DFT/TDDFT Investigation of the Modulation of Photochromic Properties in an Organoboron‐Based Diarylethene by Fluoride Ions

The diarylethene derivative 1,2‐bis‐(5′‐dimesitylboryl‐2′‐methylthieny‐3′‐yl)‐cyclopentene ( 1 ) containing dimesitylboryl groups is an interesting photochromic material. The dimesitylboryl groups can bind to F^?, which tunes the optical and electronic properties of the diarylethene compound. Hence, the diarylethene derivative 1 containing dimesitylboryl groups is sensitive to both light and F^?, and its photochromic properties can be tuned by a fluoride ion. Herein, we studied the substituent effect of dimesitylboron groups on the optical properties of both the closed‐ring and open‐ring isomers of the diarylethene molecule by DFT/TDDFT calculations and found that these methods are reliable for the determination of the lowest singlet excitation energies of diarylethene compounds. The introduction of dimesitylboron groups to the diarylethene compound can elongate its conjugation length and change the excited‐state properties from π→π* transition to a charge‐transfer state. This explains the modulation of photochromic properties through the introduction of dimesitylboron groups. Furthermore, the photochromic properties can be tuned through the binding of F^? to a boron center and the excited state of the diarylethene compound is changed from a charge‐transfer state to a π→π* transition. Hence, a subtle control of the photochromic spectroscopic properties was realized. In addition, the changes of electronic characteristics by the isomerization reaction of diarylethene compounds were also investigated with theoretical calculations. For the model compound 2 without dimesitylboryl groups, the closed‐ring isomer has better hole‐ and electron‐injection abilities, as well as higher charge‐transport rates, than the open‐ring isomer. The introduction of dimesitylboron groups to diarylethene can dramatically improve the charge‐injection and ‐transport abilities. The closed isomer of compound 1 ( 1 C ) has the best hole‐ and electron‐injection abilities, whereas the charge‐transport rates of the open isomer of compound 1 ( 1 O ) are higher than those of 1 C . Importantly, 1 O is an electron‐accepting and ‐transport material. These results show that the diarylethene compound containing dimesitylboryl groups has promising potential to be applied in optoelectronic devices and thus is worth to be further investigated.     

Reversible on/off conductance switching of single diarylethene immobilized on a silicon surface

The chemical functionalization of hydrogen-terminated silicon(111) surfaces with photochromic diarylethene using an ethylene anchoring group was achieved. Conductive atomic force microscopy measurements showed the current changes on modified silicon electrodes caused by light-induced isomerization of the diarylethene between an open form and a closed form.     

A Combination of Selective Light Reflection and Fluorescence Modulation in a Cholesteric Polymer Matrix

Summary: The phase behavior and optical properties of a cholesteric ternary copolymer, containing nematogenic phenylbenzoate, cholesteric, and photochromic diarylethene side groups, and its mixture with 2 wt.‐% fluorescent dopant were studied. The investigation of the kinetics of a photochemical opening‐cycle process of the photochromic groups in the cholesteric mixture proved the energy transfer from the fluorescent dopant to the photochromic diarylethene groups. It was shown that the fluorescence intensity of the fluorescent dopant could be controlled by the portion of the “closed” form of the diarylethene groups. During the photocyclization of the photochromic groups a “degeneration” of the selective light reflection of the cholesteric matrix is observed.

Fluorescence‐resonance energy transfer makes possible the process of photosensitization of the back ring‐opening photoreaction of the photochromic diarylethene groups in the cholesteric polymer matrix.     

Post‐Modification of the Ethene Bridge in the Rational Design of Photochromic Diarylethenes

Fine‐tuning of the molecular structure of organic bistable compounds to improve their photochromic performance or to introduce additional functions remains an important issue in the development of photoresponsive materials. Diarylethenes bearing heterocyclic moieties belong to the most intensively studied class of organic photochromes due to their excellent photochemical properties. A huge number of diarylethenes have been synthesized so far. Analysis of the literature data shows that there are very worthy examples of diarylethenes developed by the Irie and Feringa groups, which can be the common starting material for a number of diarylethenes functionalized in hetaryl moieties. We refer to these structures as photochromic diarylethene precursors. These diarylethenes have proved to be very useful in the construction of functional molecules with desired properties. On the other hand, in our groups, we have elaborated on diarylethene precursors with modifiable ethene bridges. In this review, we have collected examples of such structures and their chemical modifications, leading to the improvement or fine‐tuning of photochromic switching.     

Crystal engineering of photochromic diarylethene single crystals

Photochromic performance of diarylethene single crystals was controlled by crystal engineering using non-covalent aromatic-aromatic interactions as the directional intermolecular force. A diarylethene derivative with two pentafluorophenyl groups, 1,2-bis(2-methyl-5-pentafluorophenyl-3-thienyl)perfluorocyclopentene (1a), formed stoichiometric co-crystals with benzene (Bz) and naphthalene (Np) by aryl-perfluoroaryl interactions. Face-to-face pi-stacking interactions between the pentafluorophenyl groups of 1a and the aromatic molecules are responsible for 2:1 and 1:1 stoichiometric compositions in 1a/Bz and 1a/Np co-crystals, respectively. The diarylethene underwent thermally stable and photoreversible photochromic reactions in a homo-crystal of 1a and co-crystals 1a/Bz and 1a/Np. The absorption spectra of the photogenerated closed-ring isomers varied depending on the conformation of the diarylethene molecules packed in the crystals. The diarylethene 1a also formed 1:1 stoichiometric co-crystals with different kinds of diarylethenes, 1,2-bis(2-ethyl-5-phenyl-3-thienyl)perfluorocyclopentene (2a) and 1,2-bis[2-methyl-5-(1-naphthyl)-3-thienyl]perfluorocyclopentene (3a). Both co-crystals 1a/2a and 1a/3a showed photochromism. Although 1a, 2a, and 3a underwent efficient photocyclization reactions in their homo-crystals, highly selective photocyclization reactions of 2a or 3a were observed in the co-crystals. The selective reactions were confirmed by HPLC and X-ray crystallography. Excited energy transfers from 1a to 2a and from 1a to 3a are considered to occur and cause the selective reactions.     

Diarylethene-based multi-controllable switch with pyridinepiperazine-linked hydroxyquinoline group

Asymmetrical diarylethene with pyridinepiperazine-linked hydroxyquinoline group was synthesized and its multi-controllable switching behaviors induced by light and chemical stimuli were investigated. The diarylethene exhibited a notable fluorescence enhancement with added TFA in the solution. When triggered by Fe³⁺, its fluorescence intensity notably decreased, accompanied with a color change from light blue to dark, indicating its excellent selectivity to Fe³⁺ over many other metal ions. Moreover, a 1:1 binding stoichiometry between Fe³⁺ and the derivative was obtained according to Job's plot and ESI-MS analysis.     

Self-assembled phenylethynylene bis-urea macrocycles facilitate the selective photodimerization of coumarin

There is much interest in designing molecular sized containers that influence and facilitate chemical reactions within their nanocavities. On top of the advantages of improved yield and selectivity, the studies of reactions in confinement also give important clues that extend our basic understanding of chemical processes. We report here, the synthesis and self-assembly of an expanded bis-urea macrocycle to give crystals with columnar channels. Constructed from two C-shaped phenylethynylene units and two urea groups, the macrocycle affords a large pore with a diameter of ～9 ?. Despite its increased size, the macrocycles assemble into columns with high fidelity to afford porous crystals. The porosity and accessibility of these channels have been demonstrated by gas adsorption studies and by the uptake of coumarin to afford solid inclusion complexes. Upon UV-irradiation, these inclusion complexes facilitate the conversion of coumarin to its anti-head-to-head (HH) photodimer with high selectivity. This is contrary to what is observed upon the solid-state irradiation of coumarin, which affords photodimers with low selectivity and conversion.     

A multi-controllable selective fluorescent turn-on chemosensor for Al3+ and Zn2+ based on a new diarylethene with a 3-(4-methylphenyl)-1H-pyrazol-5-amine Schiff base group

A novel diarylethene containing a 3-(4-methylphenyl)-1H-pyrazol-5-amine was synthesized. Its multi-responsive properties induced by UV/Vis lights and metal ions were studied in detail by absorption and fluorescence emission spectroscopy. It showed excellent fluorescence sensing ability for Al³⁺ and Zn²⁺ with very low detection limit. In addition, based on the multi-responsive characteristics, a logic circuit was constructed by using both UV/Vis lights and chemical species stimuli as inputs and fluorescence intensity as outputs. Moreover, the diarylethene was successfully applied to effectively detect Al³⁺ and Zn²⁺ in actual water samples.     

The effects of phenyl-bridged glucosyltriazolyl groups on the properties of water-soluble diarylethene derivatives

Three water-soluble diarylethenes with glycosyltriazolyl groups and variable phenyl units have been synthesized, and their properties, such as water solubility, photochromism, and fluorescence have been discussed. Upon alternating irradiation with UV and visible light, each of the diarylethenes exhibited favorable photochromism and functioned as fluorescent switches in water. Experimental results showed that their water solubility as well as absorption features, cyclization quantum yields, photoconversion ratios, thermal stability, and fatigue resistance exhibited a strong correlation with the number of phenyl-bridged glucosyltriazolyl groups in diarylethene systems. The absorption maximum, thermal stability, and fatigue resistance were significantly improved, but the water solubility and fluorescent modulation efficiency were suppressed with the increase of the number of phenyl-bridged glucosyltriazolyl groups for these diarylethene derivatives.     

Photoswitching CO2 Capture and Release in a Photochromic Diarylethene Metal–Organic Framework

We demonstrate herein a promising pathway towards low‐energy CO₂ capture and release triggered by UV and visible light. A photosensitive diarylethene ligand was used to construct a photochromic diarylethene metal–organic framework (DMOF). A local photochromic reaction originating from the framework movement induced by the photoswitchable diarylethene unit resulted in record CO₂‐desorption capacity of 75 % under static irradiation and 76 % under dynamic irradiation.     

Photochromic reactions of diarylethenes in single crystals with intermolecular O--H...N hydrogen-bonding networks

The crystal structures and photochromic performance of a single crystal of a diarylethene derivative possessing carboxyl groups, 1,2-bis(5-carboxyl-2-methyl-3-thienyl)perfluorocyclopentene (1 a), and cocrystals of 1 a with 4,4'-, 2,4'-, and 2,2'-bipyridines were examined. In crystal 1 a, a discrete cyclic structure was observed, in which four 1 a molecules are linked through hydrogen bonds between the carboxyl groups. In the homocrystal, photoreactive and photoinactive conformers of 1 a exist in the ratio of 1:1. In the cocrystals of 1 a with bipyridines, O--HN-type hydrogen bonds between 1 a and pyridyl groups were formed, and all 1 a molecules are fixed in a photoreactive conformation. Both the homocrystal 1 a and the cocrystals showed photochromic performances, and color variation from bluish-violet to cyan was observed, depending on the conformation of the packed diarylethene molecules.     

A highly selective fluorescence switch for Cu2+ and Fe3+ based on a new diarylethene with a triazole-linked rhodamine 6G unit

A new diarylethene compound with a triazole-linked rhodamine 6G unit attached to the imino group (1O) was designed and synthesized. According to the test results, the solution color and fluorescence color of diarylethene can be modulated by lights and metal ions. The solution color could change from colorless to light purple when irradiated with UV light. When Cu²⁺ was added to the diarylethene solution, the color of diarylethene solution became blue, the fluorescence color turned from dark to bright yellow. Although the solution color did not change by adding Fe³⁺, its fluorescence color varied from dark to yellow. Moreover, it was found that the complex ratio of the diarylethene to Cu²⁺ was 1:1 and the binding stoichiometry of the diarylethene to Fe³⁺ was also 1:1 based on the data of NMR, MS, and other experiments. Based on these findings, photochromic figure of the diarylethene with UV/Vis light, Cu²⁺ and Fe³⁺ was constructed. Furthermore, the logic circuit was designed by input signals (ultraviolet stimulus, visible light stimulus, Cu²⁺ (or Fe³⁺) and EDTA) and an output signal (fluorescent intensity at 566?nm (or 575?nm)).     

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.     

含有两个香豆素的α,ω-双发色团长链化合物的光环合加成反应

本文研究了含有两个香豆素的α,ω双发色团长链化合物在溶液中的光环合加成反应, 结果表明: 标题化合物有着不同于单体香豆素的光化学行为, 即它能在低浓度及非极性溶剂内发生分子内的光二聚反应。此外, 通过高分辨核磁共振谱对产物立体化学的研究发现, 反应无论是在极性溶剂或非极性溶剂中进行或反应中是否加入了三重态敏化剂。所得产物均为顺式头-头分子内二聚结构。对反应机制进行了初步讨论。     

Fatigue and photochromism S1 excited state reactivity of diarylethenes from QTAIM and the stress tensor

The realization of technologically relevant functional systems from idealized photochromic compounds remains elusive due to the double requirement that such switches must possess both highly efficient photo‐isomerization reactivity and extremely low fatigue over a large number of switching cycles. Nowadays, improvements of the switching properties in complex diarylethene structures are mainly attained on a “trial and error” basis through chemical substitutions aimed at tuning the chemical properties of the core of the diarylethene. Therefore, we present new guiding principles to analyze the first excited state reactivity of diarylethenes based on the quantum theory of atoms in molecules (QTAIM) including the stress tensor. This approach straightforwardly provides consistent theoretical justification to partner the already successful symmetric substitution patterns obtained from experiments. The guiding principles provided by QTAIM and stress tensor suggest more complex asymmetric patterns should be included for the systematic design of new technologically relevant functional compounds. The stress tensor trajectory T_σ(s) analysis is used to characterize the photochromism reaction as reusable and the fatigue reaction as irreversible and find candidate sites for alteration by future experiment.     

Coumarin-conjugated cyclodextrins: remarkable enhancement of the chemical-to-light energy transfer efficiency

One or two coumarin units were incorporated to the primary face of γ-cyclodextrin (γ-CD), and the resultant coumarin derivatives were employed to harvest the chemical energy generated in the reaction of bis(trichlorophenyl)oxalate with hydrogen peroxide. In comparison with the coumarin without CD cavity for molecular recognition, the coumarin–CD conjugates demonstrated much higher chemiluminescence inetensity, indicating that the CD moiety remarkably improves the chemical energy transfer.     

Photoswitching of conductance of diarylethene-Au nanoparticle network

A network composed of gold nanoparticles covered with diarylethene dithiophenols was prepared on an interdigitated nanogapped gold electrode to show the reversible photoswitching of the conductance due to the photochromism of the diarylethene molecules induced by UV and visible light.     

The effects of heteroaryl ring on the photochromism of diarylethenes with a naphthalene moiety

Three new asymmetrical naphthalene-containing diarylethenes with different heteroaryl groups have been synthesized to investigate the heteroaryl effects on their properties. The three diarylethenes exhibited distinctive photochromism with good thermal stability, which may be attributed to the different heteroaryl effects. Their cycloreversion quantum yields increased in the order of 2-methylbenzofuran < 2-methylbenzothiophene < 1,2-dimethylindole, while the cyclization quantum yields exhibited a reverse trend. Compared to indole and benzothiophene, the benzofuran moiety could effectively shift the absorption maximum to a shorter wavelength and notably enhance the cyclization quantum yield and fluorescence quantum yield of the diarylethene. The results indicated that the category of heteroaryl groups played a vital role during the process of photoisomerization of naphthalene-containing diarylethene derivatives.     

Synthesis of a new diarylethene diradical which has extended π-conjugated chains from the 2,5-position of one thiophene ring

A diarylethene diradical having a new switching unit for intramolecular magnetic interaction was synthesized. The photoswitching unit has an extended π-conjugated chain in one aryl unit, and two nitronyl nitroxide radical are placed at both ends of the π-conjugated chain. The diarylethene moiety is located in the middle of the chain. This diarylethene is designed to change the hybrid orbital from sp² to sp³ at the 2-position of the thiophene ring when this diarylethene undergoes a photochromic reaction. But the new diradical compound did not undergo photocyclic reaction upon irradiation with UV light. The photochemical behavior is perturbed by a resonant quinoid structure which stabilizes the open-ring isomer.     

Absolute asymmetric photocyclization in chiral diarylethene co-crystals with octafluoronaphthalene

A photochromic diarylethene containing naphthyl groups formed a chiral crystal when co-crystallized with octafluoronaphthalene, although both molecules are achiral, and underwent highly enantioselective photocyclization owing to the conformational confinement in the crystal.