Redox-assisted ring closing reaction of the photogenerated cyclophanediene form of bis(ferrocenyl)dimethyldihydropyrene with interferrocene electronic communication switching

We synthesized ferrocene-attached dimethyldihydropyrene (DHP) derivatives and investigated their photochemical and redox behaviors. For bis(ferrocenylethynyl)dimethyldihydropyrene (1), reversible photoisomerization between the closed DHP form (1c) and the open CPD form (1o) occurred in high yields upon alternate irradiation of visible (578 nm) light and UV (303 nm) light, whereas no photoisomerization proceeded for bis(pentamethylferrocenylethynyl)dimethyldihydropyrene (2). 1 exhibited reversible switching of electronic communication between the ferrocene (Fc) moieties by photoisomerization of the DHP moiety and demonstrated a novel ring closing reaction induced by oxidation of the Fc moieties. The magnitude of electronic communication, deltaE0' (the difference between the redox potentials of two Fc's), was 63 mV in 1c and 16 mV in 1o, indicating that the electronic communication through the spacer is enhanced in the more developed pi-conjugation of the DHP moiety. The rate constants of the ring closing reaction from 1o+ to 1c+ and from 1o2+ to 1c2+ were estimated at 3.7 and 0.50 s(-1), respectively, by the simulation of cyclic voltammograms.     

Photonic switching of photoinduced electron transfer in a dihydropyrene-porphyrin-fullerene molecular triad

Photonic control of photoinduced electron transfer has been demonstrated in a dimethyldihydropyrene (DHP) porphyrin (P) fullerene (C(60)) molecular triad. In the DHP-P-C(60) form of the triad, excitation of the porphyrin moiety is followed by photoinduced electron transfer to give a DHP-P(*)(+)-C(60)(*)(-) charge-separated state, which evolves by a charge shift reaction to DHP(*)(+)-P-C(60)(*)(-). This final state has a lifetime of 2 micros and is formed in an overall yield of 94%. Visible (>or=300 nm) irradiation of the triad leads to photoisomerization of the DHP moiety to the cyclophanediene (CPD). Excitation of the porphyrin moiety of CPD-P-C(60) produces a short-lived (<10 ns) CPD-P(*)(+)-C(60)(*)(-) state, but charge shift to the CPD moiety does not occur, due to the relatively high oxidation potential of the CPD group. Long-lived charge separation is not observed. Irradiation of CPD-P-C(60) with UV (254 nm) light converts the triad back to the DHP form. Thermal interconversion of the DHP and CPD forms is very slow, photochemical cycling is facile, and in the absence of oxygen, many cycles may be performed without substantial degradation. Thus, light is used to switch long-lived photoinduced charge separation on or off. The principles demonstrated by the triad may be useful for the design of molecule-based optoelectronic systems.     

Mechanistic insights into the photochromism of trans-10b,10c-dimethyl-10b,10c-dihydropyrene derivatives

A series of dimethyldihydropyrene derivatives was studied to elucidate the photochemical mechanism associated with the switching between the dimethyldihydropyrene (DHP, closed) and metacyclophanediene (CPD, open) forms of the molecule. Quantum yields of ring opening and closure, fluorescence quantum yields and lifetimes, as well as laser flash photolysis studies were performed to establish the effect of substituents on the switching efficiency. Ring opening of the DHPs occurs from the first singlet excited state. The low quantum yields for the ring opening reaction observed (< or =0.042) are a consequence of the low rate constant (< or =1.7 x 10(7) s(-1)) for this process. The quantum yields for ring closure of the CPD were determined for select compounds and were of the order of 0.1-0.4. These results show that the efficiency for ring opening of this class of compounds is intrinsically low, but can be modulated to some extent by the introduction of substituents. These properties should be taken into account when considering what type of photoswitching devices DHPs might be useful for.     

A highly regioselective synthesis of 1‐n‐methyl‐spiro‐[2,3′“]‐oxindole‐spiro‐[3,2”]indane‐1“, 3”‐dione‐4‐arylpyrrolidines through 1,3‐dipolar cycloaddition protocol

2‐Arylidene‐1,3‐indanediones undergo a regioselective 1,3‐dipolar cycloaddition reaction with the azomethine ylide derived from isatin and sarcosine by decarboxylative route affording a series of 1‐N‐methyl — spiro[2.3′“]oxindole‐spiro[3.2”]indane‐1“,3”‐diones‐4‐aryl pyrrolidines. The structures were established by spectroscopic techniques as well as single crystal X‐ray analysis. Density functional theory at B3L YP/6‐31G^* and the semi empirical AM₁ calculations were employed to rationalize the observed results. The experimental regioselectivity of 1,3‐dipolar cycloadditions could be corroborated nicely with the computed Fukui frontier orbital energies and reaction energies.     

pH‐Responsive Pillar[6]arene‐based Water‐Soluble Supramolecular Hexagonal Boxes

We describe the preparation of the first water‐soluble pH‐responsive supramolecular hexagonal boxes (SHBs) based on multiple charge‐assisted hydrogen bonds between peramino‐pillar[6]arenes 2 with the molecular “lid” mellitic acid ( 1 a ). The interaction between 2 and 1 a , as well as the other “lids” pyromellitic and trimesic acids ( 1 b and 1 c , respecively) were studied by a combination of experimental and computational methods. Interestingly, the addition of 1 a to the complexes of the protonated form of pillar[6]arene 2 , that is, 3 , with bis‐sulfonate 4 a or 4 b , immediately led to guest escape along with the formation of closed 1 a₂2 supramolecular boxes. Moreover, the process of the openning and closing of the supramolecular boxes along with threading and escaping of the guests, respectively, was found to be reversible and pH‐responsive. This study paves the way for the easy and modular preparation of different SHBs that may have myriad applications.     

Behavior of 3‐benzylamino‐5‐aryl‐2(3H)‐furanones towards some nitrogen nucleophiles

Ring closure of 2‐N‐benzylamino‐3‐aroylpropionic acids ( 3 ) with acetic anhydride afforded 3‐N‐benzylamino‐5‐aryl‐2(3H)‐furanones ( 4 ). The reaction of the furanones ( 4 ) with benzylamine in benzene was found to be time dependent. Thus refluxing the reaction mixture for 1 h only afforded the open‐chain amides ( 5a‐c ). When the reaction was conducted for 3 h the 2(3H)‐pyrrolones ( 6 ) were obtained. Hydrazine hydrate affected ring opening of the furanones to give the hydrazides ( 5d‐f ). Also, semicarbazide converted ( 4 ) into the corresponding semicarbazide derivatives ( 5g‐i ). The hydrazides ( 5d‐f ) were reacted with benzoyl chloride to give the corresponding diaroylhydrazines ( 5j‐l ). The open‐chain derivatives ( 5 ) were converted into a variety of heterocycles: isothiazolones ( 7 ), dihydropyridazinones ( 8 ), 1,3,4‐oxadiazoles ( 9 ) and 1,2,4‐triazole derivatives ( 10 ) via cyclization reactions.     

Heteroanalogues of PCBM: N‐Bridged Imino‐PCBMs for Organic Field‐Effect Transistors

Crossing the bridge : Two isomeric iminofullerenes, [5,6]‐open azafulleroid 1 and [6,6]‐closed aziridinofullerene 2 , were prepared by cycloaddition of an organic azide to C₆₀. These “azalogues” enable the study of the effects of the bridging atom in a fullerene cage, that is, C₆₀‐like (5,6‐open) versus PCBM‐like (6,6‐closed), as a function of their π systems (PCBM=[6,6]‐phenyl‐C₆₁‐butyric acid methyl ester).

     

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.

     

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.     

Diarylethene Self‐Assembled Monolayers: Cocrystallization and Mixing‐Induced Cooperativity Highlighted by Scanning Tunneling Microscopy at the Liquid/Solid Interface

Stimulus control over 2D multicomponent molecular ordering on surfaces is a key technique for realizing advanced materials with stimuli‐responsive surface properties. The formation of 2D molecular ordering along with photoisomerization was monitored by scanning tunneling microscopy at the octanoic acid/highly oriented pyrolytic graphite interface for a synthesized amide‐containing diarylethene, which underwent photoisomerization between the open‐ and closed‐ring isomers and also a side‐reaction to give the annulated isomer. The nucleation (K_n) and elongation (K_e) equilibrium constants were determined by analysis of the concentration dependence of the surface coverage by using a cooperative model at the liquid/solid interface. It was found that the annulated isomer has a very large equilibrium constant, which explains the predominantly observed ordering of the annulated isomer. It was also found that the presence of the closed‐ring isomer induces cooperativity into the formation of molecular ordering composed of the open‐ring isomer. A quantitative analysis of the formation of ordering by using the cooperative model has provided a new view of the formation of 2D multicomponent molecular ordering.     

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.     

Photoswitchable,DNA‐Binding Helical Peptides Assembled with Two Independently Designed Sequences for Photoregulation and DNA Recognition

Diarylethene‐bridged peptides were developed to photoregulate biomolecular interactions. The peptides are made up of diarylethene‐bridged and DNA‐binding regions at their N‐ and C termini, respectively. The two regions could be independently designed and combined as desired. The α‐helicities of the peptides were photoregulated in on/off or off/on manners, and the manner depended on the positions of two ornithine (Orn) residues for cross‐linking reaction at the diarylethene‐bridged region. In the case of the on/off manner, when the diarylethene structure adopted the open form on the peptides, the peptides folded into stable α‐helices. Upon UV irradiation, the diarylethene moiety isomerized to its closed form to destabilize the helical structures. Quartz crystal microbalance (QCM) analysis showed that the open isomer strongly associated with a target DNA, as compared with the closed one. When the closed‐form peptide existing in the DNA complex was irradiated with a fluorescent lamp in the middle of the QCM monitoring, the frequency change (ΔF) was enhanced by the diarylethene photoisomerization.     

Visible‐Light‐Induced Morphological Changes of Giant Vesicles by Photoisomerization of a Ruthenium Aqua Complex

Visible‐ and red‐light responsive vesicles were prepared by incorporating a ruthenium aqua complex having two alkyl chains on tridentate and asymmetrical bidentate ligands (proximal‐ 2 : [Ru(C₁₀tpy)(C₁₀pyqu)OH₂]²⁺, C₁₀tpy=4′‐decyloxy‐2,2′;6′,2“‐terpyridine, C₁₀pyqu=2‐[2′‐(6′‐decyloxy)‐pyridyl]quinoline). The ruthenium complex of proximal‐ 2 with closed alkyl chain geometry and a cylinder‐like molecular shape exhibited photoisomerization to distal‐ 2 with an open alkyl chain geometry and a cone‐like shape, both in an aqueous solution and in vesicle dispersions. We observed that light irradiation of giant vesicles containing proximal‐ 2 induced diverse morphological changes.     

Synthesis of novel asymmetric dendritic‐linear‐dendritic block copolymers via “living” anionic polymerization of ethylene oxide initiated by dendritic macroinitiators

The first synthesis of asymmetric dendritic‐linear‐dendritic ABC block copolymers, that contain a linear B block and dissimilar A and C dendritic fragments is reported. Third generation poly(benzyl ether) monodendrons having benzyl alcohol moiety at their “focal” point were activated by quantitative titration with organometallic anions and the resulting alkoxides were used as initiators in the “living” ring‐opening polymerization of ethylene oxide. The reaction proceeded in controlled fashion at 40–50 °C affording linear‐dendritic AB block copolymers with predictable molecular weights (M_w = 6000–13,000) and narrow molecular weight distributions (M_w/M_n = 1.02–1.04). The propagation process was monitored by size‐exclusion chromatography with multiple detection. The resulting “living” copolymers were terminated by reaction either with HCl/tetrahydrofuran or with a reactive monodendron that differed from the initiating dendron not only in size, but also in chemical composition. The asymmetric triblock copolymers follow a peculiar structure‐induced self‐assembly pattern in block‐selective solvents as evidenced by size‐exclusion chromatography in combination with multi‐angle light scattering. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5136–5148, 2007     

Enhanced Oxidation of Simvastatin at a Multi‐Walled Carbon Nanotubes‐Dihexadecyl Hydrogen Phosphate Composite Modified Glassy Carbon Electrode and the Application in Determining Simvastatin in Pharmaceutical Dosage Forms

A sensitive electrochemical method for the determination of simvastatin (SV) was established, based on the enhanced oxidation of SV at a multi‐walled carbon nanotubes‐dihexadecyl hydrogen phosphate composite modified glassy carbon electrode (MWNTs‐DHP/GCE). The voltammetric studies showed that MWNTs instead of DHP or GCE could effectively catalyze the oxidation of SV. The dependence of oxidation current on SV concentration was explored under optimal conditions, which exhibited a good linear relationship in the range of 1.0×10^?7–7.5×10^?6 M. The detection limit of SV was also examined and a low value of 5.0×10^?8 M was obtained for 5 min accumulation (σ=3). This electrode was applied to the detection of SV in drug forms and the results were in accordance with those obtained by UV spectroscopy.     

Graphene Enhances the Shape Memory of Poly (acrylamide‐co‐acrylic acid) Grafted on Graphene

Acrylamide and acrylic acid are grafted on graphene by free‐radical polymerization to produce a series of graphene–poly(acrylamide‐co‐acrylic acid) hybrid materials with different contents of graphene. The materials demonstrate shape memory effect and self‐healing ability when the content of graphene is in the range of 10%–30% even though poly(acrylamide‐co‐acrylic acid) itself had poor shape memory ability. The permanent shape of the materials can be recovered well after 20 cycles of cut and self‐healing. The result is attributed to the hard–soft design that can combine nonreversible “cross‐link” by grafting copolymer on graphene and reversible “cross‐link” utilizing the “zipper effect” of poly(acrylamide‐co‐acrylic acid) to form or dissociate the hydrogen‐bond network stimulated by external heating.

     

Photoswitchable “Turn‐on” Fluorescence Diarylethenes: Substituent Effects on Photochemical Properties and Electrochromism

A series of “turn‐on” fluorescence diarylethenes derived from 2,3‐bis(2‐methylbenzo[b]thiophen‐3‐yl)‐5,6‐dihydro‐4H‐thieno[2,3‐b]thiopyran‐4‐one ( 1 ) with alkyl and acetyl substituents were synthesized. The photochemical and photophysical properties of these derivatives, including the photoreaction of crystalline 1 , were thoroughly investigated to reveal substituent effects on their properties. The results indicated that alkyl substituents did not significantly affect the absorption and emission spectra of the diarylethenes. However, large absorption and emission wavelength shifts were observed for the diarylethene with an acetyl substituent due to extension of π–π conjugation. Significantly, all of the fluorescent ring‐closed forms of the compounds isomerized to their ring‐open forms in the presence of Cu²⁺ in the dark. EPR results provide clear evidence for the formation of the compound 1 radical cation intermediate that might be generated in the reaction between c‐ 1 and Cu²⁺. DFT calculations found that the ground‐state activation energy for ring‐opening of 1^.+ was approximately 9.2 kcal mol^?1 lower than that of 1 without Cu²⁺, such that a Cu²⁺‐catalyzed oxidative cycloreversion reaction at room temperature might be possible.     

Photo‐/Baso‐Chromisms and the Application of a Dual‐Addressable Molecular Switch

Two typical molecular switches of spiropyran (SP) and benzoxazine (OX) were fused by sharing an indole to achieve a new dual‐addressable molecular switch (SP‐OX‐NO₂). Through proper molecular modification with NO₂, the transformation from merocyanine (MC) to ring‐closed SP or ring‐closed OX can be controlled separately with visible light or base stimuli in solution, respectively, and these processes are verified by UV‐vis and NMR spectroscopy as well as control experiments. The cis‐merocyanine (cis‐MC) form is involved in the basochromic process in solution. DFT calculation suggests that the bidirectional switching property of the fused SP‐OX molecular switch can be controlled separately, when the OX isomer is more stable than the deprotonated SP isomer. Because of the significant color variations in solution, the simple dual‐addressable switch has been further successfully applied to construct a multicolor reversible display on paper.     

Reversible Control over Molecular Recognition in Surface‐Bound Photoswitchable Hydrogen‐Bonding Receptors: Towards Read–Write–Erase Molecular Printboards

The synthesis of an anthracene‐bearing photoactive barbituric acid receptor and its subsequent grafting onto azide‐terminated alkanethiol/Au self‐assembled monolayers by using an Cu^I‐catalyzed azide–alkyne reaction is reported. Monolayer characterization using contact‐angle measurements, electrochemistry, and spectroscopic ellipsometry indicate that the monolayer conversion is fast and complete. Irradiation of the receptor leads to photodimerization of the anthracenes, which induces the open‐to‐closed gating of the receptor by blocking access to the binding site. The process is thermally reversible, and polarization‐modulated IR reflection–absorption spectroscopy indicates that photochemical closure and thermal opening of the surface‐bound receptors occur in 70 and 100 % conversion, respectively. Affinity of the open and closed surface‐bound receptor was characterized by using force spectroscopy with a barbituric‐acid‐modified atomic force microscope tip.     

Solvent‐Induced Cluster‐to‐Cluster Transformation of Homoleptic Gold(I) Thiolates between Catenane and Ring‐in‐Ring Structures

Supramolecular ensembles adopting ring‐in‐ring structures are less developed compared with catenanes featuring interlocked rings. While catenanes with inter‐ring closed‐shell metallophilic interactions, such as d¹⁰–d¹⁰ Au^I–Au^I interactions, have been well‐documented, the ring‐in‐ring complexes featuring such metallophilic interactions remain underdeveloped. Herein is described an unprecedented ring‐in‐ring structure of a Au^I‐thiolate Au₁₂ cluster formed by recrystallization of a Au^I‐thiolate Au₁₀ [2]catenane from alkane solvents such as hexane, with use of a bulky dibutylfluorene‐2‐thiolate ligand. The ring‐in‐ring Au^I‐thiolate Au₁₂ cluster features inter‐ring Au^I–Au^I interactions and underwent cluster core change to form the thermodynamically more stable Au₁₀ [2]catenane structure upon dissolving in, or recrystallization from, other solvents such as CH₂Cl₂, CHCl₃, and CH₂Cl₂/MeCN. The cluster‐to‐cluster transformation process was monitored by ¹H NMR and ESI‐MS measurements. Density functional theory (DFT) calculations were performed to provide insight into the mechanism of the “ring‐in‐ring? [2]catenane” interconversions.