Closed-ring Isomer Research Articles

A Visible-Light-Responsive Fluorescent Diarylethene Having a Betaine Structure.

As a new molecular scaffold of photoswitchable fluorophore, we developed a photochromic diarylethene containing a betaine structure based on pyridinium N-enolate. A facile reaction of a pyridyl-substituted dithienylperfluorocyclopentene derivative with octafluorocyclopentene constructed the betaine structure. The introduction of the betaine moiety provided the diarylethene molecule with bathochromically shifted optical absorption and fluorescing ability, thus enabling the molecule to function as a visible-light-sensitive turn-off mode photoswitchable fluorophore. The molecule in the open-ring form emitted bright blueish green fluorescence. Upon irradiation with 405 nm light, the molecule underwent cyclization isomerization to form the closed-ring isomer and the fluorescence intensity significantly decreased. The turn-off mode fluorescence photoswitching was observed not only in solution but also in polymer films.

Aza-Diarylethenes Undergoing Both Photochemically and Thermally Reversible Electrocyclic Reactions.

Exploring novel molecular photoswitches plays a crucial role in the field of photo-functional materials chemistry. In this study, we synthesized aza-diarylethenes with benzothiophene-S,S-dioxide as apart of hexatriene structureand investigated their photochromic properties. Unlike previously reported aza-diarylethenes, which exhibit fast thermally reversible photochromism, the compounds synthesized here exhibited pseudo-photochemically reversible photochromism. Due to their thermal stability, we successfully isolated the colored isomer. X-ray crystallographic analysis revealed for the first time that the colored isomer adopts a closed-ring structure with a bond between carbon and nitrogen atoms. Remarkably, these aza-diarylethenes exhibited not only photochemical ring-closing and ring-opening reactions but also thermal ring-closing and ring-opening reactions, driven by a thermal equilibrium between the open- and closed-ring isomers. This behavior, unprecedented for common diarylethenes, was elucidated through kinetic analysis, revealing an energy-level diagram for the thermal equilibrium between these isomers. Furthermore, 1H NMR spectroscopy revealed that both photochemically and thermally generated closed-ring isomers adopt the same structure, which was well explained based on the reaction mechanism of photochemical and thermal ring-closing reactions. These findings not only advance the field of aza-diarylethenes but also inspire future research in the development of new photoswitches.

A Hexaarylbiimidazole-Terarylene Hybrid: Visible-to-NIR-II Absorption via Sequential Photochromic Reactions.

A synergetic interaction between two or more photochromic chromophores has a potential to achieve advanced photochemical properties beyond conventional photochromic molecules and to realize photochemical control of complex systems using only a single molecule. Herein, we report a hybrid photochromic molecule consisting of hexaarylbiimidazole (HABI) and terarylene that exhibits multi-state photochromism. The biphotochrome hybrid shows four-state photochromic reaction involving sequentially proceeding photoreactions. The UV or visible light irradiation to the biphotochrome leads to the C-N bond breaking reaction of the HABI in preference to the ring-closing reaction of the 6π-electron system in the terarylene unit, leading to two terarylene radical molecules. The photogenerated terarylene radical further exhibits the 6π-electrocyclization reaction by UV irradiation. The delocalized π-radical on the closed-ring form of the terarylene is efficient to enhance the photosensitivity to the NIR-I and -II region. Furthermore, a recombination reaction of radicals between the open- and closed-ring isomers of terarylene affords an unprecedented photochromic dimer as a structural isomer of the initial molecule. This is a consequence of the sequential hybrid photochromic system involving the HABI and terarylene units.

Invisible photochromism and on-off switch based on thiophene-imidazole fused derivatives: Synthesis, properties and applications

Three novel diarylethene fluorescent molecules based on a thiophene-imidazole core have been successfully synthesized. Their photophysical properties, including photocyclization and ions response characteristics, as well as quantum chemical calculations were systematically investigated. All compounds exhibited significant invisible photochromism upon photocyclization, resulting in remarkable fluorescence “turn-off” properties. This phenomenon can be attributed to photoisomerization, specifically the transition from the open-ring isomer to the closed-ring isomer during this process, which was confirmed by 1H NMR data analysis. Moreover, AT-DPIo demonstrated high selectivity toward I− with pronounced changes in absorption from colorless to yellow and fluorescence from bright yellow to dark, enabling a limit of detection for I− at 9.75 × 10−7 mol L−1. These exceptional sensing properties make it suitable for rapid “naked-eye” detection of I− using test strips.

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene derivative

Near-infrared two-photon absorption and excited state dynamics of a fluorescent diarylethene (fDAE) derivative were investigated by time-resolved absorption and fluorescence spectroscopies. Prescreening with quantum chemical calculation predicted that a derivative with methylthienyl groups (mt-fDAE) in the closed-ring isomer has a two-photon absorption cross-section larger than 1000 GM, which was experimentally verified by Z-scan measurements and excitation power dependence in transient absorption. Comparison of transient absorption spectra under one-photon and simultaneous two-photon excitation conditions revealed that the closed-ring isomer of mt-fDAE populated into higher excited states deactivates following three pathways on a timescale of ca. 200 fs: (i) the cycloreversion reaction more efficient than that by the one-photon process, (ii) internal conversion into the S1 state, and (iii) relaxation into a lower state (S1’ state) different from the S1 state. Time-resolved fluorescence measurements demonstrated that this S1’ state is relaxed to the S1 state with the large emission probability. These findings obtained in the present work contribute to extension of the ON–OFF switching capability of fDAE to the biological window and application to super-resolution fluorescence imaging in a two-photon manner.Graphical

Coumarinyl(pyrrolyl)ethenes for the detection of Pd2+, CN− and F− ions, test strips based on them and logic gates

New coumarinyl(pyrrolyl)ethenes with a 1,3-thiazole bridge were synthesized. When irradiated at 365 nm, a hexatriene-cyclohexadiene rearrangement of the ring-opened form into a colored non-fluorescent ring-closed isomer occurs. The back reaction takes place when exposed to visible light at 540 nm. Furthermore, the compound with terminal 2,4-di-tert-butyl-6-hydroxybenzylidene receptor group represents a multifunctional sensor exhibiting selective chromogenic activity towards palladium(II) ions and CN− and F− anions. This probe interacts with Pd2+ cations with a noticeable naked eye effect - a change in the solution color from yellow to orange. The complexation stoichiometry is 1:1 and the detection limit was found to be 4.5 × 10−7 mol L−1. Similarly, CN− and F− anions cause a contrast naked eye effect with a change in the color of the solution from yellow to rose-red. For convenient detection of the above ions, test strips containing 4-(2-(2-(2,4-di-tert-butyl-6-hydroxybenzylidene)hydrazinyl)-5-(2-oxo-2H-chromen-3-yl)thiazol-4-yl)-1,5-dimethyl-1H-pyrrole-2-carbonitrile have been developed. In addition, a combinatorial molecular logic gate was designed based on multi-controllable photoswitching properties of the obtained probe.

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.

Crystallographic, spectroscopic, and computational characterization of a diiodonaphthoquinone diarylethene photochrome: halogen bonding and photocrystallography

We report the synthesis and characterization of 2,3-bis(5-iodo-2-methylthiophen-3-yl)naphthalene-1,4-dione and its ring-closed isomer.

Light-Driven Reversible Multicolor Supramolecular Shuttle.

Light-driven multicolor supramolecular systems mainly rely on the doping of dyes or a photo-reaction to produce unidirectional luminescence. Herein, we use visible light to drive the bidirectional reversible multicolor supramolecular shuttle from blue to green, white, yellow, up to orange by simple encapsulation of spiropyran-modified cyanostilbene (BCNMC) by the macrocyclic cucurbit[8]uril (CB[8]) monomer. The resultant host-guest complex displayed enhanced fluorescence properties, i.e. the multicolor fluorescence shuttle changed from blue to orange in the dark within 2 hours and reverted to the original state upon visible light irradiation for 30 s. Benefiting from the sensitivity of the spiropyran moiety to light, it can spontaneously isomerize from the ring-opened state to a ring-closed isomer in aqueous solution, and this photo-isomerization reaction is a reversible process under visible light irradiation, leading to the multicolor luminescence supramolecular shuttle as a result of intramolecular energy transfer. In addition, the light also drove the reversible conversion of the topological morphology of the host-guest complex from two-dimensional nanoplatelets to nanospheres. Different from the widely reported molecular rotaxane "shuttle", the spiropyran supramolecular shuttle confined in the macrocyclic host CB[8] not only modulated a reversible topological morphology by light but also exhibited multicolor luminescence, which was successfully applied in programmed and rewritable information encryption.

Diarylethene Photoswitches Undergoing 6π Azaelectrocyclic Reaction: Disrotatory Thermal Cycloreversion of the Closed-Ring Isomer.

Gaining insight into the dynamics of electrocyclic reactions is very important from both fundamental and application perspectives. In this study, we developed novel diarylethene photoswitches that undergo 6π azaelectrocyclic reaction. We found that they exhibit fast thermally reversible type (T-type) photochromism, in contrast to the fact that common diarylethenes exhibit photochemically reversible type (P-type) photochromism. The quantum chemical calculations revealed that the fast T-type photochromism originates from the unprecedented disrotatory thermal cycloreversion of the closed-ring isomer. Our results provide useful information not only for the dynamics of the 6π azaelectrocyclic reaction but also for the further development of diarylethene photoswitches utilizing the 6π azaelectrocyclic reaction.

Light-Modulated Morphological Transformation of Spiropyran Derivative from Nanosphere to Nanorod.

The construction of tunable morphological systems has important implications for understanding the mechanism of molecular self-assembly. In this study, we report a spiropyran derivative M1 with light-responsive assembly morphology, which can be tuned from nanosphere to nanorod by ultraviolet light irradiation. The absorption spectra show that M1 molecules are transformed from closed-ring (SP) isomers into open-ring (MC) isomers and start to form H-aggregates with increasing irradiation time. Density functional theory calculations indicate that MC-MC isomers possess stronger binding energy than SP-SP isomers. The MC isomers may thus facilitate the dissociation of the SP-SP aggregates and promote the change of self-assembled morphology with the aid of stronger π-π stackings and dipole-dipole interactions. Our research gives an effective method for modulating the morphology of assemblies, with great potential for applications in smart materials. This article is protected by copyright. All rights reserved.

Optical Control of Invertebrate nAChR and Behaviors with Dithienylethene-Imidacloprid.

Photopharmacology involving azobenzene or dithienylethene has changed established methods of studying receptor functions, allowing for increasing the spatiotemporal resolution. There are no photopharmacological tools available for the invertebrate nicotinic acetylcholine receptor (nAChR). Here, we report a photochromic ligand, dithienylethene-imidacloprid (DitIMI), using a dithienylethene photoswitch embedded in the nAChR agonist imidacloprid. It was found that DitIMI displayed good photochromism and fluorescence switching behaviors upon irradiation with UV/vis light in aqueous solution. We demonstrated that open-DitIMI has low spontaneous in vitro and in vivo activity but can be photoisomerized to a highly active closed-form. Surprisingly, the photoswitchable DitIMI showed a large difference in insecticidal activity between the open and closed forms, in which the insecticidal activity against Aphis craccivora of the ring-closed isomers for DitIMI was 355 times that of the corresponding ring-open isomers. This photoisomerization can further be translated to photomodulation of neuron membrane potential and behavioral responses of living mosquito larvae and American cockroaches. The photomanipulation of nACh neurotransmission opens new avenues to understanding inhibitory circuits in intact animals.

Selective control of donor-acceptor Stenhouse adduct populations with non-selective stimuli

Selective control of donor-acceptor Stenhouse adduct populations with non-selective stimuli

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.

Electronic Influences on the Dynamic Range of Photoswitchable Dithienylethene-Thiourea Organocatalysts.

Thiourea-based organocatalysts bearing a photoswitchable dithienylethene (DTE) core and a wide range of substituents were prepared and extensively tested for their ability to accelerate the Michael reaction between acetylacetone and trans-β-nitrostyrene. There is a strong correlation between the Hammett parameter of the modulating groups and catalytic activity following UV irradiation. Electron-withdrawing groups afford the largest reactivity difference between the catalysts in their ring-open form and their ring-closed isomer, with evidence for electronic coupling between the two halves in both oDTE and cDTE.

Photo-cyclization, photo-ring opening and thermo-ring opening reaction of cationic diarylethene adsorbed on the clay nanosheet surface

The effect of the clay nanosheet surface on the photo-cyclization, photo-ring opening and thermo-ring opening reaction of 1,2-bis(2′-methyl-5′-(N-methylpyrid-4"-yl)-thien-3′-yl)perfluorocyclopentene dichloride (DAE2+) were investigated. The quantum yield of photo-cyclization on the clay surface was smaller than that in water. The decrease in quantum yield could be explained by the change in the existence ratio of two conformers of the open-ring isomer. The thermo-ring opening rate of DAE2+ was accelerated by the adsorption onto the surface, while the quantum yield of the photo-ring opening was suppressed by the adsorption. A temperature effect experiment on the thermo-ring opening reaction was carried out and the thermodynamic parameters were calculated from the Arrhenius plot to discuss the opposite effect of the surface on photo- and thermo-ring opening. Both activation energy (Ea) and the frequency factor (A) for thermo-ring opening reaction decreased by the adsorption on the surface, compared to those in water. Thus, the acceleration of the thermo-ring opening was derived from the change of Ea and could be interpreted as the reactive CC single bond elongation due to the molecular distortion by the adsorption. Moreover, the cause of the suppression of the photo-ring opening reaction was considered as the frequency reduction of the active vibration mode for the C-C cleavage that was brought by the immobilization on the clay surface. While both the closed-ring isomer of DAE2+ and the clay surface have flat structures, the open-ring isomer of DAE2+ is distorted. Several effects on the closed- and open-ring isomers of DAE2+ could be crucial factor to determine the reaction efficiency on the nanosheet surface.

Controlling Diradical Character of Photogenerated Colored Isomers of Phenoxyl-Imidazolyl Radical Complexes.

We propose a rational method for evaluating the diradical character of the photochromic phenoxyl-imidazolyl radical complex (PIC) derivatives based on their radical-radical coupling reaction rates. PIC consists of an imidazole ring, a phenoxyl ring, and a bridging unit that structurally connects them. The C-N bond formed between the imidazole and phenoxyl rings can be dissociated photochemically in a homolytic manner. The photochromism of PIC differs significantly from other photochromic molecules in that the transient colored open-ring isomer has a diradical character. The colored open-ring isomer returns promptly to the initial colorless closed-ring isomer by the intramolecular radical recombination reaction. By changing the aromaticity and substitution position of the bridging unit, it is possible to control the degree of contribution of the open-shell diradical and closed-shell quinoidal structures to the open-ring isomer. Systematic investigation of the photochromic reactions of several PIC derivatives revealed that the half-life of the open-ring isomers reflects the diradical character. Thus, the radical recombination reaction rate of the open-ring isomer of the PIC derivatives is an excellent parameter of the diradical character.

Energy profile of formal 1,2-dyotropic rearrangement of diarylethenes

Diarylethenes with thiophene substituents belong to photoswitchable compounds (photoswitches or photochromes). Upon UV irradiation, their colorless open-ring isomers (DAE-o) convert to the colored closed-ring isomers (DAE-c), while the back reaction is induced only by visible light irradiation. A multiple photoswitching of diarylethenes usually results in irreversible photorearrangement of DAE-c to the so-called annulated isomers DAE-a, that are stable thermally and photochemically. In the present communication, structures of a series of diarylethenes as well as their isomers were optimized on the B3LYP/6-31G(d) level of theory. It was disclosed for the first time, that DAE-a destabilized relatively DAE-c by 1.71–14.00 kcal/mol. These results are important for design of photocontrollable molecules and materials, operated in the oneway (permanent manner).

Synthesis, structure–property relationships and absorbance modulation of highly asymmetric photochromes with variable oxidation and substitution patterns

Asymmetric 1,2-diarylperfluorocyclopentenes with “inverse” photochromism (due to shorter conjugation path in the closed-ring isomer), rapid switching and large absorption modulation are reported.

Photoswitchable Nitrogen Superbases: Using Light for Reversible Carbon Dioxide Capture.

Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N‐heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring‐opened and ring‐closed states is accompanied by substantial pK a shifts of the NHIs by up to 8.7 units. Since only the ring‐closed isomers are sufficiently basic to activate CO2 via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light‐controlled capture and release of CO2.