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We have demonstrated in the present report that dielectric microparticles exhibited orbital rotation in the light field of non-coaxially configured two counter-propagating laser beams both in numerical simulations and experiments. A series of computational simulations indicated that when irradiated with two non-coaxially counter-propagating parallel laser beams with same intensity distributions in the absence of thermal (Brownian) motion, a microparticle did not exhibit orbital rotation due to the symmetry of the optical field. However, the computations predicted that a microparticle exhibited one directional orbital rotation in the presence of thermal motion because of the symmetry breaking of the optical force acting on the particle. This spontaneous orbital rotation was experimentally demonstrated for 1-µm dielectric particles in water at room temperature.

Despite the excellent optoelectronic properties of halide perovskites, the ionic and electronic defects adversely affect the stability and durability of perovskites and their devices. These defects, intrinsic or produced by environmental factors such as oxygen, moisture, or light, not only cause chemical reactions that disintegrate the structure and properties of perovskites but also induce undesired photoluminescence blinking to perovskite quantum dots and nanocrystals. Blinking is also caused by the nonradiative Auger processes in the photocharged quantum dots or nanocrystals. Herein, we find real-time suppression of halide vacancy-assisted nonradiative exciton recombination and photoluminescence blinking in MAPbBr3 and MAPbI3 perovskite quantum dots by filling the vacancies using halide precursors (MABr and MAI). Also, halide vacancy filling increases the photoluminescence quantum efficiencies and lifetimes of the quantum dots. We estimate the rates of halide vacancy-assisted nonradiative recombination at 1 × 108 s-1 for MAPbBr3 and 1.9 × 109 s-1 for MAPbI3 quantum dots. The real-time blinking suppression using the halide precursors and statistical analysis of the ON/OFF blinking time reveal that the halide vacancies contribute to the type-A blinking through charging and discharging. Conversely, the blinking of the quantum dots after halide vacancy filling is dominated by the type-B mechanism.

Photomechanical molecular crystals are promising materials as photon-powered artificial actuators. To design and control the photomechanical responses, the spatiotemporal distribution of photoproducts in crystal could be a key factor in addition to molecular structures, molecular packings, illumination conditions, crystal morphology, crystal size, and so on. In this study, we have found that single crystals of 2,5-distyrylpyrazine shows a smooth photomechanical expansion, and the photochemical reaction proceeds heterogeneously on the optical length scale by observation of the changes in color, fluorescence, and birefringence. Moreover, we have performed a numerical simulation to reproduce the experimental results and revealed that both the cooperativity effect and the surface effect in crystal are essential for the heterogeneous progress of the photochemical reaction. Our results would provide a framework for analyzing the heterogeneous reaction dynamics on optical length scale in photoresponsive molecular crystals and a benchmark for future studies of photomechanical molecular crystals.

This article reviews optical manipulation coupled with photochemical/photothermal responses of nanometer sized materials including molecular systems, polymers, and inorganic nanoparticles. After the introduction, section 2 overviews the optical trapping of nanometer sized molecular systems including early-stage studies, such as trapping of polymer chains, micelles, and molecular aggregates in solution at room temperatures. Then, the conformation control of macromolecule assemblies and gels by optical force are introduced, followed by micro-fabrications achieved by combining optical trapping and photochemical reactions. Section 3 summarizes studies on the evaluation of optical force acting on nanometric molecular systems using fluorescence correlation techniques. Approaches to control optical force by using photochemical reactions are show in section 4, where the absorption band of target materials are modified through photochromic reactions, leading to micromechanical motion of small particles synchronizing with the photochemical reactions. Section 5 overviews photothermal effect in optical manipulation such as natural convection, Marangoni convection and thermophoresis, and applications of the thermal effects to develop new methods of micromanipulation achieved by combining optical force and photothermal responses.

Photocontrol of mechanical motions of small objects has attracted much attention to develop mesoscopic remote actuators. For this purpose, photoinduced morphological changes of molecules, molecular aggregates, and crystals have been extensively studied in the field of chemistry and materials science. Here, we propose direct use of momenta of light (i.e. radiation force) to control the motion of small objects, through photochromic reactions of pyranoquinazoline (PQ) derivatives. PQ is colorless in visible wavelength region while it is in closed form and undergoes photochemical ring-opening reactions to form colored isomers upon UV light irradiation; the open-ring isomers return to the colorless closed isomers mainly through the thermal back reaction. In the experiment, individual polymer microparticles with diameters of 7 μm incorporating PQ were trapped by optical tweezers. When the trapped microparticle was irradiated with UV light, the microparticle was pushed along the axis of light propagation about a...

Photoswitchable turn-on mode fluorescent molecules have been so far successfully used in super-resolution fluorescence microscopies. Here, we report on fluorescence photoswitching of 1,2-bis(2-ethyl-6-phenyl-1-benzothiophene-1,1-dioxide-3-yl)perfluorocyclopentene (1) by irradiation with single-wavelength visible (420 nm < λ < 470 nm) light, the wavelength of which is longer than the 0-0 transition of open-ring isomer 1a, without UV light excitation. By absorbing very weak hot bands or Urbach tails 1a underwent a cyclization reaction to produce fluorescent closed-ring isomer 1b. Both cyclization and cycloreversion reactions of 1 took place with the visible light in the far off-resonance region of the absorption edge. Based on numerical simulations of the formation process of 1b from 1a by irradiation with 450 nm light, weak absorption coefficients at 450 nm in n-hexane and CCl4 were estimated to be 0.084 and 0.19 M-1 cm-1, respectively. The reversible fluorescence photoswitching with the single visible light is advantageously applicable to super-resolution fluorescence imaging in biological systems.

When plasmonic nanoparticles are placed on a highly refractive semiconductor substrate, we can expect three different effects: (i) resonance mode splitting, (ii) asymmetric light scattering based on the split modes, and (iii) site-selective nanoetching due to plasmon-induced charge separation (PICS) at the nanoparticle–semiconductor interface. Here, we develop novel photofunctional materials by taking advantage of those three effects. More specifically, we control the asymmetric scattering of Ag nanocubes on TiO2 by PICS, so as to develop materials for photodrawing of one-way visible translucent images and multicolor scattering images. The one-way visible translucent images, which are translucent scattering images visible only from the back side, are drawn by anaerobic bottom-selective etching of the Ag nanocubes. For drawing the multicolor scattering images, a scattering color of Ag nanocubes is changed from yellow to green by the anaerobic bottom-selective etching and from yellow to red by aerobic nonse...

A diarylethene–fluorene dyad connected by an ester bond (DE-FL) and random and alternative copolymers with diarylethene and fluorene moieties in their side chains (poly(DE1x-co-FL1y) and poly(DE1-alt-FL2)) were synthesized, and their fluorescence on/off switching properties were investigated. All compounds exhibited reversible photochromism and fluorescence on/off switching in a solution upon alternating irradiation with ultraviolet and visible light. Although the fluorescence intensity of DE-FL linearly decreased with increasing photocyclization conversion of the diarylethene, the fluorescence intensity of poly(DE1x-co-FL1y) at the same molar fraction of diarylethene/fluorene significantly decreased with photocyclization conversion. Moreover, the fluorescence intensity of the polymers in the photostationary state was much lower than that of DE-FL. These results indicate that the fluorescence on/off switching properties of the polymers were improved by introducing diarylethene and fluorene moieties into t...

Femtosecond transient absorption (TA) and picosecond time-resolved fluorescence (TRF) spectroscopies were applied to the charge separation (CS) dynamics of 10-cyano-9,9′-bianthryl (CBA) in a normal polar solvent, acetonitrile (Acn), and in a highly viscous room temperature ionic liquid (IL), N,N-diethyl-N-methyl-N-(methoxyethyl)ammonium tetrafluoroborate (DemeBF4). The primary CS took place within the ultrafast sub-100 fs time range in both solvents, which was completely independent of diffusive solvation. Subsequent viscosity-dependent spectral evolution was observed by the TA measurement in the picosecond range which was ascribed to the structural relaxation. A red shift of the TRF spectrum in the picosecond to nanosecond range was observed in DemeBF4 which was due to the diffusive solvation in the CS state. Interestingly, integrated fluorescence intensity decayed more rapidly than TA in the IL, while they decayed simultaneously in Acn. It was concluded that diffusive solvation decreases the radiative t...

A new fluorescent photochromic diarylethene that can be activated by irradiation with 405 nm light was synthesized. The turn-on mode switching of fluorescence with visible light is favorable for application to biological systems. The fluorescence quantum yield of the photogenerated closed-ring isomer was as high as 0.8 in less or medium polar solvents, and even in polar acetonitrile the yield was higher than 0.6.

Local molecular states and biological materials in small spaces ranging from the microscale to nanoscale can be modulated for medical and biological applications using the photothermal effect (PTE). However, there have been only a few reports on exploiting the collective phenomena of localized surface plasmons (LSPs) to increase the amount of light-induced heat for the control of material states and the generation of macroscopic assembled structures. Here, we clarify that microbeads covered with a vast number of Ag nanoparticles can induce a large PTE and generate a submillimeter bubble within several tens of seconds under the synergetic effect of the light-induced force (LIF) and photothermal convection enhanced by collective phenomena of LSPs. Control of the phase transition induced by such a “collective photothermal effect” enables rapid assembling of macroscopic structures consisting of nanomaterials, which would be used for detection of a small amount of proteins based on light-induced heat coagulation.

By using confocal and wide-field microscopic techniques we have investigated fluorescence behaviors of a single perylenediimide derivative, N,N′-dipropyl-1,6,7,12-tetrakis(4-tert-butylphenoxy)-3,4,9,10-perylenetetracarboxydiimide (BP-PDI), in ca. 200–300 nm sized droplets of n-octane covered with surfactant molecules. The guest dye did not show distinguishable blinking in the droplets prepared with a nonionic surfactant, Triton X-100, while frequent blinking was observed in the droplets with a cationic surfactant, trimethylstearylammonium chloride (STAC). The survival time of the guest dye before the permanent photodegradation was much longer in the droplets with Triton X-100 than those with STAC. On the basis of the analysis with a model assuming the transition from the S1 state to the dark state (DS) and the permanent photodegradation during the dark state, it was deduced that the probability of the transition from the S1 state to the DS in the droplet with STAC, 7.2 × 10–6, was significantly larger tha...

Fluorescence correlation spectroscopy was applied to the measurement of local temperature in the vicinity of gold nanoparticles adsorbed on the surface of glass substrate under the photoexcitation at a wavelength of 633 nm. From the diffusion coefficient of fluorescent guest dyes and the temperature dependence of the viscosity of a solution, the increase in the temperature was estimated. It was revealed that the temperature ca. 5 μm from the gold nanoparticles linearly increased with an increase in the incident laser power and an increase in the number of gold nanoparticles on the substrates. Temperature elevation coefficients of single gold nanoparticles with 100 and 150 nm diameters under the irradiation at 633 nm were, respectively, estimated to be (2.3 and 6.9) × 10–3 K·kW–1·cm2. These values could be interpreted on the basis of the absorption coefficients of gold nanoparticles and the thermal conduction in the solution.

We have established a new platform to control the rate of spontaneous emission (SE) of organic molecules in the visible-light region using a combination of a two-dimensional (2D) photonic crystal (PC) slab made of TiO(2) and a single-molecule measurement method. The SE from single molecules of a perylenediimide derivative was effectively inhibited via a radiation field controlled by the 2D PC slab, which has a photonic band gap (PBG) for transverse-electric (TE)-polarized light. The fluorescence lifetimes of the single molecules were extended up to 5.5 times (28.6 ns) by the PBG effect. This result appears to be the first demonstration of drastic lifetime elongation for single molecules due to a PBG effect.

Controlling chemical reactions within a small space is a significant issue in chemistry, and methods to induce reactions within a desired position have various potential applications. Here we demonstrate localized, efficient photopolymerization by radiation pressure. We induced a one-photon UV polymerization of liquid acrylate solutions in the optical-trapping potential of a focused near-IR (NIR) laser beam, leading to the confinement of solidification to a minute space with dimensions smaller or equal to one-fifth of the wavelength of the NIR laser. Our approach can produce solidification volumes smaller than those achievable with conventional one-photon polymerization, thus enabling the production of tiny polymeric structures that are smaller than the diffraction limit of the trapping light. This is the first demonstration of a radiation pressure effect on a photochemical reaction.

We report the synthesis and structure of iridium(I) complexes with N,N′-disubstituted perimidine carbene ligands and halides or carboxylate ligands. Iodo-, chloro-, and acetate-Ir(carbene)(cod) com...

Single-molecule tracking (SMT) was applied to the evaluation of a microscopic heterogeneous structure by observing the diffusivity of guest molecules in phenyl-bridged periodic mesoporous organosilica films with one-dimensional (1D) nanoscale channels filled with template surfactant micelles. SMT clearly visualized the trajectories of individual fluorescence dyes, N,N′-dipropyl-1,6,7,12-tetrakis(4-tert-butylphenoxy)-3,4,:9,10-perylenetetracarboxdiimide (BP-PDI) and N-(2,6-diisopropylphenyl)-N′-(n-octyl)-terrylene-3,4:11,12-tetracarboxidiimide (TDI), encapsulated in the channels of the mesoporous material. The 1D trajectory indicated successful formation of bundled channel structures and encapsulation of the guest molecules into the channels. The encapsulation ratio of the guest dyes was strongly dependent on template surfactants. The difference was discussed in terms of the affinity between the guest molecules and surfactants. Statistical analysis for the diffusion of the guest molecules also revealed tha...

The temperature dependence of the fluorescence behaviors of 9,9′-bianthryl (BA) in the charge-transferred (CT) state in imidazolium ionic liquids (IL) was investigated by means of steady-state as well as time-resolved detections. At ambient and higher temperatures, the emission peak of BA in ILs shifted to longer wavelengths with decreasing temperature, which is a phenomenon commonly observed in normal polar solvents. On the other hand, the emission peak shifted toward shorter wavelengths with decreasing temperature below ca. 290 K. Time-resolved fluorescence (TRF) spectroscopy was carried out to investigate the dynamic Stokes shift, i.e., the time-dependent red-shift of the fluorescence peak. In highly viscous ILs, the time constant of the dynamic Stokes shift becomes comparable or longer than the lifetime of the CT state. In such a case, the ground state of BA is recovered before the completion of the solvation process. It is concluded that the origin of the blue-shifted emission at lower temperatures i...

The photodissociation dynamics of a hexaarylbiimidazole (HABI) derivative with two pyrenyl groups was investigated by time-resolved transient absorption spectroscopy and fluorescence measurements. Transient absorption spectroscopy revealed that photodissociation took place in the wide time region of

Excited state dynamics of 9,9'-bianthryl (BA) in 1-butyl-3-methylimidazolium ionic liquids with three different types of anions was investigated by means of femtosecond to nanosecond transient absorption (TA) spectroscopy and time-resolved fluorescence (TRF) measurements. TA spectroscopy revealed that charge transfer (CT) occurred multiexponentially in the time scale of hundreds of picoseconds while TRF measurement revealed that red-shift of the BA fluorescence peak extended into the nanosecond regime. It is concluded that an energy relaxation process slower than the CT reaction, which could be the solvation dynamics in the product state, is observed.

Fluorescence correlation spectroscopy (FCS) was applied to the quantitative evaluation of the local heating in small domains

Femtosecond near-infrared laser microscope was developed with a home-built cavity-dumped chromium:forsterite laser as a light source centered at 1.26 microm. Optimization of the pulse duration achieved 35 fs fwhm at the sample position of the microscope after passing through a 100x objective. This system was applied to the detection of multiphoton fluorescence of some organic microcrystals. Excitation intensity dependence and the interferometric autocorrelation detection of the fluorescence clearly demonstrated that simultaneous three- and four-photon absorption processes are responsible for the production of the excited state for perylene and anthracene microcrystals, respectively. The spatial resolution along the optical axis and its dependence on the order of the multiphoton process were also discussed.