Photochromism of Diarylethene Molecules and Crystals: Memories

Review pubs.acs.org/CR

Photochromism of Diarylethene Molecules and Crystals: Memories, Switches, and Actuators Masahiro Irie,*,† Tuyoshi Fukaminato,‡ Kenji Matsuda,§ and Seiya Kobatake∥ †

Research Center for Smart Molecules, Rikkyo University, Nishi-Ikebukuro 3-34-1, Toshima-ku, Tokyo 171-8501, Japan Research Institute for Electronic Science, Hokkaido University, N20, W10, Kita-ku, Sapporo 001-0020, Japan § Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Department of Applied Chemistry, Graduate School of Engineering, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan ‡

5.4. Chemical Reactivity and Bioactivity 5.5. Multichromophoric and Supramolecular Systems 5.6. Liquid Crystals 5.7. Metal Nanoparticles 5.8. Other Properties 6. Photochromic Polymers 6.1. Polymers Having Diarylethenes in the Main Chain 6.2. Polymers Having Diarylethenes in the Side Chain 7. Crystalline Photochromism 7.1. Dichroism 7.2. X-ray Crystallographic Analysis 7.3. Quantum Yield 7.3.1. Cyclization 7.3.2. Cycloreversion 7.3.3. Temperature Dependence 7.4. Multicolored Systems and Supramolecular Structures 7.4.1. Multicolored Crystals 7.4.2. Selective Photoreaction with Linearly Polarized Light 7.4.3. Supramolecular Architectures 7.4.4. Nano-Layered and Nano-Mosaic Periodic Structures 7.5. Stereoselective Photoreaction 7.6. Inorganic Crystals 7.7. Nanocrystals 8. Surface Properties 8.1. Surface Wettability 8.2. Selective Metal Deposition 8.3. Subwavelength Nanopatterning 9. Photomechanical Response 10. Diarylethene Relatives 11. Conclusion Author Information Corresponding Author Notes Biographies Acknowledgments

CONTENTS 1. Introduction 2. Reaction Mechanism 2.1. Theoretical Study 2.2. Reaction Dynamics 2.2.1. Cyclization Reactions 2.2.2. Cycloreversion Reactions 2.2.3. Triplet Mechanism 3. Properties 3.1. Quantum Yield 3.1.1. Photocyclization Quantum Yield 3.1.2. Solvent Effect in Photocyclization Quantum Yield 3.1.3. Photocycloreversion Quantum Yield 3.2. Thermal Stability 3.3. Fatigue-Resistant Property 3.4. Chiral Control 3.5. Electrochemistry 3.6. Gated Reactivity 3.7. Fluorescent Property 3.7.1. Fluorescent Diarylethenes 3.7.2. Fluorescent Diarylethenes Having a Fluorescent Unit 4. Memory 4.1. Infrared (IR) Absorption, Raman Scattering, and Refractive Index Readout 4.2. Three-Dimensional Optical Memory 4.3. Near-Field Optical Memory 4.4. Holographic Optical Memory 4.5. Multiwavelength Recording 4.6. Single-Molecule Memory 4.7. Logic Circuit 5. Switches 5.1. Optical Properties 5.2. Magnetism 5.3. Electric Conductance © 2014 American Chemical Society

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Received: May 19, 2014 Published: December 16, 2014 12174

dx.doi.org/10.1021/cr500249p | Chem. Rev. 2014, 114, 12174−12277

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molecules that undergo the thermally irreversible photochromic reaction are limited within a few families of compounds. The thermal irreversibility is an essential and indispensable property for the use of photochromic molecules in optical memories, switches, and actuators (or molecular machines). Chemical bond rearrangement during the phototransformation induces electronic as well as geometrical structure changes of the molecules. The molecular structure changes can be applied to various photonic devices. The electronic structure changes can be applied to optical memory media and various photoswitching devices. On the other hand, the geometrical structure changes can be applied to light-driven actuators and others. For such applications, photochromic molecules should fulfill the following requirements: (1) thermal stability of both isomers, (2) fatigueresistant property, (3) high sensitivity, (4) rapid response, and (5) reactivity in solid state. A new class of photochromic molecules, which fulfills the above requirements simultaneously, is a “diarylethene” family. The diarylethenes are derivatives of stilbene. When phenyl rings of stilbene are replaced with five-membered heterocyclic rings with low aromatic stabilization energy, such as thiophene or furan rings, both open- and closed-ring isomers become thermally stable and coloration/decoloration cycles can be repeated many times. The best performance of the diarylethenes is summarized as follows. (1) Both isomers are thermally stable: well-designed derivatives have a half-life time at room temperature longer than 400 000 years (section 3.2). (2) Coloration/decoloration cycles can be repeated more than 10 000 times (section 3.3). (3) The quantum yield of coloration is close to 1 (100%). (4) Both coloration and decoloration reactions take place in a picosecond time region. (5) Many diarylethenes undergo photochromic reactions in the single crystalline phase. The photochromic diarylethene was serendipitously discovered during the course of study on photoresponsive polymers a quarter of a century ago.7,8 Various types of polymers having photoisomerizable chromophores, such as spiropyran, azobenzene, or stilbene, in the side or main chains, have been prepared in an attempt to change their conformation by photoirradiation.9 A polymer having stilbene units in the main chain can be prepared by radical 1,4-polymerization of 2,3-diphenylbutadiene.10 Although the polymer changed the conformation in the absence of oxygen, it converted to poly-9,10-dimethylenephenanthrene by hydrogen elimination in the presence of oxygen. To avoid the phenanthrene formation, 2,3-dimesitylbutadiene

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1. INTRODUCTION Photochromism is defined as a reversible transformation of a chemical species between two isomers having different absorption spectra induced in one or both directions by photoirradiation.1−5 Typical examples of photochromic molecules are shown in Scheme 1. Upon irradiation with ultraviolet Scheme 1. Typical Examples of Photochromic Molecules

(UV) light, the upper two molecules, azobenzene 1 and spiropyran 2, convert from left-side isomers to right-side ones and change color from pale yellow and colorless to orange and blue, respectively. The photogenerated right-side isomers are thermally unstable, and the colors disappear in the dark at room temperature. These traditional molecules are classified into Ttype (thermally reversible) photochromic molecules.6 On the other hand, more recently invented lower two molecules, furylfulgide 3 and diarylethene 4, both of which change color from colorless to red, are classified into P-type (thermally irreversible, but photochemically reversible) photochromic ones. In these P-type photochromic molecules, the photogenerated right-side isomers are thermally stable and hardly return to the left-side isomers in the dark at room temperature. Although vast numbers of photochromic molecules have been so far reported, Scheme 2. Synthesis of a Polymer Having Diarylethene Units

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was designed, and its synthesis by the reduction of 2,4,6trimethylacetophenone was examined. Yet, the synthesis failed because of the bulky size of the mesityl group. To reduce the steric hindrance, the mesitylene was replaced with 2,5dimethylthiophene. Next, 2,3-di(2,5-dimethyl-3-thienyl)butadiene (7) was successfully synthesized from 2,5-dimethyl3-acetylthiophene (5) and polymerized to poly(2,3-di(2,5dimethyl-3-thienyl)butadiene) (8) according to 1,4-addition radical polymerization, as shown in Scheme 2. The polymer was unexpectedly found to undergo a thermally irreversible photochromic reaction. The yellow closed-ring isomer generated upon UV irradiation was stable even at 100 °C and returned to the initial colorless open-ring isomer upon irradiation with visible light. The amazing result led us to study the photochemistry of the monomer unit, 2,3-di(2,5-dimethyl-3-thienyl)butene, and its derivatives in detail. This is the course of serendipitous discovery of photochromic diarylethene.11 The first paper on diarylethenes was published in 1988,7 and a comprehensive review entitled “Diarylethenes for Memories and Switches” appeared in 2000.12 Although several review papers,13−22 which describe diarylethene photochromism in general or focus on special topics, have been published, they are not able to cover vast numbers of research on diarylethenes and their derivatives from fundamental physicochemical aspects to various applications in optical memories, switches, and molecular machines. In this Review, development of the studies on diarylethene molecules and crystals over the past 10 years is described. Synthesis of this family of compounds is not described here, because just recently a review paper22 on the synthesis was published. A typical example of electronic and geometrical structure changes of a diarylethene is shown in Figure 1. The chemical structures of the open- and closed-ring isomers of 1,2-bis(2,5dimethyl-3-thienyl)perfluorocyclopentene (4) and the absorption spectral change are shown in Figure 1a and b, respectively. In the left-side open-ring isomer 4a, π-conjugation is localized in each thiophene ring, and the spectrum is similar to a substituted thiophene. On the other hand, in the right-side closed-ring isomer 4b, the π-conjugation delocalizes throughout the molecule, and the HOMO−LUMO gap becomes small. As a result, the spectrum shifts to a longer wavelength. The red color of the closed-ring isomer is ascribed to the delocalization of the πconjugation. At the same time, the geometrical structure of the molecule changes. As can be seen from Figure 1c (top-view), the height of the triangle shape (blue broken line) increases from 0.49 to 0.56 nm and the base width decreases from 1.01 to 0.90 nm. The side view indicates that the thickness of the molecule is reduced when the molecule isomerizes from the open-ring isomer to the closed-ring one. The diarylethene molecule undergoes a small but distinct anisotropic shape change upon photoisomerization. As described above, photoinduced color change is the most pronounced phenomenon observed in photochromic molecules. Figure 2 shows the photos of color changes of diarylethenes in solution as well as in the single crystalline phase. When toluene solutions of the derivatives are irradiated with UV light, the colorless solutions turn yellow, orange, red, violet, blue, cyan, and green. The colorless single crystals of diarylethenes also turn yellow, red, blue, and green upon irradiation with UV light. The chemical structures are shown below the photos. The color change is ascribed to the electronic structure change of the diarylethenes from the open- to the closed-ring isomers. The

Figure 1. (a) Chemical structures of the open- and closed-ring isomers of 1,2-bis(2,5-dimethyl-3-thienyl)perfluorocyclopentene (4), (b) absorption spectra of the open- (black line) and the closed-ring (red line) isomers, and (c) top- and side-views of the geometrical structures of the isomers in crystals. The two isomers were isolated and independently recrystallized.

color is controlled by the length of π-conjugation. When thiophene rings are attached to the ethene moiety at 2-position, π-conjugation of the closed-ring isomers is localized in the central part. The short π-conjugation results in yellow color. On the other hand, π-conjugation delocalizes throughout the molecule in the closed-ring isomer when thiophene rings are attached to the ethene moiety at 3-position. The π-conjugation further extends when phenyl groups are substituted at 5- and 5′positions of the thiophene rings. The long π-conjugation shifts the absorption band of the closed-ring isomers to longer wavelengths, resulting in violet and blue colors. The colors disappear by irradiation with visible light. The photoinduced coloration/decoloration cycles can be repeated many times without decomposition of the molecules. In the following, detailed studies on diarylethene photochromism are described.

2. REACTION MECHANISM 2.1. Theoretical Study

Photoinduced cyclization and cycloreversion reactions between 1,3,5-hexatriene (HT) and cyclohexadiene (CHD) are a textbook example for electrocyclic reactions.23 The photochromism of diarylethenes belongs to the reversible electrocyclic reactions of the central 6π-electron systems. The well-studied HT/CHD pair provides a useful framework for describing the basic reaction mechanism of diarylethenes.24,25 The photochemical reaction processes are explained by assuming reaction paths through conical intersections (CIs).26 12176


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Figure 2. Color changes of diarylethene derivatives (a) in toluene solutions and (b) in the single crystalline phase upon photoirradiation.

on the potential surfaces. Among the conical intersections CI3 is the most important, because it is the only one that provides a pathway toward both open- and closed-ring minima on the ground state. CI3 has weakly coupled three electrons and an electron belonging to an allyl fragment. To understand the reaction mechanism, a complete active space self-consistent field (CASSCF) study of the potential energy surface topology and a molecular mechanics-valence bond (MMVB) computation of the dynamics on the model

Figure 3 shows S0 and S1 potential energy surfaces of a model diarylethene, 1,2-bis(cyclopenta-1,3-dien-2-yl)ethene, along the reaction coordinate q. On both S0 and S1 surfaces there exist two minima, which are shown as CHD and CHD* (cyclohexadienetype structures of the model diarylethene) for the closed-ring isomer and HT and HT* (hexatriene-type structure of the model diarylethene) for the open-ring isomer. Transition structures (TS0 and TS1) are also characterized on each potential surface. Several conical intersection minima (indicated by crosses) locate 12177


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Figure 5. Outline of the two reaction paths corresponding to ringopening and ring-closing reactions of diarylethenes. The letters in parentheses, c and o, represent the closed- and open-ring structures, respectively. Figure 3. Potential energy surfaces of a model diarylethene molecule. Reprinted with permission from ref 26. Copyright 2003 American Chemical Society.

(2A/1ACI(o), CI3). At the conical intersection, efficient decay to the ground-state potential energy surface takes place, leading subsequently to the closed-ring minimum. This mechanism accounts for the highly effective and ultrafast ring-closing reaction. On the other hand, the ring-opening reaction encounters the potential energy barrier 2ATS on the 2A surface before reaching the same relaxation funnel that is involved in the ring-closing reaction. At the conical intersection fast deactivation to the ground state occurs and the open-ring isomer is formed. The existence of the potential energy barrier along the pathway accounts for the temperature as well as the photoirradiation wavelength dependence of the cycloreversion quantum yield. Nakamura and co-workers27−30 also investigated the reaction mechanism in detail, especially the ring-opening reaction of 1,2bis(3-thienyl)ethene (9), based on theoretical calculations of the ground- and excited-state potential energy surfaces and conical intersections. Figure 6 shows the potential energy surfaces of the ground (1A) and two lower excited (2A and 1B) states as a function of the distance between the two reactive carbon atoms R(C−C) and the locations of the conical intersections.

diarylethene were carried out.26 A three-dimensional schematic representation of S0 and S1 potential energy surfaces of the diarylethene is shown in Figure 4. The figure derives from

Figure 4. Schematic representation of the structure of the S0 and S1 potential energy surfaces corresponding to ring-closing and ringopening reactions of the model diarylethene. Reprinted with permission from ref 26. Copyright 2003 American Chemical Society.

CASSCF calculations of stationary points and the MMVB dynamic simulation. In the dynamics simulation started from HT*, about 50% of the trajectories decay at geometries near CI3, before evolving to the open- or closed-ring forms on the ground state. Similarly, trajectories started from TS1 also decay in the region CI3. Thus, CI3 is dynamically accessible from both sides of the S1 surface. The dynamics study supports the assumption that this crossing seam CI3 is involved in both the ring-closing and the ring-opening processes. Figure 5, which is a two-dimensional simplification of the potential surfaces,26−31 outlines the reaction paths corresponding to ring-closing and ring-opening reactions of the diarylethene. Upon irradiation with UV light the open-ring isomer is excited to the allowed Franck−Condon state (1BFC(o)), which is close in energy to a 2A surface, and fast internal conversion to the 2A state takes place. The excited-state wavepacket moves down along the 2A surface and reaches the conical intersection

Figure 6. Potential energy surfaces of the ground (1A) and two lower excited (2A and 1B) states of compound 9 as a function of the distance between the two reactive carbon atoms. Reprinted with permission from ref 27. Copyright 2004 American Chemical Society. 12178


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Scheme 3 and Figures 5 and 6 outline the detailed route of the cycloreversion reaction of model diarylethene 9. Upon

Scheme 4. Diarylethene Open-Ring Isomer Has Two Conformations with Two Aryl Groups in Mirror Symmetry and C2 Symmetry, Which Are Called Parallel and Antiparallel Conformations

Scheme 3. Outline of the Cycloreversion Reaction Mechanism of Compound 927

irradiation with visible light, the closed-ring isomer is excited to the allowed 1B Franck−Condon state (1BFC(c)). The excited wavepacket moves away from the Franck−Condon region along the 1B surface and falls from the 1B to 2A surface via a conical intersection (1B/2A CI (c)) in ∼100 fs.32 The molecules excited with light of shorter wavelengths having vibrational excess energy on the 2A surface go over the energy barrier 2ATS more easily and reach 2A/1A conical intersection (2A/1A CI(o)), where fast deactivation to the ground state takes place and thus the openring isomers are formed. The above route suggests that thermal activation or excess kinetic energy on the 2A surface is required for the cycloreversion reaction to take place. This agrees with experimental observations that the cycloreversion reactions have activation energies and the cycloreversion quantum yield increases upon irradiation with light of shorter wavelengths, as described in sections 2.2.2, 3.1.3, and 3.6. Theoretical calculations of the ground- and excited-state energy surfaces were also carried out to interpret the reaction dynamics of the cyclization and cycloreversion reactions of 1,2bis(2-methyl-5-formyl-3-thienyl)perfluorocyclopentene (10)33,34 and 1,2-bis(2-methyl-5-anthryl-3-thienyl)perfluorocyclopentene (11).35 2.2. Reaction Dynamics

2.2.1. Cyclization Reactions. One of the characteristic features of diarylethenes is that they undergo very fast photoisomerization reactions, as discussed in the above theoretical analysis. To reveal the ultrafast dynamics of these systems associated with both the cyclization and the cycloreversion reactions, a large number of time-resolved spectroscopic studies have been carried out.33−60 The cyclization reaction of 1,2-bis(2,4-dimethyl-5-phenyl-3thienyl)perfluorocyclopentene (12) was investigated in the single crystalline phase with femtosecond absorption spectroscopy and ultrafast electron diffraction crystallography.36 In solution the diarylethene open-ring isomer has two conformations, photoactive antiparallel and photoinactive parallel conformations, as shown in Scheme 4. Coexistence of the two conformers prevents clear resolution of the spectral feature associated with the photocyclization reaction. In the crystalline phase, on the other hand, all molecules are fixed in an antiparallel conformation, and this simplifies the spectroscopic analysis of the reaction dynamics. Figure 7 shows the transient differential absorption spectra corresponding to the cyclization reaction in the single crystalline diarylethene upon irradiation with a femtosecond 343 nm laser

Figure 7. Transient absorption measurements of the ring-closing reaction in the single crystal of compound 12 by pumping at 343 nm (fluence = 0.3 mJ cm−2). Selected spectra for various time delays between −200 fs and +50 ps (black curves), as compared to their associated τ∞ measurements (blue curves). The 100 fs spectrum was extracted from a separated data set taken with higher resolution time steps. Reprinted with permission from ref 36a. Copyright 2011 American Chemical Society.

pulse. The formation of the spectra associated with the ringclosing reaction is clearly observed. After approximately 20 ps, the transient spectrum is the same as the final product. The subpicosecond dynamics following the photoexcitation indicates that the initially broad absorption seen at 75 fs progressively shifts from the red (635 nm) to the blue (sub-480 nm) end of the visible spectrum in the first 500 fs. The progressive spectral shift is attributed to the evolution on the excited-state potential of the open-ring isomer. The absorption 12179


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and lattice relaxation to give the spectra corresponding to the ground-state closed-ring isomer. Therefore, the decay of the excited-state absorption of the open-ring isomer is assigned to the time constant of the ring-closing reaction, that is, 5.3 ± 0.3 ps. Using femtosecond electron diffraction, the formation of the closed-ring photoproduct with a time constant of approximately 5 ps was directly observed, in addition to resolving subpicosecond structure changes associated with the formation of the reaction intermediate.36b,c The key rotational motion of the thiophene rings was analyzed on the basis of theoretical calculations. The switching behavior of 1,2-bis(2-methyl-5-phenyl-3thienyl)perfluorocyclopentene (13) and its nonfluorinated (perhydro) analogue (14) was compared using femtosecond laser spectroscopy and theoretical calculations.37,38 The dynamics after photoexcitation can be expressed in three stages: (1) preswitching due to excited-state mixing and relaxation, (2) the ring-closure, and (3) post switching related to vibrational cooling, as described above in the reaction of the single crystal of compound 12. In all stages, the fluorinated version was found to switch faster than its nonfluorinated analog. The preswitching mixing and relaxation processes of 14, the time constants of which are 70 ± 15 and 150 ± 30 fs, respectively, are accelerated to the time constants of 50 ± 10 and 120 ± 30 fs in 13. Fluorination also speeds up the rate of the cyclization reaction from 4.2 to 0.9 ps. The analysis of the dynamical processes revealed that the fluorinated switch is faster and more efficient than the nonfluorinated switch. The cyclization dynamics of 1,2-bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene (15) has also been studied in detail in solution.39−41 Figure 9 shows the time profiles of the transient absorbance in n-hexane solution excited with a femtosecond 310 nm laser pulse. The signal at 520 nm increases

band initially at 635 nm is assigned to the excited-state absorption of the open-ring molecules in the ground-state equilibrium geometry, while the absorption feature in the blue end of the spectrum range is assigned to the relaxed structure of the open-ring isomer in the excited state. The relaxation process takes place with a time constant of 200 fs. Figure 8 shows the convergence on the picosecond time scale of the transient absorption toward that of the closed-ring

Figure 8. (a) Transient absorption spectra of the closed-ring isomer in crystal 12 for time delays between 0.5 and 50 ps. (b) Transient traces for the probe wavelengths at 635 and 490 nm. Monoexponential fits are shown, with time constants of 7.3 and 5.3 ps for 635 and 490 nm, respectively. The inset demonstrates the subpicosecond behavior. Reprinted with permission from ref 36a. Copyright 2011 American Chemical Society.

photoproduct. Initially at 0.5 ps after the pulse, the excited-state absorption associated with the open-ring isomer dominates the signal. Following relaxation in the excited-state potential of the open-ring structure, the spectrum decays in concert with the growth of the ground-state absorption associated with the closedring isomer centered at 635 nm. The time constants for the rise at 635 nm due to the closed-ring isomer and the decay at 490 nm due to the open-ring isomer are 7.3 ± 0.8 and 5.3 ± 0.3 ps, respectively. It was anticipated that the rise and decay of the signals would be equally matched, as the closed-ring isomer is directly formed from the excited state of the open-ring isomer through the conical intersection, as shown in Figure 5. The discrepancy is attributed to the vibrational relaxation of the nascent closed-ring isomer. The growth of the signal at 635 nm is the convolution of the cyclization reaction, vibrational cooling,

Figure 9. Time profiles of transient absorbance of compound 15 in nhexane solution, excited with a femtosecond 310 nm laser pulse. The detection wavelength is 520 nm for (a) and (d), 420 nm for (b) and (e), and 620 nm for (c) and (f), respectively. Solid lines in each of the frames are calculated curves by taking into account the pulse duration and the time constants. The slow components in the time profiles (d), (e), and (f) show the dynamics of nonreactive parallel conformers. Reprinted with permission from ref 39. Copyright 2011 American Chemical Society. 12180


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in a few picoseconds time region. The solid line is the result calculated with a time constant of 450 fs. On the other hand, the time profiles monitored at 420 and 620 nm show decay with the time constant of 480 and 420 fs, respectively. The rise and decay profiles indicate that the excited state of the open-ring isomer having a broad absorption from 420 to 620 nm with a maximum around 500 nm converts to the closed-ring isomer with the time constant of 450 fs. The cyclization dynamics of poly(1,2-bis(2-methyl-3-thienyl)perfluorocyclopentene) (16) was investigated in chloroform.42 The polymer exhibits a very high cyclization quantum yield of 0.86, which indicates a very high contribution of an antiparallel conformation in the open-ring form in the polymer. Ultrafast transient absorption spectra were measured in the range between 430 and 760 nm with a femtosecond laser pulse (150 fs, 260 nm). The transmittance change at 620 nm, where the closed-ring isomer has absorption, was monitored as a function of probe delay. The very rapid decrease of the transmittance after the pulse indicates that the ring-closing reaction takes place on a sub-200 fs time scale. These transient spectroscopic studies in solution indicate that the cyclization reaction takes place in less than 1 ps. Although in the crystalline phase the cyclization time constant is slowed to several picoseconds, it is safe to say that the central carbon− carbon bond is made within 10 ps. The cyclization reaction of compound 4 has been studied with picosecond time-resolved Stokes and anti-Stokes Raman spectroscopies.43,44 The transient Raman spectroscopy is a promising method for investigating structural changes of the molecules during the photochromic reaction. Figure 10 shows the picosecond time-resolved Stokes Raman spectra on excitation of the open-ring form in 1-butanol. The pump and probe wavelengths are 310 nm (4 ps) and 568 nm, respectively. The probe wavelength of 568 nm is in resonance with the absorption of the closed-ring form. The time-resolved curve rises within the experimental temporal resolution ( EtO > iPrO > cyclohexyloxy. The colored isomer of the diarylethene having cyclohexyloxy substituents can return to the colorless

Figure 18. Photoirradiation wavelength dependence of cycloreversion quantum yields of compounds 12 (blue ○) and 15 (red ○) in n-hexane at 22 °C. Reprinted with permission from ref 31. Copyright 2014 The Royal Society of Chemistry. 12189


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Table 6. Photocyclization and Photocycloreversion Quantum Yields of Diarylethenes in n-Hexane

open-ring isomer in less than 1 min at 160 °C. Photochemically stable but thermally unstable performance can be applied to rewritable papers. The substituents at the reactive carbons play an important role in the photocycloreversion and thermal cycloreversion reactions. Dithiazolylethenes also show similar reactivity. 1,2-Bis(5methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene 23 has a cycloreversion quantum yield of 1.7 × 10−2. Upon introduction of methoxy groups at the reactive carbons, the cycloreversion quantum yield was found to decrease to 3.3 × 10−4 by a factor of 102.75 Diarylethenes having S,S-dioxide benzothiophene or thiophene rings also exhibit marked low cycloreversion quantum yields.141,142 The cycloreversion process was studied in detail by theoretical calculations. Figure 19 shows the potential energy surfaces calculated by the state-specific CASSCF(10,10) method with 631G basis set.27−29 The R(C−C) is defined as the distance between the two reactive carbons. The cycloreversion reaction has to overcome an energy barrier to reach the branching conical intersection. When the energy difference ΔE (=E(2Ao) − E(2Ac)) is positive, the efficiency to reach the conical intersection decreases, while it increases when ΔE is negative. The energy difference between the 2Ac and 2Ao states controls the cycloreversion reactivity. A fairly good correlation was observed in the relationship between the experimental quantum yields and the energy differences on the 2A potential surface, as shown in Figure 20.

As can be seen from Figures 5 and 19, the photocyclization process has no energy barrier, while the photocycloreversion process has an energy barrier. Temperature dependence of the photocyclization and the photocycloreversion quantum yields of compound 13 was examined in 3-methylpentane.72 The photocyclization quantum yield was independent of temperature. However, appreciable temperature dependence was observed in the photocycloreversion reaction. The activation energy was as much as 16 kJ mol−1. The activation energy was found to depend on the type of the ethene part.143 3.2. Thermal Stability

Thermal stability of the colored closed-ring isomers is one of the essential advantages of diarylethene photochromic performance. Most of the photogenerated colored isomers of diarylethenes with heterocyclic aryl groups such as thiophene, benzothiophene, or furan rings remain stable even at 100 °C. Among them, compound 23 has extraordinarily high thermal stability of the closed-ring isomer.144 The half-life time at 150 °C is 400 h. The temperature dependence of the thermal cycloreversion reaction rate shows a linear relationship between ln k and T−1. The activation energy was determined to be 142 kJ mol−1 for the thiazolyl derivative. Extrapolation of the temperature dependence suggests that the half-life time of the colored closed-ring isomer is 4.7 × 105 years at 30 °C. The large activation energy practically prohibits the thermal cycloreversion reaction at ambient temperature. 12190


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perfluorocyclopentene, allowed fast thermal cycloreversion reaction, and the half-life time of 5.4 s was observed at 20 °C.147 According to the Woodward−Hoffmann rules based on πorbital symmetries for 1,3,5-hexatriene,23 the thermal cycloreversion reaction of cyclohexadiene derivatives should proceed in a disrotatory mode. To reveal the cycloreversion mechanism in detail, thermal cycloreversion reaction was followed by in situ Xray crystallographic analysis of the closed-ring isomer crystal of compound 75 (see section 7.2). The structure of the generated open-ring isomer indicates that the thermal cycloreversion reaction proceeded in a conrotatory mode.134 The conrotatory thermal cycloreversion reaction explains the thermal stability of the closed-ring isomers when the aryl groups have low aromatic stabilization energies.8 Both photocyclization/photocycloreversion reactions of diarylethenes proceed in the conrotatory mode.148,149 The thermal stability of the colored closed-ring isomers of diarylethenes depends on the aryl groups.8 There are at least three factors, which control the thermal stability: (i) aromatic stabilization energy of the aryl groups, (ii) electron-withdrawing substituents at the aryl groups, and (iii) steric hindrance of the substituents at the reactive carbons as shown in Scheme 7. When the aryl groups are thiophene, benzothiophene, or furan heterocycles, the closed-ring isomers are thermally stable, while closed-ring isomers having phenyl, pyrrolyl, or indolyl rings are thermally unstable. This is ascribed to the increase in the energy level of the closed-ring isomers due to the loss of the aromatic stabilization energy.8 On the other hand, when electronwithdrawing substituents are introduced at the aryl groups, the closed-ring isomers become thermally unstable because the central carbon−carbon bonds in the closed-ring isomers are weakened by the electron-withdrawing substituents.150 The bulky substituents at the reacting positions lead to thermal cycloreversion reactions of the closed-ring isomers.145 The theoretically calculated bond length of the central carbon− carbon bond was found to correlate to the thermal stability.151 The thermal cycloreversion reactivity is also dependent on the corrected Taft’s steric substituent constants (Esc) of the substituents at the reacting carbons.152,153 This means that neither the hyperconjugation of the α-hydrogens in the substituent nor the polarity of the substituent affect the thermal cycloreversion reactivity.154 In all cases, the difference in enthalpy between the open- and closed-ring isomers in the ground state controls the thermal stability of the closed-ring isomers.155 It is worth noting that the substituent effect of the thermal stability in the closed-ring isomers is not directly correlated to that of the photocycloreversion quantum yield, because the potential energy surface of the ground state is different from those of the excited states, which govern the photocycloreversion quantum yields.

Figure 19. Potential energy profile of the model system at the ground (1A) and three excited (2A, 1B, and 3A) states as a function of the distance between the two reactive carbon atoms, R(C−C). The letters in parentheses, c and o, represent the closed- and open-ring structures, respectively. Reprinted with permission from ref 28. Copyright 2002 American Chemical Society.

Figure 20. Correlation between the cycloreversion quantum yields of diarylperfluorocyclopentenes and the calculated energy differences ΔE (=E(2Ao) − E(2Ac)). The calculation was carried out for the structures excluding perfluorocyclopentene as shown in the graph.29b

The closed-ring isomers of compounds 13 and 15 are thermally stable. The activation energy of the cycloreversion reaction of the former diarylethene was as large as 139 kJ mol−1.72 The thermal stability was, however, decreased by introducing bulky substituents, such as isopropyl groups, at the 2- and 2′positions of the thiophene or benzothiophene rings.135,145 The half-life times of the closed-ring isomers having the isopropyl substituents decreased to 91 and 580 days at 30 °C, respectively. The half-life times are much shorter than 1900 years estimated for the diarylethene having the methyl substituents.72 The activation energy can be estimated by calculating the transition energy barrier in the potential energy surface of the ground state using density functional theory (DFT) methods.146 Introduction of phenylethynyl groups to the reactive carbons of 4,5-dithienyl thiazole, which has a thiazole ring instead of the central

3.3. Fatigue-Resistant Property

Fatigue resistance is a necessary property for the practical use of photochromic molecules in various applications. Although enormous numbers of photochromic compounds have been reported, compounds that can repeat photoinduced coloration/ decoloration cycles more than 1000 times are limited. Some diarylethenes can undergo photoinduced cyclization/cycloreversion reactions of more than 14 000 cycles in solution and 30 000 cycles in the single crystalline phase.36,156 To improve the fatigue-resistant property, it is required to understand the fatigue mechanism. 12191


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Scheme 7. Thermal Stability of Diarylethene Closed-Ring Isomers154

π-conjugation system and bond-formation and cleavage finally produce the condensed-ring byproduct. Another possible process (route 2) is initiated by photochemical C−S bond cleavage. Radical migration and bicyclohexane formation lead to the condensed-ring byproduct. Similar C−S bond cleavage was reported for 2-methyl-2,3-dihydro-1-benzothiophene, which converts to 3,4-dihydro-2H-1-benzothiine by C−S bond cleavage.160 Although both routes can account for the formation of the byproduct, route 2 is more probable, because the byproduct formation is enhanced upon irradiation with UV light and not observed in the photocycloreversion process with visible light, which also goes through the conical intersection. Route 2 was also supported by theoretical calculations.161 The fatigueresistant compound 12, which has methyl substituents at 4- and 4′-positions of the thiophene rings, requires larger activation energy in the last step of route 2 in comparison with the energies of the less stable compounds 4 and 13. Compound 15 is one of the most fatigue-resistant diarylethenes. The fatigue-resistant property was further improved by introducing acetyl groups at 6- and 6′-positions. Ahn and coworkers162,163 evaluated the fatigue-resistant property by measuring the absorbance changes of several ethyl acetate solutions (1.0 × 10−5 M) containing bisbenzothiophenylperfluorocyclopentene derivatives upon continuous irradiation with UV light (312 nm), as shown in Table 7. Acetyl substitution dramatically increased the fatigue-resistant property of the bisbenzothiophenylperfluorocyclopentene and the sulfone derivatives. The acetyl groups are considered to suppress the photochemical C−S bond cleavage and the radical migration.

The fatigue mechanism of compound 4 was studied by isolating and analyzing photogenerated byproducts.157 Compound 4 is known to undergo photoinduced cyclization/ cycloreversion reactions in solution as well as in the single crystalline phase, as shown in the top of Scheme 8. Upon irradiation with 254 nm light, the colorless n-hexane solution of 4a turns red and a new absorption band appears at 503 nm. The band is ascribed to the closed-ring isomer. The red color completely disappears by irradiation with visible light (>400 nm). When the irradiation time with 254 nm light was prolonged, a photostable product, which cannot be bleached by irradiation with visible light, was produced. The absorption spectrum of the product is very similar to the closed-ring isomer. The stable byproduct could be isolated by high-performance liquid chromatography (HPLC) and was found by molecular mass determination to be isomeric with compound 4. The molecular absorption coefficient value (ε487 = 3.1 × 103 M−1 cm−1) in the visible region is 40% of 4b. Figure 21 shows the molecular structure of the byproduct 90 determined by X-ray crystallographic analysis. The structure is similar to the byproduct, which was produced in the case of compound 13.158 Irradiation time dependence of the byproduct formation was followed by HPLC. Although there existed some induction period in the formation of the byproduct when the n-hexane solution of 4a was irradiated with 254 nm light, the byproduct was immediately produced from 4b from the beginning of irradiation. The byproduct formation was not observed when the n-hexane solution of 4b was irradiated with 546 nm light. Compound 4 undergoes the photochromic reaction in the single crystalline phase. Formation of the byproduct was not observed in the single crystalline phase even when the crystal was irradiated with 254 nm light for a long time to reach the conversion up to 6%. The above results suggest that the byproduct is produced from 4b by irradiation with UV light. Scheme 8 shows two possible routes to produce the byproduct. Route 1 goes through the conical intersection, as shown in Figure 3. Photoexcited cyclohexadiene has a channel to produce a five-membered ring, methylenecyclopentene diradical.159 The radical migration in the

3.4. Chiral Control

Photochemical conrotatory cyclization reactions of diarylethenes produce two enantiomers of the closed-ring isomers with (R,R) and (S,S) configurations originating from the central two asymmetric carbon atoms, as shown in Scheme 9. Conrotatory cyclization reactions from P-helical (right-handed) and M-helical (left-handed) conformers of the open-ring isomers yield (R,R) and (S,S) enantiomers, respectively. In general, photocyclization reactions in solution result in the formation of two enantiomers 12192


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Scheme 8. Two Routes (1 and 2) of the Byproduct Formation

Figure 21. ORTEP drawing of byproduct 90 showing 50% probability displacement ellipsoids. Reprinted with permission from ref 157. Copyright 2000 The Chemical Society of Japan.

Table 7. Fatigue-Resistant Property of Diarylethenes upon Irradiation with Continuous UV Light162 diarylethene

X

R

byproduct formation time constant/min

14 91 92 93

S S SO2 SO2

H acetyl H acetyl

1300 ± 100 11 000 ± 1000 900 ± 50 3300 ± 200

Scheme 9. Photocyclization Reactions from P- and M-Helical Conformers of the Open-Ring Isomer Yield (R,R) and (S,S) Enantiomers of the Closed-Ring Isomer, Respectively in equal amounts. The enantiomers can be separated using chiral HPLC.164 One of the approaches to induce the diastereo differentiation in the photocyclization is to introduce an optically active substituent. The photochemical asymmetric induction was examined for the diarylmaleimide having a l- or d-menthyl group at the 2-position of benzo[b]thiophene ring (94 and 95) (Scheme 10).165 Although any asymmetric induction was not discerned in n-hexane, diastereoselective photocyclization takes place in slightly polar solvents, such as in toluene or hexane/THF mixture. In toluene at −40 °C, de (diastereomeric excess) was as high as 86.6%. The asymmetric induction in the photocyclization is ascribed to the potential energy difference between the P- and M-helical open-ring conformers in the excited state. One of the conformers is stabilized in slightly polar solvents and efficiently converts to the closed-ring isomers. Branda and co-workers166,167 employed chiral metal complex systems to induce the diastereoselective photocyclization reaction. Diarylethenes having chiral bis-oxazolines (96 and 97) form stereochemically pure copper(I) double-stranded helicates, as shown in Scheme 11. The stereoselective arrange-

ment of the four thiophene rings produces a single diastereomer upon UV irradiation. The stereoregulation was confirmed by 1H NMR spectroscopy, which shows signals corresponding to only one diastereomer. Stereoselectivity as high as 98% was observed in the case of 96. The photoswitching between the open- and closed-ring isomers by alternate irradiation with 313 nm and longer than 458 nm can modulate the optical rotation angle as large as 4−6° in dichloromethane (2.8 × 10−4 M) at 450 and 475 12193


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amorphous thin film at room temperature. The closed-ring isomers produced upon UV-irradiation in the amorphous state had 25% de. The result suggests that there exist microcrystals or short-distance regularity in the amorphous solid. Andréasson and co-workers171 reported binding of methylquinolium-appended diarylethenes to DNA and their photocyclization reactions. Significant enantioselectivity was observed for the cyclization reaction of the derivative intercalated to DNA. This enantiomeric enhancement demonstrates direct transfer of chirality from DNA to a photoproduct and is one of the rare examples of enantioselectivity in solution. Diastereo- or enantioselective photocyclization reactions are also observed in a crystalline phase. This subject will be described in section 7. Diastereo- and enantioselective photocyclization reactions described above are strongly dependent on environmental conditions (e.g., temperature, solvent, and medium). For the applications to memories and switches, it is required to develop asymmetric induction systems, which are not influenced by environmental conditions. Takeshita et al.172 prepared 1,2disubstituted [2.2]metacyclophan-1-enes (MCP-1-enes)173−176 as the enantiomeric photocyclization system (Scheme 14). Ring

Scheme 10. Chiral Diarylethenes 94 and 95165

Scheme 11. Formation of Closed-Ring Products 96b or 97b from the Open-Ring Isomers 94a or 95a and Their Complexes with Copper166

Scheme 14. Enantioselective Photochromic Reactions of [2.2]MCP-1-ene 100172

nm, where both isomers of the diarylethene are transparent. The change in the optical rotation angles can be potentially used for nondestructive readout in information storage. Feringa and co-workers168,169 reported a diastereoselective photocyclization reaction of diarylethene 98 (Scheme 12) in a gel Scheme 12. Chiral Diarylethene 98168

inversion of the phenyl groups in MCP-1-ene 100 is forbidden because of the steric hindrance between the inner methyl and the phenyl groups. MCP-1-ene 100 undergoes the enantioselective photocyclization reaction to produce tetrahydropyrene-100 (THP-100) upon UV irradiation. After several trials,177−179 optically active thiophenophan-1-ene 101 (Scheme 15) was prepared, which undergoes a diastereoselective photocyclization up to 100% de and hardly diastereomizes even at high temperature. Yokoyama et al.180,181 adopted allylic 1,3-strain as a tool182,183 to achieve highly diastereoselective photocyclizations of diaryl-

phase. Multiple hydrogen-bonding interactions between the amide groups produced gels of the open-ring isomers. The gel showed strong circular dichroism around 320 nm due to exciton coupling within the aggregates. Transmission electron microscopy showed the formation of chiral P-helical fibers. Photocyclization reactions in the chiral aggregates or in the microcrystals produced closed-ring isomers with a large de value (>96%). A diastereoselective photocyclization was also observed in a bulk amorphous state.170 Diarylethene 99 having chiral (S)-sec-butyl groups (Scheme 13) formed a stable

Scheme 15. Photochromism of Chiral Diarylethene 101178,179

Scheme 13. Chiral Diarylethene 99170

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103, as shown in Scheme 17b.184 Finally, they developed photochromic diarylethenes 104−106 (Scheme 18) that undergo a highly diastereoselective photocyclization reaction close to 100% de by introducing two chiral MOMO groups on both ends of the hexatriene moiety.185 The high selectivity is ascribed to both electronic as well as steric repulsive interactions between the two MOMO groups when the molecules take an antiparallel conformation. The de values and the conversion ratio to the closed-ring isomers are listed in Table 8.185,186 The system was

ethenes. The concept of allylic stain is shown in Scheme 16. The smallest substituent, such as hydrogen, faces the bulky Scheme 16. Concept of the Asymmetric Photocyclization Reaction Based on Allylic 1,3-Strain180,181

Table 8. Diastereoselectivity of Diarylethenes 104−106185,186 hexafluorocyclopentene that locates cis to the stereogenic carbon atom. Medium size M and largest L substituents hang down in the rather wide space. As a result, another benzothiophene would take the position close to M rather than close to L, resulting in a diastereomerically fixed helical conformation. On the basis of this concept, compound 102 (Scheme 17a) having a hydrogen atom,

compound 104 105 106

Scheme 17. Photochromism of Chiral Diarylethenes 102 and 103180,181,184

diastereomeric excess/% (conversion ratio to the closed-ring isomer/%) ethyl acetate n-hexane RT −70 °C 67 °C RT 98 (81) 100 (50) 94 (82) 98 (83) 99 (89) 96 (81) 95 (94) 89 (97) 98 (56) 92 (94)

further improved by replacing two MOMO groups with two (R)pentafluoropropanoyloxyethyl groups 107 (Scheme 18), in which diastereoselective photocyclization reaction with >99% de was observed at room temperature.187 Branda and co-workers188 prepared compound 108 (Scheme 19), which has chiral substituents in the benzothiophene aryl Scheme 19. Photochromism of Chiral Diarylethene 108188

groups. This molecule undergoes stereoselective photocyclization reaction. Helicenoid diarylethenes 109 and 110 also undergo highly diastereoselective photocyclizations, as shown in Scheme 20.189,190 Yokoyama and co-workers191 achieved ultimate diastereoselectivity (100% de) in the photocyclization reaction of a diarylethene by employing facial chirality. When one of the two surfaces of an aromatic ring (ring A) is occupied by a bulky attachment, the photocyclization can take place only when another aromatic ring (ring B) approaches from the back side of the ring A, as shown in Scheme 21. On the basis of this idea, a diarylethene 111a was designed and synthesized. This molecule takes only one antiparallel conformation and undergoes photocyclization to produce one diastereomer of the closedring isomer. In addition, no evidence of racemization of 111a was

a methyl group, and a methoxymethoxyl (MOMO) group on the stereogenic carbon atom of benzothiophene was prepared. When racemic 102a was irradiated with 313 nm light in n-hexane at room temperature, two diastereomeric 102b’s were formed. An HPLC analysis proved that de is 87%. High de values were observed in ethyl acetate and toluene (88% de). The crystals of enantiomerically pure 102a were obtained after visible light irradiation to one of the resolved 102b, and the structure was confirmed by X-ray crystallographic analysis. The de value was further improved to 94% by introducing a hydrogen atom, a methyl group, and a phenylcarbonate group, such as compound Scheme 18. Chiral Diarylethenes 104, 105, 106, and 107185−187

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These reports suggest that the electrochemical cyclization and cycloreversion reactions are common in 6π-electron systems. Branda and co-workers demonstrated that the electrochemical oxidation induces cycloreversion194 reaction of diarylethene 112 and cyclization195 reaction of diarylethene 113 (Scheme 22). They also reported the cyclization reaction of diarylethene 114 having N-methylpyridinium groups by electrochemical reduction.196 In these reactions, only one of the two reactions, cyclization or cycloreversion, can be triggered using electron transfer as the stimulus. Diarylethene 115, which undergoes reductive ring-closing and oxidative cycloreversion, was also developed.197 Hecht and co-workers198 synthesized dithiazolylmaleimide derivatives that isomerize with orthogonal stimuli. The cyclization reactions can only be accomplished electrochemically, while the ring-opening reactions can only be achieved photochemically. Figure 22 shows a possible mechanism of these reactions. The diarylethenes undergo oxidative cyclization reactions when the radical cations of the closed-ring isomers are more stable than the open-ring isomers and also that the diarylethenes undergo oxidative cycloreversion reactions when the radical cations of the open-ring isomers are more stable than the closed-ring isomers. Monoradical cation is considered to be a probable candidate of the reaction intermediate.199−206 The mechanism involving dication is also proposed.207−211 Lee et al.212 reported cycloreversion of a diarylethene in the presence of 9-mesityl-10-methylacridinium ion. Photoexcitation of 9-mesityl-10-methylacridinium ion induced the cycloreversion reaction of the dithienylethene derivative with a quantum yield of around 0.5. They attributed the high quantum yield to the catalytic radical cation mechanism. Ikeda et al.213,214 reported the photochemical and electrochemical reactions of tetrathienylethenes. Yu and co-workers215 studied in detail the reductive cyclization of dithiazolylethene having N-methylpyridinium groups. Some other attempts have also been reported to utilize the electrochromism of diarylethene derivatives.120,216−225 The group of Akita226−229 and the group of Rigaut230,231 reported that some organometallic diarylethene complexes such as 118 undergo the electrochemical cyclization and cycloreversion upon oxidation at the metal center attached at each end of the diarylethenes as shown in Figure 23. The intermediate of this reaction is proposed to be the dication located at the metal center. When these photo- and electrochromic diarylethenes are assembled on the surface of electrodes, the electrodes work as multifunctional electrodes. The application of these electrodes is intensively studied.232−234

Scheme 20. Photochromism of Chiral Diarylethenes 109 and 110189,190

Scheme 21. Molecular Modeling Concept and Molecular Design of Chiral Diarylethene 111191

observed even after refluxing in toluene (bp 110 °C) for 5 h. It was concluded that 111a undergoes ultimate 100% diastereoselective photocyclization irrespective of environmental conditions.

3.6. Gated Reactivity

Photochromic reactions, in general, proceed in proportion to the number of photons absorbed by the molecules. Such a linearresponse property is not appropriate for memory media because the recorded memories are destroyed during reading the absorbance or fluorescence changes by photons. One possible way to avoid such inconvenience is to introduce gated photochromic reactivity to the photochromic systems. Gated photochromic reactivity is the property that additional external stimuli, such as another photons, chemicals, or heat, can control the photochromic reactions. Various approaches to attain the gated reactivity have been proposed.49−51,54−58,235−246 As mentioned in section 2.2, Miyasaka et al.49−51,54−58,235 found that excitation of the closed-ring isomers of diarylethenes

3.5. Electrochemistry

Besides the photochemical reactions, diarylethenes are reported to undergo cyclization and cycloreversion reactions by electrochemical oxidation or reduction. These reactions take place also by oxidizing or reducing reagents. This phenomenon, called electrochromism, can be potentially applied to molecular-scale electronic switches. With regard to 6π-electron photochromic systems, Fox and Hurst192 reported the electrochemical cyclization reaction of a fulgide, while Kawai et al.193 reported the electrochemical cycloreversion reaction of a diarylethene. 12196


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Scheme 22. Diarylethenes That Undergo Electrochemical Cyclization and Cycloreversion Reactions194−197,199

Figure 22. Schematic diagram of the energy potentials of (a) 116a, 116b, 116a+•, 116b+• and (b) 117a, 117b, 117a+•, 117b+•. The energy unit is kcal/ mol. Zero-point energy correction was included. Reprinted with permission from ref 199. Copyright 2005 American Chemical Society.

the hydroxyl group is protected by esterification, the esterificated diarylethene derivatives undergo reversible photochromic reactions. The gated system can be applied to neurotoxic organophosphate compound 121.237 Although diarylethene 122 containing cyclobutene-1,2-dione is photochemically inactive, it undergoes reversible cyclization/ cycloreversion reactions upon photoirradiation when both carbonyl groups are protected with cyclic acetal groups.238 After deprotection of the two acetal groups, the photochromic reactivity is lost. An interesting gated photochromic diarylethene system controlled by a mild and spontaneous [4 + 2] cycloaddition reaction is developed by Branda and co-workers.239 Although compound 125, which possesses a butadiene backbone, can not undergo a photocyclization reaction, the

with a picosecond laser pulse leads to an increase in the cycloreversion yield. The increase in the cycloreversion yield is ascribed to additional reaction channels in the upper excited states. Very low cycloreversion yield by irradiation with weak light and the remarkable increase in the yield with high intensity light provide a method for an erasable optical memory system with nondestructive readout capability. Several gated reaction systems based on chemical stimuli have been developed to manipulate the photochromic reactivity, as listed in Table 9.236−252 Maleimide-type diarylethene derivatives having a N-(O-hydroxyphenyl) group 119 and 120 were designed and synthesized to introduce chemically gated reactivity.236 These derivatives are photochemically inactive because excited states of the molecules are efficiently quenched by an intramolecular proton transfer. On the other hand, when 12197


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Figure 23. Switching and color changes of organometallic diarylethene 118 that undergoes the electrochemical cyclization upon oxidation at the metal centers. Reprinted with permission from ref 228. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

workers251 demonstrated an approach for the gated photochromism by using diarylethenes 135 and 136. The photochromic reactivities of these diarylethenes are suppressed when dimesitylboron is substituted to the thiophene ring due to the presence of highly emissive π → π*/pπ(B) excited state. F− ion binding to the boron center restored the photochromic reactivity. Photocycloreversion reaction of dithienylcyclopentenes 137 strongly depends on temperature. Below ca. 120 K, the photochromic reaction is effectively suppressed.252 The temperature effect of the photocycloreversion reactions is potentially used for nondestructive readout of the memory.

Diels−Alder product 126 is able to undergo the reversible photochromic reaction. Several photochromic diarylethene derivatives, the reactivity of which is controlled by the addition of acid or base, have been reported.240−246,250 Although diarylethene 127, in which (4pyridyl)ethynyl units are attached to the 6π-electron hexatriene moiety, undergoes a normal photochromic reaction in CH2Cl2, the photochromic reactivity is strongly suppressed upon quaternarization of the pyridine moieties in any solvent.240 The gated photoreactivity is locked by the addition of trifluoroacetic acid and unlocked by diethylamine. Compounds 128 and 129 undergo typical reversible photochromic reactions, but after the addition of alkali, the photochromic reactivity is lost.241 The absence of the photochromic reactivity is attributed to the strong electron-accepting ability of the pyridinium unit. The photochromic reactivity of 3,4-bis(5-formyl-2-methyl-3thienyl)-2,5-dihydrothiophene having 8-hydroxy-2-methylquinoline moieties 130 is also controlled by the addition of base or acid.242−245 The photochromic reactivity of 131 is locked in the anion form. The deprotonated form produced by the addition of base does not show any photochromic reactivity in the absence of carbon dioxide and water. However, the photochromic reactivity is recovered when the deprotonated form converts to the protonated one upon addition of carbon dioxide and water in solution. A dithiazolylethene having a benzo[b]thiophene-1,1dioxide as the ethene bridge 132 shows similar gated photochromism.247 Although compound 132 undergoes typical photocyclization/photocycloreversion reactions upon alternate irradiation with UV and visible light, the photochromic reactivity is lost when BF3/Et2O is added. The inertness is attributed to the coordination with BF3. 1,8-Naphthalimide-piperazine-tethered dithienylethene 133 also exhibits similar gated reactivity, which forms a photoinactive conformation by the coordination with copper(II) ion.248 The photochromic reactivity of terarylene 134 can be controlled by oxidization−deoxidization reactions.249 The derivative 134 is photochemically active before oxidization, while the photochromic reactivity is strongly suppressed by oxidizing the sulfur atoms. The photochromic reactivity is reactivated by deoxidizing the S,S-dioxide moieties with NaBH4. Yam and co-

3.7. Fluorescent Property

3.7.1. Fluorescent Diarylethenes. Table 10 summarizes fluorescent diarylethenes.253−285 Reversible photoisomerization between two isomers induces an electronic structure change, and the change in the electronic structure influences the fluorescent property. Although most diarylethenes are nonfluorescent in both open- and closed-ring isomers, diarylethenes having benzothiophene as the aryl groups exhibit weak fluorescence in both isomers.47,48,104,253−255,286 The open-ring isomer of compound 15 emits blue fluorescence at 420 nm, while the closed-ring isomer exhibits a very weak but distinct emission at 630 nm.47,48 The closed-ring isomer of the bisbenzothienylethene derivative having 2,4-diphenylpheny substituents at 6and 6′-positions of the benzothiophene rings 138 emits red fluorescence at 650 nm in the amorphous state upon excitation with 530 nm light.253,254 Upon irradiation with visible (>450 nm) light, the 650 nm fluorescence disappears and a new fluorescence band appears at 450 nm, which is attributed to the fluorescence from the open-ring isomer. Diarylethene dimer 56, in which diarylethenes are connected with a diyne, also emits fluorescence in the isomer having one closed-ring form.104 Many fluorescent diarylethene derivatives were prepared by introducing fluorescent units, such as indole,256,287 furan,77,288 oxazole,89,289 pyrazole,88 pyrrole,81,257 oligothiophene,221,258 coumarin,259−261 and naphthalene290,291 as the aryl group. 2Substituted indoles are well-known to exhibit strong fluorescence.292,293 Diarylethene derivatives having a 2-substituted 12198


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Table 9. Gated Reactivity

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Table 9. continued

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Table 9. continued

indole 117 and 140 were synthesized. Both derivatives show reversible fluorescence switching along with the photochromic reactions in n-hexane upon alternate irradiation with UV and visible light. The fluorescence quantum yields of the open-ring isomers 117 and 140 were determined to be 0.046 and 0.063, respectively. The open-ring isomers of diarylethene derivatives having furan rings as the aryl groups 25 and 26 also emit fluorescence in hexane at 438 and 382 nm (Φf ≈ 0.03), respectively.77 Fluorescence quantum yields of these derivatives are much less than the values of their model compounds, such as 1-methyl-2-phenylindole (Φf = 0.85) and 1-(9-anthryl)-2-(2furyl)ethene (Φf = 0.47).294 The decrease is attributed to competition between the fluorescent process and the photocyclization reaction. Diarylethene derivatives 150 having oligothiophenes as the aryl units exhibit reversible fluorescence switching upon alternate irradiation with appropriate UV and visible light.221,258 Coumarinyl(thienyl)thiazoles having a coumarin chromophore as the aryl unit 151−155 also modulate fluorescence both in solution and in polymer matrixes upon photoisomerization.259−261 The open-ring isomers of bis-2-thienylethene derivatives exhibit weak fluorescence.90,262−265,267,295,296 Characteristic fluorescence property of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene (156) was studied in detail in solution as well as in the single-crystalline phase.262−264 The open-ring isomer emits fluorescence (Φf = 0.01) at 500 nm in 3methylpentane solution when excited at 313 nm. The fluorescence intensity decreases along with the photocyclization reaction. The isolated closed-ring isomer is nonfluorescent. Fluorescence spectrum attributable to the intermolecular interaction was observed in the single crystal. X-ray crystallographic analysis and polarized fluorescence measurement revealed that the fluorescence property is strongly affected by the intermolecular interaction between two neighboring thiophene rings. 1,2-Bis[4-methyl-2-(2-pyridyl)thiazolyl]perfluorocyclopentene (157) shows a broad emission band

centered around 500 nm upon excitation with 352 nm UV light.265 The emission intensity is reversibly changed upon alternate irradiation with UV and visible (>450 nm) light. The open-ring isomer of 1,2-bis(4-methyl-2-phenyl-5-oxazolyl)perfluorocyclopentene (37), in which oxazole rings are attached to the ethene moiety at the 5-position exhibits relatively intense blue-green fluorescence (Φf = 0.19).90 The fluorescent property of diarylmaleic anhydride and diarylmaleimide derivatives, such as 71 or 162, strongly depends on solvent polarity.40,125,297 The fluorescence spectrum of 71 shows remarkable Stokes shifts depending on solvent polarity, while the absorption spectral shift is rather small. The fluorescence maximum in n-hexane at 488 nm shifts to 560 nm in THF. At the same time, the fluorescence intensity dramatically decreases. The solvent effect on the fluorescence and the cyclization quantum yield is attributed to intramolecular CT character of the molecules, as described in section 3.1.2. Tian and co-workers268−271 prepared fluorescent photochromic compounds having diiminopyrroline 163, tetraazaporphyrin 164, or phthalocyanine 165. The open-ring isomers of the latter two derivatives exhibit near-IR emission at around 700 nm, while the closed-ring isomer is nonfluorescent. Favaro and co-workers143 prepared diarylethene derivatives 166 and 167 having various penta-atomic rings instead of perfluorocyclopentene, and studied their photochromic and fluorescent properties. The closed-ring isomers of these derivatives emit fluorescence, while the fluorescence is lost in the open-ring isomers. Liu et al.272 and Li et al.273 developed “turn-off” or “turn-on” fluorescent diarylethene derivatives 168−177, in which the fluorescence intensity changes upon photocyclization reaction. Upon UV irradiation the fluorescence of compound 169 decreases from 0.114 to 0.009, while that of compound 170 increases from 0.002 to 0.130. Ahn and co-workers274 developed a “turn-on” photochromic fluorescent probe for live cell imaging. They found that the diarylethene derivative 178 undergoes a cyclization reaction upon UV irradiation and the closed-ring 12201


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Table 10. Fluorescent Diarylethenesa

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isomer gives a fairly high fluorescence at 580 nm (Φf ≈ 0.11). The fluorescence switching contrast was estimated to be ∼97:1. The cyclization and cycloreversion quantum yields were determined to be 0.25 and 0.17, respectively. The high fluorescence contrast and excellent reversibility make the compound an excellent fluorescent probe for live cell imaging. In addition, the compound showed no toxicity to cells even in the millimolar concentration range. Figure 24 shows the photoswitching of images of live cells using compound 178. The derivatives having thienothiophenes 179 and 180 and dithienothiophene 181 as the ethene bridge emit weak fluorescence [179, Φf (λex = 330 nm) = 0.10; 180, Φf (λex = 330 nm) = 0.06; and 181, Φf (λex = 330 nm) = 0.14] in the openring forms.275 When fluorescent chromophores such as imidazole,298,299 naphthalimide,276,300 2,1,3-benzothiadiazole,277 β-diketonate ligand,278 thienylpyridine-bis(alkynyl)borane,301 or coumarine279 are introduced at the central bridging unit, compounds 182−191 show reversible fluorescence photoswitching. Aggregation of diarylethene derivatives affects the fluorescent property. Fluorescence intensity becomes stronger in the closed-

ring isomer when dicyano-diarylethene 192 forms colloidal or amorphous state.280 Bis(diarylethene)sexithiophene derivatives 58 and 193 show temperature-dependent aggregation, and their fluorescent property depends on the aggregate formation.281 Fluorescence enhancement was observed in a supramolecular self-assemble system 194.282 The fluorescence intensity of the self-assemble system was found to be 5-fold stronger than that of each component. Such fluorescence enhancement was observed not only in solution, but also in solid as well as in nanoparticles. The fluorescence quantum yield of compound 157 in acetonitrile solution was as low as 0.005, while it increased to 0.2 in the solid state.266 The colloidal aqueous suspension containing nanoparticles of 157 emits appreciable fluorescence (Φf = 0.017) and undergoes efficient photochromic reactions (Φo→c = 0.20 and Φc→o = 0.96). Nanocrystals of compound 156 grown in sol−gel thin film also exhibit fluorescence.302 A photoswitchable probe 195 was prepared for living cells imaging using an amphiphilic diarylethene, which has both hydrophilic and hydrophobic chains at each end of the diarylethene unit.283 This molecule forms stable vesicle nanostructures in aqueous solution and exhibits reversible fluorescence photoswitching, 12205


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Figure 24. Photoswitching of images of live HeLa cells. Upon UV irradiation, a cytoplasmic fluorescent signal appeared. Subsequent visible light irradiation completely abolished the signal. Reprinted with permission from ref 274. Copyright 2012 The Royal Society of Chemistry.

Figure 25. Photographs of 1,4-dioxane solutions containing 201 and 204 (a) before and (b) after irradiation with 365 nm light upon excitation with 488 nm blue light (left-side solution containing 201) and 532 nm green light (right-side solution containing 204). Reprinted with permission from ref 141. Copyright 2011 American Chemical Society.

although the fluorescence quantum yield is very low (Φf ≈ 0.007). When sulfur atoms of compound 15 are oxidized to sulfones (92), both the open- and the closed-ring isomers give fluorescence (Φf(open) = 0.025, Φf(closed) = 0.011).162,163,284 By introducing acetyl groups at the 6- and 6′-positions, the fluorescence quantum yield of the closed-ring isomers was found to increase (Φf(closed) = 0.036).162,163 S,S-Dioxide derivatives of terarylenes 199 and 200 also exhibit relatively high fluorescence (Φf ≈ 0.35) in the closed-ring isomers.142 The fluorescent performance of the sulfone derivatives was dramatically improved by introducing both short alkyl chains at 2- and 2′-positions and phenyl or thiophene rings at 6- and 6′positions of the benzothiophene-1,1-dioxide groups, such as 201−205.141,285 The fluorescence quantum yields of the closedring isomers increased close to 0.9 and the absorption coefficients larger than 4 × 104 M−1 cm−1. The phenylsubstituted derivatives 201−203 undergo photocyclization reactions (Φo→c = ∼0.5) to produce yellow closed-ring isomers, which emit brilliant green fluorescence at around 550 nm in 1,4dioxane (Φf = 0.87) upon excitation with 488 nm light (Figure 25, left). In n-hexane, the fluorescence quantum yield of the derivative was found to reach 0.92. Any absorption intensity decrease of the closed-ring isomers was not observed even after 100 h storage in the dark at 80 °C. The closed-ring isomers slowly returned to the initial open-ring isomers upon irradiation with visible (λ > 480 nm) light (Φc→o = ∼10−4). When the phenyl substituents are replaced with thiophene rings, such as compounds 204 and 205, the absorption bands of the closedring isomers shift to longer than 500 nm. The closed-ring isomers exhibit brilliant red-orange fluorescence at around 620 nm (Φf = 0.61−0.78) upon excitation with 532 nm light (Figure 25, right). The closed-ring isomer of 204 in 1,4-dioxane exhibits excellent fatigue-resistant property under irradiation with visible light (λ > 440 nm) superior to the stability of Rhodamine 101 in ethanol. Most fluorescent diarylethenes show “turn-off” fluorescence switching. Initially the open-ring isomers are fluorescent, but they convert to nonfluorescent state upon UV irradiation. The “turn-off” switching is difficult to apply to super-resolution microscopy, which requires an absolutely dark background in the

initial state. The above sulfone derivatives exhibit “turn-on” fluorescence switching. They are initially nonfluorescent and dark under irradiation with visible light (λ > 450 nm), while they are activated to emit green or red fluorescence upon irradiation with UV light. The derivatives are useful for PALM (photoactivated localization microscopy) or STORM (stochastic optical reconstruction microscopy), as well as RESOLFT (reversible saturable optical fluorescence transition), superresolution bioimaging.303 3.7.2. Fluorescent Diarylethenes Having a Fluorescent Unit. In inherently fluorescent diarylethenes, the fluorescent process and the photochromic reaction compete with each other. When the fluorescence quantum yield is high, the photoisomerization reaction is suppressed. A possible approach to prepare a molecule having both an efficient photochromic reactivity and a high fluorescent quantum yield simultaneously is to combine photochromic and fluorescent chromophores in a molecule. Fluorescence resonance energy transfer (FRET) or intramolecular electron transfer (IET) mechanisms are adopted for the molecules to reversibly switch the fluorescence. Table 11 summarizes diarylethene derivatives having fluorophores such as anthracene,35,304−311 2,4,5-triphenylimidazole,312 tetraphenylporphyrin,313,314 fluorene,315,316 polythiophene,317−319 polyphenylenevinylene,318−320 metal complexes,64,321−342 BODIPY,343 fluorescein,344 rhodamine,345−349 napthalimide,350−357 perylenebisimide,358−373 or others.374−384 Highly fluorescent diarylethene 206, in which two diarylethene moieties are linked to a fluorescent bis(phenylethynyl)anthracene, was prepared.304 The fluorescence quantum yield was as high as 0.83 for the open-ring isomer and 0.001 for the closed-ring isomer. The dye exhibits a laser emission, and the intensity can be reversibly switched by alternative irradiation with 313 nm light and λ > 500 nm light. When fluorescent chromophores are directly linked to a diarylethene unit, the fluorescence is in general suppressed and photochromic reaction quantum yields decrease. Osuka et al.313 prepared diarylethene 12206


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Table 11. Fluorescent Diarylethenes Having Fluorophoresa

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and 220 emitted right- or left-handed circularly polarized fluorescence (CPF) with relatively large dissymmetry factors in the order of 10−2 and also modulated the fluorescence along with the photochromic reactions. Jares-Erijman and co-workers350 reported photochromic photoswitchable FRET (pcFRET) systems 250 and 251 having a diarylethene unit, where Lucifer Yellow was used as a fluorescence donor and connected to the diarylethene chromophore using a long alkyl chain. The closed-ring isomer of the diarylethene unit has an absorption band overlapping the fluorescence band of the donor, while the absorption band of the open-ring isomer locates in a wavelength region shorter than the fluorescence band. When the diarylethene is in the open-ring form, the donor’s fluorescence is not perturbed by the diarylethene unit and gives strong fluorescence. On the other hand, when the diarylethene converts to the closed-ring form, energy transfer from the donor fluorophore to the acceptor diarylethene closed-ring isomer takes place, and the fluorescence is quenched. Highly fluorescent photoswitchable chromophores 207, 208, 256, and 257, in which fluorescent bis(phenylethynyl)-

derivative 214 having tetraphenylporphyrin moieties. It was found that close attachment of the porphyrin chromophores to the diarylethene unit leads to loss of its photochromic reactivity, while insertion of a spacer unit between the diarylethene and the porphyrin unit ensures the photochromic reactivity and the corresponding fluorescence switching. A diarylethene-fluorene conducting copolymer 216 was synthesized.315 Reversible photochromism was observed in solution as well as in solid film, and its photoluminescence was found to change along with the photochromism. Wolf, Branda, and co-workers317 prepared polythiophene 217 having diarylethenes in the side groups. The fluorescence of the polythiophene was fully quenched when only 14% of the openring isomers converted to the closed-ring isomers. Akagi and coworkers318,319 synthesized photoresponsive liquid crystalline poly(p-phenylene-vinylene) and poly(bithienylene-phenylene) by introducing diarylethene moieties into the side groups, such as compounds 218−221. The macroscopically aligned film emitted linearly polarized fluorescence with notable dichroic ratio, and the fluorescence was modulated by irradiation with UV and visible light. The cast films of chiral π-conjugated polymers 219 12212


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Figure 26. (a) Relative fluorescence quantum yields as a function of dielectric constant for the open-ring isomer 266a (○) and the closed-ring isomer 266b (●). (b) Nondestructive fluorescence readout capability of both open- and closed-ring isomers. Fluorescence intensities at 568 nm upon excitation at 532 nm of 266a (○) and 266b (●) were plotted against irradiation time. Reprinted with permission from ref 370. Copyright 2011 American Chemical Society.

It is favorable to employ S,S-dioxide diarylethene derivatives as electron acceptors and fluorescent perylenebisimide chromophores as electron donors to avoid spectral overlap.366−370 The fluorescence spectrum of the perylenebisimide unit does not overlap with the absorption spectra of both isomers of S,Sdioxide diarylethene derivatives in 263−266. The dyads 264 and 265 exhibit reversible fluorescence quenching with a highly efficient switching ratio (Φf,O/Φf,C ≈ 7).367,369 Although the molecules showed fluorescent quenching based on pcIET mechanism, it was found that visible light causes the photocyclization reaction of the diarylethene unit because of an unexpected triplet route. Upon irradiation with visible (>500 nm) light, where only the perylenebisimide absorbs, the photocyclization reaction takes place efficiently. To avoid the triplet route, the absorption band of the perylenebisimide unit was shifted to longer wavelengths by introducing bulky substituents.371 When the S1 state of the perylenebisimide unit is lower than the T1 state of the diarylethene unit, the triplet route is anticipated to be lost. After several trials, it was found that molecule 266 satisfies the conditions.370 In 266, the fluorescence quantum yield of the closed-ring isomer dramatically decreases when the dielectric constant of the solvent increases above 5, while that of the open-ring isomer remains almost constant with an increase in the dielectric constant of the solvent (Figure 26a). In polar solvents, therefore, the fluorescence is reversibly switched by alternate irradiation with visible (445 nm) and UV (365 nm) light. From femtosecond time-resolved transient and fluorescent lifetime measurements, it was revealed that the fluorescence quenching is attributed to the intramolecular electron transfer. The fluorescence intensities of both isomers stay constant even after 2 h under irradiating with 532 nm laser light (2.5 mW cm−2) (Figure 26b). Nondestructive fluorescence readout based on the pcIET mechanism was successfully demonstrated in dyad 266. Berberich and Würthner further improved the fluorescence switching performance by optimizing the diarylethene unit, as shown in 267.365 In dichloromethane the high fluorescence quantum yield (0.95) of the open-ring isomer decreases to 0.05 upon photocyclization of the diarylethene unit. The emission intensities of both the open- and the closed-ring isomers remained constant over the time range of more than 3 h of irradiation at 600 nm (6.9 mW cm−2), and the switching cycles showed increased photostability as compared to 261. Compound 267 has capability of both nondestructive fluorescent readout and photostability. Aggregate formation affected by the photochromic reaction causes color changes in fluorescence.372,373 Self-assembled and organogel systems also exhibit unique properties, such as

anthracene or perylenebisimide (PBI) and a diarylethene switching unit are linked through an adamantyl spacer, have been synthesized and used for single-molecule fluorescence photoswitching.306,307,358,359 The diarylethene unit undergoes the reversible photoisomerization between the open- and the closed-ring isomers. The chromophores having the open-ring isomer give strong fluorescence upon excitation with 488 nm light, while the fluorescence is quenched when the diarylethene unit converts to the closed-ring isomer. The fluorescence quenching is ascribed to the energy transfer from the excited state of fluorescent unit to the closed-ring isomer. Similar fluorescence photoswitching systems 248 and 252, which have both fluorescent and photochromic units, were prepared by Hell and co-workers.345,353 The chromophore having the rhodamine dye showed a large fluorescent quantum yield (Φf ≈ 0.52) and a large fluorescence modulation ratio (94%) in ethanol solution. The emission wavelength and efficiency of compound 252 strongly depend on solvent polarity. The compound shows a positive solvatochromism effect in the emission maxima, and their quantum yield decreases as the solvent polarity increases from cyclohexane to dioxane. Köhler and co-workers362 prepared a photoswitchable fluorescent triad 260, in which two fluorescent perylenebisimides are connected to a diarylethene unit, and demonstrated the key functionalities of an optical transistor, gating and amplification, by exploiting the fluorescence photoswitching based on a pcFRET mechanism. The fluorescence switching by pcFRET mechanism has an inherent drawback. The quenching by the intramolecular energy transfer to the closed-ring isomer induces undesirable ringopening (cycloreversion). To avoid the cycloreversion reaction, it is required to separate the absorption spectra of both isomers of the photochromic unit and the fluorescence spectrum of the fluorescent unit and employ photochromic intramolecular electron transfer (pcIET) as the fluorescence quenching mechanism. Würthner, Scandola, and co-workers363−365 prepared diarylethene-perylenebisimide dyad 261 and diaryletheneterylenebisimide (TBI) dyad 262. Dyad 261 undergoes reversible photochromic reactions upon alternate irradiation with UV and visible light. The fluorescence quantum yield of the open-ring isomer is almost constant against the solvent polarity, while that of the closed-ring isomer decreases with increasing the solvent polarity. The result indicates that the fluorescence of the perylenebisimide unit is quenched by pcIET mechanism. However, dyad 261 suffers from low fluorescence quantum yield and low photostability. To improve the property, they designed and synthesized dyad 262, which has a high fluorescence quantum yield and exhibits repeatable fluorescence switching upon alternate irradiation with UV and visible light. 12213


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Figure 27. IR spectra of 273 in CCl4 solution: the open-ring isomer 273a (left) and the closed-ring isomer 273b (right). The concentration in both cases was 5.4 × 10−2 M. Adapted with permission from ref 392. Copyright 1999 Elsevier.

property. In this section, optical memories that use diarylethenes as the recording media are described.

multiple switching, or morphology changes, in addition to reversible fluorescence switching associated with the photochromism.354,355,384 Luminescent nanohybrid systems, such as silica nanoparticles (NPs), upconverting NPs, quantum dots (QDs), or polymerdots (PDs), have been recently developed and widely applied to optical data storage, bioimaging, and superresolution imaging.346,385−391 Hell and co-workers346 prepared photoswitchable fluorescent silica NPs encapsulated with compound 248. Super-resolution fluorescence imaging (∼80 nm) was demonstrated using the photoswitchable fluorescent NPs. Jares-Erijman and co-workers387 prepared photoswitchable QDs (psQD) based on pcFRET mechanism. Diarylethene derivatives were covalently linked to an amphiphilic polymer that self-assembles with the lipophilic chains surrounding commercial hydrophobic core−shell CdSe/ZnS QDs. The psQD retains all desirable properties of the original QD (broad excitation bands, narrow emission bands and photostability), and the brightness of the emission can be modulated by light. Branda and coworkers389 employed NaYF4 nanocrystals doped with lanthanide ions to remote-control photoswitching of diarylethenes using NIR light (980 nm). The upconversion nanoparticles are favorable in biological systems because the systems are transparent in red/NIR regions and harmful UV light is not required to switch the chromophores.

4.1. Infrared (IR) Absorption, Raman Scattering, and Refractive Index Readout

For the application to optical memory media, an indispensable issue is nondestructive readout property. If electronic absorption spectral changes in UV and visible regions are used as the recording and readout signals, the recording information is destroyed during the readout process. One of the available methods to avoid the destructive readout is to use IR absorption or Raman spectral changes as the readout signals. When the difference between the open- and closed-ring isomers can be detected by using IR light, the recording information can be read many times without destruction. Zerbi and co-workers392 revealed that some diarylethenes having thiophene rings exhibit remarkable IR spectral changes accompanying the photochromic reactions. For example, 1,2-bis[5-(4-methoxyphenyl)-2-methyl3-thienyl]perfluorocyclopentene (273) has a strong IR absorption peak at 1495 cm−1 only in the closed-ring isomer, as shown in Figure 27, and the IR intensity changes can be used for the nondestructive readout. Uchida et al.393,394 found that compound 15 and bis(2-methyl-6-phenyl-1-benzothiophen-3yl)perfluorocyclopentene (274) having benzothiophene rings also show large IR spectral changes upon photoisomerization. The spectral changes can be applied to image recording. Initially the film was homogeneously irradiated with UV light. A mask pattern, in which 702 is written, then was placed on the film and irradiated with visible light. Figure 28 shows (a) visible and (b) IR images of the recorded film. The IR image was monitored by IR light at 1590 cm−1, where only the closed-ring isomer has strong absorption. Substituent effect on the IR spectra was theoretically studied in detail,395 and it was found that diarylethene derivatives with electron-donating substituents exhibit larger difference in the spectra between two isomers due to the push−pull−push effect. On the basis of this fundamental knowledge, several examples of nondestructive IR readout performance were demonstrated using diarylethene-doped films,393,396 diarylethene-polymer

4. MEMORY Photochromic molecules can be applied to optical memory media. The photochromic optical memory is based on a photochemical (or photon-mode) recording method, which differs from the heat-mode recording employed in current optical media. The photon-mode recording has various advantages over the heat-mode recording in terms of resolution, speed of writing, and multiplex recording, such as wavelength, polarization, and phase. Diarylethenes are the most promising photochromic molecules for the photon-mode recording media because of their high thermal stability, picoseconds response time, high photoisomerization quantum yield, and excellent fatigue-resistant 12214


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Photochromic polyurethane (276) containing two kinds of photochromic moieties was prepared by polyaddition polymerization of 1,2-dithienylethenes having end-capped hydroxyl groups with 4,4′-diisocyanate dicyclohexylmethane, as shown in Scheme 23.403 The Raman spectra showed distinct signal difference between the open- and the closed-ring isomers. When the sample contains the two photochromic monomers in the ratio of 1:7, the four states, open−open, open−closed, closed− open, and closed−closed, can be distinguished in the Raman spectra, as shown in Figure 29, and readout nondestructively

Figure 28. (a) Visible and (b) infrared images of the recorded photochromic film containing 4 wt % diarylethene 274. The IR image was monitored by IR light at 1590 cm−1, where only the closed-ring isomer has strong absorption. Adapted with permission from ref 393. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 29. Raman spectra of the four states of PPU-276 in the region 1700−1000 cm−1 (open−open, black line; closed−open, pink line; open−closed, blue line; closed−closed, purple line). Scheme 23 shows the four chemical structure of PPU-276. Reprinted with permission from ref 403. Copyright 2011 The Royal Society of Chemistry.

films,397,398 amorphous films,399 and liquid crystalline films. One of the attractive features of the IR readout is multistate recording.400−402 A film containing three kinds of diarylethenes, 1,2-bis(3,5-dimethyl-2-thienyl)perfluorocyclopentene (275), compounds 4 and 13, was prepared and used for image recording. Upon UV irradiation, the three derivatives changed to their closed-ring isomers having different absorption spectra not only in UV/vis region but also in the IR spectral region. Visible light of appropriate wavelengths selectively bleached closed-ring isomers, and three bits eight states recording was achieved. The eight states could be readout nondestructively by using IR light of appropriate wavenumbers. This technique is useful for rewritable high-density optical image recording with nondestructive readout capability.

using a 1064 nm laser, which does not induce any photochromic reaction, as the excitation line source. The setup used for the readout is shown in Figure 30. The signals detected by the two photodiodes can discriminate the four states by a comparison of the two signals. It is worth noting that the readout is simply based on the presence or absence of signals to the photodiodes and not on the intensity. 3D recording, hologram recording, and waveguide-type switching devices based on refractive index changes of diarylethenes have been reported.21,404−415 The refractive index changes can be used for nondestructive readout, because the changes can be detected by light longer than the absorption

Scheme 23. Molecular Structures of Photochromic Polyurethane PPU-276 Containing Two Kinds of Diarylethene Units403

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nm) was used as the recording light source, while the memory was erased by irradiation with green light (520 nm). The memories were readout using refractive index changes in the IR region. Figure 31 shows a transmission image of pit arrays

Figure 31. Transmission microscopy image of pit arrays recorded in diarylethenes by LD light (33 mW) with pulse width of 2 ns, where pitch between recorded pits was 3 μm and exposure times were 0.4, 0.3, 0.2, and 0.1 s for the lines from the left. Reprinted with permission from ref 420. Copyright 2005 The Japan Society of Applied Physics.

Figure 30. Schematic illustration of the simple readout process based on the Raman signals. Reprinted with permission from ref 403. Copyright 2011 The Royal Society of Chemistry.

recorded by the IR LD light (33 mW), where pitch between recorded pits is 3 μm and exposure times are 0.4, 0.3, 0.2, and 0.1 s for the lines from the left.420 A supermultilayered type medium, which increases the reflectance from the pits, was developed for the 3D recording.420 An increase in the sensitivity makes high-speed recording possible. Kawata and co-workers421 developed a compact fiber laser, which can generate a pulse with a high peak power of 5.3 kW, and successfully applied it to TPA recording on a diarylethenes medium. Belfield and co-workers422,423 demonstrated 3D two-photon recording and two-photon fluorescence readout using photochromic polymer composites containing a mixture of compound 15 and a fluorene derivative. Two layers of bits separated by 50 μm in the Z direction (depth) were written in the polymer film by TPA of 800 nm light. Readout was carried out by monitoring the emission of the fluorene. The readout based on the resonance energy transfer was proved to be useful as a nondestructive readout method. A loss of the initial fluorescence emission was less than 20% of the initial emission even after 10 000 readout cycles. Multiphoton absorption was successfully applied to induce both cyclization and cycloreversion reactions of 1,2-bis(5-{4[N,N-bis(4-methylphenyl)amino]phenyl}-2-methyl-3-thienyl)perfluorocyclopentene (277) amorphous film using a single near-infrared femtosecond laser pulse at 1.28 μm with 35 fs full width at half-maximum.424 A high intensity laser pulse (70 Gbit/inch2. Other recording methods using the near-field optics, such as SIL,414 Super-RENZ,427 or bent cantilever fiber probes,428,429 also have been tried and applied to ultrahigh density recording with nondestructive readout capability.

were stable at room temperature in the dark for more than 2 months, and no remarkable degradation in resolution of reconstructed image after 12 write/erase cycle was found. 4.5. Multiwavelength Recording

Multiwavelength (or multifrequency) storage is a possible approach to increase the recording density and data capacity. N-wavelength storage can increase the recording density by 2(N−1) times in comparison with single-wavelength storage. For example, in single-wavelength storage, unrecorded and recorded dots are coded by {(0),(1)}, but in three-wavelengths storage, they are coded by {(000), (001), (010), (011), (100), (101), (110), (111)}. The recording density is 4 times larger than that of single-wavelength storage. Photochromic materials with different absorption bands can be used for the multiwavelength storage system.440−442 PMMA film containing three kinds of diarylethene derivatives 279, 280, and 281 having different absorption bands in the closed-ring isomers was prepared by spin coating on a glass substrate, and three-wavelength photon-mode optical storage was carried out (Scheme 24).441 Initially the film was colored homogeneously by irradiation with UV light. and then recorded dots were marked by irradiation with three laser beams, 532, 650, and 780 nm. The reflectivity of the recorded dot was much higher than that of the unrecorded region, and the reflectivity change was used as the readout signal. The reflectivity ratios between the recorded dot and unrecorded region of 532, 650, and 780 nm recording curves were 50%, 65%, and 30%, respectively. All of the readout signals recorded by the threewavelength lasers have a relatively high S/N ratio and no crosstalk.

4.4. Holographic Optical Memory

Holographic recording is one of the characteristic optical memory technologies. It possesses large capacity and enables high speed input and output capability of information. To a great extent, the holographic memory performance depends on the recording media. Photochromic diarylethenes are promising candidates for the media because of their high resolution, fatigue resistance, thermal irreversibility, and self-development properties. Various kinds of diarylethene-doped polymer films have been examined for the holographic recording.409,430−439 As an example, Pu and co-workers439 demonstrated multiplexing optical recording on a 1-[2-methyl-5-(p-N,N-dimethylaminophenyl)-3-thienyl]-2-[2-methyl-5-(3-methoxyphenyl)-3thienyl]perfluorocyclopentene (278)/PMMA polymer film

4.6. Single-Molecule Memory

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other hand, when Tg of the matrix is high (∼100 °C), such as PMMA, the histograms show a peak. The peak indicates that the photoreaction takes place only after several excitation processes and the quantum yield of the photochromic reaction is not constant. The appearance of the peak in the histogram suggests that the quantum yield of the molecule increases with an increasing number of absorbed photons. In other words, the molecule remembers the number of absorbed photons and undergoes the reaction after absorbing a certain number of photons. A possible explanation of the abnormal histograms is given by a multilocal minima model, as shown in Figure 36.359,443−445 The polymer chains surrounding the molecule provide steric hindrance. The steric hindrance modifies the potential surfaces of both ground and excited states. In the absence of such steric hindrance as in low Tg matrixes the molecule undergoes one-step photoreaction, which leads to a constant quantum yield. In high Tg matrixes, multilocal potential surfaces prevent the one-step photoreaction and require multistep photoexcitations to reach the final reaction process. It requires many steps for the molecule to undergo the reaction from open- (or closed-) to closed- (or open-) isomers. Therefore, the quantum yield depends on the number of excitation. The result provides a new insight into the photoreaction of molecules in polymer matrixes. Single-molecule fluorescence photoswitching with nondestructive readout capability was demonstrated by using a poly(methyl acrylate) (PMA) film containing dyad 266.370 The fluorescence quenching is based on an IET mechanism from the perylenebisimide donor to the diarylethene acceptor units, as described in section 3.7. Figure 37 shows the wide-field fluorescence microscopy images taken on the same area upon excitation with 532 nm as the fluorescence excitation light source. Initially, the PMA film containing the nonfluorescent closed-ring isomer shows a dark image under irradiation with only 532 nm laser light. The fluorescence “OFF” state changes to the fluorescence “ON” state upon UV light irradiation. After stopping UV light irradiation, the fluorescence “ON” state remains stable during irradiating with 532 nm laser light. The fluorescence “ON” state turns back to the fluorescence “OFF” state when the sample is irradiated with 440−490 nm light. Both fluorescence “ON” and “OFF” states remain stable during excitation with 532 nm laser light. This means that non-

Scheme 24. Photochromism of Diarylethenes 279, 280, and 281441

performances of photochromic molecules is that the isomerization can be detected even at the single-molecule level. An appealing feature of single molecules is their small size. If a single diarylethene molecule would work as one bit memory, then ultimately high-density (1 P bit/inch2) optical memory could be realized. As the first step to approach this goal, a highly fluorescent photoswitchable molecule 207 was synthesized, and the photoswitching behavior at the single-molecule level was studied.306,307 Figure 34 shows a typical example of the singlemolecule photoswitching. The diarylethene molecule undergoes a reversible photoisomerization from the open- to the closed-ring isomer, and the fluorescence of the anthracene chromophore is reversibly quenched by a FRET mechanism. Although in solution the fluorescence intensity gradually changed upon irradiation with UV and visible light, digital on/off switching between two discrete states was observed at the single-molecule level. The digital switching confirmed that the fluorescence photoswitching takes place at the single-molecule level. Durable fluorescent diarylethene derivative 256 was synthesized, and a detailed analysis of single-molecule photochromic reactions was carried out.358,359 The time trace of the fluorescence was measured under irradiation with both UV (325 nm) and visible (488 nm) light. The histograms of the response times are shown in Figure 35. When the polymer matrix has a low Tg near room temperature, such as poly(n-butyl methacrylate) (PnBMA), the histogram has an exponential shape. The exponential shape indicates that the quantum yield of the photochromic reactions is constant. On the

Figure 34. (a) Schematic illustration of fluorescence photoswitching based on the photochromic intramolecular energy transfer. (b) Images of the single-molecule fluorescence photoswitching of four-individual diarylethenes. (c) Digital on/off fluorescence photoswitching based on the photoisomerization between 207a and 207b. Reprinted with permission from ref 306. Copyright 2002 Macmillan Publishers Ltd. 12218


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Figure 35. Histograms of (left) on-time and (right) off-time in PnBMA (top) and PMMA (bottom) containing compound 256. The histograms were constructed from the time trace of fluorescence intensity of 256 in the polymer matrixes irradiated with both 488 and 325 nm light. Reprinted with permission from ref 359. Copyright 2007 American Chemical Society.

pathways, all of which result in distinct changes to cubic nonlinearity for specific regions of the spectrum. Therefore, the complex has potential to be used in construction of multi-input logic gates responding to diverse stimuli across the broad spectral range. All-photonic molecular devices, which use only optical input stimuli and generate optical output signals, are a promising system for the logic gates. One of the first examples of such molecular all-photonic logic systems was reported by Gust and Andréasson group.456−458 All-photonic inputs and outputs have been implemented with a photochromic triad 283 composed of a diarylethene photochrome and two chemically identical fulgimide photochromes, as shown in Scheme 26. The triad 283 performs the functions of the following operations: AND, XOR (exclusive OR), INT (inhibit), half-adder, half-subtractor, multiplexer, demultiplexer, encoder, decoder, keypad lock, and logically reversible transfer gate.458 Although various molecular logic circuits have been so far constructed, it is hard for such molecular systems to be practically used in real optical computing.

Figure 36. A schematic diagram of the potential energy surfaces of a diarylethene (a) in the gas phase and (b) in the polymer matrix. Adapted with permission from ref 359. Copyright 2007 American Chemical Society.

destructive fluorescence readout is achieved even at the singlemolecule level. 4.7. Logic Circuit

Molecular switches based on diarylethene photochromism allow us to perform logic operations including AND, XOR, NOR, INHIBIT, and a half-adder. Adoption of bistable as well as multistable photochromes offers the possibility of performing multi-input logic operations.446−458 Tian et al.452 constructed a four optical outputs system responding to four inputs by using a diarylethene derivative having pyridyl groups, which responds to Zn2+, protons, and alternating UV/vis light irradiation. The absorption and fluorescence properties including the intensity and the peak wavelengths of the compound reversibly change by such input signals. Complicated molecular switches with four optical outputs responding to four inputs were proved using a single diarylethene compound. Samoc, Humphrey, and co-workers455 synthesized a binuclear metal alkynyl complex having a diarylethene bridge 282, which can afford six stable and switchable states and possesses distinct NLO properties, as shown in Scheme 25. The complex is composed of independently addressable modules that respond orthogonally to protic (alkynyl ligand ⇄ vinylidene ligand), electrochemical (metal-centered redox: RuII ⇄ RuIII), and photochemical (diarylethene: ring-opening ⇄ ring-closing) stimuli. The six states can be interconverted along seven

5. SWITCHES Molecular switches can convert from one state to another and change their chemical as well as physical properties by external stimuli, such as photons, electrons (or holes), or chemicals.459 Among the stimuli, photon is the most convenient because of easy and rapid on/off switching operation and remote control capability. Diarylethenes, which interconvert between two discrete open- and closed-ring form states upon alternate irradiation with UV and visible light, can be used as key elements of various light-driven molecular switches. 5.1. Optical Properties

Photoinduced cyclization and cycloreversion reactions of diarylethenes belong to electrocyclic reactions between hexatriene and cyclohexadiene structures, as described in section 2. The open-ring isomer having a hexatriene structure is colorless in most cases, while the closed-ring isomer having a cyclohexadiene 12219


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Figure 37. Single-molecule fluorescence photoswitching in a poly(methyl acrylate) (PMA) thin film containing dyad 266. (a−g) Wide-field fluorescence images of 266 embedded in the PMA thin film. (h) Time trace of fluorescence intensity of a single molecule of 266, represented as a white circle in panels a−g. Green, violet, and blue bars indicate excitation periods with 532 nm laser light, UV light, and visible light, respectively. Reprinted with permission from ref 370. Copyright 2011 American Chemical Society.

Scheme 25. Interconversion of Compound 282 by Chemical, Electrochemical, and Photochemical Stimuli455

a planar structure and its π-electrons are delocalized throughout the molecule. The difference in the two electronic structures brings about the physical property changes, which have been widely used to switch the physical and chemical functions of the molecular systems. In this section, the switching of the optical properties of the diarylethene molecular systems will be described.

structure has a color of yellow, red, or blue, depending on the molecular structure. Not only the color but also various physical properties are different between the open- and the closed-ring isomers because of the difference in the electronic and geometrical structures. A striking feature of the molecular systems is that while the π-systems of the two aryl rings are discontinued in the open-ring isomer, the closed-ring isomer has 12220


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Scheme 26. A Triad That Uses Only Optical Input Stimuli and Generates Optical Output Signalsa

a

The designations FG and DTE indicate fulgimide and dithienylethene moieties, respectively.458b

Scheme 27. Diarylethenes That Show Refractive Index Changesa

a

Δn was determined at 1.5 μm.462,465

Because of delocalization of π-electrons, the closed-ring isomer has higher polarizability and refractive index.412,414,460−467 Refractive index has been measured for amorphous neat film of compound 284 (Scheme 27).462 While the open-ring isomer 284a has a refractive index of 1.551 at 817 nm, the value of the closed-ring isomer 284b increases to 1.615. The variation of the refractive index is as large as 0.064. Bertarelli and co-workers465 reported ellipsometric study on the photoinduced refractive index change (Δn) between the open- and closed-ring isomers of a series of diarylethenes 13, 273, 285−287 (Scheme 27) in the transparency region from 800 to 1700 nm. Although Δn of compound 13 is lower than 10−3, Δn increases to (3 ± 1) × 10−3 for compound 273 at 1.5 μm. Compound 287 showed the best Δn as large as 10−2 between 800−1000 nm. Donor substituents on diarylethenes give rise to an increase in Δn. Kim and co-workers410,411,468 applied this difference to waveguide-type optical switches. They prepared a Mach− Zehnder modulator, which is composed of polycarbonate doped with compound 15 in the core and a thick light blocking metal layer on it, as shown in Figure 38.411 The metal layer was opened on one arm of the modulator, so that only one arm of the

Figure 38. Configuration of the Mach−Zehnder modulator using compound 15. (a) Au layer (200 nm), (b) UV curable VTC-2 (25 μm), (c) polycarbonate doped with compound 15 (2 μm, 37 wt %), (d) Cyclotene 3022 (benzocyclobutene) (18 μm), (e) silicon wafer. Reprinted with permission from ref 411. Copyright 2002 AIP Publishing LLC.

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modulator can be irradiated by modulation light, thus allowing a differential phase shift. The optical modulator exhibited an extinction ratio of about −12 dB at a wavelength of 1.55 μm. Holographic memory431,433,438,439 and optical gratings409 were also fabricated on the basis of the refractive index change. Nonlinear optical properties of diarylethene derivatives are controlled by photoirradiation.296,336,337,455,469−476 When πconjugation length between donor and acceptor chromophores is switched by photoisomerization, there appears a difference in the polarizability of the electrons, and consequently the NLO property significantly changes. Le Bozec and co-workers336,470 measured electric-field induced second harmonic generation (EFISH) for compound 288 (Scheme 28) and showed that the

Scheme 29. Resonance Structure of the Open- and ClosedRing Isomers of a Diarylethene Diradical

Scheme 28. A Diarylethene Derivative That Shows Photoswitching of Nonlinear Optical Properties470 electrons. In the closed-ring isomer strong antiferromagnetic interaction is expected to operate, while the magnetic interaction in the open-ring isomer should be very small. On the basis of this principle, switching of magnetic interaction has been demonstrated.481−497 Two radicals are introduced to a diarylethene, as shown in compound 290 (Scheme 30), in which 1,2-bis(2-methyl-1benzothiophen-3-yl)perfluorocyclopentene 15 is employed as a photochromic spin coupler and nitronyl nitroxides as spin sources.481 The measurement of magnetic susceptibilities revealed that the spins of 290b have a remarkable antiferromagnetic interaction (2J/kB = −11.6 K), although the interaction between the two spins is small in the open-ring isomer 290a (2J/ kB = −2.2 K). The photoinduced change in magnetism agrees well with the prediction that the open-ring isomer has an “OFF” state and the closed-ring isomer has an “ON” state. The magnitude of the switching ratio is estimated to be more than 150-fold based on the measurement and simulation of ESR spectrum of diarylethene diradical 291.487 The switching direction was reversed by using a bis(2thienyl)ethene structure. Using a bis(2-thienyl)ethene diradical 292, switching direction is reversed; the open-ring isomer has an “ON” state, and the closed-ring isomer has an “OFF” state.489 Figure 39 shows the array of diarylethenes used for the switching. The ESR spectra of 293(OO) and 293(OC) are 5-line spectra, suggesting that the exchange interaction between the two nitronyl nitroxide radicals is much weaker than the hyperfine coupling constant. Yet, the spectrum of 293(CC) has a clear 9line spectrum, indicating that the exchange interaction between the two spins is much stronger than the hyperfine coupling constant. The result indicates that each diarylethene chromophore serves as a switching unit to control the magnetic interaction.488 Switching of bulk magnetism has been achieved by intercalating diarylethenes into layered magnetic systems.498−500 Kojima and co-workers500 prepared cobalt layered double hydroxides (Co-LDHs) intercalated by diarylethene sulfonate, as shown in Figure 40. The Curie temperature Tc of the layered magnetic system was measured to be 9 and 20 K for the openand closed-ring isomers, respectively. The delocalization of πelectrons in the closed-ring isomer is the origin of the increase in the Curie temperature. Spin-crossover phenomenon is an attracting target to control the spin state in bulk materials. When photochromic ligands are incorporated into spin-crossover complexes, photoisomerization of the ligands would change the spin state of the metal

value of μβ (where μ is the dipole moment and β is the quadratic hyperpolarizability coefficient) is 200 × 10−48 esu for the openring isomer, while it increased to 4220 × 10−48 esu for the double closed-ring isomer. Around 20 times enhancement upon photocyclization was observed. When diarylethene has a chiral center, optical rotation changes along with the photoisomerization.172,177,188,189,477−479 The change of the optical rotation is observed in the wavelength range where no absorption band exists. Therefore, detection of the optical rotation can be used for nondestructive readout in optical memory. When a helicene-like molecule is used, the difference becomes very large. Branda and co-workers188 reported that the difference in specific rotation according to the isomerization of compound 108 is as large as 8698° at 373 nm. Birefringence can be controlled by photoirradiation.480 In a sol−gel film, diarylethene molecules do not change their orientation. When linearly polarized light is used for the isomerization, there appears linear dichroism in the visible absorption and birefringence in the near-infrared region. This technique is also a possible candidate for nondestructive readout in optical memory. 5.2. Magnetism

There is a characteristic feature in the electronic structural changes of diarylethenes with regard to magnetic interactions.481 Scheme 29 shows an example of open- and closed-ring isomers of radical-substituted diarylethenes. While there is no resonant closed-shell structure for 289a, there exists 289b′ as a resonant quinoid-type closed-shell structure for 289b. In the open-ring isomer 289a, the bond-alternation is discontinued at the 3position of the thiophene rings, but in the case of the closed-ring isomer 289b, the ground electronic state has no unpaired 12222


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Scheme 30. Diarylethene Diradicals Showing Photoswitching of Magnetic Interaction481,487,489

Figure 39. Diarylethene dimer 293 that shows photoswitching of magnetic interaction. X-band ESR spectra measured at room temperature in benzene (9.32 GHz): (a) 293(OO), (b) 293(CO), and (c) 293(CC). Reprinted with permission from ref 488. Copyright 2001 American Chemical Society.

complexes. This concept is called ligand-driven light-induced spin-change (LD-LISC). Several spin-crossover complexes using diarylethenes as photochromic ligands have been prepared.501−504 Photoinduced spin change was monitored for the Fe(II) complex with diarylethene ligand 294 (Scheme 31).501 An increase in the magnetization was observed upon irradiation with 355 nm light due to the LD-LISC effect. Nihei et al.503 synthesized Fe(II) spin-crossover complex with a diarylethene moiety 295. The complex showed bidirectional spin-state switching by light-induced excited-spin-state trapping (LIESST) effect and LD-LISC effect induced by the photochromism of the diarylethene. Single-molecule magnets (SMMs), which behave as nanosized magnets below blocking temperature, have recently attracted

much attention. The control of SMM behavior using a diarylethene photochromic unit has been carried out.505,506 Figure 41 shows an example of SMMs made of Mn4 cluster linked by a diarylethene ligand 296 having two carboxylate groups.505 Upon visible irradiation the Mn4 cluster linked by the closed-ring isomer underwent a cycloreversion reaction at the diarylethene moiety, which results in the drastic change in the magnetic behavior. The change in the magnetic behavior is attributed to the significant increase in the antiferromagnetic through-space interunit interaction caused by the change in the crystal packing structures. 5.3. Electric Conductance

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on both the ionization potential barrier for carrier injection and the mobility of carriers. The ionization potential of the closedring isomer is smaller than that of the open-ring isomer. Therefore, the carrier injection can be controlled by photoisomerization. Tsujioka and Masuda512 prepared a device by depositing a diarylethene having diphenylamino groups 298 and a hole injection/transport layer (N,N′-di(naphthalene-1-yl)N,N′-diphenyl-benzidine (α-NPB)) between magnesium cathode and ITO anode, as shown in Figure 43, to reveal the mechanism of carrier injection and transport characteristics of diarylethene switching devices. By introducing the hole transport layer, the current was effectively controlled upon photoirradiation. Various types of photocurrent switching devices have been developed by Tsujioka and co-workers.518−520 Aida and co-workers521 prepared hexabenzocoronene graphitic nanotubes appended with diarylethene pendants 299, as shown in Figure 44. Photoisomerization of the diarylethene pendants on the nanotubes controls the photoconductivity in the graphite-like bilayer wall. Photoactive electrodes can be constructed by surface modification by diarylethenes. Uosaki and co-workers522 prepared diarylethene-modified Si(111) surface and used for the photoactive electrode. The current density was found to change upon alternate UV and visible light irradiation. The properties of field-effect transistors523−525 and photovoltaic cells526 have also been examined by introducing diarylethenes. Photoswitching of conductance of a single molecule is a longstanding dream in the field of molecular electronics. Feringa and co-workers527 reported one-way photoswitching of a diarylethene molecule using a gold nanogap prepared by a mechanically controllable break junction technique, as shown in Figure 45. A diarylethene with thiol groups 300 was prepared, and its conductance was measured along with the photoisomerization. By irradiation with visible light to the closed-ring isomer, the resistance increased about 3 orders of magnitude. Although in solution UV irradiation can induce the reverse cyclization reaction, the cyclization reaction was prohibited when the molecule was connected to gold. Nuckolls and co-workers528 prepared a diarylethene-bridged single-walled carbon nanotube (SWNT) 301 and 302, as shown in Figure 46. SWNT was oxidatively cut through a lithographic mask defined with an electron beam. The amide bond then was formed between diarylethene diamine and carboxylic acids that terminate the SWNT. By irradiation with UV light, the conductance increased by more than 5 orders of magnitude. The pyrrole-based compound is known to undergo a T-type thermally reversible photochromic reaction. Therefore, the high

Figure 40. Schematic representation of the photoinduced conversion between Co-LDHs with the open-ring isomer of diarylethene (left) and Co-LDHs with the closed-ring isomer of diarylethene (right). Reprinted with permission from ref 500. Copyright 2006 American Chemical Society.

Conjugated polymers having photochromic diarylethenes in the main chain have been prepared to photocontrol the electrical conductivity.315,507−510 The closed-ring isomers are considered to provide higher conductivity than the open-ring isomers from the viewpoint of their smaller HOMO−LUMO gap and fully conjugated π-systems. A diarylethene-oligophenylene-fluorene polymer was prepared, and its change in conductivity upon photoirradiation was investigated.507 The conductivity of the polymer having the closed-ring isomers is higher than that having the open-ring isomers. Meerholz and co-workers510 synthesized oxetane-functionalized dithienylethene 297 and prepared a photoprogrammable light-emitting device, as shown in Figure 42. The monomer 297 (XDTE) was cross-linked using a photoinitiated cationic ring-opening polymerization, and a flexible photochromic layer was prepared. A hole transport layer (XTPD) based on triphenylamine dimer was introduced between poly(3,4-ethylenedioxythiophene) (PEDOT):polystyrene-4-sulfonate (PSS) and XDTE polymer. By changing the HOMO level of XTPD, the ON/OFF ratio could be optimized. Electrical conductivity before and after photoisomerization was measured, and the ON/OFF ratio was found to be as high as 3000. Thin layer films of neat diarylethenes were also used to control electrical current.511−517 The electrical conductivity is dependent

Scheme 31. Diarylethene Iron(II) Complexes Showing Ligand-Driven Light-Induced Spin-Change Effect (LD-LISC)501,503,504

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Figure 41. Proposed mechanism for the occurrence of the antiferromagnetic ordering in 296b upon irradiation with visible light, where the arrows indicate the magnetization of SMM aligned on an easy axis. Reprinted with permission from ref 505. Copyright 2009 American Chemical Society.

isomerization reactions can provide information on the conductance of a single molecule of each isomer. van Wees and co-workers536−539 reported the increase of the apparent height of spots of single diarylethene molecules along with the cyclization reaction, which is caused by the conductance change of the molecules. Reversible photoswitching of STM apparent height was observed using asymmetric diarylethene molecules 309 embedded in insulating dodecanethiols on Au(111).539 The closed-ring isomer was observed as a bright protrusion. Upon irradiation with visible light the protrusion disappeared, and by subsequent irradiation with UV light the protrusion was reconstructed, as shown in Figures 49 and 50. The asymmetric structure enables reversible cyclization/cycloreversion reactions upon alternate irradiation with UV and visible light, even when the molecules are placed on the gold substrate. Several other attempts have been carried out to control singlemolecule conductance, such as STM-break junction,540,541 conductive AFM,542 and others.543−546 The use of metal complexes is also proposed.547,548 Theoretical study on the photoswitching of conductance of a single diarylethene molecule has been extensively carried out. Not only the molecule itself but also the effect of gold electrode549−560 or carbon nanotube electrode561−563 was investigated. Yoshizawa and co-workers551−553 studied the conductance of diarylethenes using a nonequilibrium Green’s function method combined with density functional theory. The

conductance state returns to the low conductance state in the dark, and the on/off cycle can be repeated. Figure 47 shows a new method to control electrode-molecule binding.529 Two electrodes were connected by a molecular wire as follows. In the first step, the electrodes and iodobenzenethiol are bound to each other, and in the second step, a molecular wire, which is a π-conjugated molecule covered with α-cyclodextrin 303, is introduced. In the last step, p-diiodostilbene 304 and diarylethene residue 305 bridge the molecular wires. Current− voltage characteristic measurement revealed that the current changes as large as 20-fold upon alternate irradiation with UV and visible light. Gold nanoparticle networks are also used to make molecular devices for conductance photoswitching.530−535 Gold nanoparticles were reacted with dithiolated diarylethenes 306−308 to form networks and placed onto interdigitated gold electrodes, as shown in Figure 48. Reversible conductance photoswitching was observed by alternate irradiation with UV and visible light. When bis(2-thienyl)ethene dithiol was used as the switching units, such as 308, switching direction is reversed; the open-ring isomer has an “ON” state, and the closed-ring isomer has an “OFF” state. Diarylethenes isolated in host self-assembled monolayers (SAMs) on Au(111) surface were observed by scanning tunneling microscopy (STM). STM measures apparent height that is a convolution of electronic and topographic characteristics. The change of the apparent height along with photo12225


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Figure 42. (a) Layer arrangement of the photoswitchable device. (b) Switching dynamics of the optimized device containing compound 297, simultaneously measuring the absorption of the photochromic layer (black line, left axis) and the current density (blue line, right axis). The applied voltage was 6 V. (A) Under UV irradiation (312 nm; OFF → ON). (B) Under irradiation with orange light (590 nm; ON → OFF). Adapted with permission from ref 510. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

isomers is on the order of magnitude 102. It was also recognized that the choice of anchoring site of the diarylethene plays an important role in the control of electron transport. 5.4. Chemical Reactivity and Bioactivity

Switching of pKa has been reported by using photoisomerization of diarylethenes.564−567 Lehn and co-workers564 utilize photoswitching of π-conjugation to control the effect of the electronwithdrawing group on the acidity of a phenol-substituted diarylethene, as shown in Scheme 32. Although the open-ring isomer 310a has a pKa value of 10.5, the closed-ring isomer 310b decreases the pKa value to 9.3. By photoisomerization of the diarylethene, the increase in the dissociation constant by a factor of 16 was observed. Irie and co-workers565 reported a pKa switching system using diarylethenes containing 2,5-diaryl-3thienyl group. In the open-ring isomer 311a the phenol unit interacts with the electron-donating methoxy group, but in the closed-ring isomer 311b the phenol group interacts with the electron-withdrawing pyridinium group. The pK a value decreased from 10.2 to 9.0 when the open-ring isomer was converted to the closed-ring isomer. Chemical reactivity and catalytic activity can be photoswitched using the photoisomerization of diarylethenes.568−576 Branda and co-workers571 synthesized fulvene derivatives 312 and 313 (Scheme 33). These molecules undergo reversible Diels−Alder

Figure 43. Structure of a device composed of diarylethene 298 having diphenylamino groups and N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (α-NPB) layers. Reprinted with permission from ref 512. Copyright 2003 AIP Publishing LLC.

π-electron conjugation and HOMO−LUMO gap are two important parameters that determine the conductance of molecules. The higher is the delocalization, the faster is the electron transport. The higher conductivity of the closed-ring isomer is ascribed to the π-electron delocalization in the system and the narrow energy gap. The theoretical calculation predicts that the ratio of conductance for the closed- and the open-ring 12226


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Figure 46. (a) Molecular bridges between the ends of an individual SWNT electrode. (b) Switching between conjugated and nonconjugated molecular structures. Reprinted with permission from ref 528. Copyright 2007 American Chemical Society.

Figure 44. (a) Molecular structures of the isomers of diarylethene 299 for nanotube formation. (b) Schematic representation of nanotubes of the open-form and the closed-form with a coaxial configuration where a graphite-like bilayer is laminated by molecular layers of open-form (yellow) and closed-form diarylethenes (green), respectively. Reprinted with permission from ref 521. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 45. Photochromic molecular switch between two Au contacts in (a) closed state and (b) open state. By exposing the molecule to light of wavelength in the range 500 nm < λ1 < 700 nm, the molecule switched from 300b to 300a. Reprinted with permission from ref 527. Copyright 2003 The American Physical Society.

reaction with a dienophile to generate diarylethene derivatives 314a and 315a, which cyclize upon irradiation with UV light. The photogenerated 314b and 315b can be stably isolated and become inactive to the retro-Diels−Alder reaction. Visible light irradiation induces the cycloreversion reaction and regenerates the cyclohexene moiety. The photogenerated cyclohexene can undergo the retro-Diels−Alder reaction to release the dienophile. When two compounds 314b and 315b are mixed, appropriate wavelength of light only induces reaction on either 314b or 315b, so that selective release becomes possible. Branda and co-workers572 also reported the control of coppercatalyzed cyclopropanation of olefins with diazoesters, as shown

Figure 47. Self-organized interconnect method for molecular devices. 304 and 305 function as a conduction wire and as an optical switching device, respectively. Molecule 303 is a π-conjugated molecule covered with α-cyclodextrin. Reprinted with permission from ref 529. Copyright 2006 American Chemical Society.

in Scheme 34. When the open-ring isomer 316a is used as a ligand of the copper complex, the reaction showed significant enantioselectivity, but the complex composed of purified closed12227


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Figure 48. Schematic drawing of a diarylethene−gold nanoparticle network. The diarylethene dithiols bridge the gold nanoparticles and make a network between nanogapped interdigitated gold electrodes. Diarylethene molecules and gold nanoparticles make a conducting path between the electrodes. Adapted with permission from ref 531. Copyright 2008 American Chemical Society.

Energy transfer and electron transfer are important phenomena in many biological systems and electronic materials. Photoswitchings of energy and electron transfer processes using diarylethenes have been reported by several groups.314,361,577−582 Coudret, Launay, and co-workers577 reported that the intervalence band due to intramolecular electron transfer between ruthenium center can be detected for the closed-ring isomer, but not for the open-ring isomer. He and Wenger580 synthesized diarylethenes with appended amine units and investigated the behavior of the oxidized radical cations by electrochemical and optical spectroscopic methods. The mixed valence states of the closed-ring isomers were dependent on the distance between two nitrogen atoms. A new emerging field is the photoswitching of bioactivity.583−586 Branda and co-workers584 reported photocontrol of paralysis of C. elegans by diarylethene 114 having Nmethylpyridinium groups, as shown in Figure 51. C. elegans is one of the simplest organisms with a nervous system, having 302 neurons, so that it has been extensively used as a model for neuroactivity and metabolism. Moreover, its transparent nature makes it an ideal candidate for monitoring color changes in vivo. Paralysis is induced by cyclization of the diarylethene by irradiation with UV light and restored by cycloreversion reaction with visible light. The paralysis in C. elegans is likely a result of interruption of the metabolic electronic pathways due to the unique electron-accepting ability of the closed-ring isomer.

Figure 49. (a) A mixed self-assembled monolayer (SAM) incorporating dodecanethiol and diarylethene 309. Individual diarylethene molecules are isolated within the dodecanethiol matrix. Scanning in constantcurrent mode shows the more conductive “on” state of diarylethene as a different apparent height as compared to the apparent heights of the “off” state of diarylethene. (b) Chemical structures of the “on” state (left) and the “off” state (right) of the asymmetric diarylethene 309. Reprinted with permission from ref 539. Copyright 2013 The Royal Society of Chemistry.

5.5. Multichromophoric and Supramolecular Systems

ring isomer 316b did not bring about the enantioselectivity. More recently, Neilson and Bielawski576 synthesized a diarylethene-annulated N-heterocyclic carbene complex, which shows reduced catalytic activity of hydroboration of alkenes and alkynes by up to an order of magnitude upon UV irradiation.

Several multichromophoric systems have been synthesized, as shown in Scheme 35 (refs 103, 107, 117, 120, 121, 220, 271, 587−589, and 604). These systems can provide multimode switching between more than two states depending on the 12228


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structures.95,171,590−600 Reinhoudt and co-workers590−592 synthesized diarylethene-tethered cyclodextrin 321 (Scheme 36) and used this compound as a photoswitchable host for mesotetrakis(4-sulfonatophenyl)porphyrin (TSPP). In the open-ring isomer, the two intramolecularly linked β-cyclodextrin cavities have a certain amount of flexibility to bind TSPP tightly, while the binding is much less favorable in the photogenerated closedring isomer. The binding constant of the open-ring isomer was 35 times larger than that of the closed-ring isomer. Hecht and coworkers594 synthesized hydrogen-bonded diarylethene 322. The binding affinity of the closed-ring isomer to complementary melamine receptors was larger than that of the open-ring isomer. Supramolecular structures containing DNA sequences were also investigated.171,597−600 Supramolecular structures constructed by metal complexation or binding of small molecule were studied (refs 166, 265, 299, 322, 323, 331, 475, 476, and 601−608). Andréasson et al.602 demonstrated geometrical structure control of a porphyrin dimer by photoinduced cyclization/cycloreversion reaction of a pyridine-appended diarylethene ligand 323 (Scheme 37). The geometrical structural change of the porphyrin dimer induces the absorption and the fluorescence spectral changes. Because the monitoring light does not influence the ratio of the two isomers of the diarylethene unit, nondestructive readout of the absorption spectral change can be achieved. The geometrical structure change of diarylethenes has been applied to construct molecular machines.609−611 Scheme 38 shows a ternary complex 324 interconnected by a bridging rotary module using a diarylethene.609 The cyclization/cycloreversion reactions of the diarylethene induce the scissoring motion of the chiral ferrocene derivative. Yokoyama and co-workers612 prepared dendrimers having a diarylethene core 325 (Scheme 39). When they are incorporated into polycarbonate films, the thick wall of dendron provides fluidity around the core diarylethene and maintains the favorable cyclizable antiparallel conformation, resulting in high conversion ratio to the closed-ring isomer even after many cyclization/ cycloreversion cycles. Takagi and co-workers613−616 prepared clay-diarylethene hybrid materials, and Iwamoto and co-workers617 prepared mesoporous silica-diarylethene hybrid materials. The unique reactivities in mesoporous silicas and liposomes have also been reported.619 Preparation of organic nanoparticles will be described in section 7.7.618

Figure 50. (a) Cropped sequential STM images of the molecular photoswitching of compound 309 in an area of 100 nm2. The apparent height is reduced upon irradiation with visible light for 13 min (f and g). UV light irradiation was used to switch diarylethene molecules back to the “on” state (h and i). In (j), the molecular conductance is switched back to its “off” state. (b) The variation of apparent height as a function of time and irradiation. Reprinted with permission from ref 539. Copyright 2013 The Royal Society of Chemistry.

number of diarylethene units. All diarylethene units are not equally reactive. Photocyclization of one part of the molecule impedes the photoreactivity of the remaining other units. In most of the multichromophoric systems, fully closed-ring isomers are hardly prepared, as described in section 3.1.1.119,121 Geometrical structural changes along with photoisomerization can be used for controlling hydrogen-bonded supramolecular Scheme 32. Diarylethenes Exhibiting Photoswitching of pKa564,565

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Scheme 33. Fulvene Derivatives That Undergo Diels−Alder Reactions with a Dienophile To Produce Photoisomerizable Diarylethenes571

appearance of the CD signal suggests that while the open-ring isomers do not form a chiral structure, the closed-ring isomers assembled into a chiral nanostructure (Scheme 42). The difference in the aggregated structure is attributed to the difference in the molecular planarity. Another example is the control of the lower critical solution temperature (LCST). Photoinduced LCST transition was observed in diarylethene 329 (Scheme 42). Aggregation of diarylethene dyes has also been extensively studied.281,355,372,373,636−656

Scheme 34. A Diarylethene Showing Photoswitching of Catalytic Activity for Stereoselective Cyclopropanation572

5.6. Liquid Crystals

When photochromic diarylethenes are doped in a liquid crystal, the properties of the liquid crystal can be photochemically controlled. Chiral diarylethenes can be used as chiral dopants in liquid crystalline systems.318,657−666 The liquid crystal of 4cyano-4′-pentylbiphenyl (K-15) exhibits a nematic texture even when a small amount of chiral diarylethene 330 is added (Scheme 43).657 Upon irradiation with UV light, the nematic texture changes to a cholesteric fingerprint texture. The fingerprint texture returns again to the nematic texture by irradiation with visible light. The change of the texture is attributed to the larger helical twisting power of the closed-ring isomer than that of the open-ring isomer. Lemieux and co-workers667−669 reported photoswitching of a ferroelectric liquid crystal. They doped diarylethene 331 (Scheme 43) to a ferroelectric liquid crystal composed of MDW950 and PhP1. By increasing the temperature, the ferroelectric SmC* phase showed a phase transition to a nonferroelectric SmA* phase. Upon irradiation with UV light, the SmC*−SmA* phase transition temperature decreased significantly, due to the geometrical structure change of the diarylethene dopant. Some diarylethene derivatives form liquid crystals. Mehl and co-workers670−674 synthesized diarylethene 332 (Scheme 44), which has cyanobiphenyl groups as mesogens via oligomethylene chain spacers. The open-ring isomer showed smectic C phase between 33.3 and 74.5 °C and showed nematic phase between 74.5 and 78.5 °C. In the photostationary state containing the closed-ring isomer, smectic X phase appeared between 26.5 and 54.6 °C and nematic phase between 54.6 and 75.9 °C. The liquid crystalline properties can be modulated by photoirradiation. Some other liquid crystalline systems, in which diarylethenes are covalently incorporated into mesogenic molecules, were also reported.675−677

Photocontrol of large supramolecular structures has been examined. Hydrogen-bonded self-assemblies containing photochromic diarylethenes have been constructed, and their photochemical reactivities were investigated (refs 168, 169, 282, 354, and 620−631). Diarylethene 326 (Scheme 40) having quadruple hydrogen-bonding moieties forms supramolecular polymers, and their particle size was found to increase when the diarylethenes undergo the photocyclization reaction.622 Yagai and co-workers628 prepared diarylethene 327 (Scheme 41) equipped with two monotopic melamine hydrogen-bonding sites. Upon mixing it with oligothiophene-functionalized ditopic cyanurate (OTCA) in a nonpolar solvent, AA-BB-type suprastructure copolymers bearing photoswitchable moieties in their main chains and extended π-systems as the side chains were formed through H-aggregation of the oligothiophene side chains. Upon irradiation with UV light, a transition occurred from the Haggregated state to nonaggregated monomeric oligothiophene side chains. The H-aggregated helical nanofibers were regenerated upon irradiation with visible light. The results indicate that the hierarchical organization can be effectively controlled by photoirradiation. Solvophobic effect was observed in photochromism of diarylethenes.632−635 The molecules with hydrophilic side chain and hydrophobic core self-assemble into nanostructures in water. Amphiphilic diarylethene 328 (Scheme 42) performs different aggregation behavior between the open- and the closedring isomers. When asymmetric methyl groups were introduced in the amphiphilic side chains, exciton-coupled type CD signal appeared upon irradiation with UV light in water. The

5.7. Metal Nanoparticles

Photochromic behavior of diarylethenes on surfaces of metal nanoparticles has been investigated.310,678−681 Gold and silver nanoparticles covered with diarylethenes 333 having an alkanethiol group were prepared, and their photochromic 12230


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Figure 51. Fluorescence microscopy images of C. elegans incubated with (a) 10% DMSO and photoswitch 114b (12 mM) (b) within the first 10 min and (c) after 60 min. Optical microscopy shows the photoswitch being interconverted between (d) colorless 114a and (e) blue 114b with light of wavelengths longer than 490 nm and UV light (365 nm) for 20 and 2 min, respectively. (f) The number of mobile, nonresponsive, and paralyzed nematodes for samples that have been treated with the ring-open or the ring-closed form of the photoswitch as compared to controls after 60 min incubation and (g) for samples exposed to varying amounts of 114b. Reprinted with permission from ref 584. Copyright 2009 American Chemical Society.

chromic ring-opening reaction is proportional to the square of the irradiation fluence. The localized surface plasmon on the gold nanoparticle caused the two-photon reaction. Upconversion characteristics of nanoparticles of lanthanide complexes are attractive because the nanoparticles can emit light by excitation with NIR light.389−391,695−697 Surface plasmon resonance of thin gold films is very sensitive to the change in refractive index of materials on the surface. Fiber optics based on the surface plasmon resonance provides a sensitive way to measure the change in the resonance wavelength. The shift of surface plasmon resonance was used for nondestructive readout.698,699 Diarylethene molecules were covalently linked to a chemically modified gold surface, and the photoisomerization was detected by the change in the refractive index by surface plasmon resonance. The use of magnetic iron oxide nanoparticles was also investigated.700

reactivity was explored, as shown in Figure 52. In general, the photoexcited states of organic dyes on noble metal nanoparticles are quenched due to a surface plasmon resonance, but photochromic reactions of diarylethenes can take place even on the noble metal surfaces. The very fast reaction rate of diarylethenes, which is comparable to the quenching rate, enables the diarylethenes to undergo the photoisomerization.310 The shift of the local surface plasmon resonance has been observed along with the photoisomerization of diarylethene that covers the gold nanoparticles.682−691 Kobatake and co-workers used relatively large gold nanoparticles covered with diarylethene polymer 334 and observed a shift of plasmon resonance, as shown in Figure 53.683 The shift is attributed to the change in the refractive index of the photochromic chromophores. Enhancement of electromagnetic field at the surface of gold nanoparticles or gold nanodimer induces two-photon-induced photochromic reaction of diarylethenes.692−694 Tsuboi et al.692,693 found that the two-photon ring-opening reaction of diarylethenes was induced by near-infrared continuous-wave laser light, when an ultrathin polymer film doped with diarylethenes in its closed form was coated onto a goldnanoparticle-integrated glass substrate. The yield of the photo-

5.8. Other Properties

Matsuda and co-workers701 reported the photoinduced change in molecular ordering of diarylethene dimer 335 at a solution− HOPG interface studied by STM, as shown in Figure 54. Twodimensional ordering of diarylethene derivatives carrying pyrene 12231


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Scheme 35. Structures of Multichromophoric Diarylethenes

Scheme 36. Diarylethenes Whose Supramolecular Structures Can Be Controlled by Photoirradiation590−592,594

the ordering of the closed-ring isomers. Several other STM

moiety formed at octanoic acid−HOPG interface was observed by STM. Upon irradiation with UV light, a new ordering appeared and returned to the original ordering upon subsequent irradiation with visible light. The new ordering was assigned to

studies on diarylethene molecules placed on the surfaces of Si(100) and Au(111) have been reported.702−705 12232


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Scheme 37. Isomeric Forms of Pyridine-Appended Diarylethene Photoswitch 323 and the Diarylethene-porphyrin Dimer Complex (Ar = 3,5-di(octyloxy)phenyl, R = Si(C6H13)3)602

the applied electric field. The change in the polarization induced by the photochromic reaction of the molecules in the dielectric causes an unbalanced electric field and induces an electric current flow in the external circuit; thus the energy can be taken out. Photochromic electret can directly convert the photon energy into electric energy. Morimoto and Irie707 prepared imidazoline-substituted diarylethene 336 and reported photomodulation of the dielectric properties, which is based on the intermolecular proton transfer in one-dimensional hydrogen-bonded chains, as shown in Figure 56. Other aspects of diarylethene properties, such as solvatochromism,708 X-ray photoemission spectra (XPS spectra) of open- and closed-ring isomers on Au(111),709 and applicability of the photoswitchable dye to probe water convection in high magnetic field, were also studied.710

Scheme 38. Structure of a “Molecular Reacher” (Ar = 3,5dioctyloxyphenyl)609

Scheme 39. Structure of Dendrimers Having a Diarylethene Core612

6. PHOTOCHROMIC POLYMERS Photochromic polymers are useful for various types of applications. Polymers having diarylethene derivatives in the main chain (refs 42, 51, 96, 209, 224, 284b, 315, 316, 320, 383, 397, 398, 403, 507−509, 543, 570, 612, and 711−722) or the side groups (refs 318, 319, 386, 412, 460, 682−686, and 723−735) have been prepared. Polymer films, in which diarylethene derivatives are dispersed or embedded, have also been used for photonics applications (refs 359, 381, 409, 411, 423, 463, 468, 636, 638, 643, 655, and 736−748).

Bertarelli and co-workers706 proposed a new concept, “photochromic electrets”, and carried out a principle experiment by using 1,2-bis(2-methyl-5-(p-cyanophenyl)-3-thienyl)perfluorocyclopentene (285). The photochromic electret is composed of a capacitor with a dielectric material doped with the photochromic dye, which is sandwiched between plates, as shown in Figure 55. By performing a poling process, the photochromic molecules rearrange with dipole moment along

6.1. Polymers Having Diarylethenes in the Main Chain

Table 12 summarizes polymers having diarylethenes in the main chain. Diarylethene polymer 16 exhibited high cyclization quantum yield, Φ = 0.86.96,713 The cyclization reaction takes place in a sub-200 fs time scale, as described in section 2.2.1.42 12233


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Scheme 40. A Diarylethene Derivative Having Two Ureidopyrimidinone Groups at Both Endsa

a

The closed-ring isomer 326b formed a supramolecular polymer.622

Scheme 41. Melamine-Appended Diarylethene 327 and Oligothiophene-Functionalized Cyanurate (OTCA)a

a

Dodecylated cyanurate (dCA) was used as a reference.628

Scheme 42. Amphiphilic Diarylethenes That Self-Assemble in Water Due to the Hydrophobic Effect632−634

The Bertarelli group also prepared polyurethanes 276 and polyesters 339 having diarylethenes in the main chains, as shown in Scheme 23.403,543,715 Photochromic reactivity of the diarylethenes in the main chains of these polymers remains similar in solution because of relatively low Tg of the polymers. The polyurethanes were used for multistate optical memories403 and optical metrology,714 and the polyester for volume phase holographic grating715 and light-controlled resistance modulation.543 π-Conjugation polymers having diarylethenes in the main chain have been prepared by Pd-catalyzed cross-coupling,315,316,507 Wittig condensation,320,509,716,718 Horner reaction,717 Knoevenagel condensation,383 or Friedländer condensation.508 Such π-conjugation polymers can modulate fluorescence315,316,320,383,718 and electric property507−509 by the photochromic reactions. Park and co-workers have designed diarylethene polymer 346 showing fluorescence in the neat polymer film.383 Fluorescence switching ratio of more than 10 times was observed by alternating irradiation with UV and visible light. 1,2-Bis(2-methyl-1-benzothiophen-3-yl)perfluorocyclopentene substituted with 3,4-ethylenedioxythiophene underwent electrochemical oxidation to afford polymer films 348 deposited on an ITO glass.224 The electrochemical deposition of the diarylethene derivative offers a convenient

Scheme 43. Diarylethene Dopants for Liquid Crystals657,667,668

Scheme 44. A Diarylethene Liquid Crystal670,673

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Figure 52. Diarylethene-metal nanoparticles.

sexithiophene polymers. On the other hand, in the closed state, oxidation results in the formation of a stable dication, which cannot undergo α,α-dimerization to produce polymers. 6.2. Polymers Having Diarylethenes in the Side Chain

Table 13 summarizes diarylethene polymers bearing diarylethene chromophores in the side groups. The diarylethene polymers have been prepared by Pt-catalyzed hydrosilylation of a diarylethene connected vinyl olefin with silsesquioxanes or polysiloxane,723 ring-opening metathesis polymerization of bicyclic olefins with a diarylethene chromophore,724−726 radical polymerization of vinyl monomer with a diarylethene chromophore,460,718,727−733 Pd-catalyzed cross-coupling or Stille coupling of aromatic compounds with a diarylethene chromophore,318,319,734,735 and polymer reactions.387,729 Modification of the end groups of polymers has been carried out using living radical polymerization.412,682−686,712

Figure 53. Gold nanoparticles covered with diarylethene polymer 334.

deposition method to prepare diarylethene polymer films. Uchida et al.52,397,398,719,720 successfully prepared cross-linked network diarylethene polymers by oxidation polymerization of diarylethenes having phenol moieties. The prepolymers prepared by oxidation polymerization, which are soluble in organic solvents, are casted on a glass plate, and continuous oxidation polymerization leads to the cross-linked polymers. A highdensity diarylethene main-chain homopolymer 351 can be also synthesized by the reaction of compound 15 and chloromethyl methyl ether in the presence of TiCl4.721 The polymerization proceeds through chloromethylation, followed by Friedel− Crafts alkylation. The polymerization method is efficient to prepare polymers of various molecular weights in high yield. The polymer (Mw = 40 000) exhibits characteristic photochromic properties in solution as well as in solid film. The benzothiophene ring of the polymer can be oxidized to the benzothiophene-1,1-dioxide ring using 3-chloroperbenzoic acid.284b The closed-ring form of the oxidized polymer is fluorescent (Φf = 0.093). A diarylethene with terthiophene as the aryl groups shows electropolymerization.570 In the open state, the terthiophene cation radical formed by oxidation reacts readily via α,α-dimerization to form perfluorocyclopentene bridged

7. CRYSTALLINE PHOTOCHROMISM Diarylethenes can undergo reversible photochromic reactions even in the crystalline phase.749−751 The crystalline photochromism of diarylethenes was also serendipitously discovered during the course of synthesis of diarylethene derivatives having thiophene oligomers.132 The diiodo-derivative of compound 20, 1,2-bis(5-iodo-2,4-dimethyl-3-thienyl)perfluorocyclopentene, was prepared as the starting compound for the synthesis. Microcrystals of the iodo-derivative were accidentally found to change the color from colorless to red under UV light, which is daily used for thin-layer chromatography. At first, iodine released by the action of the UV light was thought to be the origin of the red color. But, the assumption was immediately proved not to be true, because microcrystals of compound 20, which has no iodine, also exhibit the color change from colorless to red under UV light.749 Since then, many diarylethene derivatives were carefully examined and found to undergo crystalline photochromism. This is the course of finding the crystalline photochromism in diarylethenes. Upon irradiation with ultraviolet light, the colorless crystals change to yellow, red, blue, or green, depending on the molecular 12235


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Figure 54. Sequence of STM images at the interface of octanoic acid-HOPG upon UV and visible irradiation of compound 335(o-o) (100 × 100 nm2, the same areas of the sample): (a) before UV irradiation (Iset = 10 pA, Vbias = −2.0 V); (b) after UV irradiation for 5 s (Iset = 10 pA, Vbias = −2.5 V); and (c) after successive visible irradiation (Iset = 10 pA, Vbias = −2.5 V). Reprinted with permission from ref 701. Copyright 2008 American Chemical Society.

Figure 55. Scheme of the photochromic electrets and its working principle. The poling operation aligns the dipoles. Reprinted with permission from ref 706. Copyright 2012 American Chemical Society.

Figure 56. Structure of imidazoline-substituted diarylethene 336 and schematic illustration of proton transfer in a 1D hydrogen-bonded chain of imidazoline. Reprinted with permission from ref 707. Copyright 2011 The Royal Society of Chemistry.

structure of the diarylethenes, as shown in Figure 2b. The coloration takes place not only in the surface layer but also in the crystal bulk. UV light penetrates the crystal into a depth of millimeters and induces the photochromic reaction. The colors of the crystals are due to the formation of closed-ring isomers. The colors remain stable in the dark, but disappear by irradiation with visible light. The molecular structures and colors of diarylethenes showing photochromism in the crystalline phase are summarized in Table 14 (refs 68, 71, 72, 76, 82, 83, 87, 91, 92, 134, 257, 262, 263, 265, 294, and 751−785). Photoinduced coloration/decoloration cycles of the crystals can be repeated more than 30 000 times while maintaining the photochromic performance and the bulk crystal shape.36 The response times of the coloration/decoloration are less than 10 ps even in the crystalline phase.36,786

crystal is to observe the photoirradiated colored crystal under polarized light using a polarizing microscope. Because molecules are regularly oriented in the crystal, absorption anisotropy is observed. Figure 57 shows a typical example of the colored crystal 4 observed under polarized light.68 Before photoirradiation, the crystal is colorless. Upon irradiation with UV light, the crystal turns red at a certain angle. When the crystal is rotated as much as 90°, the color becomes weak. The clear dichroism indicates that each closed-ring isomer is regularly oriented in the crystal and the photochromic reaction proceeds in the crystal lattice. Figure 58 shows the polarized absorption spectra of the photogenerated closed-ring isomer in single crystal 4 and the polar plots of absorbance at 535 nm. The orientation of the photogenerated closed-ring isomer can be evaluated by an order parameter, S = (A|| − A⊥)/(A|| + 2A⊥). The S value for single crystal 4 was determined to be 0.84 at 535 nm. Such a high order parameter indicates that the diarylethene undergoes the

7.1. Dichroism

A simple way to discern whether the photochromic reactions take place in the crystalline phase without destruction of the 12236


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Table 12. Research Categories of Diarylethene Polymers in the Main Chain

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Table 12. continued

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Table 13. Research Categories of Diarylethene Polymers Having Diarylethene Chromophores in the Side Chain

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Table 13. continued

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Table 13. continued

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Table 14. Diarylethenes Showing Photochromism in the Crystalline Phase

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Table 14. continued

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closed-ring isomer is regularly oriented in the crystal and that the photochromic reaction proceeds in the crystal lattice. Figure 59e shows the molecular packing diagrams in the rhombus-shaped crystal. The molecular packing indicates that the transition moment vectors of the yellow and blue color are perpendicular to each other. According to the molecular orbital calculation of the closed-ring isomer of compound 15,94 the transition moment vector of the longest absorption band is in the long axis of the closed-ring isomer, and the second band is perpendicular to that. The electronic transition moment vectors of the longest absorption band (600 nm) and the second band (465 nm) of the closed-ring isomer are assigned to the long- and short-axes of the closed-ring isomer, respectively.

Figure 57. Photographs of single crystal 4 under polarized light before ((a) θ = 30°, (b) θ = 120°) and after ((c) θ = 30°, (d) θ = 120°) irradiation with 366 nm light. θ is defined in Figure 58a. Adapted with permission from ref 68. Copyright 1999 American Chemical Society.

7.2. X-ray Crystallographic Analysis

X-ray crystallographic analysis was carried out to reveal the geometrical structure change of diarylethene 4 during the photochromic reaction in the crystal. The open-ring form in the crystal has an antiparallel conformation. The distance between the reactive carbons of hexatriene moiety is 0.3576(2) nm at 293 K.68 Figure 60 shows the structures of the diarylethene 4 observed by X-ray crystallographic analysis before and after the photocyclization reaction.148 In situ X-ray crystallographic analysis of the photocyclization reaction indicates that the photocyclization reaction takes place in the conrotatory mode. Almost all atoms remain in the same positions even after the reaction except the sulfur and reactive carbon atoms. This small structural change allows the molecule to undergo the photochromic reaction in the crystal. The molecular structure of the photogenerated closed-ring isomer was found not to be the same as that of the closed-ring isomer in the crystal of the closed-ring isomer.788 The single crystal of the closed-ring isomer was prepared from the solution containing isolated closed-ring isomer. Figure 61 shows the structural difference between the photogenerated closed-ring isomer in the open-ring form crystal and the closed-ring isomer in the closed-ring form crystal. The central C1−C10 bond lengths of the closed-ring isomers observed in both the open- and the closed-ring form crystals are the same, but the distance between S1 and S2 in the open-ring form crystal is larger than that in the closed-ring form crystal. The structure of the closedring isomer in the open-ring form crystal is distorted. The structure difference is reflected in the absorption spectrum of the closed-ring isomer. The maximum of the photogenerated closedring isomer in the open-ring form crystal shifts to longer wavelengths. The red shift is ascribed to the strained structure. The red shift was also confirmed by the theoretical calculations of the spectra by the DFT method. The photocycloreversion reaction of the closed-ring isomer of 4 in the crystal was followed by in situ X-ray crystallographic analysis.149 The b axis and the unit cell volume increased as much as 0.0267 nm and 0.0434 nm3, respectively. The occupancy factor for the open-ring isomer converged to 0.094(2). This means that about 9% of the closed-ring isomers converted to the open-ring isomers by photoirradiation in this experiment. The diagonal orientation of photogenerated sulfur and reactive carbon atoms indicates that the cycloreversion reaction proceeded topochemically in a conrotatory mode. The distance between the reactive carbon atoms was 0.333(3) nm, which is slightly shorter than the value for the open-ring isomer in the crystal, 0.3576(2) nm. The distance between two sulfur atoms was 0.542 nm, which is smaller than the distance between two sulfur atoms of the open-

Figure 58. Polarized absorption spectra of the red color in single crystal 4: (a) direction of polarizer, (b) polarized absorption spectra, and (c) polar plots of absorbance at 535 nm. Reprinted with permission from ref 68. Copyright 1999 American Chemical Society.

photochromic reaction in the crystal lattice. In some cases, the high order parameter maintains up to high conversion.775,787 Clear two-colored dichroism was observed in the crystal of 1,2bis(2-methyl-6-nitro-1-benzothiophen-3-yl)perfluorocyclopentene (444).780 The rhombus-shaped single crystal of the diarylethene turns green upon irradiation with UV light. The green color of the crystal disappears upon irradiation with visible light. When the color of the crystal is observed under polarized light, the crystal exhibits yellow at a certain angle. When the crystal is rotated up to 90°, the color turns blue, as shown in Figure 59. The yellow color reappears at 180°. The clear twocolored dichroism from yellow to blue also indicates that each 12244


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Figure 59. Photographs of single crystal 444 under polarized light (a,b) before and (c,d) after irradiation with 366 nm light. (e) Molecular packing. (f) Electronic transition moments at 465 nm (yellow) and 600 nm (blue). Adapted with permission from ref 780. Copyright 1999 American Chemical Society.

Figure 62. F0 − Fc difference Fourier electron density maps through peaks Q1 and Q2 after heating the closed-ring form crystal of compound 75 at 100 °C for 256 h. Reprinted with permission from ref 134. Copyright 2000 American Chemical Society.

maps after heating the closed-ring form crystal of 75 at 100 °C for 256 h. Four new peaks appeared around the sulfur atoms and the reactive carbon atoms. The new peaks are assigned to those of the open-ring isomer. The diagonal orientation of the four peaks indicates that the thermal cycloreversion reaction also proceeded in a conrotatory mode.134

Figure 60. ORTEP drawings of photoirradiated crystal 4. Black and red molecules indicate the open- and closed-ring isomers, respectively. Adapted with permission from ref 148. Copyright 2000 The Chemical Society of Japan.

7.3. Quantum Yield

7.3.1. Cyclization. The quantum yields of photocyclization reactions of diarylethenes in the single-crystalline phase were measured using a polarizing microscope connected with a photodetector.69 Table 15 shows the photocyclization/cycloreversion quantum yields in crystal together with those in solution. The cyclization quantum yields in crystal are almost twice as large as those in solution. The increase in the yields is ascribed to the conformation of the open-ring isomer. The openring isomer in solution has two conformations, antiparallel and parallel, in almost equal amount as shown in Scheme 4. The photocyclization reaction can proceed only from the antiparallel conformation. Very fortunately, in most cases all molecules in crystals are packed in the antiparallel conformation. Therefore, the cyclization quantum yields in crystal become close to 1 (100%).69,781 This means that photon energy absorbed in the crystal is quantitatively used for the cyclization reaction. Figure 63 shows the correlation between the cyclization quantum yields of diarylethene crystals and the distances between the reactive carbon atoms of the diarylethenes in the crystals.781 When the distance is larger than 0.42 nm, the photocyclization reaction in crystals does not take place. The reaction process has been theoretically analyzed on the basis of ab initio and DFT methods assuming the most stable initial

Figure 61. Conformations of the molecular structures of the closed-ring isomers 4 (a) in the open-ring form crystal and (b) in the closed-ring form crystal. Reprinted with permission from ref 788. Copyright 2002 The Chemical Society of Japan.

ring isomer in the crystal, 0.596 nm. The photogenerated openring isomer in the closed-ring form crystal has a conformation more planar than that in the most stable conformation.68 The reaction mode in the thermal cycloreversion reaction of the closed-ring isomer of 75 was also examined by in situ X-ray crystallographic analysis.134 According to the Woodward− Hoffmann rules,23 the thermal cycloreversion reaction of cyclohexadiene derivatives should proceed in a disrotatory mode. Figure 62 shows the difference Fourier electron density 12245


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Table 15. Photocyclization and Photocycloreversion Quantum Yields in n-Hexane and in Crystal

Figure 63. Relationship between the photocyclization quantum yield and the distance between the reacting carbons. 1, 1,2-bis(2,4,5trimethyl-3-thienyl)perfluorocyclopentene (452); 2, compound 450; 3, 1,2-bis(2-methyl-6-formylbenzothiophen-3-yl)perfluorocyclopentene (453); 4, compound 451; 5, compound 15; 6, 1-(2-methylbenzothiophen-3-yl)-2-(2,6-dimethylbenzothiophen-3-yl)perfluorocyclopentene (454); 7, 1,2-bis(2,6-dimethylbenzothiophen-3yl)perfluorocyclopentene (455); 8, compound 12; 9, compound 75; 10, compound 428; 11, compound 4; 12, compound 429; 13, compound 13; 14, compound 273. Reprinted with permission from ref 781. Copyright 2002 The Royal Society of Chemistry.

between reactive carbon atoms are less than 0.40 nm. Although the diarylethene 456 connected at 1-position of anthracene undergoes photochromism in the crystalline phase, diarylethenes 457 and 458 connected at 2- and 9-positions of anthracene do not exhibit any photochromism in the crystalline phase. To know the reason why 457 and 458 cannot undergo the cyclization reaction, molecular structures and conformations were examined in detail by X-ray diffraction analysis and theoretical DFT calculations. In 456, π-conjugation delocalizes throughout the molecule because of low dihedral angles between the anthracene and the thiophene rings, while the anthracene substituents are isolated electronically in 457 and 458 because of the large dihedral angles. Upon irradiation with UV light, the excited energy of the anthracene substituents in 456 can extend to the central part and induce the photocyclization reaction. On the other hand, the excitation energy is localized in the anthracene substituents in 457 and 458 and cannot induce the cyclization reaction of the central part. The electron density at the reactive carbon atoms in a LUMO level controlled by the conformation of the substituents also plays an important role in the crystalline photochromism. Other examples, which do not obey the general rule, were observed in diarylethenes having 5-thiazolyl groups265 or a 2-thienyl heteroaryl group.792b 7.3.2. Cycloreversion. The quantum yields of photocycloreversion reactions in the crystalline phase are also summarized in Table 15. The values for crystals 4, 20, and 13 are 0.10, 0.12, and 0.017, which are almost similar to those in nhexane. The values for crystals 75, 428, 435, and 12 are 0.027, 0.029, 0.044, and 0.027, which are larger than those in n-hexane by a factor of 2−3. The large quantum yields are ascribed to a restricted conformation of the photogenerated closed-ring isomers in the open-ring form crystal. The photogenerated isomers are in the constrained forms, which are different from the most stable closed-ring isomer conformation. The quantum yields in the crystal depend on both the molecular structure and the crystal packing mode. A good example that distinguishes the two factors is provided by polymorphic crystals. Diarylethene 273 has four polymorphic crystals: α, β, γ, and δ, when recrystallized from n-hexane.777 The four crystals have different space groups, unit cell volumes,

geometry and the relaxation from the Franck−Condon states.30,791 When the distance is larger than 0.42 nm, the reactive carbon atoms in the excited state are separated from each other. Most photoexcited molecules in the antiparallel conformation fixed in the crystal lattice with the distance of reactive carbon atoms less than 0.40 nm readily undergo photocyclization reactions. Exceptional examples, which do not obey the above general rule, were observed in diarylethene derivatives having anthracene substituents shown in Scheme 45.792a These three derivatives are fixed in the antiparallel conformation in crystal, and the distances 12246


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Scheme 45. Dithienylethenes Having Anthracene Substituents792a

and in crystal. The conformation of the photogenerated closedring isomers in crystal has some strain energy, as shown in Figure 61,788 which may decrease the activation energy. The absence of the activation energy in the photocyclization process indicates that the molecules are in a favorable conformation for the reaction, and the crystal lattice scarcely disturbs the rotation of the thiophene rings.36

conformations of the molecules, and distances between the reactive carbon atoms. The thiophene rings in all four crystals have the antiparallel conformation, and the distances between the reactive carbons are 0.348−0.370 nm. The four polymorphic crystals gave almost similar cyclization quantum yields, around 0.95−1.0. The cycloreversion quantum yields of α- and γ-crystals (0.0064 and 0.0061, respectively) are similar to that in n-hexane (0.0052). On the other hand, the yields of β- and δ-crystals (0.026 and 0.027, respectively) are much larger than the yields in solution, by a factor of 5. The difference is ascribed to πconjugation length, or the conformation of the phenyl and thiophene rings of the closed-ring isomers. When the πconjugation is limited to two thiophene rings, the cycloreversion quantum yield is relatively large. On the other hand, when the πconjugation extends to the p-methoxyphenyl rings, the quantum yield decreases as shown in Figure 64.

7.4. Multicolored Systems and Supramolecular Structures

7.4.1. Multicolored Crystals. In one-component photochromic crystals, molecules interconvert only between two (colorless and colored) states. On the other hand, in multicomponent systems composed of different kinds of photochromic compounds, reversible multimode switching between more than two states can be constructed by combining two states of each component. Although various types of multicolored photochromic systems, such as a mixture of chromophores, multicomponent dimers and trimers, and terpolymers, have been reported, ideal systems are multicolored single crystals because of their high durability, high efficiency of photocoloration, and molecular scale high resolution. Although it is not easy to prepare a molecule that is composed of three chromophores with different colors, yellow, red, and blue,117 a mixed crystal composed of three different kinds of molecules can be readily prepared by choosing suitable molecules. In crystals each molecule is regularly orientated and randomly arranged. In other words, a part of molecule A is replaced with B and C in the crystal of A, and molecules A, B, and C are oriented under a certain direction. The preparation of mixed crystals requires host A and guest B and C molecules of similar size and shape. A full-color photochromic mixed crystal composed of three kinds of diarylethenes exhibiting three primary colors, such as yellow, red, and blue, was prepared.752 The molecular structures are shown in Scheme 46a. The combination of diarylethenes 4 and 273 gave a mixed crystal, which turns red and blue.793 Recrystallization of a mixture of 275, 4, and 273 (275:4:273 = 21:53:26 in molar ratio) from acetonitrile afforded a single crystal composed of 275, 4, and 273 in the molar ratio of 2.4:97.4:0.2. The crystal lattice of 4 provided the sites where 275 and 273 can sit. Figure 65a shows a photograph of the three-component crystal. Upon irradiation with light of appropriately selected wavelengths, the colorless crystal turns yellow, red, and blue. These colors are due to the closed-ring isomers of 275 (yellow), 4 (red), and 273 (blue) generated in the photoirradiated crystal. Three diarylethenes having similar molecular size, 459, 23, and 131 (Scheme 46b), can readily mix with each other in a single crystal. These three chromophores 459, 23 and 13 exhibit yellow, red and blue colors, respectively, upon UV irradiation. Mixed crystals composed of the three diarylethene derivatives in almost equal amounts have been successfully prepared, as shown in Figure 65b.759,794 The colorless crystal turned purplish black upon irradiation with 370 nm light. Upon irradiation with >620

Figure 64. Geometrical structure changes and photocyclization/ cycloreversion quantum yields of (a) crystal 273-α and (b) crystal 273-δ. Adapted with permission from ref 777. Copyright 2003 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

7.3.3. Temperature Dependence. Temperature dependence of photocyclization/photocycloreversion reactions of compounds 13, 428, and 429 was measured in 3-methylpentane solution as well as in the crystalline phase.72 In both solution and crystal, no appreciable temperature dependence in the cyclization quantum yields was observed. The activation energy in the photocyclization process is almost zero. In the photocycloreversion reaction, however, the quantum yield decreased with decreasing temperature.72 The activation energies of compounds 13 and 428 in solution and in crystal were determined to be 16 and 5−10 kJ mol−1, respectively. The activation energy in crystal was smaller than that in solution. The lower activation energy in crystal indicates that the photogenerated colored isomers have different reactivity in solution 12247


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Scheme 46. Molecular Structures of Diarylethenes That Can Form Three-Component Mixed Crystals752,794

Figure 65. Photographs of partially colored mixed crystals (a) 275/4/ 273 and (b) 459/23/13. The irradiation wavelength is as follows: (i) λ = 405 and 578 nm, (ii) λ = 370 and 458 nm, (iii) λ = 405 and 492 nm, (iv) λ = 370 nm, (v) λ > 620 nm, (vi) λ = 435 and >690 nm, (vii) λ = 435 nm. Adapted with permission from refs 752 and 794. Copyright 2003 and 2007 American Chemical Society.

nm light, the color of the crystal then turned yellow. On the other hand, by partially bleaching with light of wavelengths of 435 and >690 nm, it turned red. When irradiated with 435 nm light, the black crystal turned blue. X-ray crystallographic analysis revealed that 459 and 13 are packed in the crystal lattice of 23. These crystals exhibit various colors, such as colorless, yellow, red, blue, orange, purple, green, and black, upon irradiation with light of appropriate wavelengths. All of the colors are thermally stable and are completely bleached by irradiation with visible light. Such multicolored photochromic crystals have the potential for application to multifrequency three-dimensional optical memory media and photoswitchable full-color displays. 7.4.2. Selective Photoreaction with Linearly Polarized Light. One of the characteristic properties of single crystals is that the reaction of each molecule can be controlled with linearly polarized light. When diarylethene molecules are packed in a herringbone structure, in which long axes of the molecules are oriented perpendicular to each other, irradiation with linearly polarized light can induce selective photoreactions of the molecules aligned in one direction, because the closed-ring isomer has an electronic transition moment in the direction of the long axis of the molecule. Diarylethene 273 formed herringbone-type crystal structures suitable for selective photoreactions with linearly polarized light.777 Figure 66a shows the molecular packing. The long axes of the diarylethene molecules are oriented into two directions A and B that are perpendicular to each other. The colorless crystal was irradiated with nonpolarized 370 nm light to give the bluecolored crystal. When the colored crystal is irradiated with linearly polarized light (λ > 570 nm) parallel to the direction A, the colored closed-ring isomers along the polarized light are preferentially bleached. On the other hand, when the colored

Figure 66. (a) Molecular packing of 273 and (b) molecular structure of 62. Adapted with permission from ref 777. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

crystal is irradiated with linearly polarized light parallel to the direction B, the molecules oriented to the direction B are selectively bleached. A similar selective photoisomerization induced with linearly polarized light was observed in a single crystal of diarylethene dimer 62 (Figure 66b), in which two diarylethene chromophores are covalently linked via a fluorene spacer.114 Because of the welldefined bond angle of the fluorene moiety, the long axes of the two diarylethene units in the dimer structure are oriented almost perpendicularly to each other, and a herringbone-type molecularpacking structure is constructed in the crystal. The crystal undergoes photochromism and turns blue by irradiation with nonpolarized UV light. Upon irradiation with polarized visible (λ > 570 nm) light parallel to one of the transition moments, the closed-ring isomers along the polarized light are selectively bleached. 12248


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7.4.3. Supramolecular Architectures. Hydrogen-bonding, metal-coordinating, and π−π interactions have been widely used as noncovalent intermolecular interaction in the crystal engineering for the construction of various types of supramolecular structures of organic or organometallic crystals, as described in section 5.5. Photochromic diarylethenes bearing carboxyl groups at the ortho, meta, and para positions of both terminal phenyl groups have been synthesized.621 Figure 67

Figure 68. π−π intermolecular interaction in (a) 435/Bz crystal and (b) 435/Np crystal. Adapted with permission from ref 778. Copyright 2003 American Chemical Society. Adapted with permission from ref 782. Copyright 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

7.4.4. Nano-Layered and Nano-Mosaic Periodic Structures. A single crystal of diarylethene 434 has a unique molecular-packing structure in which photoreactive and photoinactive conformer layers are alternately stacked.776 Figure 69a shows ORTEP drawings of the conformers. Both conformers A and B adopt an antiparallel conformation, but the distances between the reactive carbon atoms are different, 0.365 and 0.493 nm for the conformers A and B, respectively. Only conformer A satisfies the conditions for the photocyclization reaction. The conformer A can efficiently photoreact, while the conformer B cannot. The photoreactivity of the molecules in each layer is controlled by the molecular conformations. Stoichiometric diarylethene cocrystals 435/75 and 435/446 were prepared by recrystallization of a 1:1 (molar ratio) mixture.782,805 Figure 70 shows the molecular packing diagrams in cocrystals 435/75 and 435/446. The crystal 435/75 has a layered structure in which unimolecular layers of 435 and 75 are alternately stacked. The phenyl rings of 75 and the pentafluorophenyl rings of 435 are not stacked with each other. On the other hand, in cocrystal 435/446, the pentafluorophenyl groups of 435 and the naphthyl groups of 446 are stacked by intermolecular aryl−perfluoroaryl interactions. The diagrams viewed from the a-, b-, and c-axes indicate that 435 and 446 molecules are packed in a three-dimensional alternating arrangement to form a mosaic-like structure in the cocrystal. Cocrystals 435/75 and 435/446 showed photochromism. Upon irradiation with UV light, the colorless cocrystals of 435/75 and 435/446 turn blue and green, respectively. In both cocrystals 435/75 and 435/446, components 75 and 446 selectively convert to the closed-ring isomers, while photocyclization of 435 is strongly suppressed. The highly selective photoreaction of 75 and 446 was confirmed by in situ X-ray analysis. In homocrystals of each compound, efficient photocyclization reactions with quantum yields of 1 take place.778 The molecular packing mode in crystal controlled the reactivity of component molecules. As a result of the selective photocyclizations, the colored and colorless molecules, which have different refractive indices, are arranged periodically at the molecular level in the UV-irradiated crystals. Such photoreversible periodic refractive index changes

Figure 67. X-ray structures of one-dimensional linear chains of carboxylsubstituted diarylethenes. Reprinted with permission from ref 621. Copyright 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

shows the crystal structures of these crystals. The diarylethenes are linked by the intermolecular hydrogen bonds between the carboxylic acids. The interaction affected the dihedral angle between the thiophene and phenyl rings in the diarylethene molecules. The diarylethene crystals show photochromism in the crystalline phase upon irradiation with UV light. The absorption maxima of the closed-ring isomers show quite large differences depending on the position of the substituent: 560 nm (462), 600 nm (461), and 640 nm (460). The difference is attributed to the dihedral angles between the thiophene and phenyl rings, which affect the π-conjugation length. Noncovalent aromatic−aromatic interactions play an important role in biological recognition, host−guest binding in supramolecular assemblies, crystal engineering, and other molecular recognition processes.795−803 Stacking between aryl and perfluoroaryl units is a special class of aromatic−aromatic interactions and has been studied extensively since the first report by Patrick and Prosser in 1960.804 A diarylethene derivative 435 having two pentafluorophenyl groups cocrystallized with aromatic hydrocarbons, such as benzene (Bz) and naphthalene (Np), by recrystallization from n-hexane.778,782 Figure 68 shows the π−π interactions observed in cocrystals 435/Bz and 435/Np. A linear chain structure composed of 435 and Bz in the ratio of 2:1 was observed in cocrystal 435/Bz. On the other hand, a discrete sandwiched structure composed of 435 and Np molecules was formed in crystal 435/Np. The cocrystals of 435/Bz and 435/Np and a single crystal of 435 undergo photochromic reactions. These three crystals exhibited absorption maxima at different wavelengths (λmax = 630 nm (435), 620 nm (435/Np), and 555 nm (435/Bz)). The spectral shift is ascribed to the difference in the conformation of the diarylethene molecules in the crystals induced by the intermolecular aryl− perfluoroaryl interactions. 12249


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Figure 69. (a) ORTEP drawings and (b) molecular packing diagrams of crystal 434. Blue and red molecules in the packing diagrams indicate photoreactive and photoinactive conformers A and B, respectively. Reprinted with permission from ref 776. Copyright 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 70. Molecular packing diagrams of cocrystals (a) 435/75 and (b) 435/446. Red, blue, and green molecules indicate molecules 435, 75, and 446, respectively. (a) Adapted with permission from ref 805. Copyright 2003 Royal Society of Chemistry (RSC) on behalf of the European Society for Photobiology, the European Photochemistry Association, and RSC. (b) Adapted with permission from ref 782. Copyright 2004 The Japan Chemical Journal Forum and Wiley Periodicals, Inc.

ring isomers yield the (R,R) and (S,S) enantiomers, respectively. In general, the photocyclization in solution results in the formation of the two enantiomers in equal amounts, even when a chiral substituent is introduced into the diarylethene, as described in section 3.4.165 On the other hand, diastereo- or enantioselective photocyclization reactions can take place in crystal.806−809 Diarylethenes having (S) or (R)-3-methyl-1penten-1-yl substituent 463 (Scheme 47) were synthesized and crystallized to form a chiral crystal with orthorhombic space group P212121.806,807 The open-ring isomers are packed in the crystals in only one conformer of the possible two atropisomers. (S)-463a is packed in the crystal as P-helical conformation. These diarylethenes undergo photochromism both in solution and in the single-crystalline phase. In crystal only one closed-ring diastereomer was produced, while the diastereoselection was not

in the crystalline nanostructures have a potential application as new photonic nanodevices. 7.5. Stereoselective Photoreaction

In crystals, topochemical reactions take place because of restriction of molecular movement in the crystalline phase. When diarylethene molecules are symmetrically packed, the molecules undergo photochromism in the crystalline phase to result in a formation of two enantiomers of the closed-ring isomers with (R,R) and (S,S) configurations originating from two asymmetric carbon atoms at the reacting positions, as shown in Scheme 9. Enantioselective reactions may take place in chiral crystals containing chiral auxiliaries, such as chiral host molecules or covalently bonded chiral substituents. Conrotatory cyclization reactions from P-helical (righthanded) and M-helical (left-handed) conformers of the open12250


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Scheme 47. Photochromism of Diarylethenes Having (S)- or (R)-3-Methyl-1-penten-1-yl Substituent806,807

photocyclization reactions in the single-crystalline phase. The enantioselective photocyclization reaction was also proved by in situ X-ray crystallographic analysis. The enantiomeric excess (ee) in the crystalline-phase photoreactions was higher than 94%. Achiral diarylethene, 1,2-bis(2-methyl-5-(1-naphthyl)-3thienyl)perfluorocyclopentene 446, formed a chiral cocrystal with octafluoronaphthalene by intermolecular aryl−perfluoroaryl interactions.811 In the chiral cocrystals, highly enantioselective photocyclization took place due to the conformational confinement.

observed in solution. (S)-463a underwent photocyclization reaction to produce diastereomer (S,R,R)-463b in crystal as confirmed by HPLC analysis and X-ray crystallography. The diastereomeric excess (de) value was >95%. Absolute asymmetric photochromism, in which achiral compounds produce chiral products in the absence of any external chiral agents, was demonstrated using chiral single crystals of an achiral diarylethene.810,811 Recrystallization of a diarylethene with m-formylphenyl substituents 464 (Scheme 48) Scheme 48. Enantioselective Photochromic Reactions of a Diarylethene Having m-Formylphenyl Substituents in Single Crystals810

7.6. Inorganic Crystals

Coordination-driven self-assembly is also an important strategy to construct supramolecular architectures.812,813 Several diarylethene metal complexes have been synthesized.166,296,577,601,814 Munakata and co-workers815−818 prepared Cu(I), Ag(I), Mo(II), and Rh(II) complexes with cis-1,2-dicyano-1,2-bis(2,4,5-trimethyl-3-thienyl)ethene (192) and examined their photochromic behavior in the crystalline phase. Figure 71 shows the structures

from acetonitrile solution gave colorless needle crystals.810 X-ray crystallographic analysis showed that the crystals have monoclinic chiral space group P21. In addition, two kinds of crystals, (P)-464a and (M)-464a, which are mirror images of each other, were obtained from the same batch. In crystal (P)464a, all open-ring isomers are restricted to a P-helical conformation, and all molecules are fixed to a M-helical conformation in crystal (M)-464a. Both crystals (P)-464a and (M)-464a underwent photochromism. Although no enantioselectivity was observed in solution during photoreaction, an enantioselective formation of the closed-ring isomer was discerned in crystals. The chiral crystal of (P)-464a gave optically pure (R,R)-464b enantiomer, and the chiral crystal of (M)-464a gave optically pure (S,S)-464b enantiomer by topochemical

Figure 71. Schematic illustration of the crystal structure for (a) [Cu(DE)2(ClO4)], (b) [Ag2(DE)(XCO2)2] (X = C2F5,C3F7, C4F9), (c) [Mo2(XCO2)4(DE)](benzene) (X = CF3), and (d) [Rh2(XCO2)4(DE)2] (X = CH3). DE shows cis-1,2-dicyano-1,2bis(2,4,5-trimethyl-3-thienyl)ethene (192). X and benzene were omitted for clarity. 12251


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photochromism in the crystalline phase. The magnetic data and ESR spectra show obvious changes by the removal of methanol.

of the complexes. Ag(I), Cu(I), and Mo(II) coordination polymers with 192 exhibit a reversible photochromic reaction in the crystalline phase, whereas the coordination of 192 to Rh(II) does not show any photochromic reaction. This is ascribed to the parallel conformation of the diarylethene units in the Rh(II) complex. Complexes composed of symmetric bidentate and asymmetric monodentate diarylethene ligands having one or two pyridyl groups and M(hfac)2 (M = Zn2+, Mn2+, and Cu2+; hfac = hexafluoroacetylacetonate) were synthesized, and their crystal structures and photochromic performance in the singlecrystalline phase were studied.819−824 In the crystals, the bidentate diarylethene ligand forms linear-chain coordination polymers, while the monodentate diarylethene ligand forms discrete 1:2 complexes. Figure 72 shows the linear chain

Scheme 49. Photochromism of Diarylethene Co Complexes 465 and 466 in the Crystalline Phase825

7.7. Nanocrystals

Diarylethene nanoparticles have attracted considerable attention because of their characteristic properties different from those in solution and in crystal. Nanoparticles can be dispersed in colloidal aqueous solutions or incorporated in thin layer matrices, avoiding problems of light scattering or shallow light penetration into bulk materials. The nanoparticles can be fabricated by both “bottom-up” and “break-down” approaches. Typical “bottom-up” approach is known as a reprecipitation method from photochrome solutions. The nanoparticles of a diarylethene derivative (1,2-bis(2-methyl-5-p-hydroxyphenyl-3thienyl)perfluorocyclopentene (467)) have been prepared by the reprecipitation method.826 The THF solution containing 467 was injected into distilled water with vigorous stirring at certain temperature, and then THF was removed by nitrogen gas. The spectroscopic and spectrokinetic studies showed that the diarylethene aggregates exhibited different photochromic behavior as compared to the isolated molecules, indicating the formation of H-type aggregation. Park and co-workers382 have designed and synthesized a multifunctional molecule, which exhibits both fluorescence emission and photochromism. High-contrast (>10) on/off fluorescence switching was successfully implemented in the size-tuned neat nanoparticles of diarylethene attached with 1cyano-1-(4′-methylbiphenyl)-2-phenylethylene moiety and also in a PMMA film highly loaded with the diarylethene. Figure 73 shows the FE-SEM image of the nanoparticles. Monodispersed and size-controlled diarylethene nanoparticles composed of 12 were also fabricated by the reprecipitation method.827−829 After the injection of a THF solution of the diarylethene, the size of the nanoparticles was measured by a dynamic light scattering. During the nucleation growth of the diarylethene nanoparticles, microwave or light irradiation was used to control the size and size distribution of the nanoparticles. Powder XRD pattern of the nanoparticles was similar to that of the bulk crystal. The absorption spectra of the nanoparticles upon irradiation with UV light showed absorption maximum between the solution and the bulk crystal, depending on the size of the particles. Polystyrene-encapsulated diarylethene 12 nanocrystals were also fabricated by soap-free emulsion

Figure 72. Linear chain structures of coordination polymers: (a) 323/ Zn(hfac)2, (b) 323/Mn(hfac)2, and (c) 323/Cu(hfac)2. Reprinted with permission from ref 820. Copyright 2004 American Chemical Society.

structures of the bidentate diarylethene ligand crystals composed of compound 323. Although the absorption spectra of the colored closed-ring isomers are similar in solution, spectra of the UV irradiated colored crystals are different among the complexes with Zn2+, Mn2+, and Cu2+, reflecting the molecular conformation of the diarylethene ligands in the crystals. Co, Cu, Zn complexes with 1,2-bis(2-methyl-5-(carboxylic acid)-3-thienyl)perfluorocyclopentene were synthesized and characterized by X-ray diffraction analysis.825 The diarylethene serves as a bis-monodentate ligand that bridges metal centers to generate one-dimensional polymers. The Co and Cu complexes exhibit photochromism in the crystalline phase. Upon heating at 75 °C, it was found that methanol is released from the complex 465 and the rearrangement to 466 was observed, as shown in Scheme 49. The desolvated form of the Co complex also shows 12252


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growth beyond the desired nanoparticle size. On the other hand, the laser-ablation method yields reproducible results, because the fragmentation does not proceed further once the laser is turned off, and the nanoparticle colloidal solutions are stable.831 Colloidal nanoparticles can be prepared using the laser-ablation method as follows. Micrometer-sized crystals of 1,2-bis(5ethoxy-2-(2-pyridyl)thiazolyl)perfluorocyclopentene (468) were put in a cuvette containing an aqueous solution of dodecyltrimethylammonium bromide, and the mixture was stirred vigorously and exposed to 355 nm beam of a nanosecond Nd:YAG laser, at which wavelength the diarylethene absorbs strongly. The initial colorless opaque suspension was gradually transformed to a purple transparent colloidal solution. Under 355 nm UV laser irradiation, degradation of the diarylethene occurred. To avoid the degradation the laser beam was set to 532 nm, where the closed-ring isomer has absorption, and an additional light source (continuous wave cw UV lamp) irradiated the suspension during the laser-ablation procedure. Under this condition, no turbidity was observed, and the absorbance did not decrease. The cyclization quantum yield of colloidal nanoparticles is more efficient than that in CHCl3 solution. It can be attributed to the crystalline character packed into antiparallel conformation of the colloidal nanoparticles. The microcrystals of 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (75) and 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene (156) were fabricated by crystal growth from sol−gel thin film.302 The solutions containing solvent, dye, silicon alkoxides (an equimolar mixture of tetramethoxysilane and methyltrimethoxysilane), and water were heated. The resulting sols were deposited at room temperature by spincoating onto thin microscope slides. Thus, the films of around 0.5 μm thick were prepared. These nanocomposite coatings were then stabilized by annealing at 80 °C. The nanocrystals grown in the sol−gel film are shown in Figure 75. Upon irradiation with UV light, the nanocrystals turned clearly blue.

Figure 73. Molecular structure of diarylethene 269 having 1-cyano-1(4′-methylbiphenyl)-2-phenylethylene and FE-SEM image of the fluorescent nanoparticles of the diarylethene (275 ± 75 nm) prepared from a THF/water mixture. Adapted with permission from ref 382. Copyright 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

polymerization.830 The polystyrene-encapsulated diarylethene nanocrystals are of monodisperse and spherical shape, as shown in Figure 74. The nanocrystals exhibited photochromism upon

Figure 75. Optical transmission images of diarylethene 75 nanocrystals grown in sol−gel film (a) before photoirradiation and (b) after UV irradiation. Reprinted with permission from ref 302. Copyright 2011 Elsevier.

Figure 74. SEM and TEM images of polystyrene-encapsulated diarylethene 12 nanocrystals. The sizes were (a and b) 110 nm and (c and d) 553 nm, respectively. (e and f) SEM images of a cast film of polystyrene-encapsulated diarylethene nanocrystals on a silicon substrate with different scales. Reprinted with permission from ref 830. Copyright 2011 The Royal Society of Chemistry.

8. SURFACE PROPERTIES 8.1. Surface Wettability

Surface property, especially surface wettability, plays an important role in various applications relating to adsorption, adhesion, dewetting, lithography, and biomedical engineering. There are two factors that control wettability on solid surfaces, chemical and topological factors.832 Superhydrophobic surfaces, such as the leaves of the lotus plant, are governed by the topological factor.833 If surface wettability can be controlled by external stimuli such as light, temperature, pH, and solvents,834 such materials may find various applications.

UV and visible light irradiation, and the size-dependence photochromic properties of the diarylethene nanocrystals were maintained even after encapsulation. The “break-down” approach is laser-ablation of bulk materials. Nakatani, Yu, Asahi, and co-workers266,618,831 have reported the fabrication of diarylethene nanoparticles and their properties. The reprecipitation method requires a very sophisticated control of experimental conditions, to avoid continuous nucleation 12253


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Uchida and co-workers835−844 demonstrated the change in the surface topology by the crystal growth of a photochromic diarylethene upon irradiation with UV light. Figure 76 shows

Figure 77. Schematic illustration of the formation of the surfaces that show lotus and petal effects. SEM images of the microcrystalline surfaces are also shown in (a) and (b). Adapted with permission from ref 838. Copyright 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

the irradiated film for 1 h at 70 °C, small fibrils grow sparsely within 1 h. After additional heating in the dark, larger rod-shaped crystals grow while fine fibrils melt and finally the surface is covered only with rod crystals. The surface is photoisomerized by UV light irradiation again. After storing the film at 50 °C for 1 h in the dark, the surface is covered with rod crystals and also newly formed fibrils. When the water droplet is carried out on the surface, the droplet stays pinned onto the surface with superhydrophobic character (contact angle = 154°), even if the surface is turned upside down. This phenomenon is the pinned effect or petal effect.847 A surface that changes wettability between hydrophilic and superhydrophobic by photoirradiation can be fabricated using a diarylethene with large polarity.839 The microcrystalline surface of 1-(2,5-dimethyl-1,1-dioxo-3-thienyl)-2-(2,5-dimethyl-3thienyl)perfluorocyclopentene (470) shows the contact angle of 150°, as shown in Figure 78. Upon alternating irradiation with UV and visible light, the contact angle changes between hydrophilic 80° and superhydrophobic 150°. Rapid topological changes can be attained using a polymorphic diarylethene.848−850 1,2-Bis(2-methyl-6-nitro-1-benzothiophen3-yl)perfluorocyclopentene (444) undergoes crystallization of the needle-like crystals from the amorphous film in PMMA at 130 °C. The crystallization can be controlled by the presence of the closed-ring isomer produced by the photochromic reaction. The contact angles are 82° and 117° in the amorphous and crystallized films, respectively. The formation of a superhydrophobic surface by crystal growth can be fabricated by heating at 130 °C and subsequently at 180 °C. The surface has a contact angle of 154° showing superhydrophobicity.

Figure 76. Contact angles of a water droplet on a microcrystalline film of diarylethene 469. (a) Time profile of the contact angle during the fibril generation process after UV irradiation. (b) Profile during fibril disappearance upon irradiation of the colored fibril structured surface with visible light. Adapted with permission from ref 835. Copyright 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

changes of the contact angle of a water droplet on a microcrystalline film of 1,2-bis(2-methoxy-5-trimethylsilyl-3thienyl)perfluorocyclopentene (469). The contact angle with water changes from 120° to 92° upon UV light irradiation for 1− 2 min. After UV light irradiation for 10 min, fibril crystals appear on the crystal surface and increase the contact angle to 163°. Such a superhydrophobic surface is known as the lotus effect. The fibril crystal’s surface returns to the flat state by irradiation with visible light. The reversible changes in the contact angle are attributed to a reversible formation of the fibril crystals and a change in the surface roughness. Uchida and co-workers838 have also reported the fabrication of surfaces that show the lotus and petal effects using the same diarylethene compound by heating. Figure 77 shows an illustration of the formation of the lotus and petal surfaces. Upon UV irradiation on the surface of a coated film of diarylethene 469, the closed-ring isomer is formed only on the irradiated side of the surface. After storage in the dark, the surface is covered with closely packed small fibrils. The contact angle and sliding angle of a water droplet are (162.9 ± 0.8)° and 2.0°, respectively. This is called the lotus effect.845,846 After storage of 12254


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Figure 79. Chemical structures of two isomers of diarylethene 471 and selective Mg deposition on the diarylethene film: (a) colorless diarylethene film formed on glass substrate and a photomask, (b) partially colored diarylethene film upon UV irradiation with the photomask, and (c) Mg films selectively deposited on colored areas without the photomask. Reprinted with permission from ref 853. Copyright 2009 The Royal Society of Chemistry.

Figure 78. SEM images (×1000) of microcrystalline surface of diarylethene 470 (a) before UV irradiation, (c) after UV irradiation for 10 min and without for 20 min, and (e) after next visible light irradiation for 1.5 h. After (a), the temperature was always kept at eutectic temperature of 67 °C. Water droplet on the surfaces of (a), (c), and (e) are shown in (b) (contact angle = 150.4°), (d) (81.1°), and (f) (150.3°), respectively. Scale bar: 10 μm. Reprinted with permission from ref 839. Copyright 2011 American Chemical Society.

8.2. Selective Metal Deposition

Selective metal deposition means that metal vapor atoms are deposited selectively on a hard diarylethene surface but not on a soft surface. Tsujioka and co-workers851−853 found that metal atoms are selectively deposited on photochromic diarylethene (1,2-bis[2-methyl-5-(5-trimethylsilyl-2-thienyl)-3-thienyl]perfluorocyclopentene (471)) film depending on the photoisomerization state. Figure 79 shows selective Mg deposition.853 The metal vapor is deposited only on the colored part. No metal deposition was observed on the surface of the open-ring isomer. The selective metal deposition was found to originate from the difference in the glass-transition temperature (Tg), or more precisely, in the activity of the surface molecular motion between the two states of the photoisomerization of the amorphous diarylethene film.851 Tg of the amorphous diarylethene in the colorless state is 32 °C, whereas it is 95 °C in the colored state. When Mg atoms are evaporated onto the colorless diarylethene surface, weak van der Waals interactions and the active molecular motions of the diarylethene surface cause active migration and rapid desorption of Mg atoms from the surface. Figure 80 shows photoisomerization dependence on the metal deposition.854 The areas indicated by the word “Photochromism” are colored into four levels upon UV irradiation, as shown in Figure 80b. The numbers 1−5 indicate the degrees of the color (Figure 80a and c). Mg atoms are deposited on the diarylethene film in the area numbers 1, 2, and 3, which contains more than 60% of the closed-ring isomer. In contrast, Mg films are not formed in area numbers 4 and 5.

Figure 80. Deposition threshold behavior for the photoisomerization conversion of diarylethene 471. (a) Four-level colored areas in the word “Photochromism”. (b) The corresponding Mg-deposited areas. (c) The absorption spectra in areas (1)−(5). Reprinted with permission from ref 854. Copyright 2009 The Royal Society of Chemistry (RSC) on behalf of the Center National de la Recherche Scientifique (CNRS) and RSC.

The metal atom deposition (or desorption) behavior is affected by the metal deposition rate,854,855 substrate temperature,856 the gas pressure during the metal evaporation,857 the surface viscosity,748 and metal species855,856 in addition to Tg. 12255


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the polymer chain can be controlled by doping diarylethene molecules into polymer matrices. The uncolored state on 5%diarylethene-doped polystyrene film did not form Mg-deposited film, while the colored state formed Mg metal film. Selective metal deposition was also demonstrated on a phase-separated polymer blend surface, which consists of polystyrene (PS) and a polystyrene-block-polybutadiene copolymer (PS-BR).859 Pb was evaporated onto the phase-separated surface without an evaporation mask and was selectively deposited on the PS phase but not on the PS-BR phase. Selective metal deposition was also performed on diarylethene crystals.860 The selective metal deposition can be applied to electronic devices. Tsujioka and co-workers applied it to the preparation of microsized thin film fuses (μ-fuses)856,861 and dual-function diffraction gratings862,863 with laser scanning and maskless metal deposition. Figure 82 shows the preparation and demonstration of μ-fuse array using laser scanning and maskless Pb deposition.856 An amorphous layer of diarylethene 471 on the glass substrate was irradiated with UV light (λ = 365 nm) to prepare a whole colored surface, and then uncolored lines were prepared on it using laser spot scanning (λ = 635 nm). Colored lines then were formed by scanning a violet laser spot (λ = 410 nm) between the uncolored lines as shown in Figure 82a. Finally, Pb is evaporated onto the whole area without a shadow mask. Figure 82b and c shows the μ-fuse array obtained and the Pb pattern of the single μ-fuse, respectively. When a voltage was applied to the device, the bridged part was broken as shown in the right figure. Figure 82d shows current−voltage characteristics of the device. The current was interrupted at around 300 μA at 2.4 V. These results indicate that the μ-fuse has a very low interrupt (highly sensitive) current level (Iint). Iint can be controlled by adjusting the cross section of the Pb-bridged part. Figure 82e shows Iint increasing exponentially with the cross section. The diffraction grating is one crucial optical element consisting of a periodic structure with metal lines, grooves, or fine surface structures, or a periodic modulation of the refractive index on a substrate. A dual-functional diffraction grating prepared by the selective metal deposition method shows different diffractions in transmission and reflection.862,863 Such dual-functional gratings

The metal can easily deposit with high deposition rate, low temperature, high pressure, and/or high viscosity. The selective metal deposition was observed for not only Mg but also Zn, Mn, and Pb depending on the deposition rate.855,856 The multiple-metal patterning with maskless evaporation via a single vacuum process was demonstrated as shown in Figure 81.

Figure 81. Multiple-metal patterning with maskless evaporation via a single vacuum process based on the selective deposition of the diarylethene film. Reprinted with permission from ref 855. Copyright 2010 The Chemical Society of Japan.

The blue-colored letters “Zn”, “Mg”, and “Mn” were drawn on a diarylethene film by UV irradiation through a photomask. The isomerization ratios to colored state for the domains of the letters “Zn”, “Mg”, and “Mn” were adjusted to 90%, 70%, and 50%, respectively, by controlling UV irradiation. Zn, Mg, and Mn metals were successively evaporated to the substrate. Selective metal deposition was also achieved on diarylethenedoped polymer surfaces.746,747 Tg of the polymers is largely affected on the metal deposition behavior.748,858 The mobility of

Figure 82. (a) Preparation of μ-fuse array using selective Pb deposition. (b) μ-fuse array. (c) μ-fuse. The sample was observed with reflected light, and the bright color indicates the Pb-deposited area. (d) Current-interrupt characteristics of the μ-fuse. (e) Cross section dependence of the interrupt current level (Iint). Reprinted with permission from ref 856. Copyright 2012 The Japan Society of Applied Physics. 12256


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9. PHOTOMECHANICAL RESPONSE It is of particular interest from both scientific as well as technological points of view to have molecular systems make mechanical motion based on geometrical structure changes of individual molecules induced by chemical or physical stimuli and link the motion to macroscopic mechanical work. This is a longstanding dream since the first report by Merian866 in 1966. In biological systems, molecular-scale sliding movement of actin− myosin proteins is artfully linked to macroscopic mechanical work of muscles.867 Although elaborate molecular machines, 868,869 such as molecular shuttles, 870 molecular muscles,871,872 molecular elevators,873 and molecular rotors,874,875 have been extensively studied, attempts to link the molecular-scale motion of these man-made devices to macroscopic mechanical work have failed. Any certain methodology how to rationally design the molecular machines to perform macroscopic mechanical work has not yet been developed. On the other hand, various types of polymers9d,876−880 and carbon nanotubes881 have been reported to transfer molecular phenomena into macroscopic mechanical movement of materials. The movement, however, relies not on individual molecular behavior but on the response of bulk materials. The photoinduced contraction of liquid crystal elastomers,878−880 for example, is attributed to the order−disorder phase transition of the liquid crystal materials. Extensive studies on photochromism of diarylethene single crystals by Irie and co-workers have made a breakthrough in this area.774,882 Macroscopic mechanical movement of materials based on molecular-scale structure changes of individual molecules was realized in molecular crystals of diarylethene derivatives.20,775,883−888 In general, photochromic reactions rarely occur in crystals, because in crystals large geometrical structure change is prohibited. For example, azobenzene and most of the spiropyran derivatives cannot undergo photochromism in the singlecrystalline phase. Fortunately, the geometrical structure change of diarylethene derivatives during the photochromic reactions is small, and therefore many diarylethene derivatives can undergo photochromism even in the crystalline phase, as described in section 7. As evidenced by X-ray crystallographic analysis, a diarylethene molecule shrinks in size upon the ring-closing reaction and expands upon the ring-opening reaction.774 Molecular crystals are densely packed organized systems. Therefore, if such a molecular structure change takes place in the crystal, the crystal shape is anticipated to follow the shape change of component diarylethene molecules. In other words, the change of molecular shape can be directly linked to the macroscopic shape deformation of the crystal. The most convenient way to know the crystal shape change is to measure the surface morphology by using an atomic force microscope (AFM).774 Figure 84a shows the morphology changes of (100) surface of compound 12 upon irradiation with ultraviolet (365 nm) and visible (>500 nm) light measured with atomic force microscopy. Upon irradiation with UV light, new steps appear on the (100) single crystal surface, and the steps disappear upon irradiation with visible light. The step height, about 1 nm, corresponds to one molecular layer, as shown in Figure 84b. The surface morphology change is attributed to photoinduced contraction of the long axis of each diarylethene molecule regularly packed in the single crystal. This result indicates that the geometrical structure change of each molecule is linked to the shape deformation of the crystal surface.

showed different diffraction in reflection and transmission and could be reprogrammed upon light irradiation. They have great potential as optical elements for various future optical devices and systems. 8.3. Subwavelength Nanopatterning

Optical lithography is a key technology for fabricating nanostructures. Various sophisticated techniques have been developed to increase the resolution of nanopatterning. Recently, super-resolution fluorescence imaging techniques, such as STED (Stimulated Emission Depression) or RESOLFT, by using photoswitchable fluorescent dyes have attracted much attention, as described in section 3.7.1. The technique can be extended to nanopatterning.864,865 Figure 83a shows the principle of the

Figure 83. (a) The photochromic layer turns transparent upon exposure to λ1 and opaque upon exposure to λ2. When irradiated with a node at λ2 coincident with a peak at λ1, a subwavelength transparent region is formed through which photons at λ1 penetrate, forming a nanoscale optical writing beam. (b) Scanning electron micrograph of subwavelength lines using absorbance modulation. Adapted with permission from ref 864. Copyright 2009 AAAS.

nanopatterning. On top of the recording photoresist layer, a thin photochromic film is placed. The film adopts two isomeric forms that interconvert upon absorption of UV (λ1) and visible (λ2) light. Both colors are simultaneously applied in an interference pattern that overlaps peaks at λ1 with nodes at λ2. Absorption at λ1 makes the film transparent at that wavelength, but regions exposed to λ2 revert to the initial isomer and continue to absorb at λ1. A stable nanoscale transparent aperture is formed. Photons at λ1 penetrate this aperture, forming a nanoscale write beam that can pattern the underlying photoresist. This technique can confine light to spatial dimensions far smaller than the wavelength used. Figure 83b shows the result. The photochromic layer containing 1,2-bis(2-methyl-5-(5-methyl-2-thienyl)-3-thienyl)perfluorocyclopentene (472) is illuminated by two overlapping standing waves with periods of 350 nm (λ2 = 633 nm) and 170 nm (λ1 = 325 nm), respectively. The scanning electron microscopic image of lines indicates that the line width is around 30 nm, which is far smaller than writing wavelength 325 nm. Although this demonstration utilizes one-dimensional standing waves, it is anticipated to extend to two-dimensional peaks and nodes, which can be generated with diffractive microoptics. 12257


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Figure 84. (a) AFM images of the (100) crystal surface of compound 12 (A) before photoirradiation, (B) after irradiation with 366 nm light for 10 s, and (C) after irradiation with visible light (λ > 500 nm). (b) Molecular packing of the molecules. Adapted with permission from ref 774. Copyright 2001 AAAS.

In a densely packed crystal, strain energy is considered to directly influence the shape of the bulk crystal. Figure 85 shows the color and anisotropic shape changes of three single crystals: 8 8 2 , 8 8 4 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (75), 1,2-bis(5-ethyl-2-phenyl-4thiazolyl)perfluorocyclopentene (473), and 1,2-bis(5-methyl-2phenyl-4-thiazolyl)perfluorocyclopentene (23). When crystals 75 and 473 with size of around 10 μm prepared by sublimation are irradiated with 365 nm light, their color and shape change. Crystal 75 turns its color from colorless to blue, while crystal 473 changes from colorless to red. Both crystals transform the shape from square to lozenge. Although the color changes are different, both crystals show similar shape deformation. The result indicates that the deformation of crystal shape is ascribed to the geometrical structure change of component molecules and the electronic structure change does not influence the shape deformation. To elucidate the relation between the crystal shape deformation and geometrical structure changes of individual molecules, X-ray crystallographic study was carried out.882,884 Figure 86 shows the molecular packing of the crystal 75 before UV irradiation. The shape deformation from square to lozenge indicates that the crystal contracts along the c-axis and expands along the b-axis. In fact, it was found by in situ X-ray crystallographic analysis of a micrometer-sized crystal that baxis expands from 10.971 to 11.026 Å and c-axis contracts from 10.585 to 10.542 Å.884 Figure 87 shows the geometrical structure change of 75 upon photoisomerization determined by X-ray crystallographic analysis. The height of the molecule increases from 0.61 to 0.73 nm, which is the direction of b-axis. On the other hand, the thickness of the molecules is reduced when the molecule converts from the open- to the closed-ring form. The cofacial packing of molecules along the c-axis, as shown in Figure 86c, indicates that the thin layers of closed-ring isomers allow the molecules to be stacked one-by-one, resulting in contraction along the c-axis. The molecular structure change of diarylethene molecules directly influences the crystal shape. Figure 85c shows another type of crystal shape deformation.882 Upon UV irradiation, the rectangular crystal 23 contracts along the long axis of the crystal and reverts to the initial shape upon irradiation with visible light. When a rod-like crystal was prepared from compound 23, the crystal bends toward the direction of

Figure 85. Chemical structures and deformation of three kinds of diarylethene crystals upon UV (365 nm) and visible (λ > 500 nm) light irradiation. (a) 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (75), (b) 1,2-bis(5-ethyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (473), and (c) 1,2-bis(5-methyl-2-phenyl-4thiazolyl)perfluorocyclopentene (23). Adapted with permission from ref 882. Copyright 2007 Macmillan Publishers Ltd.

incident UV light. This effect is due to the gradient in the extent of the photoisomerization from the surface caused by the high absorbance of the crystal. The shrinkage of the irradiated part of the crystal causes the bending as observed in a bimetal. The bent rod-like crystal again becomes straight upon irradiation with visible light. The rod-like crystal can launch a tiny silica particle as shown in Figure 88. The relatively large Young’s modulus of the crystal (around 10 GPa) can hit the silica particle as if it were a tennis ball. For practical applications, the crystal actuators should have sufficient durability and substantial mechanical properties. The above single-component diarylethene crystals lack the durability 12258


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Figure 86. Molecular packing of a single crystal of compound 75 before UV irradiation. The red arrows indicate the direction of contraction and blue arrows the direction of expansion of the crystal upon UV irradiation.

and break in less than 100 deformation cycles. To improve the fatigue-resistant property, multicomponent crystals were prepared. Multicomponent crystals composed of 1-(5-methyl-2-phenyl4-thiazolyl)-2-(5-methyl-2-p-tolyl-4-thiazolyl)perfluorocyclopentene (474) and 1,2-bis(5-methyl-2-p-tolyl-4thiazolyl)perfluorocyclopentene (475) were found to exhibit superior fatigue-resistant property.885 Microcrystals were prepared by recrystallization from ethanol. Plate-like crystals were mainly formed from the ethanol solution containing only 474,

Figure 88. A shot of a silica microparticle (diameter: 90 μm) by a crystal rod of 1,2-bis(5-methyl-2-phenyl-4-thiazolyl)perfluorocyclopentene (23) upon irradiation with 365 nm light. Reprinted with permission from ref 883. Copyright 2008 The Chemical Society of Japan.

Figure 87. Chemical structures of the open- and closed-ring isomers of compound 75 and top- and side-views of the geometrical structures of the openand closed-ring isomers in crystals. The two isomers were isolated and independently recrystallized. 12259


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Figure 89. Chemical structures and photoreversible crystal shape changes of (a-i and a-ii) mixed crystals of 474 and 475, and (b) 476 upon irradiation with UV (365 nm) and visible (>500 nm) light. Adapted with permission from refs 885 and 888. Copyright 2012 and 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

amounts of the two component molecules suggests that intermolecular interaction among the component molecules is weakened in the mixed crystals, and the weakened intermolecular interaction is considered to improve the durability of the crystal. A long rod-like mixed crystal (3 mm in length) composed of 474 and 475 showed curling into a hairpin shape upon irradiation with UV light, as shown in Figure 89a-ii. The crystal remained crystalline even after the curling and returned again to its straight shape upon irradiation with visible light. The two-component mixed crystal is flexible. A noticeable feature of the actuators is

while formation of rod-like crystals became dominant when 475 was added in an amount exceeding 30 mol %. In the ethanol solution containing equimolar amounts of 474 and 475, rod-like crystals composed of 474 (63 mol %) and 475 (37 mol %) grew. The rod-like crystals can repeat bending cycles upon irradiation with UV and visible light, as shown in Figure 89a-i, more than 1000 times. The crystal surface remained clear even after 1000 cycles, and no damage was discerned. The melting point of the mixed crystals showed the minimum of 131 °C at the molar ratio of 1:1. The low melting point of the crystal containing equimolar 12260


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Figure 90. Photomechanical work of a molecular crystal cantilever made of the cocrystal (446-NpF). Illumination of light was carried out below the crystal. The weights of the lead ball and the crystal cantilever were 46.77 and 0.17 mg, respectively.

In situ X-ray crystallographic analysis revealed that the macroscopic deformation of these crystals is due to the anisotropic deformation of the crystal lattice induced by the geometrical shape changes of the component diarylethene molecules upon photoisomerization. The molecular crystals link the geometrical structure changes of molecules in the molecular world to macroscopic mechanical movement of the materials and perform the mechanical work in the macroscopic world. Similar shape deformation of single crystals upon photoirradiation was also found for other diarylethenes, 477887 and 77775 (Scheme 50), azobenzene,889 salicylideneaniline,890 furylfulgide,891 and anthracene derivatives.892

the performance at low temperature. The rod-like crystal exhibits light-driven bending even at 4.6 K, and the bending was found to take place in less than 5 μs. The two-component cocrystal composed of a diarylethene derivative, 1,2-bis(2-methyl-5-(1-naphthyl)-3-thienyl)perfluorocyclopentene (446), and perfluoronaphthalene (NPF) also exhibited light-driven bending upon irradiation with UV and visible light.886 The rectangular plate-like crystal turned its color from colorless to blue and bent moving away from the light source. The bending ceased when the illumination light was switched off, and the crystal kept the bent shape in the dark. Upon visible light irradiation, the blue color disappeared, and the crystal returned to the initial straight shape. The rectangular plate-like cocrystal 446-NPF was fixed at the edge of a glass plate as a cantilever arm, and a lead ball was loaded onto the crystal, as shown in Figure 90.886 The crystal and the ball weigh 0.17 and 46.77 mg, respectively. Upon irradiation with UV light, the heavy ball is lifted as high as 0.95 mm. The cantilever arm performs lifting work, and the amount of the work is as large as 0.43 μJ. The photogenerated maximum stress was estimated to be 44 MPa. A photochromic diarylethene crystal (thickness:width:length = 1.5:11:320 μm) composed of 1-(2-methyl-5-(4-(1naphthoyloxymethyl)phenyl)-3-thienyl)-2-(2-methyl-5-phenyl3-thienyl)perfluorocyclopentene (476) was found to twist upon irradiation with UV light, accompanied by a color change from colorless to blue, as shown in Figure 89b.888 The twisting and the blue color remain stable in the dark. By irradiation with visible light, the blue color disappears and the twisted crystal relaxes back to the original shape in a few second. The reversible twisting upon alternate irradiation with UV and visible light could be repeated for more than 30 cycles. Two types of twisting, lefthanded and right-handed helices, were observed in almost equal amounts. The twisting action of the crystals is attributed to the anisotropic shape change of the unit cell upon photoisomerization of the diarylethene molecule. The twisting shape change can be explained on the basis of the molecular-scale change in the unit cell shape.

Scheme 50. Single Crystals Composed of the Following Diarylethenes 477 and 77 Exhibit Shape Deformation upon Alternate Irradiation with UV and Visible Light775,887

10. DIARYLETHENE RELATIVES Most of the diarylethenes described in the previous sections have heterocyclic aryl groups, such as thiophenes or benzothiophenes, and ethene bridges, such as perfluorocyclopentene, maleic anhydride, or maleimide. Both the aryl groups and the ethene bridges can be optionally replaced with various aromatic as well as nonaromatic cyclic systems. A cyclic bridge is introduced to prevent trans−cis photoisomerization from competing photocyclization reaction. The original diarylethene derivatives have maleic anhydride or maleimide as the ethene bridge.7,12 The diarylethene derivatives having maleic anhydride or maleimide undergo photochromic reactions in less polar solvents, but the reactivity is strongly suppressed in polar solvents, such as methanol or acetonitrile. To 12261


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Scheme 51. Diarylethene Relatives Having Various Kinds of Ethene Bridges

Krayushkin and co-workers895−898 and others899−913 employed various types of heterocyclic systems as the ethene bridges as shown in Scheme 51. When the ethene bridge is perfluorocyclopentene, most of the derivatives are nonfluorescent or very weakly fluorescent in both open- and closed-ring isomers. However, diarylethene derivatives having a phosphoruscontaining or silylated five-membered ring, such as compounds 481 and 482, or dithiole-2-thione, such as compound 483, can exhibit fluorescence in the closed-ring isomers.899,900 When five-membered heterocyclic rings, such as thiophene, thiazole, oxazole, or imidazole, are used as the ethene bridges, such as compounds 484, 485, 479, 486, 478, 480, the closed-ring isomers become thermally unstable.147,895,903 The colors of the closed-ring isomers disappear even in the dark in contrast to stable colors of closed-ring isomers of diarylethenes having a perfluorocyclopentene bridge, as described in section 3.2. The thermal stability depends on the type of bridges. The half-life time of the closed-ring isomer of compound 485 was 11 h at 20 °C in toluene, while the half-life time of compound 486 increased to 14 days by replacing the thiophene ring with the thiazole ring. The difference is attributed to the aromatic stabilization energy difference between the two heterocyclic rings.903 Although the absorbance of the closed-ring isomer of compound 487 decreased to 80% of the initial value after 15 h at 80 °C, the absorbance of the closed-ring isomer of compound 488 remained stable even after 40 h at 80 °C. The oxidation of the sulfide bond to sulfone decreases the aromatic stabilization

provide photochromic reactivity even in polar solvents, the electron-withdrawing ethene bridges were replaced with perfluorocycloalkenes. 1,2-Bis(2-methyl-1-benzothiophen-3-yl)perfluorocycloalkenes with four-, five-, and six-membered perfluorocycloalkene rings were prepared, and their photochromic performance was examined in various solvents.156 The photochromic cyclization/cycloreversion reactions were observed not only in less polar methylcyclohexane but also in polar methanol or acetonitrile. Among the three derivatives, compound 15 (five-membered ring) showed the highest fatigueresistant property. The methylcyclohexane solution containing compound 15 (1.2 × 10−4 mol dm−3) was irradiated with UV light (300 < λ < 400 nm) until the photostationary state was attained, and then the color due to the closed-ring isomer was bleached completely by irradiation with visible light (>450 nm). The coloration/decoloration can be repeated for more than 14000 cycles, keeping 90% of the performance. The perfluorocyclopentene bridge can be replaced with a nonfluorinated cyclopentene ring.37,38,893,894 Although the photochromic performance is similar, the fatigue-resistant property of the derivatives having nonfluorinated cyclopentene bridge is inferior to that of the derivatives having perfluorocyclopentene. The fluorinated version was found to switch faster than its nonfluorinated analogue by about a factor of 4.7, as described in section 2.2. The absorption maximum showed a hypsochromic shift by replacing the fluorines with hydrogens. 12262


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Scheme 52. A Diarylethene Derivative That Undergoes a Nucleophilic Reaction in the Closed-Ring Isomer912

AUTHOR INFORMATION

energy of the benzothiophene ring and stabilizes the closed-ring isomer.907 Cyclobutene-1,2-dione, cyclopentenone, and indenone and its ethylene acetal are also employed as the ethene bridges, such as compounds 489, 490, and 491. 1,3-Disubstituted imidazolium can also be used as the ethene bridge. The imidazolium bridge converts to imidazolinium upon photoisomerization by irradiation with UV light. The imidazolium closed-ring isomer exhibited characteristic solvato- and ionochromism. The imidazolium closed-ring isomer 495b undergoes the nucleophilic reaction with sodium methoxide, while the imidazolium openring isomer 495a was inert to the reaction, as shown in Scheme 52.912 When chiral naphthoquinone is used as the bridge, the closed-ring isomer 492b is produced in a remarkable diastereomeric excess (98%) with high conversion ratio (96%).911 The modification of the ethene bridge provides additional chemical and physical properties to the diarylethene family.

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

11. CONCLUSION The original diarylethene has maleic anhydride as the ethene bridge and 2,5-dimethylthiophene as the aryl groups. In the second version of diarylethenes, the maleic anhydride bridge was replaced with perfluorocyclopentene, and various types of aryl groups were introduced into the aryl groups. The photochromic performance, especially fatigue-resistant property, was dramatically improved by choosing suitable aryl groups, such as benzothiophene or 2,4-dimethyl-5-phenylthiophene. The thermally irreversible, fatigue-resistant, ultrafast, and highly sensitive photochromic reactivity of the diarylperfluorocyclopentene derivatives was confirmed by extensive experimental studies and also by theoretical calculations. When a dienophile is introduced in the bridge, such as dithienylfulvene, the molecule can control Diels−Alder cycloaddition reactions with dienes by photoirradiation. In general, both isomers of diarylethenes are not fluorescent, but sulfone derivatives of bisbenzothienylethenes were found to emit strong fluorescence in the closed-ring form. Turn-on fluorescence property was given to the diarylethene family. Appropriate chemical modification of the diarylethene 6π-elecron system can provide potential various applications of the derivatives. In addition, many of the diarylethene derivatives undergo photochromism even in the single-crystalline phase. The photoisomerization of diarylethene molecules in crystals was successfully linked to macroscopic shape changes of bulk crystals. The shape changes can be applied to micro-size light-driven actuators. The outstanding performance of the diarylethenes offers great potential for advancing future optics and optoelectronic technologies and also certain promise for applications to biological science and technologies.

Masahiro Irie received his B.S. (1966) and M.S. (1968) degrees from Kyoto University and his Ph.D. (1973) degree in radiation chemistry from Osaka University. In 1968, he joined the Faculty of Engineering, Hokkaido University, as a research associate and started his research on photochemistry. In 1973, he moved to Osaka University and was promoted to associate professor at the Institute of Scientific and Industrial Research in 1978. In 1988, he was appointed professor at the Institute of Advanced Material Study, Kyushu University; in 1996, he was reappointed professor of chemistry at the Faculty of Engineering. In 2007, he moved to Tokyo and is now specially appointed professor at Rikkyo University. He has been conducting research on photochromic molecular systems for the last 35 years. In the middle of the 1980s he discovered thermally irreversible and fatigue-resistant photochromic diarylethenes, the detailed studies of which are described in this Review. His current interest focuses on the development of light-driven molecular crystal actuators based on single-crystalline photochromism of diarylethenes and turn-on fluorescent diarylethenes for superresolution fluorescence microscopy. 12263


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Seiya Kobatake received his B.S. (1991), M.S. (1993), and Ph.D. (1996) degrees from Osaka City University. From 1996−1997, he engaged as a postdoctoral researcher at the Institute of Polymer Science, The University of Akron, OH. In 1998, he joined the group of Prof. Dr. M. Irie, as a postdoctoral researcher of Core Research for Evolutional Science and Technology (CREST) project, Japan Science and Technology Corporation. From 2000−2004, he was a research associate at the Graduate School of Engineering, Kyushu University. He moved to Osaka City University as associate professor at the Graduate School of Engineering in 2004, and was promoted professor in 2011. Concurrently, he worked as a researcher of PRESTO, Japan Science and Technology Agency in 2006−2010. He received the Chemical Society of Japan Award for Young Chemists (2002) and the Asian and Oceanian Photochemistry Association Prize for Young Scientist (2010). His current research interests include the photochromism of diarylethene crystals and their application.

Tuyoshi Fukaminato received his B.S. (1999), M.S. (2001), and Ph.D. (2004) degrees from Kyushu University. In 2004, he joined the Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, as a research associate and started his research on photochemistry. In 2008, he moved to the Research Institute for Electric Science, Hokkaido University. He was also a JSPS researcher from 2001 to 2004 and a researcher of the PRESTO (Precursory Research for Embryonic Science and Technology) project sponsored by JST (Japan Science and Technology Agency) from 2008 to 2012. His research interests are focused on the single-molecule fluorescence photoswitching and the photocontrol of biological functions.

ACKNOWLEDGMENTS We wish to express our thanks to the following for support: CREST of Japan Science and Technology Corporation, Japan, PRESTO of Japan Science and Technology Agency, Japan, Grant-in-Aid for Scientific Research on Priority Areas “New Frontiers in Photochromism (471)” from the Ministry of Education, Culture, Sports and Science and Technology, Japan (MEXT), and MEXT-Supported Program for the Strategic Research Foundation at Private Universities. REFERENCES (1) Brown, G. H. Photochromism; Wiley-Interscience: New York, 1971. (2) Kelly, J. M.; McArdle, C. B.; Maunder, M. J. de F. Photochemistry and Polymeric Systems; RSC: Cambridge, 1993. (3) (a) Irie, M. Photoreactive Materials for Ultrahigh-density Optical Memory; Elsevier: Amsterdam, 1993. (b) Irie, M.; Yokoyama, Y.; Seki, T. New Frontiers in Photochromism; Springer: Tokyo, 2013. (4) Crano, J. C.; Guglielmetti, R. J. Organic Photochromic and Thermochromic Compounds; Plenum Press: New York and London, 1998. (5) (a) Bouas-Laurent, H.; Dürr, H. Pure Appl. Chem. 2001, 73, 639. (b) Dürr, H.; Bouas-Laurent, H. Photochromism: Molecules and Systems; Elsevier: Amsterdam, 2003. (6) Recently, new T-type photochromic compounds, hexaarylbiimidazoles (HABI), have also been developed. Abe, J. In New Frontiers in Photochromism; Irie, M., Yokoyama, Y., Seki, T., Eds.; Springer: Tokyo, 2013; p 161. (7) Irie, M.; Mohri, M. J. Org. Chem. 1988, 53, 803. (8) Nakamura, S.; Irie, M. J. Org. Chem. 1988, 53, 6136. (9) (a) Irie, M.; Hayashi, K. J. Macromol. Sci., Chem. 1979, A13, 511. (b) Irie, M.; Menju, A.; Hayashi, K. Macromolecules 1979, 12, 1176. (c) Irie, M.; Hirano, K.; Hashimoto, S.; Hayashi, K. Macromolecules 1981, 14, 262. (d) Irie, M. Adv. Polym. Sci. 1990, 94, 28.

Kenji Matsuda received his B.S. (1992), M.S. (1994), and Ph.D. (1997) degrees from the University of Tokyo. In 1994, he joined the Faculty of Science, the University of Tokyo, as a research associate, and in 1995 he moved to Kyushu University at Institute for Fundamental Research of Organic Chemistry. In 1998 he moved to the Graduate School of Engineering at the same university and was promoted to associate professor in 2004. In 2008 he was appointed as full professor at the Graduate School of Engineering, Kyoto University. From 2001−2002 he was a JSPS researcher at the University of Illinois. He received the Chemical Society of Japan Award for Young Chemists (2003) and Nozoe Memorial Award for Young Scientist (2006). His research interest is in physical organic chemistry for molecular electronics and highly functional materials. 12264


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