Straightforward Fabrication of Double Roughness Structures on a Microcrystalline Film of a Diarylethene Derivative.

Double roughness structure mimicking the surface of a lotus leaf was prepared using a newly synthesized diarylethene having a six-membered perfluorocyclohexene ring. The cubic-shaped crystals of the open-ring isomer, with sizes of approximately 7 µm, appeared immediately following solution casting. Upon UV irradiation, each cubic crystal was covered with needle-shaped crystals of the closed-ring isomer to form double roughness structures within 1 h. This structure could bear the continuous impact of water droplets.

Phototunable Cell Killing by Photochromic Diarylethene of Thiazoyl and Thienyl Derivatives.

We report a unique phototunable cell killing technique using diarylethene molecules as photo-isomerizing-molecular switches. These molecules were delivered to DNA in the cell nucleus due to closed-form generated by UV light, and then blue light triggered cell killing. A UV light irradiation switches the open form, having no DNA intercalation activity, to the closed form to induce intercalation in DNA. This isomer, thus prepared ready for the action, exerts photocytotoxicity upon the subsequent blue light irradiation. Molecular biological analysis clarifies that photocytotoxicity is due to DNA double-strand breaks. Since cell death is observed only when irradiated with light where both the open- and closed-ring isomers have absorption, the possible mechanism of cell death is assumed to be due to the repeated photocyclization and photocycloreversion reactions of the diarylethene molecules, which induce irreparable damage to DNA. This unique photo-controllable action in a cell system can provide the basis of a novel scheme of phototherapy.

All-Visible (>500 nm)-Light-Induced Diarylethene Photochromism Based on Multiplicity Conversion via Intramolecular Energy Transfer.

Photoswitching molecules that reversibly switch upon visible-light irradiation are some of the most attractive targets for biological and imaging applications. In this study, we found a diarylethene (DAE) derivative having a covalently attached perylenebisimide (PBI) unit (DAE-PBI dyad) underwent an unexpected cyclization reaction upon irradiation with green (500-550 nm) light, where the DAE unit has no absorbance. The photoreactivity was enhanced in solvents containing heavy atoms and in the presence of oxygen. As inferred from the solvent dependence and the calculated excited-state energies of DAE and PBI units, it was suggested that the probable mechanism for this unique visible-light-induced cyclization reaction is multiplicity conversion based on intramolecular energy transfer from the excited singlet state of the PBI unit to the triplet state of DAE units (i.e., DAE-1[PBI]* → 3[DAE]*-PBI). Such a unique photoreaction mechanism with the assistance of oxygen will pave the way for new molecular design for the development of visible-light switching molecules.

Photoinduced cytotoxicity of photochromic symmetric diarylethene derivatives: the relation of structure and cytotoxicity.

Photopharmacology has been attracting attention for the development of drugs with fewer side effects and lower toxicity by introducing a photoswitch structure in the drug and controlling its spatiotemporal effects by light irradiation. Ideally, to achieve precise spatiotemporal control, it is desirable to use photoresponsive molecules that act as anticancer agents based on molecular switch mechanisms at the molecular level. However, very few reports on photoinduced cytotoxicity have used photoresponsive molecules with simple structures. Here, we investigate the photoinduced cytotoxicity of twelve diarylethene derivatives having thiazole or pyridine rings in their molecules and evaluate them in terms of molecular structure and size. Our results provide insight into molecular design principles for diarylethene with a simple structure toward achieving precise control based on molecular-level switch mechanisms.

Molecular crystalline capsules that release their contents by light.

Here, we present single crystalline capsules of a photoresponsive molecule produced by simple recrystallization from organic solutions without direct human processing. During the crystal growth process, a movie was taken of the capsule taking in the organic solution. The capsules responded rapidly (<1 s) to the UV light stimuli and released the captured solution or solute. In principle, they can take in any substance dissolved in organic solvents, and their size can be controlled. Moreover, the capsule can be broken by multi-photon excitation using a near-infrared laser within the biological window. Furthermore, because the molecular packing in the crystal is unidirectional, the response can be controlled by the polarization of the light. This study shows the new potential of photoresponsive molecules.

Determining Factor of the Quantum Yield of the Cyclization Reaction via Triplet States for Dye-Attached Diarylethene.

The difference in the quantum yields of the cyclization reaction of two isomers of dye-attached diarylethene via the triplet state observed in the experiment [J. Phys. Chem. C 2009, 113, 11623-11627] was theoretically examined by quantum chemical calculations. By evaluating the spin-orbit couplings, we found that the ratio of the rate constants from the S1 state to the T2 state between two isomers agreed with that of the experimental quantum yield of the cyclization reaction. The difference in the spin-orbit couplings is due to the difference in the delocalization of the orbitals between diarylethene and dye. We further found that after the intersystem crossing took place the cyclization reaction via the triplet state occurred in the experiment due to the low energy barrier (∼10 kcal/mol) for the reaction.

Spontaneous Combustion of 2-Bromo-3-Methoxythiophene: A Study on Reaction Pathways and Energetics by Quantum Chemical Calculations.

Reaction pathways and energetics for the dimerization and trimerization reactions of 2-bromo-3-methoxythiophene (2Br-3Met) molecules are investigated using hybrid density functional theory (DFT) calculations to obtain insight into the oligomerization reaction observed in the spontaneous combustion of pure liquid 2Br-3Met. The calculations show that the carbon-bromine bond in a 2Br-3Met molecule elongates easily, and the trans addition of this C-Br bond to a double bond in the neighboring 2Br-3Met molecule occurs easily at room temperature, reflecting the evaluated activation energy of ΔHa = 12.46 kcal/mol (enthalpy) or ΔGa = 35.68 kcal/mol (Gibbs free energy, 298.150 K and 1 atm). The formation process of trimers is calculated in a similar way. A model for the explanation of spontaneous combustion is proposed; large oligomers of the 2Br-3Met molecule are produced spontaneously following the initial formation of dimers or trimers. UV-vis spectra and vibration spectra are obtained for related molecular species, which show reasonable agreement with the experimental results.

Cyclization from Higher Excited States of Diarylethenes Having a Substituted Azulene Ring.

The cyclization reaction of diarylethenes having an azulene ring occurs only via higher excited states. Novel diarylethenes having an azulene ring with a strong donor or acceptor were synthesized and examined in these reactions. A derivative having an electron-donating 1,3-benzodithiol-2-ylidenemethyl group at the 1-position of the azulene ring showed photochromism, whereas neither a derivative having a π-conjugated electron-donating group at the 3-position of the azulene ring nor derivatives having a π-conjugated electron-withdrawing group at the 1- or 3-position of the azulene ring showed any photochromism. The photoreactivities of these compounds were explained by calculating forces and bond orders on the excited states using density functional theory (DFT) and time-dependent (TD)-DFT.

Photoinduced swing of a diarylethene thin broad sword shaped crystal: a study on the detailed mechanism.

We report a swinging motion of photochromic thin broad sword shaped crystals upon continuous irradiation with UV light. By contrast in thick crystals, photosalient phenomena were observed. The bending and swinging mechanisms are in fact due to molecular size changes as well as phase transitions. The first slight bending away from the light source is due to photocyclization-induced surface expansion, and the second dramatic bending toward UV incidence is due to single-crystal-to-single-crystal (SCSC) phase transition from the original phase I to phase IIUV. Upon visible light irradiation, the crystal returned to phase I. A similar SCSC phase transition with a similar volume decrease occurred by lowering the temperature (phase IIItemp). For both photoinduced and thermal SCSC phase transitions, the symmetry of the unit cell is lowered; in phase IIUV the twisting angle of disordered phenyl groups is different between two adjacent molecules, while in phase IIItemp, the population of the phenyl rotamer is different between adjacent molecules. In the case of phase IIUV, we found thickness dependent photosalient phenomena. The thin broad sword shaped crystals with a 3 µm thickness showed no photosalient phenomena, whereas photoinduced SCSC phase transition occurred. In contrast, large crystals of several tens of µm thickness showed photosalient phenomena on the irradiated surface where SCSC phase transition occurred. The results indicated that the accumulated strain, between isomerized and non-isomerized layers, gave rise to the photosalient phenomenon.

Crystal Growth Technique for Formation of Double Roughness Structures Mimicking Lotus Leaf.

Bio-inspired functional materials have received much attention for their potential to provide sustainable and advanced materials. The lotus effect has proven to be one of the most remarkable biomimetic effects since it was discovered by Barthlott. A superhydrophobic surface with the ability to bounce water droplets is the origin of the self-cleaning mechanism that keeps the surface clean by removing dust using water droplets moving with momentum. We have developed a crystal growth technique (CGT) of photochromic diarylethenes over the past decade, and from this, we fabricated a surface structure that closely resembles the natural lotus leaf's characteristic of controlling the Laplace pressure and clarified the importance of the double roughness structure of the surface. The bouncing ability is also discussed in terms of the characteristic size of the double roughness structure theoretically. Moreover, this work clarifies the exquisiteness of the double roughness structure of the leaf. We also show that the CGT is a versatile technique with the potential to fabricate desired structured surfaces.

Aggregation-induced emission effect on turn-off fluorescent switching of a photochromic diarylethene.

Background: Diarylethenes are well-known photochromic compounds, which undergo cyclization and cycloreversion reactions between open- and closed-ring isomers. Recently, diarylethene derivatives with photoswitchable fluorescent properties were prepared. They are applicable for fluorescence imaging including bio-imaging. On the other hand, a new system called "excited state intramolecular proton transfer (ESIPT)" is reported. In the system, absorption and emission bands are largely separated due to the proton transfer, hence it showed strong fluorescence even in the crystalline state. We aimed to construct the photochromic system incorporating the ESIPT mechanism. Results: A diarylethene incorporating a fluorescent moiety that exhibit ESIPT behavior was prepared. The ESIPT is one of the examples which express the mechanisms of aggregation-induced emission (AIE). This compound emits orange fluorescence with a large Stokes shift derived from ESIPT in aprotic solvents such as THF or hexane, while it exhibits only a photochromic reaction in protic solvents such as methanol. In addition, it shows turn-off type fluorescence switching in an aprotic solvent and in crystals. The fluorescence is quenched as the content of closed-ring isomers increases upon UV light irradiation. Conclusions: A diarylethene containing an ESIPT functional group was prepared. It showed fluorescent turn-off behavior during photochromism in aprotic solvents as well as in crystalline state upon UV light irradiation. Furthermore, it showed AIE in THF/water mixtures with blue-shift of the emission.

Object Transportation System Mimicking the Cilia of Paramecium aurelia Making Use of the Light-Controllable Crystal Bending Behavior of a Photochromic Diarylethene.

The design of an object transportation system exploiting the bending behavior of surface-assembled diarylethene crystals is reported. A photoactuated smart surface based on this system can transport polystyrene beads to a desired area depending on the direction of the incident light. Two main challenges were addressed to accomplish directional motion along a surface: first, the preparation of crystals whose bending behavior depends on the direction of incident light; second, the preparation of a film on which these photochromic crystal plates are aligned. Nuclei generation and nuclear growth engineering were achieved by using a roughness-controlled dotted microstructured substrate. This system demonstrates how to achieve a mechanical function as shown by remote-controlled motion along a surface.

Photosalient Effect of Diarylethene Crystals of Thiazoyl and Thienyl Derivatives.

The photoresponse of diarylethene crystals is found to depend on the intensity of UV light, that is, photoinduced bending is switched to photosalient phenomena by increasing the light intensity. The change in the size of the crystal unit cell upon UV irradiation is larger for asymmetric diarylethenes with thiazole and thiophene rings than that for the corresponding symmetric diarylethenes. As a result, the crystals of an asymmetric diarylethene show much more drastic photosalient effects than those of the corresponding symmetric diarylethene crystals upon UV irradiation. It is also found that the crystals of diarylethene, which have not previously been reported to exhibit a photosalient effect, show photosalient phenomena upon irradiation with strong UV light. Furthermore, the dependence of photosalient phenomena on the size and shape of the crystals is reported.

Photochromic Crystalline Systems Mimicking Bio-Functions.

Photoresponsive crystalline systems mimicking bio-functions are prepared using photochromic diarylethenes. Upon UV irradiation of the diarylethene crystal, the photogenerated closed-ring isomers self-aggregate to form needle-shaped crystals on the surface. The rough surface shows the superhydrophobic lotus effect. In addition, the rose-petal effects of wetting, the anti-reflective moth-eye effect, and a double-roughness structure mimicking the surface of a lotus leaf are observed by controlling the heating procedures, UV irradiation processes, and molecular structural modification. By changing the molecular structure, a superhydrophilic surface mimicking a snail shell can be generated. We also find the crystal of a diarylethene derivative that shows a photosalient effect. The effect is observed partly due to the hollow structure of the crystal. It is demonstrated that a photo-response similar to the response of impatiens plant to stimulation is observed by packing small beads in the hollow. These photoresponsive functions are unique, and they demonstrate a macroscopic response by means of microscopic molecular movement induced by light. In the future, such a molecular assembly system will be a promising candidate for fabricating photoresponsive architectures and soft robots.

N-C Axially Chiral Anilines: Electronic Effect on Barrier to Rotation and A Remote Proton Brake.

N-Aryl-N-methyl-2-tert-butyl-6-methylaniline derivatives exhibit a rotationally stable N-C axially chiral structure and the rotational barriers around an N-C chiral axis increased with the increase in electron-withdrawing character of para-substituent on the aryl group. X-ray crystal structural analysis and the DFT calculation suggested that the considerable change of the rotational barriers by the electron effect of para-substituents is due to the disappearance of resonance stabilization energy caused by the twisting of para-substituted phenyl group in the transition state. This structural property of the N-C axially chiral anilines was employed to reveal a new acid-decelerated molecular rotor caused by the protonation at the remote position (remote proton brake).

Photosalient Phenomena that Mimic Impatiens Are Observed in Hollow Crystals of Diarylethene with a Perfluorocyclohexene Ring.

A diarylethene with a perfluorocyclohexene ring formed hollow crystals by sublimation under normal pressure. Upon UV irradiation of the crystals, they showed remarkable photosalient phenomena and scattered into small pieces. The speed of the flying debris released from the crystal by UV irradiation exceeded several meters per second. To clearly show a photosalient effect resembling the scattering behavior of Impatiens on a smaller scale, small fluorescent beads (1-µm diameter) were inserted into the hollow crystal. Consequently, scattering of the beads was observed as UV irradiation caused deformation and bursting of the hollow structure. This phenomenon is unique to hollow crystals, and the ability to effectively induce remarkable photosalient phenomena is similar to the behavior of hollow-structured Impatiens in nature.

Fractal Surfaces of Molecular Crystals Mimicking Lotus Leaf with Phototunable Double Roughness Structures.

Double roughness structure, the origin of the lotus effect of natural lotus leaf, was successfully reproduced on a diarylethene microcrystalline surface. Static superwater-repellency and dynamic water-drop-bouncing were observed on the surface, in the manner of natural lotus leaves. Double roughness structure was essential for water-drop-bouncing. This ability was not observed on a single roughness microcrystalline surface showing the lotus effect of the same diarylethene derivative. The double roughness structure was reversibly controlled by alternating irradiation with UV and visible light.

Photosalient Effect of a Diarylethene with a Perfluorocyclohexene Ring.

Crystals of a diarylethene with a perfluorocyclohexene ring exhibit a remarkable photosalient effect upon UV light irradiation that is attributed to the structural changes that occur when going from open- to closed-ring isomers in the crystalline state, together with the existence of two conformers with different photoconversions compared with those of a perfluorocyclopentene derivative. Our current results give a design principle for molecular structures so as to achieve the photosalient effect for photochromic crystals.

Photoinduced reversible formation of a superhydrophilic surface by crystal growth of diarylethene.

When visible light is irradiated onto the melted microcrystalline-surface of a diarylethene having ionic structures by UV irradiation, it induces crystal-growth of the open-ring isomer of the diarylethene; consequently, the surface covered with lumpy crystals shows superhydrophilicity that can be reversibly controlled by alternating irradiation with UV and visible light.

The synthesis of optically active N-C axially chiral tetrahydroquinoline and its response to an acid-accelerated molecular rotor.

Optically active atropisomeric N-(2,5-di-tert-butylphenyl)-1,2,3,4-tetrahydroquinoline with an N-C chiral axis was prepared via a catalytic enantioselective reaction. The addition of methane sulfonic acid to this axially chiral quinoline dramatically lowered the barrier to rotation around the chiral axis.