Preparation, characterization, and electrochemical properties of a new series of hybrid dendrimers containing a viologen core and Frechet 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.     

Synthesis and optical and electrochemical properties of glucose-cored ferrocenyl dendrimers

Triazole-based ferrocenyl glycoconjugates 1, 2, and 3 were synthesized by regiospecific copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC) of azidoferrocenyl derivatives with glucose pentaacetylide. Higher generation ferrocenyl glycoconjugates form the stable ferrocenium cation and exhibit increased light harvesting property as revealed from cyclic voltammogram studies and ultraviolet–visible spectrum respectively due to the presence of more ferrocenyl and triazolyl units than the lower generation dendrimer.     

Thermal behaviour and electrochemical properties of bis(trifluoromethanesulfonyl)amide and dodecatungstosilicate viologen dimers

We report the synthesis and characterization of dimeric viologen salts (1',1'-(alkane-1,n-diyl)bis(1-ethyl-4,4'-bipyridinium) with n = 4-10) with bis(trifluoromethanesulfonyl)amide (bistriflimide, Tf(2)N(-)) as a counteranion. For n = 4, 5 and 6, and for the nonylviologen cation (1,1'-dinonyl-4,4'-bipyridinium) we also prepared salts with the totally inorganic dodecatungstosilicate anion, SiW(12)O(40)(4-), featuring a poly-charged surface and nanosized dimensions. The materials have been characterized by means of calorimetric techniques, X-ray diffraction and solid state NMR and a comparison is made with analogous monomeric viologen salts exhibiting smectic mesophases. A strong odd-even effect is observed in the melting points and in the thermal behaviour of the bistriflimide dimeric systems, similar to what was reported for dipolar calamitic liquid crystal dimers, although the studied viologen dimers are not mesomorphic. By increasing the size of the counteranion we have observed a destabilization of the crystal phases and of the mesophases in favour of a glassy amorphous state. Implications on the design of novel ionic liquid crystals are discussed. The electrochemical behaviour in solution has been investigated by cyclic voltammetry measurements: interestingly, the odd-even effect is clearly visible also in the redox potentials. The spin-pairing of the viologen radical cations formed at each end of the dimer is responsible for the observed redox trend. Insights on the structure of the spin-paired dimer have been obtained by DFT calculations.     

Functional Janus dendrimers containing carbazole with liquid crystalline,optical and electrochemical properties

ABSTRACT

Novel liquid crystalline Janus dendrimers that combine a mesogenic block and an electroactive block have been synthesised. The mesogenic block is based on two third-generation Percec-type dendrons bearing six or eight terminal dodecyloxy alkyl chains, whereas the electroactive blocks are formed by one or two carbazole units. The liquid crystal behaviour was investigated by polarised-light optical microscopy, differential scanning microscopy and X-ray diffraction. The Janus dendrimers with one electroactive unit exhibited cubic or columnar liquid crystal phases, whereas the Janus dendrimers with two electroactive units did not show liquid crystalline behaviour. The UV-vis absorption and emission properties of the Janus dendrimers were investigated. The spectra suggested the existence of π-π stacking and the formation of aggregates in the solid state. Electrodeposition of the carbazole-containing dendrimers afforded semi-globular particles in which the number of electropolymerizable units and the flexible or rigid character of the linker have a decisive influence in the particle size.     

Electrochemical and guest binding properties of Frechet- 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.     

Synthesis,photophysical, and electrochemical properties of triazolyl dendrimers with thiazolylchalcone surface unit

Synthesis of novel triazolyl dendrimers with thiazolylchalcone surface group and triazole as bridging unit has been achieved in good yields by click chemistry through convergent approach. All the triazolyl dendrimers showed an absorption band between 293 and 336?nm and an emission band between 430 and 435?nm. The intensity of absorption and emission bands increases on increasing the number of dendritic wedges and triazole units. Triazolyl dendrimers exhibited quasi-reversible behavior in cyclic voltammetry. A shift in the reversible potential was observed in cyclic voltammetry on increasing the dendritic wedges in the triazolyl dendrimers.     

Synthesis and electrochemical characterization of donor-acceptor phenylazomethine dendrimers

The synthesis of the first donor-acceptor phenylazomethine dendrimers (4 and 5) is described. A convergent method is used via the condensation of aromatic ketones with an appropriately functionalized tri(aminophenyl)-s-triazine promoted by titanium (IV) chloride. Cyclic voltammetry investigations show a donor-acceptor behaviour due to the presence of the electron acceptor s-triazine core and the donor ability of the conjugated peripheral butoxybenzene units.     

Investigating carbazole jacketed precursor dendrimers: sonochemical synthesis, characterization, and electrochemical crosslinking properties

Precursor carbazole terminated dendrons and dendrimers up to generation four (G4-D) were synthesized using a convergent approach. Sonication as a means of facilitating organic reactions in dendrimer chemistry was explored resulting in very facile and very fast (up to 50x) reaction times compared to those using traditional reflux conditions. The limits of peripheral group functionality were explored as a function of generation. The electrochemical cross-linking of the dendrimers as thin films revealed unusual cyclic voltammetry (CV) behavior depending upon the generations, which were significantly different from their linear counterpart, Poly(N-vinylcarbazole) (PVK). G1-D showed a higher extent of intermolecular cross-linking while G4-D showed a higher extent of intramolecular cross-linking. The formed films were optically clear and possess superior energy band gap properties making them an alternative candidate over PVK for future hole-transport layer materials in electro-optical devices.     

Synthesis and properties of aramid dendrimers

The synthesis, characterisation and properties of simple low molecular weight aromatic amide dendrimers is described. The insights into the problems inherent in the synthesis of such systems provided by molecular modelling studies are discussed along with possible strategies for circumventing them. Preliminary results concerning blends of aramid dendrimers with poly(hexamethyleneadipamide) are presented.     

Spectral additive properties of phosphorus-containing dendrimers

The FT-Raman and FTIR spectra of 12 generations of phosphorus-containing dendrimers:

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with terminal aldehyde and PCl groups have been recorded and analyzed. Their spectral pattern is determined by the ratio T/R (T, the number of terminal groups; R, the number of repeating units). The influence of the encirclement on the band frequencies and intensity was studied and due to the predictable, controlled and reproducible structure of dendrimers the information usually inaccessible was obtained. Bands assigned to the core, repeated units and terminal groups of dendrimers were separated by the differential spectroscopy method. The strong band at 1600 cm⁻¹ show marked changes of optical density in dependence of the aldehyde (CHO) or azomethyne (CHN) substitution in aromatic ring. From the differential IR and Raman spectra of dendrimers it follows that for the generations higher than 6, the steric congestion disturbs the conformations of the terminal groups. The rather rigid repeated units with little conformational flexibility define the perfect microstructure of phosphorus-containing dendrimers up to 11 generation. FT-Raman and FTIR spectroscopy provides the unique detailed information about the structure of technologically relevant materials, which could not be obtained before with any other technique.     

Synthesis, characterization and electrochemical behavior of unsymmetric transition metal-terminated biphenyl ethynyl thiols

The synthesis of the biphenyl alkynyl thiols and thioesters R′-CC-C₆H₄-C₆H₄-SR (3: R′ = SiMe₃, R = C(O)Me; 4: R′ = SiMe₃, R = H; 5: R′ = H, R = C(O)Me) from I-C₆H₄-C₆H₄-SC(O)Me (1) is described. Molecules 1 and 5 have been used as starting materials in the synthesis of mono- and heterobimetallic transition metal complexes of type L_nM′-CC-C₆H₄-C₆H₄-SR (7: L_nM′ = Fc, R = C(O)Me; 8: L_nM′ = Fc, R = H; 10: L_nM′ = (Ph₃P)Au, R = C(O)Me; 14: L_nM′ = FcPPh₂-Au, R = C(O)Me; Fc = (η⁵-C₅H₅)(η⁵-C₅H₄)Fe; FcPPh₂ = (η⁵-C₅H₅)(η⁵-C₅H₄PPh₂)Fe). While complex 7is accessible by the Sonogashira cross-coupling of Fc-CCH (6) with 1, molecules 10 and 14 can be prepared by treatment of the thioester 5 with (Ph₃P)AuCl (9) and FcPPh₂-AuCl (13), respectively.The molecular solid state structures of 3, 7, 10 and 13-15 have been determined by single crystal X-ray crystallographic analysis. Typical features of these species are their linear M-CC-C₆H₄-C₆H₄-SR structure and the lack of coplanarity of the biphenyl arene rings. The overall length of these complexes are 13.345(2) Å for 3 (molecule A), 15.146(3) Å for 7, 15.705(2) Å for 10 (molecule A) and 15.649(4) Å for 14. The thioester groups are pointing away from the ferrocene building block. In 7 a linear 1D chain is set-up by π-interactions between two independent molecules of 7. Characteristic for 15 is the formation of a Au₂I₂ ring, while 13 is monomeric.All compounds were studied with cyclic voltammetry. Characteristic are the reversible ferrocene Fe(II)/Fe(III) redox wave, the irreversible reduction of Au(I) to Au(0), the oxidative cleavage of the S-C(O)Me sulfur-carbon (3, 5, 7, 10 and 14) and of the sulfur-hydrogen bond (4 and 8), respectively. Electronic effects extending from the -SH-end group to the ferrocene unit resulting in considerable shifts of the redox potential of the latter entity are found. Coordination of Au(I) at the FcPPh₂ moiety also results in a shift of the redox potential of the ferrocene group indicative of an electron withdrawing effect of the Au(I) species.     

Host-guest complexes between an aromatic molecular tweezer and symmetric and unsymmetric dendrimers with a 4,4'-bipyridinium core

We have investigated the spectroscopic and electrochemical behavior of symmetric and unsymmetric first-, second-, and third-generation dendrimers comprising an electron-acceptor 4,4'-bipyridinium core (viologen type) and electron-donor 1,3-dimethyleneoxybenzene (Fréchet-type) dendrons. The quite strong fluorescence of the symmetrically and unsymmetrically disubstituted 1,3-dimethyleneoxybenzene units of the dendrons is completely quenched as a result of donor-acceptor interactions that are also evidenced by a low-energy tail in the absorption spectrum. In dichloromethane solution, the 4,4'-bipyridinium cores of the investigated dendrimers are hosted by a molecular tweezer comprising a naphthalene and four benzene components bridged by four methylene units. Host-guest formation causes the quenching of the tweezer fluorescence. The association constants, as measured from fluorescence and (1)H NMR titration plots, (i) are of the order of 10(4) M(-1), (ii) decrease on increasing dendrimer generation, and (iii) are slightly larger for the unsymmetric than for the symmetric dendrimer of the same generation. The analysis of the complexation-induced shifts of the temperature-dependent (1)H NMR signals of the host and guest protons confirms that the bipyridinium core is positioned inside the tweezer cavity and allows the conclusions that (i) shuttling of the tweezer from one to the other pyridinium ring is fast (DeltaG < 10 kcal/mol), (ii) in the case of the unsymmetric dendrimers, the less substituted pyridinium ring is preferentially complexed in apolar solvents, and (iii) complexation of the 4,4'-bipyridinium core proceeds by clipping for the symmetric dendrimers and by threading in the case of unsymmetric ones. Host-guest formation causes a displacement of the first reduction wave of the 4,4'-bipyridinium unit toward more negative potential values, whereas the second reduction wave is unaffected. These results show that the host-guest complexes between the tweezer and the dendrimers are stabilized by electron donor-acceptor interactions and can be reversibly assembled/disassembled by electrochemical stimulation.     

Self‐assembly of a new class of amphiphilic poly(amidoamine) dendrimers and their electrochemical properties

Amphiphilic poly(amidoamine) (PAMAM) dendrimers consisting of a hydrophilic dendrimer core and hydrophobic aromatic dansyl or 1‐(naphthalenyl)‐2‐phenyldiazene (NPD) shells have been synthesized. These amphiphilic dendrimers from the zero generation to the third generation self‐assemble into vesicular aggregates in water. The self‐assembly behavior of these dendrimers strongly depends on their generations. The generation dependence has been further investigated by an exploration of their electrochemical properties. For the PAMAM–NPD aggregates, the photoisomerization process leads to a change in the aggregate size. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 5512–5519, 2005     

Synthesis and photophysical properties of phosphole-cored dendrimers

Phosphole-cored dendrimers having poly(benzyl ether) units through the third generation have been synthesized. The dendrimers display intense blue photoluminescence, the quantum yield increasing with the increasing generation of the dendron units. The optical properties are easily tuned by oxidation of the phosphorus atom of the phosphole ring.     

Synthesis and electrochemical properties of dendrimers containing meta-terphenyl peripheral groups and a 4,4′-bipyridinium core

Fréchet-type poly(arylether) dendrons carrying m-terphenyl peripheral groups were synthesized up to second generation by convergent methodology. Simple quarternisation of 4,4′-bipyridine with the dendritic bromides afforded the corresponding dendrimers containing a 4,4′-bipyridine core. The electrochemical parameters were obtained for all the dendrimers and the half-wave potentials of both the first and second redox processes shift to less-negative values as the dendrimer generation increases.     

Specific vapor sorption properties of phosphorus-containing dendrimers

Specific combination of guest sorption properties was observed for phosphorus-containing dendrimers, which distinguish them from ordinary polymers and clathrate-forming hosts. The sorption capacity for 30 volatile guests, binding reversibility, guest desorption kinetics and guest exchange, glass transition behavior and ability to be plasticized with guest were studied for phosphorus dendrimers of different generations (G(1)-G(4) and G(9)) using quartz crystal microbalance sensor, FTIR microspectroscopy, atomic force microscopy, simultaneous thermogravimetry and differential scanning calorimetry combined with mass-spectrometry of evolved vapors. The dendrimers were found to have a different selectivity for different homological series of guests, high glass transition points without plasticization with guest even at high temperatures and saturation levels, moderate guest-binding irreversibility and ability both for effective guest exchange and independent guest sorption. These properties constitute an advantage of the studied dendrimers as receptor materials in various applications.     

Complexes of unsymmetric bis-hydrazide ligands: crystal structures and properties

Two unsymmetric bis-aroyl-hydrazines, N′-(2-hydroxybenzoyl)isonicotinohydrazide (L1) and N′-(2-hydroxybenzoyl)nicotinohydrazide (L2), were synthesized through reactions of salicyl hydrazide with isonicotinoyl chloride and nicotinoyl chloride, respectively. Reactions of metal salts with L1 or L2 gave three new complexes, [Cd(L1)₂(SCN)₂] _n (1), [Zn(L1)₂Cl₂]?·?H₂O (2), and [Zn(L2)₂Cl₂] (3). Complex 1 features a 1-D double-chain structure built by SCN^– bridging six-coordinate Cd^II centers while 2 and 3 are mononuclear Zn^II complexes. In 1–3, isomeric ligands L1 and L2 coordinate with metal ions in a terminal coordination mode. Ligands L1 and L2 through O–H···N and N–H···O hydrogen-bonding interactions in 1–3 are crucial for the structure extension into 3-D supramolecular structures of 1 and 2, or 2-D sheet of 3. Complexes 1–3 emit interesting blue-green luminescence. Thermal behaviors of 1–3 as well as the specific rotation of 2 were also investigated.     

Syntheses and NLO properties of metal alkynyl dendrimers

Metal-containing (particularly organometallic) dendrimers are briefly introduced, the origin of nonlinear optical (NLO) effects in molecules is briefly summarized, and the syntheses of metal alkynyl dendrimers and progress in employing metal alkynyl dendrimers as NLO materials are reviewed.