1 Thermal stability of both isomers
2 Fatigue‐resistance
3 High sensitivity
4 Rapid response
5 Reactivity in the solid state.
Molecular photoswitches belonging to the diarylethene family fulfill the above requirements simultaneously. Diarylethenes are derivatives of stilbene. When the phenyl rings of stilbene are replaced with five‐membered heterocyclic rings, such as thiophene or furan rings, both open‐ and closed‐ring isomers become thermally stable and photoinduced coloration/decoloration cycles can be repeated many times. The best photoswitching performance of well‐designed diarylethenes is summarized as follows.
1 Both isomers are thermally stable: half‐life times at room temperature are as long as 470 000 years at 30 °C.
2 Photoinduced coloration/decoloration can be repeated for more than 104 cycles.
3 The quantum yield of cyclization (coloration) reaction is close to 1 (100%).
4 Response times of both coloration and decoloration reactions are less than 20 ps.
5 Many of diarylethene derivatives undergo photoswitching even in the single crystalline phase.
1.2 Discovery of Diarylethene Molecular Photoswitches
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 (λmax ∼ 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