Viologen Residue Research Articles

Prediction of cathodic E1/21 and E1/22 values for viologen-containing conjugated unimers and dimers from calculated pKb values of the aromatic substituents

A library of 18 conjugated, rod-like compounds with either one or two viologen residues were synthesized and characterized electrochemically. Containing up to 8 aromatic/heterocyclic rings in conjugation, the members of the library differ in the substitution pattern of electron-withdrawing or -donating groups on the aromatic substituents of the viologen units. The first and second half-wave potentials of each member were found to be linearly correlated with the calculated pKb values of the aromatic end-groups. This relationship will enable the half-wave potentials of related, novel, substituted viologen species to be predicted using a simple, empirical formula.

Preparation, Characterization, and Electrochemical Properties of a New Series of Hybrid Dendrimers Containing a Viologen Core and Fréchet and Newkome Dendrons

Five new 4,4'-bipyridinium (viologen) core dendrimers containing a Frechet (Fn, n = 1-3, first to third generation) and a Newkome (Nn, n = 1-3) dendron linked to each of the termini of the viologen residue were prepared and characterized. These macromolecules (FnNn) were prepared according to synthetic methodology already developed by our group. The electrochemical behavior of these dendrimers is characterized by the stepwise reduction of the viologen nucleus (V(2+)/V(+) and V(+)/V). The recorded half-wave potentials are affected by dendron growth in the three surveyed solvent media (dichloromethane, tetrahydrofuran, and acetonitrile). The size of the Newkome dendron has a more pronounced effect on the half-wave potentials than the size of the Frechet dendron. However, increasing the size of the Frechet dendron diminishes the magnitude of the cathodic potential shifts resulting from Newkome dendron growth. The largest dendrimers investigated (F1N3 and F2N3) exhibit quasi-reversible voltammetric behavior. The diffusion coefficients of these molecules were also determined using pulse gradient stimulated echo NMR techniques.

Encapsulation of Ferrocene and Peripheral Electrostatic Attachment of Viologens to Dimeric Molecular Capsules Formed by an Octaacid, Deep‐Cavity Cavitand

In aqueous media the deep-cavity cavitand octaacid 1 forms stable dimeric molecular capsules 1(2), which are stabilized by hydrophobic effects. In this work we investigate the binding interactions in aqueous solution between these capsules and the redox active guests, ferrocene (Fc) and three 4,4'-bipyridinium (viologen) dications: methyl viologen (MV(2+)), ethyl viologen (EV(2+)), and butyl viologen (BV(2+)). Using NMR spectroscopic and electrochemical techniques we clearly show that the hydrophobic Fc guest is encapsulated inside 1(2). An interesting effect of this encapsulation is that the reversible voltammetric response of Fc is completely eliminated when it resides inside the 1(2) capsular assembly, a finding that is attributed to very slow electrochemical kinetics for the oxidation of Fc@1(2). Diffusion coefficient measurements (PGSE NMR spectroscopy) reveal that all three viologen guests are strongly bound to the dimeric capsules. However, the (1)H NMR spectroscopic data are not consistent with encapsulation and the measured diffusion coefficients indicate that two viologen guests can strongly associate with a single dimeric capsule. Furthermore, the (V(2+))(2)*1(2) complex is capable of encapsulating ferrocene, clearly suggesting that the viologen guests are bound externally, via coulombic interactions, to the anionic polar ends of the capsule. The electrochemical kinetic rate constants for the reduction of the viologen residue in the V(2+)*1(2) complexes were measured and found to be substantially lower than those for the free viologen guests.

A Host-Induced Intramolecular Charge-Transfer Complex and Light-Driven Radical Cation Formation of a Molecular Triad with Cucurbit[8]uril

The host-guest chemistry of systems containing a molecular triad Ru(bpy) 3-MV (2+)-naphthol complex (denoted as Ru (2+)-MV (2+)-Np, 1) and cucurbit[8]uril (CB[8]) is investigated by NMR, ESI-MS, UV-vis, and electrochemistry. The Ru (2+)-MV (2+)-Np guest and CB[8] host can form a stable 1:1 inclusion complex, in which the naphthalene residue is back-folded and inserted together with the viologen residue into the cavity of CB[8]. The selective binding of Ru (2+)-MV (2+)-Np guest with beta-CD and CB[8] host is also investigated. We find that CB[8] binds the Ru (2+)-MV (2+)-Np guest stronger than beta-CD. Upon light irradiation, a MV (+*) radical cation stabilized in the cavity of CB[8] accompanied by the naphthalene residue has been observed. This novel system may open a new way for design and synthesis of photoactive molecular devices.

Host−Guest Chemistry and Light Driven Molecular Lock of Ru(bpy)3−Viologen with Cucurbit[7−8]urils

Host-guest chemistry and photoinduced electron-transfer processes have been studied in the systems containing Ru(bpy)3 complex covalently linked to viologen as a guest molecule and cucurbit[n]urils (n = 7, 8) as host molecules in aqueous solution. The Ru(bpy)3-viologen complex, [Ru(2,2'-bipyridine)2(4-(4-(1'-methyl-4,4'-bipyridinediium-1-yl)butyl)-4'-methyl-2,2'-bipyridine)]Cl4 (denoted as Ru2+-MV2+, 1) was shown to form stable 1:1 inclusion complexes with cucurbit[7]uril (CB[7]) and cucurbit[8]uril (CB[8]). The binding modes are slightly different with CB[7] and CB[8]. CB[7] preferentially binds to part of the viologen residue in 1 together with the butyl chain, whereas CB[8] preferentially encloses the whole viologen residue. Photoinduced intramolecular electron transfer from the excited-state of the Ru moiety to MV2(+) which is inserted into the cavity of the CBs occurred. Long-lived charge-separated states Ru3(+)-MV(+*) were generated with the lifetimes of 280 ns with CB[7] and 2060 ns with CB[8]. This shows that CBs can slow down the charge recombination within supramolecular systems, and the difference in lifetimes seems to be due to the difference in binding modes. In the presence of a sacrificial electron donor triethanolamine, light-driven formation of a dimer of MV(+*) inside the CB[8] cavity was observed. This "locked" molecular dimer can be "unlocked" by molecular oxygen to give back the original form of the molecular dyad 1 with the MV2(+) moiety inserted in the cavity of CB[8]. The processes could be repeated several times and showed nice reversibility.

Electrochemical and Guest Binding Properties of Fréchet- and Newkome-Type Dendrimers with a Single Viologen Unit Located at Their Apical Positions

Several new series of dendrimers containing a single redox-active 4,4'-bipyridinium (viologen) residue were synthesized and characterized. In these dendrimers, the viologen group is covalently attached to the apical position of a Newkome- or Frechet-type dendron, ranging in size from first to third generation of growth. The half-wave potentials corresponding to the two consecutive one-electron reductions of the viologen residue are affected by the size of the dendritic component. The size effects are more pronounced in the Newkome-type dendrimers and seem to result from the polarity contrast between the microenvironments provided by the solution and the internal phase of the dendrimer. Unlike in many other dendrimers having a redox-active core, the voltammetric behavior remains fast (reversible) even in third generation dendrimers. Pulse gradient stimulated echo NMR diffusion coefficient measurements on the Newkome-type dendrimers reveal that their hydrodynamic radii are relatively invariant in solvents of widely different polarities (dichloromethane to dimethyl sulfoxide). The host-guest binding interactions between the viologen residue in these dendrimers and the crown ether host bis-p-phenylene-34-crown-8 were also investigated. While in Newkome-type dendrimers the growth of the dendron caused a substantial attenuation of the binding constant values, this size effect was not observed in the Frechet-type dendrimers. These electrochemical and binding measurements underscore some of the structural differences between these two common types of dendritic architectures.

Unusual electrochemical properties of unsymmetric viologen dendrimers.

A new series of redox-active dendrimers containing a single 4,4'-bipyridinium (viologen) group covalently attached to the focal point of Newkome-type dendrons (first to third generation) has been prepared and characterized. The electrochemical properties of these unsymmetric dendrimers show two unusual aspects. First, the electrochemical kinetics for viologen reduction remains fast from the first to the third dendrimer generation. Second, the values of the half-wave potentials reveal that dendrimer growth favors the generation of positive charge in the viologen residue in CH(2)Cl(2), THF, and CH(3)CN solutions, while slightly disfavors it in DMSO.

Synthesis and electrochemical properties of cavitands functionalized with 4,4′-bipyridinium units

Four new brominated resorcinarene cavitand derivatives ( 3– 6 ) and the already known tetrakis(bromomethyl)cavitand 2 have been synthesized, starting from the tetramethylcavitand 1 . These brominated cavitands constitute excellent building blocks for the covalent assembly of new structures containing one ( 8 ), two ( 9 and 10 ), three ( 11 ) and four ( 7 ) monoquaternized bipyridine units attached to the upper rim of the cavitand bowl. The free terminal nitrogens in these structures can be methylated to yield compounds with one to four 4,4′-bipyridinium (viologen) units attached to the cavitand ( 12– 16 ), or treated with monobromocavitand 3 to produce oligomeric structures containing from two to five cavitand units connected by viologen residues ( 17– 21 ). Compounds 8– 16 , 18 and 20 show the electrochemical reactivity anticipated from their reducible units (mono- or diquaternized bipyridines) with no detectable level of electronic communication between them. Surprisingly, compounds 17 , 19 and 21 exhibit more complicated electrochemical behavior, which simplifies and becomes more reversible as the temperature increases.

Enhancement effects of l-tyrosine esters on photosensitized charge separation using ruthenium(II) complex- and viologen-containing polymers

Ruthenium(II) complex- and viologen-containing polymers, which consist of the bis(2,2′-bipyridine) ruthenium(II) complex coordinated with imidazolyl residues on poly(1-vinylimidazole) (RuVPIm) and partially quaternized poly(1-vinylimidazole) (RuVQPIm) and viologen covalently linking these polymers through a hexylene spacer, have been synthesized. These polymers have two kinds of ruthenium(II) complex residues, the 557Ru(II) complex compartmentalized by the viologen residues and polymer backbone and a 640Ru(II) complex exposed to the bulk solution, which show emission maxima at 557 and 640 nm, respectively. The influence of l-tyrosine esters with hexyl (C 6Tyr), octyl (C 8Tyr), and dodecyl (C 12Tyr) groups on the luminescence properties and photosensitized charge separation has been investigated in methanol. The luminescence intensities of these polymers decreased with the addition of the l-tyrosine esters, indicating that the quenching of the ruthenium(II) complex residues increased. The initial rates of the viologen radical formation was enhanced by the addition of the l-tyrosine esters. These results were attributed to a mediated effect of the l-tyrosine esters which acted as a mediator for photoinduced electron transfer from the photoexcited ruthenium(II) complex to viologen residue. Furthermore, the l-tyrosine esters undergoing interaction with a solvophobic domain where 557Ru(II) complex residues were compartmentalized acted as the mediator for only the quenching reaction, while they undergo an interaction with the polymer backbone mediating both the quenching reaction and the back reaction in the photosensitized charge separation.

Photosensitized charge separation using ruthenium(II) complex and viologen-containing polymers — effects of quaternization and length of alkyl side-chain

A series of the ruthenium(II) complex and viologen-containing partially quaternized poly(1-vinylimidazole)s with various degrees of quaternization (RuVQPIms) and various lengths of the alkyl side-chains [CnRuVQPIms: n=4(butyl),8(octyl),12(dodecyl),16(hexadecyl)] have been synthesized and characterized using UV–Vis absorption and luminescence spectroscopies. The effects of quaternization and length of the alkyl side-chains on the photosensitized charge separation using these metallopolymers have been investigated in methanol. The photosensitized charge separation reaction took place through an intrapolymer process for all systems. In the cases of the RuVQPIms having a low degree of quaternization and short alkyl side-chain, the initial rate of the viologen radical formation linearly depended on the polymer concentration with two stages, while the RuVQPIm having a high degree of quaternization and long alkyl side-chain showed only one-tage dependence. Further, the rate of the second stage was larger than that of the first stage. The dependence with two stages would be caused by aggregation at a high polymer concentration through the interpolymer van der Waals interaction. The stronger interaction induced by increases in the number and length of the alkyl side-chains on the polymer backbone decreased the flexibility of the viologen residues; consequently, the enhancement of the rate in the second stage was not observed.

Photosensitized charge separation using partially quaternized poly(1-vinylimidazole)-bound ruthenium(II) complex and viologen

Photosensitized charge separation using metallopolymers with both a photosensitizer and an electron acceptor on the same polymer backbone has been investigated in methanol. These metallopolymers consist of the bis(2,2′-bipyridine)ruthenium(II) complex as the photosensitizer and alkylviologen as the electron acceptor linked covalently to the imidazolyl residues on partially quaternized poly(1-vinylimidazole) through a hexylene spacer (RuQPImC 6VC 16) or a butylene spacer (RuQPImC 4VC 16). The viscosity and luminescence studies showed that these polymers, having two luminescence maxima at 560 and 650 nm, underwent interpolymer interaction between alkyl side chains through van der Waals interaction with increasing concentration of the polymer. The interpolymer interaction was weak at low concentration of the ruthenium(II) complex ([Ru(II)]<ca. 2.0×10 mol dm −3), which hardly affected the luminescence properties, while at high concentration, it became strong, which led to acceleration of the quenching of ruthenium(II) complex residue with the viologen residue. For both the RuQPImC 4VC 16 and RuQPImC 6VC 16, the rates of viologen radical formation increased with increasing concentration of the ruthenium(II) complex residue in two stages. Such a unique dependence can be explained by the interpolymer van der Waals interaction. Furthermore, it was found that the interpolymer van der Waals interaction depended significantly on length of the alkylene spacer.

Intra-polymer photosensitized charge separation using a series of ruthenium(II) complex and viologen-containing polymers

Spectroscopic and thermal properties of a series of ruthenium(II) complex and viologen-containing polymers based on poly( 1-vinylimidazole) and partially quaternized poly( 1-vinylimidazole) and photosensitized charge separation using these polymers have been investigated. These polymers have the structures in which bis(2,2-bipyridine)rutneuium(II) complexes are coordinated with the imidazolyl residues and the alkyl viologen is covalently linked to the imidazolyl residue through a hexylene spacer on the same polymer backbone. These polymers have two maximum wavelengths in the luminescence spectra. 560 nm and 65() nm, which correspond to emissions of the ruthenium( II) complex residue compartmentalized by viologen residues and the alkyl side chains exposed to the bulk solution, respectively. The luminescence intensity at 560 nm decreases with decreasing viologen content. For photosensitized charge separation, the rates of viologen radical formation depend linearly on the initial concentration of the ruthenium(II) complex and viologen residue, indicating that the reactions proceed through an intra-polymer process.

Synthesis and characterization of a partially quaternized poly(1-vinylimidazole) bound to a ruthenium(II) complex and a viologen moiety

Three metallopolymers having both a photosensitizer and an electron acceptor on the same polymer backbone were prepared and characterized by UV-Vis absorption and fluorescence spectroscopies and thermal measurements as differential scanning calorimetry (DSC) and thermogravimetric differential analysis (TG/DTA). These metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complexes as a photosensitizer and a viologen species as an electron acceptor linked covalently to the imidazolyl residues on partially quaternized poly(1-vinylimidazole) through an alkyl spacer. DSC curves showed a glass transition temperature at ca. 230°C and an exothermic peak around 170°C. The exothermic peak was assigned to the cleavage of the alkyl spacer between the viologen and the quaternized imidazolium segment. UV-vis absorption spectra showed that these polymers have three absorption maxima, the metal-to-ligand charge transfer band (MLCT) of the ruthenium(II) complex residue being in the visible region and two electronic transition bands deriving from the viologen residue and the imidazolyl ring in the UV region. These absorption maxima are independent of the length of the alkyl group in the viologen residues and the spacer between the imidazolyl and the viologen segment. However, the luminescence spectra depend significantly on the length of alkyl group and the spacer, demonstrating that part of the ruthenium(II) complex residues are quenched by viologen residues. Furthermore, quenching studies revealed that these polymers contain at least four ruthenium(II) complex residues on the same polymer backbone.

Synthesis and characterization of a partially quaternized poly(1-vinylimidazole) bound to a ruthenium(II) complex and a viologen moiety

Three metallopolymers having both a photosensitizer and an electron acceptor on the same polymer backbone were prepared and characterized by UV-Vis absorption and fluorescence spectroscopies and thermal measurements as differential scanning calorimetry (DSC) and thermogravimetric differential analysis (TG/DTA). These metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complexes as a photosensitizer and a viologen species as an electron acceptor linked covalently to the imidazolyl residues on partially quaternized poly(1-vinylimidazole) through an alkyl spacer. DSC curves showed a glass transition temperature at ca. 230°C and an exothermic peak around 170°C. The exothermic peak was assigned to the cleavage of the alkyl spacer between the viologen and the quaternized imidazolium segment. UV-vis absorption spectra showed that these polymers have three absorption maxima, the metal-to-ligand charge transfer band (MLCT) of the ruthenium(II) complex residue being in the visible region and two electronic transition bands deriving from the viologen residue and the imidazolyl ring in the UV region. These absorption maxima are independent of the length of the alkyl group in the viologen residues and the spacer between the imidazolyl and the viologen segment. However, the luminescence spectra depend significantly on the length of alkyl group and the spacer, demonstrating that part of the ruthenium(II) complex residues are quenched by viologen residues. Furthermore, quenching studies revealed that these polymers contain at least four ruthenium(II) complex residues on the same polymer backbone.

Synthesis and characterization of a partially quaternized poly(1-vinylimidazole) bound to a ruthenium(II) complex and a viologen moiety

Three metallopolymers having both a photosensitizer and an electron acceptor on the same polymer backbone were prepared and characterized by UV-Vis absorption and fluorescence spectroscopies and thermal measurements as differential scanning calorimetry (DSC) and thermogravimetric differential analysis (TG/DTA). These metallopolymers consist of bis(2,2′-bipyridine)ruthenium(II) complexes as a photosensitizer and a viologen species as an electron acceptor linked covalently to the imidazolyl residues on partially quaternized poly(1-vinylimidazole) through an alkyl spacer. DSC curves showed a glass transition temperature at ca. 230°C and an exothermic peak around 170°C. The exothermic peak was assigned to the cleavage of the alkyl spacer between the viologen and the quaternized imidazolium segment. UV-vis absorption spectra showed that these polymers have three absorption maxima, the metal-to-ligand charge transfer band (MLCT) of the ruthenium(II) complex residue being in the visible region and two electronic transition bands deriving from the viologen residue and the imidazolyl ring in the UV region. These absorption maxima are independent of the length of the alkyl group in the viologen residues and the spacer between the imidazolyl and the viologen segment. However, the luminescence spectra depend significantly on the length of alkyl group and the spacer, demonstrating that part of the ruthenium(II) complex residues are quenched by viologen residues. Furthermore, quenching studies revealed that these polymers contain at least four ruthenium(II) complex residues on the same polymer backbone.