b Comprehensive Analysis Center, ISIR, Osaka University, Mihogaoka, Ibaraki 567-0047, Japan;
c BSFR, Institute of High Energy Physics Chinese Academy of Sciences, Beijing 100049, China
Supramolecular chemistry  has been one of the most attractive research topic of modern chemistry,and is still receiving rapid progresses in multidisciplinary fields,such as supramolecular photochirogenesis [2, 3],organic electronic devices [4, 5] and biocompatible materials . Stimuli-responsive supramolecular systems is particularly intriguing because they provides an attractive approach for creating novel materials that are capable of responding to environmental changes [7, 8, 9]. Comparing to temperature (T),pH and other stimulating factors [10, 11, 12],light is considered as the most useful external stimulus for dynamically controlling the morphology and functionality of supramolecular assemblies because of its non-invasivity and convenience for use.
Photochromism,known as a photoinduced reversible transformation between two isomers that have different absorption spectra,offers a fascinating tool for manipulating molecular and supramolecular geometry and photo- and electronic properties . Photochromism phenomenon is commonly found in inorganic compounds,organic compounds and many biologic systems . Among the variety of synthetic organic photochromic compounds,such as azobenzenes,diarylethenes (DAEs),spiropyrans,thiophenefulgides and hemithioondigos [15, 16, 17],DAEs are particularly appealing because of their accessible modification, thermal stability,high fatigue resistance over many cycles of photoswitchingandlargechangesintheir opticalandelectronicproperties upon photoisomerization [14, 15]. The fundamental of the photochromismofdiarylethenes is reversiblephoto-inducedring-opening and ring-closing isomerization. The substitution effects on the absorption spectra and quantum yields have been extensively investigated,which provides detailed information and guidance for designing novel photoswitches with properties desired . Hence, photochromic diarylethenes compounds are the most promising candidates as photoresponsive molecular building blocks to assemble photoresponsive supramolecular systems.
This review will summarize recent studies on photoresponsive supramolecular systems with diarylethenes as photoswiching units,as well as the applications of such systems in different areas, including photoresponsive supramolecular systems in construction of artificial biological molecular systems,as "smart" materials exhibiting mechanic properties upon photo irradiation and as photoresponsive chemosensors. 2. Diarylethenes as photoresponsive building blocks in supramolecular systems
The pioneering study concerning photochromic diarylethenes can be traced back to 1980s ,and the excellent properties of this kind of compounds attracted significant attention from chemists. The photochromic properties of diarylethene derivatives correlated to their chemical structure have been thoroughly investigated over the 40 years development. Generally speaking, both open and closed forms are stabilized by the aromatic stabilization energy of the heterocyclic aromatic moiety,and the introduction of electron-withdrawing groups on the aromatic rings often decreases the thermal stability of the closed forms. The conversion ratio between the open- and closed-isomers in the photostationary states,a critical parameter for their applications, could be optimized by rationally choosing appropriate substituent(s) and frameworks to offer ideal conversion ratios of > 95%, which makes it an excellent building block for stimuli responsive supramolecular architecture . The construction of stimuliresponsive supramolecular self-assembly has recently attracted significant interest because these assembles are promising candidates for smart materials that could find applications in chemistry,biology and material science [20, 21, 22]. By virtue of the significant difference of absorption spectra in open and closed forms,the morphology and chemical/physical properties of diarylethenes-based supramolecular assemblies,which are thermally irreversible,can be regulated by photoirradiation [23, 24, 25, 26]. Strongly fluorescent emission in the solid state could find broad potential applications,such as fluorescent biological labels,sensors and light-emitting diodes . However,many organic chromophores and polymers that can emit strong fluorescence in dilute solutions usually show drastically reduced fluorescence efficiency in the solid state because of "concentration quenching". In order to solve the "concentration quenching" problem,Yi and coworkers have designed a series of switchable supramolecular self-assemblies based on the hydrogen bonding interaction between pyridine-containing diarylethene BTEPy and carboxylic acids  (Fig. 1). By adapting this strategy,fluorescence enhancement of BTEPy was observed in solution,solid state and as nanoparticle assembly. Especially,when BTEPy interacts with a carboxylcontaining Bodipy dye (Fig. 1),which is an attractive chromophore been extensively used for light-harvesting molecular arrays and fluorescent molecular probes ,the "concentration quenching" in the solid state can be alleviated due to the hydrogen bond formation and energy transfer process .
|Fig. 1. Photochromic process of self-assembled complex 1 and structures of compound BTEPy and Bodipy.|
The use of phototriggered conformational change of diarylethenes has been used to tune the physical properties of selfassembled supramolecular systems. Yagai and coworkers  reported merocyanine-diarylethene multi-chromophore complex between 2a and chiral diarylethene derivative 3,formed by hydrogen bonding interaction (Fig. 2),which cause a J-type aggregation of 2a. Switching between monomer and J-type aggregation is controlled through the photoisomerization of diarylethene component,and subsequently switching of the absorption band,the circular dichroism (CD) signals and fluorescence can be controlled by light input. When 2b is incorporated to the binary system of 2a and 3,H aggregation due to the assembly of 2b and 3 is formed. Photoisomerization of the ternary system allowed a switching of the system among monomer,H and J aggregations.
When the aggregation is driven by π-π interaction,the central two methyl groups of the diarylethene component may work as regulators for aggregation,because they are mobile by the rotation of thiophene rings in the open form,whereas in the closed form they are fixed. Yagai and coworkers have demonstrated that compound 4 (Fig. 3) in the flexible open form more inclines to aggregate compared to the rigid closed form . This is mainly due to the aggregation of compound 4 is driven by strong π-π stacking interaction between the two π-conjugated wings,and the steric effect of the two methyl groups of the diarylethene core of the closed isomer prevents the aggregation. Fluorescence quenching was observed for the closed isomer because of the energy transfer within aggregates and the intramolecular energy transfer from the π-conjugated moieties to the diarylethene core. The unique aggregation abilities and the fluorescence quenching properties of the closed isomer,enables them to form a phototriggered fluorescent organogel,and thus providing a new methodology in the design of smart optoelectronic materials .
|Fig. 3. (a) structures of compound 4 in the open (4o) and closed forms (4c) and (b) schematic representation of phototriggered self-assembly.Reproduced with permission from Ref. .|
To design smart material that can reversibly change shape and/ or size by photoirradiation has attracted much attention of chemists. In the recent five years,scientists have been made significant progress in this field. Zhou et al. manipulated the morphology of a self-assembled supramolecular architecture from nanofiber to nanosphere by photoirradiation . The selfassemble monomer is an amphiphilic diarylethene derivative with histamines as side chains (Fig. 4). They demonstrated that the self-assemble properties of this molecular aggregate are highly sensitive to the pH conditions .
Yi and coworkers have used a naphthalimide-based 6 as a gelator to form triple hydrogen bond with the diarylethene attached melamine derivative 7. The morphology of the selfassembly of the gelator changes from flaky texture to films upon the formation of the assembly of 8o,which finally turns into aggregated flakes upon photoirradiation to give 8c (Fig. 5). The fluorescence of the co-assemblies can be light on by UV irradiation .
|Fig. 5. Structures of compounds 7 and 8 and the morphology change with photoirradiation.Reproduced with permission from Ref. .|
A copolymer using a melamine-attached diarylethene derivative 9 has been constructed through forming multiple hydrogen bond with an oligothiophene-functionalized ditopiccyanurate 10 (Fig. 6) . The assemble behaviours can be modulated by changing external light input. When the diarylethene moiety is in the open form,the chains of the copolymer intertwine into thermodynamically stable helical nanofiber through H-aggregation dominated by interchain π-π stacking interaction. Upon UV photoirradiation,the H-aggregated assembly transforms to nonaggregation,which gives fibrils with the evaporation of the solvent .
|Fig. 6. (a) Structures of 9 in its open and closed form and compound 10, (b) schematic representation of the photoresponsive hierarchical organization of 9 and 10.Reproduced with permission from Ref. .|
Most supramolecular assemblies with diarylethenes as photoswitchable units take use of the difference in the stacking propensity between the flexible open-ring isomers and the rigid closed ring isomers to control the assembly and disassembly of the aggregates [7, 26]. The flexible open-ring isomers have two conformers,the photochemically unreactive parallel conformer and the reactive antiparallel conformer,and they are in equilibrium with each other in solution,but the former is not stable. Very recently,Yagai group demonstrated that the parallel conformation was stabilized by the cooperative π-π stacking of the perylenebisimide (PBI) chromophore 12 through the complementary hydrogen bonds (Fig. 7) . The parallel conformer of DAE and PBI building blocks coassembled to form well-defined helical nanofibers featuring J-type aggregation of PBI dyes. Upon irradiating with UV light,by the aggregate-monomer exchange and the equilibration between the parallel and antiparallel conformers,the solution assemblies converted into granular aggregates through a ring-closing photoreaction. Therefore a reversible morphology change between nanofibers and nanoparticles has been observed .
|Fig. 7. Structures of the open-ring isomer of 11 and perylenebisimide guest 12 and the binding motif between the parallel conformer of 11o and 12.|
Many biological processes and pathways,such as photosynthesis and the generation of vision,are entwined with light. Through introducing photochromic moieties into biological molecules, including short peptides,proteins,nucleosides and DNA,various artificial analogues can be constructed. The analogues are possible to control the activities and behaviours of the modified biological systems reversibly by light and bring new properties into biologic supramolecular systems . In nature,conformational changes of proteins driven by light are the fundamental of photobiological processes. These processes are usually induced by non-peptide chromophores or by modified side chains [32, 33]. Report on the polypeptide backbone itself undergoes direct photo-transformation is very rare. In order to reversibly control the biological activity of the peptidomimetics with light,by incorporating a photoswitchable diarylethene into the skeleton of the natural antibiotic Gramicidin S 13,a serials of cyclic peptidomimetic compounds 14-16 have been synthesized (Fig. 8) . These diarylethene-containing cyclic peptides show considerable differences in antimicrobial activities which is photocontrollable. This significant performance was attributed to the change of the ring size,flexibility,and strain dependent on the isomeric state of the photoswitchable unit induced by photoisomerization.
By incorporating diarylethene moiety into existing inhibitors, the activities of enzymes can be reversibly regulated with UV/ visible light. Konig and coworkers have designed a reversible photochromic 1,2-dithienylethene-based human carbonic anhydrase I inhibitor by introducing sulfonamide and copper (II) iminodiacetate Cu(ida) to give compound 17 (Fig. 9) . In this two-pronged enzyme inhibitor,sulfonamide group acted as the inhibitor while the Cu(ida) component can reversibly coordinate to the imidazole side chains of the histidine residues,which is close to the ZnⅡ active site of the enzyme,and help bring the sulphonamide inhibitor into the catalytic centre. The inhibition activity is much more effective when the 1,2-dithienylethene moiety is in the flexible ring-opening form,which allows both recognition components to bind to the enzyme and leads to a higher overall binding affinity. When the 1,2-dithienylethene moiety is in the rigid,ring-closed form,the distance of the two components is pulled away and the relative orientation of the two groups is fixed, to jointly result in a low inhibition activity . This design allows the use of visible light to activate the inhibitor,which is particularly significant because visible light has better penetration into tissue and can reduce the amount of damage caused by higher energy UV light.
|Fig. 9. Reversible photochemical ring-closing reaction of the 1,2-dithienylethene moiety inhibitor 17.|
The artificial control of DNA structure and function with light is an attractive realm in synthetic biology. Usually,small autonomous photoactive molecules,like azobenzenes ,arylvinyl derivatives  and spiropyranes ,are attached to DNA for exploring the influence of photoisomerization on the properties of the nucleic acid. Very recently,Jäschke et al. synthesized photoswitchable oligonucleotides 18,in which the nucleobase of DNA is involved in the rearrangement of chemical bonds to form an active part of the photoswitch (Fig. 10) . The target compounds contain only one alkyl group attached to the carbon atoms that form the new bond in the cyclization reaction. This features a good reversibility of photoswitching and a high quantum yield of the photoreaction between the ring-open and ring-close isomers. The study of the properties of thermal stability,helical structure and biological process (e.g. transcription) indicates that the modification does not cause much detrimental effects .
|Fig. 10. Photoswitchable DNA with a pyrimidine nucleotide as part of the photoswitch.|
Much attention has been drawn to construct molecular systems which exhibit mechanical movement based on geometrical changes of individual molecules induced by external stimuli, especially by photo-irradiation,due to their potential applications in micro- and nanomechanics. Several interesting works have been done to link the molecular-scale motions to macroscale movement of materials [40, 41, 42, 43].
A big challenge for this area is that most of the mechanical effect was too small to be practically used,and usually the behaviour is repeated only for a few tens of cycles . In 2010,Irie made big progress on this area,and developed a novel two-component cocrystal which compose of a photochromic diarylethene derivative 19 and perfluoronaphthalene with the property of photoinduced mechanical motion (Fig. 11) . By altering the irradiation wavelength from visible to UV light,the co-crystal plate exhibited a reversible bending behaviour,which could be repeated over 250 times. This mechanical bending is induced by the shape change of diarylethene molecules upon photocyclization,which caused anisotropic expansion of the crystal lattice. After modification,the thin co-crystal plate could bear a "heavy ball" weighted 614 times than that of the crystal. The thin layer could also act as an elastic modulus. According to a manual beam-test,it was measured to be as large as 11 GP,which was a much more outstanding performance compared with other organic materials .
|Fig. 11. (a) Photochromic reaction of 19 co-crystal and (b) schematic illustration of the photoinduced bending, the blue molecules are photogenerated closed-ring isomers in the crystal. Reproduced with permission from Ref. .|
Similarly,Kitagawa and co-workers designed another diarylethene crystal of 20 with the property of photo-induced twisting . Upon irradiation of UV light (e.g. 365 nm),the needle-like crystal changed its geometrical shape from thin layer to twisting (Fig. 12),with photochromic process from colourless to blue. The twisting recovered reversibly when irradiated by visible light (＞560 nm). In addition,UV irradiation from different faces of the crystal layer induces a helix of the aggregation.
|Fig. 12. Chemical structures and photoreversible crystal twisting of diarylethene 20 upon irradiation with UV and visible light. Reproduced with permission from Ref. .|
Li and coworkers have designed a novel three-dimensional diarylethene-based chiral photoswitch and investigated its application as chiral dopants for photo-tuneable cholesteric liquid crystal devices . In this chiral photoswich of 21 (Fig. 13),the dithienylcyclopentene is committed as the photochromic switching core,and the bridged binaphthyl units as chiral precursors,the aromatic rings as rigid terminals. The authors found that the helix twisting powers (HTP) of these molecules are much higher than that of the known chiral diarylethenes compounds. Photocyclization of these molecules upon light irradiation brings out dramatic variation in the helix twisting powers,and induces a change of colour of the hybrids.
|Fig. 13. Chemical structures of axially chiral dithienylcyclopentene molecules 21a–c.|
As described above,most of the practical applications of the diarylethene-containing supramolecular systems are based on the photochemical conversion between the open and closed form of diarylethene units. Diarylethene compounds can also be used to switch the fluorescence or electron transfer of organic fluorophores. Recently,the differences between the triplet energies of the open- and closed-diarylethene switches were used to turn on/ off the energy transfer process of other photochemical and photophysical process.
Browne and co-workers developed a bicomponent system which shows efficiently and reversibly switching on and off singlet oxygen (1O2) generation by alternating irradiation with UV and visible light . This system comprises a diarylethene photochromic switch and a porphyrin photosensitizer 23 (Fig. 14). The pyridine unit of photochromic switch is used to coordinate to the photosensitizer to increase the effective local concentration and hence to facilitate energy transfer efficiency. The large difference in excited state energies between the open and the closed forms of the diarylethenes is applied to control the 1O2 generation. When the diarylethene unit is in the colourless open form,efficient generation of singlet oxygen by the photosensitizer can be observed. On the other hand,irradiation with UV light at 312 nm hinders 1O2 generation because the triplet energy transfer from 23 to the closed form of the diarylethene unit competes effectively with the energy transfer to 3O2. The 1O2 generating ability of 23 can be fully recovered with visible light irradiation due to the non-efficient energy transfer between 23 and the open-form of diarylethene switches (triplet energy level of the open-form isomer is much higher than that of 23) . This investigation is quite significant,as 1O2 can be used in photodynamic therapy (PDT) which is used clinically to treat diseases through exposure of tissue to light,and if the amount of 1O2 produced is controllable,it could provide a way for efficient and selective control in PDT and limit nonspecific photodamage in the body .
|Fig. 14. Chemical structure of the photoswitch 22 and the photosensitizer 23 and schematic illustration of photochemical control of the generation of 1O2. Reproduced with permission from Ref. .|
With a similar design concept,Zhao and co-workers devised a reversible photoswitched triplet-triplet annihilation upconversion (TTA UC) system . In a TTA UC system,a photosensitizer and acceptor with matched triplet state energy level are needed. By selectively exciting the triplet photosensitizers in the presence of acceptor,the triplet excited states of the photosensitizers are populated,and the triplet energy migrates to the triplet acceptor from the photosensitizer by the triplet-triplet energy-transfer (TTET) process. Then the TTA between the triplet acceptors produces the singlet excited state,and the upconverted fluorescence can be observed . When the open form isomers of the dithienylethene derivatives were added to the TTA UC system,TTA UC emission will not be perturbed. Upon irradiation of the mixture with UV light (254 nm) to form the closed form isomer,the TTA UC emission can be completely quenched due to the efficient triplet energy transfer from the acceptor to the closed form of the dithienylethene switch,and hence the TTA between the triplet acceptors will not occur. Upon visible light irradiation of the solution,the TTA UC emission is switched on because the triplet energy level of the open form is much higher than that of the photosensitizer and acceptor,therefore the TTET and TTA process will not be hindered and TTA UC emission is recovered (Fig. 15) . External stimuli-responsive TTA UC emission will offer unprecedented spatial and temporal resolution for the practical application such as luminescence bioimaging of this newly developed technology.
|Fig. 15. Schematic illustration of photochemical control of the TTA upconversion. Reproduced with permission from Ref. .|
Photoresponsive self-assemblies with photochromic diarylethenes as photoswitch units have experienced a rapid progress in the last decade. Amphiphilicity,hydrogen binding and π-π interaction are the frequently used tools to assemble stimuliresponsive supramolecular systems. Photoisomerization of diarylethene provides an intriguing and powerful tool to tune the chemical/physical properties and to control the morphology of the supramolecular assemblies by the conformational change upon photoirradiation. Usually,when the diarylethene photoswitch unit is in its flexible open form,the supramolecular assemblies show higher aggregation ability compared to the rigid closed form. In this review,a few recently developed sophisticated supramolecular architectures with diarylethenes derivatives as photoswitch units were summarized and the design rationalities were also analyzed.
How to use the photochromic diarylethene-containing supramolecular systems as "smart materials" is a central part of research motivation. When diarylethene units are incorporated into short peptides,proteins,nucleosides and DNA,etc.,the physiological function are successfully reversibly manipulated with non-invasive light by changing the amphiphility or morphology of these biological molecules. In biological systems, the molecular-scale movements of actin-myosin proteins are artfully correlated to macroscale motion of muscles. However,to develop man-made devices which exhibit mechanical movement based on the geometrical structural changes of individual molecules induced by external stimuli still remain challenging, despite a few successful examples that rationally assemble the molecular machines to perform macroscale mechanical work upon photoirradiation have been developed. We believe that it will be a long-lasting ambition for chemists to construct such systems with greater mechanical effect. Furthermore,the practical applications of the diarylethene containing supramolecular systems should not limited to the shape change between the open and closed form of diarylethene units,using the differences between the triplet energies of the open and closed diarylethene switches to turn on/off the energy transfer process of other photochemical process will open new opportunity of applications,and the investigation of more smart stimuliresponsive supramolecular systems with special functions is surely on the way.
We thank the National Natural Science Foundation of China (Nos. 21372165,21321061 and 21402129) and State Key Laboratory of Polymer Materials Engineering (No. sklpme2014- 2-06),and Comprehensive Training Platform of Specialized Laboratory,College of Chemistry,Sichuan University for financial support.
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