiltration of liquid crystal polymers having
azobenzene groups into voids of SiO2 inverse opal films. Linearly polarized light irradiation resulted in transformation
from a random to an anisotropic molecular orientation of azobenzene side chains in the voids of the SiO2 inverse opal
film, leading to the reversible and stable shift of the reflection band to longer wavelength more than 15 nm. In order to
improve switching properties, we used copolymers with azobenzene monomer and tolane monomer, which indicate
higher birefringence, as infiltration materials into the voids. The azo-tolane copolymers were found to show the higher
birefringence than azobenzene homopolymers by the linearly polarized light irradiation. Thus, the reflection band of the
SiO2 inverse opal film infiltrated with the azo-tolane copolymers was shifted to long wavelength region more than 55 nm
by the irradiation of linearly polarized light.
Photo-tunable PBG composite material was prepared by infiltration of the polymer liquid crystal (LC) having azo-chromophores
in a SiO2 inverse opal structure. The SiO2 inverse opal film infiltrated with the polymer LC reflected a
light, which is called Bragg diffraction, corresponding to the periodicity as well as the refractive indices of the inverse
opal structure. Linearly polarized light irradiation caused the shift of the the Bragg diffraction band to longer
wavelength more than 15 nm. This is caused by the formation of uniaxially anistorpic molecular orientation of the
polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be
achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable for
various optical materials such as memory, display so on.
A photochemically tunable structural color material was prepared by infiltration of the polymer liquid crystal (LC)
having azo-chromophores in a SiO2 inverse opal structure. The SiO2 inverse opal film infiltrated with the polymer LC
reflected a light, which is called a structural color, corresponding to the periodicity as well as the refractive indices of the
inverse opal structure. Linearly polarized light irradiation caused the shift of the structural color band to longer
wavelength more than 15 nm. This is caused by the formation of uniaxially anistorpic molecular orientation of the
polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be
achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable for
various optical materials such as memory, display so on.
Three azobenzene compounds with chiral moieties were prepared. A cholesteric phase was induced by mixing each chiral azobenzene in a host nematic liquid crystal. The twisting power of the chiral azobenzene compounds was decreased by ultraviolet irradiation to cause trans - cis photoisomerization of the azobenzene compounds. In addition, a compensated nematic phase was induced by mixing of a chiral azobenzene and a non-photochromic chiral compound having opposite chiral ability to the chiral azobenzene in the host nematic liquid crystal when the twisting powers of both chiral compounds were balanced. The photoisomerization of the chiral azobenzene broke the balanced state in the twisting power, resulting in the transformation of the compensated nematic phase into a cholesteric phase. Effect of the trans-cis photoisomerization on the phase structure of liquid crystals containing the chiral compounds will be discussed.
Photoresponsive liquid crystalline polymer networks with macroscopically uniaxial molecular orientation were prepared by polymerizing mixtures of liquid crystalline mono and di- acrylates, and an azobenzene compound in a homogeneous glass cell at a nematic phase. The polymer networks showed an enantiotropic phase transition from an anisotropic phase to an isotropic phase as well as high transparency. The photoisomerization of the azobenzene compound resulted in a change in a birefringence. The photochemical change in the birefringence was investigated by using a Xe lamp and a single pulse light from a Nd:YAG laser at light sources. We observed a response time in a range of a few microseconds and a decay time in a range of a few microhundreds of seconds. The optical switching behavior depended on the cross-linking density of the liquid crystalline polymer networks.
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