Photocontrolled Intramolecular Charge/Energy Transfer and Fluorescence Switching of Tetraphenylethene‐Dithienylethene‐Perylenemonoimide Triad with Donor–Bridge–Acceptor Structure

Photochromic 1,2‐dithienylethene (DTE) derivatives with a high thermal stability and fatigue resistance are appealing for optical switching of fluorescence. Here, we introduce a donor–photochromic bridge–acceptor tetraphenylethene‐dithienylethene‐perylenemonoimide (TPE‐DTE‐PMI) triad, in which TPE acts as the electron donor, PMI as the electron acceptor, and DTE as the photochromic bridge. In this system, the localized and intramolecular charge transfer emission of TPE‐DTE‐PMI with various Stokes shifts have been observed due to the photoinduced intramolecular charge transfer in different solvents. Upon UV irradiation, the fluorescence quenching resulting from photochromic fluorescence resonance energy transfer in TPE‐DTE‐PMI has been demonstrated in solution and in solid films. The fluorescence on/off switching ratio in polymethylacrylate film exceeds 100, a value much higher than in polymethylmethacrylate film, thus indicating that the fluorescence switching is dependent on matrices.     

Synthesis,Photochromic, and Computational Studies of Dithienylethene‐Containing β‐Diketonate Derivatives and Their Near‐Infrared Photochromic Behavior Upon Coordination of a Boron(III) Center

A series of dithienylethene‐containing 1‐thienyl‐3‐aryl‐propane‐1,3‐diones (aryl=phenyl (Ph), thienyl (Th), and 4,5‐bis(2,5‐dimethylthiophen‐3‐yl)thiophen‐2‐yl (DTE‐Th)) and the corresponding boron(III) diketonates, (O^O)BR₂ (R=F, C₆F₅, and Ph), have been designed and synthesized. Their photophysical, electrochemical, and photochromic properties have been studied. Upon coordination of a boron(III) center, the closed forms of the dithienylethene‐containing β‐diketonates show near‐infrared response and the photochromic behavior was also found to be affected by the aryl substituents at the 3‐position of the β‐diketonates. Moreover, computational studies have been performed that help to provide an understanding of the effect of substituents on the photophysical and photochromic properties.     

Nondestructive Photoluminescence Read‐Out by Intramolecular Electron Transfer in a Perylene Bisimide‐Diarylethene Dyad

A novel, highly stable photochromic dyad 3 based on a perylene bisimide (PBI) fluorophore and a diarylethene (DAE) photochrome was synthesized and the optical and photophysical properties of this dyad were studied in detail by steady‐state and time‐resolved ultrafast spectroscopy. This photochromic dyad can be switched reversibly by UV‐light irradiation of its ring‐open form 3 o leading to the ring‐closed form 3 c , and back reaction of 3 c to 3 o by irradiation with visible light. Solvent‐dependent fluorescence studies revealed that the emission of ring‐closed form 3 c is drastically quenched in solvents of medium (e.g., chloroform) to high (e.g., acetone) polarities, while the emission of the ring‐open form 3 o is appreciably quenched only in highly polar solvents like DMF. The strong fluorescence quenching of 3 c is attributed to a photoinduced electron‐transfer (PET) process from the excited PBI unit to ring‐closed DAE moiety, as this process is thermodynamically highly favorable with a Gibbs free energy value of ?0.34 eV in dichloromethane. The electron‐transfer mechanism for the fluorescence quenching of ring‐closed 3 c is substantiated by ultrafast transient measurements in dichloromethane and acetone, revealing stabilization of charge‐separated states of 3 c in these solvents. Our results reported here show that the new photochromic dyad 3 has potential for nondestructive read‐out in write/read/erase fluorescent memory systems.     

Synthesis of Bifunctional Azobenzene Glycoconjugates for Cysteine‐Based Photosensitive Cross‐Linking with Bioactive Peptides

Azobenzene linker molecules can be utilized to control peptide/protein function when they are ligated to appropriately spaced amino acid side chains of the peptide. This is because the photochemical E/Z isomerization of the azobenzene N?N double bond allows to switch peptide conformation between folded and unfolded. In this context, we have introduced carbohydrate‐functionalized azobenzene derivatives in order to advance the biocompatible properties of azobenzene peptide linkers. Chloroacetamide‐functionalized and O‐allylated carbohydrate derivatives were synthesized and conjugated with azobenzene to achieve new bifunctional cross‐linkers, in order to allow ligation to cysteine side chains by nucleophilic substitution or thiol‐ene reaction, respectively. The photochromic properties of the new linker glycoconjugates were determined and first ligation reactions performed.     

Optical switching,photophysical, and electrochemical behaviors of pendant triazole‐linked indolylfulgimide polymer

Triazole‐linked 2‐indolylfulgimide polymer has been synthesized and its photochromic switching behavior has been characterized by NMR, IR, GPC, TGA, DSC, and UV–Vis spectroscopy. The synthesized photochromic polymer showed absorption peak maxima at 386 and 510 nm wherein the absorption at 510 nm was attributed to charge transfer from triazole ring nitrogen to carbonyl carbon of fulgimide unit. Fluorescence lifetime studies on exciting at 550 nm reveals triexponential behavior with fluorescence decay around 0.1, 1 and 4.2 ns, which correspond to open (E), closed (C) form of fulgimide and triazole ring, respectively. Whereas exciting at 470 nm evidences biexponential fit with fluorescence decay around 0.1 and 2.2 ns, which corresponds to the closed (C) form and triazole ring, respectively. Fluorescence decay of triazole ring was found to be influenced by the excitation wavelength. The cyclic voltammogram of open form of polymer depicts irreversible reductive wave at ?1.4 V. On illumination with 360‐nm light, the reduction wave of polymer was shifted toward less cathodic wave at ?0.9 V; this leads to formation of the closed form of fulgimide unit. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Orthogonally Self‐Assembled Multifunctional Block Copolymers

We report the synthesis of telechelic poly(norbornene) and poly(cyclooctene) homopolymers by ring‐opening metathesis polymerization (ROMP) and their subsequent functionalization and block copolymer formation based on noncovalent interactions. Whereas all the poly(norbornene)s contain either a metal complex or a hydrogen‐bonding moiety along the polymer side‐chains, together with a single hydrogen‐bonding‐based molecular recognition moiety at one terminal end of the polymer chain. These homopolymers allow for the formation of side‐chain‐functionalized AB and ABA block copolymers through self‐assembly. The orthogonal natures of all side‐ and main‐chain self‐assembly events were demonstrated by ¹H NMR spectroscopy and isothermal titration calorimetry. The resulting fully functionalized block copolymers are the first copolymers combining both side‐ and main‐chain self‐assembly, thereby providing a high degree of control over copolymer functionalization and architecture and bringing synthetic materials one step closer to the dynamic self‐assembly structures found in nature.     

Core‐Modified Rubyrins Containing Dithienylethene Moieties

Two stable core‐modified rubyrins bearing one and two dithienylethene (DTE) units ( 1 and 2 ) have been synthesized. With one “closed‐form” DTE unit, 1 shows aromaticity associated with its conjugated circuit of 26 π‐electrons. In contrast, rubyrin 2 containing one “open‐form” DTE unit has nonaromatic properties.     

Reversible Multistimuli‐Response Fluorescent Switch Based on Tetraphenylethene–Spiropyran Molecules

Two tetraphenylethene (TPE)‐functionalized spiropyran (SP) molecules with very similar structure were designed and synthesized. The two molecules exhibit aggregation‐induced emission (AIE) properties, as well as multistimuli‐responsive color‐changing properties, such as photochromism and acidchromism. The investigation of their different photochromic and acidchromic characteristics and dual‐response fluorescent switch during isomerization indicated that the different link position between TPE and SP will significantly affect the extended π‐conjugated system, resulting in completely different photochromic and acidchromic properties.     

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.     

Efficient One‐step Synthesis and Properties of Photochromic Diarylethenes Having an Indene Bridging Unit

Two novel isomeric photochromic diarylethenes with an indene bridging unit have been prepared by a simple and efficient one‐step synthesis method. Their properties, including photochromic behavior, fluorescent properties and fatigue resistance, have been investigated. These two isomeric compounds showed photochromic back‐and‐forth reactions with ultraviolet and visible light both in solution and in PMMA film. Their ring‐open forms exhibited appreciable fluorescence, which was quenched by the ring‐closed forms. All results indicated that diarylethenes derivatives with indene‐aryl bridges exhibited rather high fatigue resistance and good thermally irreversible photochromic properties.     

Photomobile Polymer Materials: Photoresponsive Behavior of Cross‐Linked Liquid‐Crystalline Polymers with Mesomorphic Diarylethenes

Cross‐linked liquid‐crystalline (LC) polymers with a mesomorphic diarylethene were prepared to demonstrate a versatile strategy for cross‐linked photochromic LC polymers as photomobile materials. Upon exposure to UV light to cause photocyclization of the diarylethene chromophore, the cross‐linked polymer films bend toward an actinic light source. By irradiation with visible light to cause a closed‐ring to open‐ring isomerization, the bent films revert to the initial flat state. Without visible‐light irradiation, the bent films remain bent even at 120 °C, indicating high thermal stability of the cross‐linked diarylethene LC polymers.     

A Second‐Coordination‐Sphere Strategy to Modulate Nickel‐ and Palladium‐Catalyzed Olefin Polymerization and Copolymerization

Transition‐metal‐catalyzed copolymerization reactions of olefins with polar‐functionalized comonomers are highly important and also highly challenging. A second‐coordination‐sphere strategy was developed to address some of the difficulties encountered in these copolymerization reactions. A series of α‐diimine ligands bearing nitrogen‐containing second coordination spheres were prepared and characterized. The properties of the corresponding nickel and palladium catalysts in ethylene polymerizations and copolymerizations were investigated. In the nickel system, significant reduction in polymer branching density was observed, while lower polymer branching densities, as well as a wider range of polar monomer substrates, were achieved in the palladium system. Control experiments and computational results reveal the critical role of the metal−nitrogen interaction in these polymerization and copolymerization reactions.     

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.     

Copolymerization of Ethylene with Sterically Hindered 3,3‐Dimethyl‐1‐Butene Using a Chain‐Walking Pd‐Diimine Catalyst

In this Communication, the copolymerization of ethylene with a sterically hindered α‐olefin comonomer, γ‐trisubstituted 3,3‐dimethyl‐1‐butene (DMB), using a chain‐walking Pd‐diimine catalyst, [(ArNC(Me) (Me)CNAr)Pd(CH₃)(NCMe)]SbF₆ (Ar2,6‐(iPr)₂C₆H₃) ( 1 ) is reported. In spite of its high steric bulkiness in the close proximity of the double bond, appreciable DMB incorporations (up to 3 mol‐%) are successfully achieved in the copolymers. The chain microstructure of the copolymers is elucidated, and the effect of DMB incorporation on polymer topology and thermal properties are examined. This work thus demonstrates the high capability of the Pd‐diimine catalyst in incorporating sterically encumbered α‐olefins.

     

Redox‐Active Metal–Organic Frameworks: Highly Stable Charge‐Separated States through Strut/Guest‐to‐Strut Electron Transfer

Molecular organization of donor and acceptor chromophores in self‐assembled materials is of paramount interest in the field of photovoltaics or mimicry of natural light‐harvesting systems. With this in mind, a redox‐active porous interpenetrated metal–organic framework (MOF), {[Cd(bpdc)(bpNDI)] ? 4.5 H₂O ? DMF}_n ( 1 ) has been constructed from a mixed chromophoric system. The μ‐oxo‐bridged secondary building unit, {Cd₂(μ‐OCO)₂}, guides the parallel alignment of bpNDI (N,N′‐di(4‐pyridyl)‐1,4,5,8‐naphthalenediimide) acceptor linkers, which are tethered with bpdc (bpdcH₂=4,4′‐biphenyldicarboxylic acid) linkers of another entangled net in the framework, resulting in photochromic behaviour through inter‐net electron transfer. Encapsulation of electron‐donating aromatic molecules in the electron‐deficient channels of 1 leads to a perfect donor–acceptor co‐facial organization, resulting in long‐lived charge‐separated states of bpNDI. Furthermore, 1 and guest encapsulated species are characterised through electrochemical studies for understanding of their redox properties.     

2‐(1 H‐1,2,3‐Triazol‐4‐yl)‐Pyridine Ligands as Alternatives to 2,2′‐Bipyridines in Ruthenium(II) Complexes

The synthesis of a variety of 2‐(1H‐1,2,3‐triazol‐4‐yl)‐pyridines by click chemistry is demonstrated to provide straightforward access to mono‐functionalized ligands. The ring‐opening polymerization of ε‐caprolactone initiated by such a mono‐functionalized ligand highlights the synthetic potential of this class of bidentate ligands with respect to polymer chemistry or the attachment onto surfaces and nanoparticles. The coordination to Ru^II ions results in homoleptic and heteroleptic complexes with the resultant photophysical and electrochemical properties strongly dependent on the number of these ligands attached to the Ru^II core.     

Novel Bisthienylethene Containing Ferrocenyl‐Substituted Naphthalimide: A Photo‐ and Redox Multi‐Addressable Molecular Switch

A new photochromic bisthienylethene system (BTE? NAFc) is reported in which the ferrocene unit (Fc) is incorporated into a naphthalimide chromophore as the central ethene bridging unit. The incorporated Fc unit in the photochromic system of BTE? NAFc has several effects on optical properties, such as fluorescence‐modulation through photoinduced electron transfer (PET), a decrease in the photochromic cyclization quantum yield, and a selective two‐step oxidation process. The ability to drive ring‐opening and ring‐closing reactions with a secondary redox‐modulation provides increased functionality to the photochromic system. Based on these meaningful photo‐ and redox‐modulation properties, five unprecedented multi‐addressable states (BTE? NAFc, BTE? NAFc⁺, c‐BTE? NAFc, c‐BTE? NAFc⁺, and BTE⁺? NAFc⁺) and gated photochromism are successfully obtained within the unimolecular BTE platform, thus providing deeper insight into photochromic systems as multifunctional outputs.     

Amphiphilic Diblock Copolymer with Dithienylethene Pendants: Synthesis and Photo‐Modulated Self‐Assembly

In this work, an amphiphilic diblock copolymer (PEG₄₃‐b‐PSDTE₂₉) bearing photochromic dithienylethene (DTE) pendants is synthesized by reversible addition fragmentation chain transfer radical polymerization. The diblock copolymer was characterized by spectroscopic methods and gel permeation chromatography. The analyses proved the well‐defined structure and narrow molecular weight distribution of the diblock copolymer. The DTE pendants could undergo reversible photoisomerization between their open and closed forms in solution when irradiated with UV and visible light as indicated by ¹H NMR and UV‐vis spectroscopy. Hollow vesicle‐like structures were formed by gradually adding deionized water to the colorless PEG₄₃‐b‐PSDTE_29open (DTE in open form) tetrahydrofuran solution. Under the same conditions, the aggregates formed in the blue PEG₄₃‐b‐PSDTE_29close (DTE in closed form) solution were colloidal spheres with solid interiors. The isomerization of DTE pendants could cause the deformation of the vesicle‐like structures. The above results demonstrate a kind of novel photo‐modulated self‐assembly behavior of the amphiphilic diblock copolymer, which could be used for drug‐delivery and other applications.

     

Ring transformations of heterocyclic compounds. XXIII. The UV irradiation product of photochromic 2,4,6‐triaryl‐1‐(spiro[2H‐l‐benzopyran‐2,2′‐indoline]‐6‐yl)pyridinium salts ‐ A phenolate betaine or a pyridinium substituted merocyanine dye?

The synthesis of the novel 2,4,6‐triaryl‐1‐(spiro[2H‐1‐benzopyran‐2,2′‐indoline]‐6‐yl)pyridiniumper‐chlorates 4 by reaction of 5 ‐nitrosalicylaldehydes 6 with 1,3,3‐trimethyl‐2‐methyleneindoline ( 7 ) to 6‐nitro‐spiro[2H‐1‐benzopyran‐2,2′‐indolines] 1 , their stannous chloride reduction to the 6‐amino derivatives 8 , followed by a 2,6‐[C₅+N] ring transformation with 2,4,6‐triarylpyrylium perchlorates 9 , is reported. UV irradiation experiments in twenty solvents of different polarity prove their photochromic properties and show that the photochemically generated negative solvatochromic dyes 5 , formed by ring opening of the benzopyran moiety of 4 , are rather merocyanine than pyridinium phenolate betaine dyes.     

Conformational Studies on Peptides of α‐Aminoxy Acids with Functionalized Side‐Chains

Peptides of homochiral α‐aminoxy acids of nonpolar side chains can form a 1.8₈‐helix. In this paper, we report the conformational studies of α‐aminoxy peptides 1 , 2 , 3 , which have functionalized side chains, in both nonpolar and polar solvents. ¹H NMR, XRD, and FTIR absorption studies confirm the presence of the eight‐membered‐ring intramolecular hydrogen bonds (the N‐O turns) in nonpolar solvents as well as in methanol. CD studies of peptides 1 , 2 , 3 in different solvents indicate that a substantial degree of helical content is retained in methanol and acidic aqueous buffers. The introduction of functionalized side chains in α‐aminoxy peptides provides opportunities for designing biologically active peptides.