Читать книгу Diarylethene Molecular Photoswitches - Masahiro Irie - Страница 11

The diarylethene molecular photoswitches were serendipitously discovered during the course of a study on photoresponsive polymers [23]. Various polymers having molecular photoswitches, such as spirobenzopyran, azobenzene, or stilbene, in the side groups or main chains have been prepared in an attempt to modulate their conformations by photoirradiation. When azobenzene chromophores are incorporated into a polymer backbone, the solution viscosity was found to reversibly change upon alternate irradiation with UV and visible light [24]. Before UV light irradiation, the polymer has a rod‐like extended conformation. Upon UV light irradiation, the azobenzene units convert from the trans‐ to the cis‐form and the isomerization kinks the polymer chain, resulting in a compact conformation and a decrease in the viscosity, as shown in Figure 1.2. Not only viscosity but also other properties, such as pH, solubility, and sol–gel phase transition temperature, were successfully modulated upon photoirradiation [23–28].

Figure 1.2 Schematic illustration of the photoinduced conformational change of a polymer having azobenzene units in the backbone.

Just like azobenzene, stilbene also undergoes the trans–cis photoisomerization reaction. The photoresponsive polymer research was extended to polymers having stilbene units. A polymer having stilbene units in the backbone can be prepared by 1,4‐addition radical polymerization of 2,3‐diphenylbutadiene, which is prepared from acetophenone, as shown in Figure 1.3 [29]. Upon irradiation with 313‐nm light, the poly(2,3‐diphenylbutadiene) efficiently underwent photocyclization reactions to produce a polymer having yellow colored dihydrophenanthrene units in a deaerated dichloromethane solution, instead of the trans–cis photoisomerization. The trans–cis photoisomerization of stilbene units in the backbone was strongly suppressed due to rigidity of the polymer chain. The dihydrophenanthrene units readily returned to the initial 2,3‐diphenyl‐2‐butene units and the yellow color disappeared in less than 10 minutes at room temperature.

Figure 1.3 A synthesis route of poly(2,3‐diphenylbutadiene) and its photochemical and thermal reactions.

On the other hand, in the presence of air the dihydrophenanthrene units converted to phenanthrene units by hydrogen elimination and the reversibility was lost. To prevent hydrogen elimination and provide reversibility even under aerated conditions, 2,3‐dimesitylbutadiene was designed (Figure 1.4A(a)). The synthesis of 2,3‐dimesitylbutadiene was attempted by photoreduction of 2,4,6‐trimethylacetophenone, as shown in Figure 1.4B(a). But, the synthesis of pinacol failed because of the bulky size of the mesityl group. To reduce steric hindrance, the mesitylene was replaced with 2,5‐dimethylthiophene (Figure 1.4A(b)). According to the synthetic route shown in Figure 1.4B(b), 2,3‐bis(2,5‐dimethyl‐3‐thienyl)butadiene was successfully synthesized from 2,5‐dimethyl‐3‐acetylthiophene. The butadiene was polymerized to poly(2,3‐bis(2,5‐dimethyl‐3‐thienyl)butadiene) by 1,4‐addition radical polymerization.

Figure 1.4 (A) Synthesis of polymers having (a) 2,3‐dimesitylbutene units and (b) 2,3‐di(2,5‐dimethyl‐3‐thienyl)butene units in the backbone. (B) (a) A synthetic route to prepare 2,3‐dimesitylbutadiene. (b) Synthetic routes and photochemical reactions of poly(2,3‐di(2,5‐dimethyl‐3‐thienyl)butadiene) and poly(2,3‐di(2,5‐dimethyl‐3‐furyl)butadiene).

The polymer having 2,3‐dithienyl‐2‐butene units was dissolved in benzene and the solution was irradiated with 313‐nm light. The colorless solution turned yellow (λ_max ∼ 430 nm) along with the formation of cyclized closed‐ring isomers. The yellow color disappeared upon irradiation with visible light. In contrast to poly(2,3‐diphenylbutadiene), the yellow color of the closed‐ring isomer units was found to remain stable overnight in the dark. The yellow closed‐ring units were stable even at 100 °C and returned to the initial colorless open‐ring isomer units with visible light. The dithienylethene unit in the polymer was unprecedentedly found to undergo a thermally irreversible photoswitching reaction. Poly(2,3‐bis(2,5‐dimethyl‐3‐furyl)butadiene) also underwent the thermally irreversible photoswitching reaction. The amazing result led us to study the photochemistry of the monomer unit, 2,3‐di(2,5‐dimethyl‐3‐thienyl)‐2‐butene and its derivatives in detail. This is the course of serendipitous discovery of diarylethene molecular photoswitches.

Since the discovery of thermally irreversible diarylethene molecular photoswitches in the middle of 1980s, various types of diarylethene derivatives have been synthesized to improve their photoswitching performance. Figure 1.5 shows a list of main diarylethene derivatives developed in Kyushu University and Rikkyo University until 2017. Upon irradiation with UV light, 2,3‐di(2,5‐dimethyl‐3‐thienyl)‐2‐butene underwent a cis–trans isomerization in addition to the cyclization reaction. To prevent the unfavorable cis–trans photoisomerization, a cyclic bridge, such as maleic anhydride or maleimide, was introduced. Although diarylethene derivatives with the maleic anhydride or maleimide bridge showed photocyclization reactivity in less polar solvents, the reactivity was strongly suppressed in polar solvents, such as methanol or acetonitrile. To provide photoswitching reactivity even in polar solvents, the ethene bridges were replaced with perfluorocycloalkenes with four‐, five‐, and six‐membered rings [30]. The 1,2‐bis(2‐methyl‐1‐benzothiophen‐3‐yl)perfluorocycloalkenes underwent reversible photoinduced cyclization/cycloreversion reactions in polar methanol and acetonitrile. Among the three derivatives having four‐, five‐, and six‐membered rings, five‐membered 1,2‐bis(2‐methyl‐1‐benzothiophen‐3‐yl)perfluorocyclopentene was found to offer the highest resistance to photofatigue. Since then, perfluorocyclopentene derivatives have been mainly studied.

Figure 1.5 Development of diarylethene molecular photoswitches.

Although diarylethene photoswitches exhibit brilliant color changes upon photoirradiation, most of them are nonfluorescent or very weakly fluorescent in both isomer forms. It was a long‐standing ambition to prepare photoswitchable fluorescent diarylethenes without attaching fluorescent chromophores to the diarylethenes. In 2011, sulfone derivatives of 1,2‐bis(2‐ethyl‐6‐aryl‐1‐benzothiophen‐3‐yl)perfluorocyclopentene were found to exhibit very strong fluorescence (fluorescence quantum yield ∼ 0.9) in the closed‐ring isomers [31]. The turn‐on mode fluorescent diarylethenes are now extensively applied to super‐resolution fluorescence microscopy in materials science and biological systems. Diarylethenes are able to switch both absorption (color) and fluorescence emission upon photoirradiation.

At first sight, the most striking phenomenon observed in molecular photoswitches is a photoinduced instantaneous color change. Figure 1.6 shows photos of the color changes of diarylethene derivatives in solution. When the toluene solutions of the derivatives are irradiated with UV light, the colorless solutions turn yellow, orange, red, violet, blue, cyan, and green. The chemical structures of the derivatives are shown below in the photos. These colors disappear upon irradiation with visible light. The photoinduced coloration/decoloration cycles upon alternate irradiation with UV and visible light can be repeated many times.

Figure 1.6 Color changes of diarylethene derivatives 1–7 in toluene upon irradiation with UV and visible light.

The color changes are ascribed to the electronic structure changes of the derivatives from the open‐ to the closed‐ring isomers. Two typical examples of the electronic structure changes are shown in Figure 1.7. 1,2‐Bis(2,5‐dimethyl‐3‐thienyl)perfluorocyclopentene (3) and 1,2‐bis(3,5‐dimethyl‐2‐thienyl)perfluorocyclopentene (1) undergo reversible electrocyclic rearrangements. The electrocyclic reactions involve rearrangements of positions of single and double bonds in a molecule. During the reactions, a new single bond is made between the central reactive carbon atoms by the cyclization reaction and the bond is broken as the ring is opened.

Figure 1.7 Chemical structures and absorption spectra of open‐ and closed‐ring isomers of (a) 3 and (b) 1 in n‐hexane.

Figures 1.7 shows the chemical structures of the open‐ and the closed‐ring isomers and their absorption spectra. In both derivatives 3 and 1, upon irradiation with appropriate wavelength of light (λ₁ or λ₃) a single bond is formed between the central reactive carbon atoms and the double bonds change the position. Upon irradiation with another wavelength of light (λ₂ or λ₄) the single bond is broken and the molecule returns to the initial structure. The color is controlled by the length of π‐conjugation. In the open‐ring isomers, two thiophene rings have no particular interaction and the spectra are comparable to substituted thiophenes. In the closed‐ring isomers, the π‐conjugation length depends on the attached position of thiophene rings to the ethene bridge. When the thiophene rings are attached to the ethene bridge at 3‐position, such as derivative 3, π‐conjugation is delocalized throughout the molecule in the closed‐ring isomer and the delocalization results in red color. The π‐conjugation is further extended when phenyl groups are substituted at 5‐ and 5′‐positions of the thiophene rings, such as 5. The long π‐conjugation shifts the absorption band to longer wavelengths, resulting in blue color. On the other hand, when the thiophene rings are attached to the ethene bridge at 2‐position, such as derivative 1, π‐conjugation is localized in the central part. The short π‐conjugation results in yellow color in the closed‐ring isomer.

The photoswitching between two discrete states and thermal irreversibility of the two states are indispensable for applications to memory media and switching devices. The bistability is a basic characteristic of diarylethenes. Although the chemical structures of the two isomers suggest photoswitching between two discrete states, in general, the absorption and fluorescence spectra gradually change upon photoirradiation in ensemble systems. Figure 1.8b shows the photoswitching performance of 1,2‐bis(2‐ethyl‐6‐phenyl‐1‐benzothiophene‐1,1‐dioxide‐3‐yl)perfluorocyclopentene (8, Figure 1.8a) in the ensemble system in 1,4‐dioxane. The gradual analog increase in the fluorescence intensity upon irradiation with UV light indicates a change in the concentrations of the two isomers. Upon UV irradiation the open‐ring isomers convert to the fluorescent closed‐ring isomers and the concentration ratio of the closed‐ring isomers increases, causing the gradual increase in the fluorescence intensity. Subsequently, upon irradiation with visible light the ratio of the closed‐ring isomers decreases, resulting in disappearance of the fluorescence. The switching between the two discrete states cannot be discerned from the photoswitching performance in the ensemble system.

Figure 1.8 (a) Chemical structures of open‐ and closed‐ring isomers of 8. (b) Fluorescence photoswitching of 8 upon irradiation with UV and visible light in the ensemble system in 1,4‐dioxane. (c) Fluorescence photoswitching of 8 at the single‐molecule level in a Zeonex polyolefin film.

Digital on/off photoswitching between two discrete states was confirmed by measuring the switching response at a single‐molecule level. Figure 1.8c shows the fluorescence photoswitching of a single molecule of derivative 8 upon alternate irradiation with UV and visible light. Upon irradiation with UV light, the fluorescence abruptly switches from the off‐state to the on‐state, while upon irradiation with visible light the on‐state abruptly returns to the off‐state. The digital photoswitching response definitely indicates that diarylethene photoswitch 8 has bistable states. The photoisomerization between two discrete isomer states expressed by the two chemical structures is experimentally evidenced.