Photosynergetic Responses in Molecules and Molecular Aggregates

In the present chapter, we first introduce the mechanism of the multiphoton-gated photochromic reactions in diarylethene and fulgide derivatives by pulsed laser excitation, where the multiple photons lead to the specific photoresponse beyond the Kasha’s rule. In the second part, we discuss the one-color control of the activation, deactivation, and fluorescence excitation in the single-molecule detection. One-color of light takes various roles resulting in the effective single-molecule tracking via the photon-material synergetic response. In the third section, photochromic reaction is applied to the controlled translational motion of nano- to micron-sized materials under the laser trapping. Integrated photoresponses of materials, such as absorption, photochemical reaction, and the photon pressure, lead to the mesoscopic motion synchronized with the photochemical reaction.

In this chapter, we describe our recent development of theoretical approach to analyze photochemisty relevant electronic states (wave functions and energies) for studying photochromic molecules with a promised numerical reliability. Our approach is based on multireference (MR) theory, which is considered to be a critical treatment to properly compute complex electronic structures arising in excited states. It characterizes the wave function by explicitly expressing it as a correlated superposition of multiple electronic configurations (or Slater determinants). We mainly review our recent development of extended multi-state complete active-space second-order perturbation theory incorporated into density matrix renormalization group reference wave functions. Its illustrative applications to molecular photochromic isomerizations, diarylethene derivatives and pentaarylbiimidazole are briefly reviewed in order to show the applicability of our computational approach.

We introduce molecular two-photon absorption and two-step excitation using quantum-mechanically entangled-photon pairs. The entangled photons possess the inherent simultaneity of photons originating from quantum correlation with energy anticorrelation, which is not shared by conventional laser light. This simultaneity enables entangled photons to achieve highly efficient two-photon absorption and two-step excitation under the condition of low light intensity. We show that ultrabroadband frequency-entangled photons can enhance the transition rates of two-photon absorption and two-step excitation more than 1000 times, compared to uncorrelated photons like laser light, and can also selectively excite a single vibrational mode in the molecular two-photon processes.

Stepwise two-photon absorption (2PA) is a well-known nonlinear optical phenomenon where two photons are sequentially absorbed to proceed a photophysical or photochemical reaction. The stepwise 2PA can produce a higher excited state with lower energy photons than the energy level of the excited state. Therefore, it can be an efficient tool to explore and realize nonlinear photoresponsive materials beyond Kasha’s rule. Moreover, if a photogenerated chemical species is used as an intermediate state of the stepwise 2PA, the power threshold to induce the stepwise 2PA can be greatly reduced compared with that of a simultaneous 2PA. We focus on the stepwise 2PA processes related to photochromic reactions because the combination of photochromic reactions and stepwise 2PA processes is beneficial to develop unique nonlinear photoresponsive materials. We introduce several anomalous phenomena beyond one-photon photophysical and photochemical reactions.

We describe the relationship between the rate constant of internal conversion and vibronic coupling constant (VCC) based on the crude-adiabatic approximation. Vibronic coupling density (VCD) is introduced to clarify the origin of vibronic couplings from the local picture. The control of vibronic couplings from pseudo-degenerate excited electronic states gives the suppression of internal conversions. We discuss the fluorescence via higher triplets (FvHT) mechanism observed in the organic light-emitting diodes (OLEDs) of 1,4-bis(10-phenylanthracene-9-yl)benzene (BD1) used as a fluorescent dopant and the aggregation-induced enhanced emission (AIEE) of 1,2-bis(pyridylphenyl)ethene (CNPPE).

Strong light fields induced on mesoscopic structures have the potential to pave the way to develop novel excitation schemes that cannot be realized using conventional light fields. In this chapter, we use various scanning near-field optical microscopy techniques to obtain near-field spatial and spectral characterizations of metal mesoscopic structures. We also describe the enhancement of molecular optical responses by mesoscopic structures.

In this chapter, we introduce multiphoton photochromic reversible reaction in a diarylethene derivative at nanometer scale much beyond the diffraction limit. For this, we have utilized nonlinear plasmonics along a plasmonic waveguide, particularly on the chemically synthesized silver nanowire. Propagating near-field surface plasmons excited by near-infrared femtosecond laser pulses successfully excite diarylethene derivatives surrounding a silver nanowire. Three-photon cyclization and two-photon cycloreversion reaction induced through the propagating surface plasmon polaritons were visualized by means of fluorescence imaging. We demonstrated one-color near-infrared laser reversible control by tuning the laser power.

The modulation of electronic or transition states plays an important role in improving the selectivity and reactivity of chemical reactions. In recent years, the strong coupling between an optical mode and excitons or molecular vibrational modes has received attractive attention as a physical phenomenon for controlling the activation energy of chemical reactions by modulating the electronic state or the vibrational state. In this study, we investigated the spectral properties of hybrid states formed by the strong coupling between plasmon and molecular/intermolecular vibrational modes in the infrared wavelength region. The strong coupling between plasmon and molecular excitonic states have been also studied for demonstrating how to confirm the formation of hybrid states. We developed a highly efficient photochemical reaction field by using modal strong coupling systems between plasmons and different optical modes.

Utilization of high density of triplets is important to overcome the dissipation of excitation energy. Triplet–triplet annihilation (TTA) has attracted attention to overcome the conventional limitation for the application of photon upconversion (UC). Synergetic approaches of experimental and computational studies on TTA have been made for efficient TTA-UC in the solid state. Binary solid fabricated by the rapid-drying casting from mixed solution enabled efficient triplet sensitization. Structural modification of emitter molecules has enhanced the performance of TTA-UC in solid. Computational studies clarified the importance of the dimensionality of the exciton diffusion process and orbital overlap in the TTA process. Various extended studies such as solid TTA-UC by near-infrared (NIR) excitation and sensitizer-fixed approach on a solid surface are also presented.

Hot carrier transfer of semiconductor nanocrystals (NCs) plays an important role for solar energy conversion. In this chapter, effects of quantum confinement of colloidally synthesized semiconductor NCs on hot carrier transfer and the carrier transfer mechanism are discussed on the basis of state-selective excitation of femtosecond transient absorption spectroscopy and initial bleach yield of band-edge state. The role of phonon emission from higher excited states on hot carrier transfer in quantum-confined NCs is revealed. In addition, carrier manipulation of a single semiconductor NC by plasmonic nanostructures is demonstrated with single particle spectroscopy. The distance dependence between a single semiconductor quantum dot (QD) and a plasmonic nanostructure on luminescence intensity and lifetime of a single semiconductor QD is discussed in terms of the electromagnetic enhancement of absorption and luminescence and energy transfer quenching by the plasmonic nanostructure.

With the growing success of halide perovskites as functional materials in solar cells, LEDs, and lasers, studies are progressing to reveal their optical and charge-carrier properties. This chapter describes the consequences of charge-carrier generation, stabilization, and recombination in perovskite nanocrystals, quantum dots, and their assemblies. The strong confinement of charge-carriers in perovskite quantum dots causes photoluminescence blinking with distinct ON and OFF events, which is due to photocharging and ultrafast Auger nonradiative recombination. Such a blinking with long OFF periods suppresses superoxide generation and oxidation of perovskites. When perovskite nanocrystals and quantum dots are closely-packed into superlattices, the carrier confinement is broken due to the narrowing of inter-particle energy levels and the formation of minibands that allow for carrier migration. This results in unexpectedly delayed photoluminescence at low intensities of excitation light, whereas at high intensities of excitation light, the ultrafast radiative recombination of charge-carriers occurs. These properties of quantum dots, nanocrystals, and assemblies of perovskites are important to be considered during the construction of devices such as solar cells, LEDs, and lasers.

The infrared (IR) region of the solar spectrum is a source of untapped potential energy. Conversion of IR-light to electrical energy or fuel would provide a plentiful energy source for modern society. Plasmonic energy conversion holds the key to conversion of solar energy through materials that efficiently absorb photons of the desired wavelength, including the IR region. Herein, we investigated plasmon-induced carrier transfer from plasmonic heavily doped semiconductor nanocrystals in a wide bandgap semiconductor to convert IR-light to energy. We discovered that efficient hot-carrier transfer proceeds from a heavily doped semiconductor nanocrystal to a wide bandgap semiconductor upon excitation of a localized surface plasmon resonance band. In addition, this material system achieved photocatalytic H2 evolution based on excitation at long wavelengths of the solar spectrum (i.e., 2500 nm). The apparent quantum yield of photocatalytic H2 evolution based on a catalyst with a plasmonic heavily doped semiconductor as a light-harvesting material represents a highly efficient conversion of IR energy to fuel. The relationship between the plasmon-induced carrier dynamics and photocatalytic activity paves the way for use of this undeveloped low-energy light as a solar energy resource.

We introduce solution-phase preparation of multinary QDs composed of less-toxic Ag-III-VI-based semiconductors and control of their physicochemical properties. The energy gaps (Egs) of multinary QDs could be adjusted by the chemical composition as well as by the particle size. The photochemical properties, including photoluminescence (PL), photocatalysis, and photocurrent generation, of prepared QDs, are discussed in terms of the Eg, the non-stoichiometric chemical composition, and the particle morphology. The PL peak was controlled from visible to near-IR wavelength regions by varying the chemical composition of QDs, in which the peak width of Ag-In-Ga-S QDs was remarkably narrowed by the removal of deep defect levels via tuning of non-stoichiometry and the surface condition. A nonlinear photoresponse induced by hot hole transfer was observed by visible light irradiation to near-IR-light-responsive Zn-Ag-In-Te QDs. The findings will provide new insights into design and fabrication of novel QD-based devices.

Photochromic molecules are one of the major targets of research among various scopes of photosynergetic approaches on molecular photochemistry. In this chapter, the authors describe progress of photosynergetic enhancement of photoreactivity of photochromic substances beyond classic photochemical stoichiometry based on one molecule/one photon hypothesis. Catalytic activity of photoproduct is one of the key concepts to achieve this and the classic concept of photoacid generators, PAGs, was far expanded for photo-Lewis Acid Generators, PLAGs. Cascade reactions involving photochromic molecules are further maximized with terarylene derivatives and amplified radio-chromic nature was rationally demonstrated, opening critical scope for the future of molecular photochemistry.

In this chapter, we describe: (1) the use of photochromic acid-generating spiropyran in (i) controlling the absorption wavelength of bisphenanthrolinodiarylethene and (ii) the on/off switching of photochromism in a diarylethene; (2) highly enantioselective light-induced chirality transfer from human serum albumin to diarylethenes and (3) the short-step assembly of (i) thermally irreversible stealth photochromic arylbutadienes and (ii) thermally reversible photochromic teraryls by way of “click” Huisgen reactions catalyzed by Ru(I) complexes.

In this chapter, we introduce singlet fission and related photophysical processes of acene derivatives such as pentacene and tetracene dimers and supramolecular assemblies with gold nanoclusters. Time-resolved spectroscopic measurements revealed the long-lived and high-yield triplet excited state through singlet fission. Moreover, light energy conversion processes such as electron transfer and singlet oxygen generation through singlet fission are discussed.

Triplet-triplet annihilation (TTA) is the most promising photon upconversion (UC) sequence, where a weak light source such as sun light can be used to frequency upconvert the visible to near-infrared light into a photon with a more useful wavelength, applicable in such as the solar cell devices. Although this anti-Stokes shift emission has long been known, and their application has been extensively investigated, less has been explored the effect on the structural modification of the relevant sensitizer and acceptor chromophores. Motivated by a beautiful and functional cylindrical structure of light-harvesting proteins, as well as recent investigation on the cyclic array of nano-wires, we prepared the derivatives of hexaarylbenzene (HAB) containing 9,10-diphenylanthracene and 2,5-diphenyloxazole units as model acceptor systems to investigate the efficiency of TTA-UC and the relevant photophysical processes. The dynamics of propeller-shaped HAB structures as well as toroidal interaction between the peripheral aromatic rings has been well investigated by combined experimental and theoretical studies on their chiroptical properties. Yet preliminary, we found that through the synergetic interaction of the peripheral chromophores, effective energy migration and hopping become possible, to facilitate the efficient TTA-UC, that can be switched on and off by modulation of the strength of toroidal interaction of the relevant HAB emitter. Further possibilities of photosynergetic effects in the TTA-UC process are also discussed.

Ion-pairing assemblies consisting of appropriately designed oppositely charged π-electronic systems give rise to various functional supramolecular assemblies including crystals and soft materials based on the anisotropic orientation of charged species through electrostatic and other weak noncovalent interactions. The introduction of photo-responsive moiety to the charged species facilitated the formation of photo-responsive ion-pairing assemblies, whose assembling modes were controlled by photoirradiation. Furthermore, pyrrole-based π-system–PtII complexes were designed and exhibited unique photo-induced excited-state dynamics.

Self-assembly of amphiphilic photochromic diarylethene having tri(ethylene glycol) monomethyl ether chains was examined in water from the viewpoint of photoinduced morphological transformation and photodriven movement of objects. Self-assembled supramolecular architectures of the amphiphilic diarylethenes undergo photoinduced macroscopic morphological transformation upon alternate irradiation with UV and visible light. The photoreversible morphological change can be rationalized as a photoinduced phase transition between the high- and low-temperature phases of the lower critical solution temperature (LCST) transition. By using a depletion force in a methylcellulose aqueous solution, an amphiphilic diarylethene hierarchically assembled into bundled fibers, which showed shrinking of more than 100 μm under visible light irradiation. Linearly polarized light induced anisotropic growth of the assembled architecture and the diffusive motion of added polystyrene beads was suppressed in the perpendicular direction to the polarized light. The movement of many objects tracing the movement of a UV-irradiation spot was achieved with the assistance of the photogenerated supramolecular architecture.

Flapping molecules (FLAP) have been developed as a new series of photofunctional systems. The molecular design is based on the concept of the rigid/flexible hybridization of π conjugated units. FLAP bearing bulky substituents works as a ratiometric viscosity probe that shows no polarity dependence, while FLAP with long alkyl chains provides a “light-melt adhesive,” a highly cohesive columnar liquid crystals that can be melt by ultraviolet light irradiation.

Fluorescence photoswitching properties of novel fluorescent photochromic diarylethene (DAE)-benzothiadiazole (BTD) dyads were studied in a solution, in the nanoparticle state, and in the single-crystalline state. The nanoparticles represent a state-of-the-art system, showing bright red emission, reversible fluorescence photoswitching upon UV-visible irradiation, complete ON-OFF contrast, excellent photostability and fatigue resistance. Most interestingly, upon UV irradiation, the nanoparticles exhibit a complete fluorescence quenching even at very low conversion (<5%) of the photochromic unit. This “giant amplification of fluorescence quenching” originates from a long-range intermolecular Förster resonance energy transfer (FRET) within each nanoparticle, leading to the quenching of ~400 fluorescent molecules for only one converted photochromic molecule. Furthermore, similar efficient fluorescence photoswitching was observed even in the single crystal of a DAE-BTD dyad.

Arbitrary and precise control of two-dimensional (2D) molecular alignment patterns over large areas play an important role for developing highly functionalized soft materials and devices. Here we demonstrate a dye-free system for 2D alignment patterning, termed “scanning wave photopolymerization (SWaP)”. SWaP utilizes a spatial light-triggered mass flow induced by scanning light to propagate the wavefront to direct molecular order. Macroscopic, arbitrary 2D alignment patterns are generated in a wide variety of optically transparent polymer films from various polymerizable mesogens with sufficiently high birefringence (>0.1) by single-step photopolymerization, without alignment layers or polarized light sources. SWaP successfully inscribed a set of 500 × 300 arrays of a radial alignment pattern with a size of 27.4 μm × 27.4 μm, in which each individual pattern is smaller by a factor of 104 than that achievable by conventional photoalignment methods.

Photosynthesis in natural systems is one of the typical examples where the interaction between multiple photons and multiple chromophores leads to the synergetic responses in three-dimensionally arranged molecular aggregates. Accordingly, the investigation of primary events in the excited state could provide important information on the progress in the artificial photosynergetic systems. Along with this line, we have prepared biohybrid light-harvesting complexes to study ultrafast dynamics of excitation energy transfer in the present project. Light-harvesting 2 complex (LH2) from photosynthetic purple bacteria is a molecular assembly, in which multiple photosynthetic chromophores are well organized. To expand high-harvesting activity of LH2, we attached a fluorophore, Alexa647, to lysine residues on the LH2 polypeptide so as to cover broad solar spectrum. Ultrafast energy transfer from Alexa 647 to the intrinsic chromophores B800 and B850 in LH2 was observed using femtosecond transient absorption spectroscopy. Detailed analysis revealed two possible energy transfer pathways. In addition, we made a LH2 mutant bearing reactive Cys residue on the N-terminal region of LH2β polypeptide. The engineered LH2 allows us to provide the defined position of Alexa647. Together with the system in which LH2 was dissolved in micellar solution, a lipid bilayer system in which the bioengineered LH2 was incorporated was applied to control the location of the external fluorophore.

Photoresponsive crystalline systems by microscopic molecular photoreactions to induce biomimetic functions are prepared using photochromic diarylethenes. Upon UV irradiation to crystals of open-ring isomer of diarylethene, needle-shaped crystals are formed on the surface by self-aggregation of the photogenerated closed-ring isomers. By using the photoinduced crystal growth technique, we prepared both superhydrophilic and superhydrophobic surfaces. Our technique is then successfully applied to prepare the double roughness surfaces by mimicking the structure of natural lotus leaf to show the bouncing ability of water droplet suggesting the importance of this double roughness structure. By a fine control of surface structures, we also prepared an object transportation system exploiting the bending behavior of surface-assembled diarylethene crystals. This system can transport polystyrene beads on a photo-actuated smart surface to a desired area by changing the light incidence irradiation directions. As further biomimetics, we prepared a hollow crystal showing photosalient phenomena. After packing small beads in the hollow, the crystal broke with scattering the included beads resembling the Impatiens.

Various types of photomechanical motion have drawn much attention because there is a potential to create photomechanical actuators from molecular-scale to macro-scale. To construct photoactuators, it is necessary to utilize a molecular assembly with a small free volume. Photochromic compounds undergo photoreversible isomerization between the original colorless isomer and the photogenerated colored isomer upon alternating irradiation with UV and visible light. Among many known photochromic compounds, diarylethenes undergo photochromic reactions even in the crystalline phase. The present review introduces recent development in the study of photomechanical crystals including crystal shape changes, bending velocity, dependence of the bending behavior on irradiation wavelength, the behavior in mixed crystal, new types of photomechanical motion, and applications. These photomechanical behaviors are based on geometrical structure changes in the crystalline phase and can be applied to macro-sized light-driven photoactuators.

Autonomous molecular-based microrobots have not been created despite progress in our understanding of the chemistry of molecular motors and machines. In the field of chemistry, the design and synthesis of molecular structures remain an ongoing and diverse challenge. The creation of a system in which molecular structures interact represents an additional challenge. In order to functionalize a molecule for a motor, it must be exposed to a specific reaction field. Synchronization of the molecules involved is also required. In this chapter, we discuss our research results on self-oscillatory flipping motions of azobenzene-containing assemblies. Briefly, reversible photoisomerization of an azobenzene derivative occurs under steady light irradiation. In coordination with phase transition events, a cycle modulated by the components involved leads to repetitive structural changes. When this azobenzene-based assembly is placed in water, the assembly exhibits autonomous swimming. This macroscopic light-powered motion is the result of the self-organization of a large number of nanometer-scale molecules and it provides an example of the potential for mechanical work to be performed by molecular assemblies. We subsequently describe the driving concept of far-from-equilibrium dynamics, namely a dissipative structure by which a spatial pattern maintains dynamic behavior to drive the mechanical motion observed.

In this chapter, the concept of the photoinduced solid–liquid phase transition and its wide area of possible application are introduced. Also, two macroscopic motions induced by the synergetic action of isomerization of azobenzene molecules are described: firstly, the crawling motion of crystals of azobenzene derivatives on a glass surface by light irradiation with two or one light sources. The motion of the crystals is continuous, and it can be generated by simple experimental setups. Secondly, the reversible bending motion of liquid crystalline polymer network consisting azobenzene monomer as a crosslinker. This polymer also changes its glass transition temperature reversibly upon light irradiations.

Crosslinked liquid-crystalline polymers (CLCPs) containing photochromic moieties show macroscopic deformation upon irradiation with light. This conversion of light energy into mechanical work is attributed to photosynergetic amplification of changes at multiscale: from nano to macro. This chapter focuses on recent progress of photomobile polymer materials to achieve versatile design as soft actuators. Three approaches are detailed: incorporation of amorphous polymers into CLCPs to control mechanical and photoresponsive properties, reshaping of CLCPs into various three-dimensional architectures through the rearrangement of network structures, and application of two-photon absorption processes to enhance the spatial selectivity of photoactuation.

A novel femtosecond pump-probe microspectroscope using a femtosecond Ti:Sapphire oscillator as a light source was developed. The fundamental specifications are the temporal resolution of 350 fs, the spatial resolution of 770 nm, and the tunable probe wavelength in the region from 500 to 900 nm. The setup has two modes of the probe light detection; one is back-scattered light detection and another conventional transmitted light detection. The back-scattering mode measurement demonstrated about 20-times higher gain of the transient signal of single nanoparticles compared to the conventional transmittance-mode one. This high-sensitivity enables the single nanoparticle ultrafast spectroscopy of organic nanomaterials which in general are low photo-durability. The femtosecond pump-probe microspectroscopy is fruitful in the researches of solid-state photochemical processes characterized by restricted molecular motions and intermolecular electronic interactions in the confined space. The potential applications of the developed microspectroscopic system are demonstrated for the picosecond excited state dynamics of nm- to μm-sized organic crystals.

We studied nanoscale photosynergetic response of molecular complexes and related inorganic and hybrid materials using single-molecule detection and spectroscopy. As examples, we report simultaneous electroluminescence and photoluminescence study of single CsPbBr3 perovskite nanocrystals which revealed the origin of blinking and of reduced quantum efficiency of electroluminescence devices. In addition, we observed linear and quadratic Stark shift on individual nanocrystals in different matrices. For novel I-III-IV semiconductor quantum dots, defect emission was found to originate from multiple sites in one particle, and the phenomenon of blinking was suppressed by multi-color excitation. In hybrid metal nanoparticle-organic dye systems, selective excitation of a localized plasmon in single gold nanorods by the polarization of light was found to enhance Förster energy transfer efficiency by two orders of magnitude. Localized plasmon was also found to cause previously unknown phenomenon of enhancement of triplet Dexter transfer in photon upconversion materials. Nanoscale study of triplet exciton diffusion in molecular solids by visualization of upconversion emission helped to uncover heterogeneity and the role of molecular orientation in the diffusion process.

Crystal formation in solution starts from nucleation in the saturated state. The quality of the product and the number, size, and structure of crystals are determined in the initial stage of the nucleation process. Understanding nucleation is the key to control crystal properties including polymorph formation. In this chapter, the fluorescence visualization of the crystal formation process probed by organic fluorescent molecules exhibiting cooperative and hierarchal photoresponses is summarized. Fluorescence observations of evaporative crystallization revealed a two-step nucleation model for both nuclei and polymorph formation. A Dibenzoylmethanatoboron difluoride complex exhibiting mechanofluorochromism and a dipyrrolyldiketone difluoroboron complex derivative displaying polymorphism-dependent fluorescence were studied. The fluorescence from the droplets showed dramatic changes depending on the molecular state, such as monomer, amorphous, and crystal polymorph. This method could be easily used to detect assembly processes by measuring the real-time fluorescence changes under ambient conditions. Furthermore, the formation of polymorphs is most likely affected by the cluster structure prior to nucleation. Therefore, insights into the nuclei precursor clusters in polymorph formation measured by fluorescence changes will enable us to predict the outcome of polymorph formation.

Charge-transfer complex exhibits versatile characteristics such as optical properties, conductivity, and magnetism. The physicochemical properties can be controlled through molecular design combining various kinds of electron donors and acceptors. In particular, the degree of charge-transfer γ and the composition ratio of electron donor to acceptor are important factors to determine physicochemical properties. On the other hand, nanocrystallized charge-transfer complex would lead to unique properties, being different from both isolated molecule and bulk crystal. However, the reprecipitation method for common organic nanocrystals cannot be employed due to low solubility of charge-transfer complex in common organic solvents. Therefore, novel nanocrystallization method for charge-transfer complex should be developed. In this chapter, nanocrystallization involving doping process of charge-transfer complex, copper 7,7,8,8-tetracyanoquinodimethane (Cu-TCNQ), will be introduced in detail towards nanoelectronics application. Cu-TCNQ is typical Mott insulator and indicates a unique resistive switching behavior. It is expected that Cu-TCNQ nanocrystals with an excess amount of Cu, namely doped Cu-TCNQ nanocrystals, would show unique resistive switching behavior because of the degree of charge-transfer γ different from bulk crystal.

Sulfone derivatives of 1,2-bis(2-alkyl-6-aryl-1-benzothiophen-3-yl)perfluorocyclopentene have been developed as a new type of turn-on mode photoswitchable fluorescent molecules for super-resolution fluorescence microscopy. Upon irradiation with ultraviolet (UV) light, the open-ring isomers undergo cyclization reactions to produce the closed-ring isomers, which emit brilliant fluorescence with high quantum yields (Φf ~ 0.9). Upon irradiation with visible light, the closed-ring isomers revert back to the initial open-ring isomers and the fluorescence vanishes. The cycloreversion quantum yields of the fluorescent diarylethenes were tuned by appropriate chemical modifications to fulfill the requirements for the different types of super-resolution imaging techniques. A turn-on mode fluorescent diarylethene that shows solvatochromism of fluorescence in the closed-ring form is potentially applicable to multicolor super-resolution imaging of microscopic environmental polarity. Unprecedented reversible photoswitching of fluorescent diarylethene molecules upon irradiation with single-wavelength visible light was found and has been successfully applied to super-resolution fluorescence imaging.

The photoisomerization between an open-ring isomer (1o) and a closed-ring one (1c) in diarylethene 1 film and the needle-shaped microcrystal growth subsequent to photoisomerization were in situ measured with an optical microscope. The 1o film was exposed to UV light on the stage of an upright-inverted microscope and optical and spectroscopic images were in situ observed in order to clarify the mechanism of microcrystal growth in the film. 1o films were prepared on a glass slide and an aluminum (Al) plasmonic chip, which is the Al-coated substrate with a wavelength-size periodic pattern and can provide the enhanced electric field to the chip surface. A plasmonic chip was expected to promote a photoisomerization and a microcrystal growth. Crystal growth of needle-shaped crystal of 1c was observed at the center of a film under the UV irradiation from upright side, but not observed under irradiation from inverted side. On the other hand, crystal growth was found at the film edge by the UV exposure even from the inverted side. Therefore, a high mobility of 1c molecules near the film surface or edge is essential for crystal growth of 1c. Further, alignment of 1c molecules also requires the platform of 1o. The conversion rate from 1o to 1c was larger on the Al grating. By the plasmonic enhanced electric field, when the attenuated UV light was exposed to the film edge from inverted side, the needle-shaped crystals were observed only inside the grating at the conversion rate above 60%. Conversion rate to 1c controlled crystal growth and therefore, crystal growth was promoted inside the grating. In summary, the conversion rate to 1c above 60%, a mobility of 1c near the film surface or edge, and the 1o underlayer platform are required for crystal growth after 1c alignment.