Positive dendritic effects on the electron-donating potencies of poly(propylene imine) dendrimers

[structure: see text] Two series of poly(propylene imine), PPI, dendrimers terminated with a redox-active donor, 4-dimethylaminobenzyl (4-DMAB), including their respective nondendronized model compounds, are reported. In these two series, a positive dendritic effect was observed for the formation of charge-transfer (CT) complexes between the dendrimers and 7,7,8,8-tetracyanoquinodimethane (TCNQ). However, the nondendronized compounds did not form CT complexes with TCNQ, even though their redox potentials are similar to those of the 4-DMAB units attached to the dendrimers.     

Polysulfurated pyrene-cored dendrimers: luminescent and electrochromic properties

We have synthesized a novel class of dendrimers, consisting of a polysulfurated pyrene core with appended poly(thiophenylene) dendrons (PyG0, PyG1, and PyG2, see Scheme 1), which exhibit remarkable photophysical and redox properties. In dichloromethane or cyclohexane solution they show a strong, dendron-localized absorption band with a maximum at around 260 nm and a band in the visible region with a maximum at 435 nm, which can be assigned to the pyrene core strongly perturbed by the four sulfur substituents. The dendrimers exhibit a strong (Phi=0.6), short-lived (tau=2.5 ns) core-localized fluorescence band with maximum at approximately 460 nm in cyclohexane solution at 293 K. A strong fluorescence is also observed in dichloromethane solution at 293 K, in dichloromethane/chloroform rigid matrix at 77 K, and in the solid state (powder) at room temperature. The dendrimers undergo reversible chemical and electrochemical one-electron oxidation with formation of a strongly colored deep blue radical cation. A second, reversible one-electron oxidation is observed at more positive potential values. The photophysical and redox properties of the three dendrimers are finely tuned by the length of their branches. The strong blue fluorescence and the yellow to deep blue color change upon reversible one-electron oxidation can be exploited for optoelectronic devices.     

Dendritic Molecular Electrochromic Batteries Based on Redox‐Robust Metallocenes

In this Concept article, we summarize and discuss recent reports on dendritic molecular electrochromic batteries. Giant dendrimers containing 3ⁿ⁺² terminal tethers (n=generation number) and terminated by first‐raw late‐transition‐metal metallocenes, permethyl metallocenes and other sandwich complexes were shown to be redox robust. Indeed, they can be oxidized and reduced without decomposition and exist under two stable oxidation states (Fe^III/II, Co^III/II). Thus, a pre‐determined number of electrons (up to 14 000) per dendrimer can be exchanged. Cyclic voltammetry showed a remarkable complete reversibility even up to 14 000 Fe and Co termini in metallodendrimers, indicating fast electron hoping among the redox sites and between dendrimers on a carbon surface covered by arylcarboxylate groups. The dendrimer sizes were measured by dynamic light scattering in solution and by AFM (subsequent to flattening in the condensed state also indicating that these metallodendrimers aggregate to form discrete nanoparticles of dendrimers, as atoms do). The metallodendrimer size varies considerably between the two redox forms due to tether extension of the cationic dendrimers upon oxidation, and a breathing mechanism was shown by atomic and electric force microscopy (AFM and EFM). When the redox potential is very negative, the reduced form is an electron‐reservoir system that can deliver a large number of electrons per dendrimer to various reducible substrates. These systems are thus potential dendritic molecular batteries with two different colors for the two redox forms (electrochromic behavior).     

Synthesis and photoluminescent properties of 1,1'-binaphthyl-based chiral phenylenevinylene dendrimers

New chiral, soluble binaphthyl derivatives that incorporate stilbenoid dendrons at the 6,6'-positions have been prepared. The synthesis of the new enantiopure dendrimers was performed in a convergent manner by Horner-Wadsworth-Emmons (HWE) reaction of the appropriately functionalized 1,1'-binaphthyl derivative (R)-1 and the appropriate dendrons (R)(2n)G(n)-CHO. Different electroactive units were incorporated in the peripheral positions of the dendrons in order to tune both the optical and electrochemical behavior of these systems. Fluorescence measurements on the chiral dendrimers reveal a strong emission with maxima between 409 and 508 nm depending upon the substitution pattern. Finally, the redox properties of the dendrimers were determined by cyclic voltammetry, showing the influence of the functional groups at the peripheral positions of the dendrimer on the redox behavior of these systems.     

Synthesis,Redox Activity of Rigid Ferrocenyl Dendrimers,and Isolation of Robust Ferricinium and Class‐II Mixed‐Valence Dendrimers

The coupling reactions of ethynylferrocene with trihalogenoarenes do not lead to ethynylferrocenyl arenes that are soluble enough to form the basis of a suitable construction of stiff ferrocenylethynyl arene‐cored dendrimers, which explains the previous lack of reports on stiff ferrocenyl dendrimers. However, rigid ferrocenyl‐terminated dendrimers have been synthesized from 1,3,5‐tribromo‐ and triiodobenzene through Sonogashira and Negishi reactions with 1,2,3,4,5‐pentamethyl‐1′‐ethylnylferrocene ( 1 a ), according to 1→2 connectivity. With compound 1 a , the construction of a soluble dendrimer ( 10 a ) that contained 12 ethynylpentamethylferrocenyl termini was achieved. Stiff dendrimer 10 a shows a single, reversible cyclic voltammetry (CV) wave (with adsorption), which disfavors the hopping heterogeneous electron‐transfer mechanism that is postulated for redox‐terminated dendrimers that contain flexible tethers. The selectivity of these Sonogashira reactions allows the synthesis of an arene‐cored dendron ( 2 c ) that contains both ethynylferrocenyl and 1,2,3,4,5‐pentamethyl‐ferrocenylethynyl redox groups, thus leading to the construction of a dendrimer ( 7 c ) that contains both types of differently substituted ferrocenyl groups with two well‐separated reversible CV waves. Upon selective oxidation, this mixed dendrimer ( 7 c ) leads to a class‐II mixed‐valence dendrimer, 7 c [PF₆]₃, as shown by Mössbauer spectroscopy, whereas oxidation of the related fully pentamethylferrocenylated dendrimer ( 7 a ) leads to the all‐ferricinium dendrimer, 7 a [PF₆]₆.     

Multifunctional triazolylferrocenyl Janus dendron: Nanoparticle stabilizer,smart drug carrier and supramolecular nanoreactor

Owing to their unique broken symmetry, amphiphilic Janus dendrimers and dendons provide fascinating properties for material, biological, pharmaceutical and biomedical applications. The integration of various organometallic moieties into these macromolecules will further offer the opportunity to form complex and intelligent architectures and materials. Here, we report a novel, simple and multifunctional Janus dendron containing redox‐reversible hydrophobic ferrocene (Fc) unit, complexing‐effective 1,2,3‐triazole ligand and biocompatible hydrophilic triethylene glycol termini. Silver and gold nanoparticles were firstly successfully prepared by using the Janus dendron as the reducing agent of Au(III) and Ag(I), and the stabilizer of the corresponding nanoparticles. The redox response of the Fc moiety was then employed to trigger the release of model drug, rhodamine B, encapsulated in supramolecular micelles formed by the self‐assembly of the Janus dendron. Finally, the precise and excellent metal‐complexing ability of the triazole group in this dendron was fully utilized to stabilize a water‐soluble Cu(I) catalyst, forming supramolecular nanoreactors for the catalysis of the copper(I)‐catalyzed azide alkyne cycloaddition click reaction in only water. The multifunctional characteristics of this dendron highlight the potential for organometallic Janus dendrimers and dendrons in the fields of functional materials and nanomedicines.     

Redox active, hybrid dendrimers containing Fréchet- and Newkome-type blocks

A new series of dendrimers was prepared by covalently attaching a Newkome dendron, a Fréchet dendron, and a redox active, aminoferrocene group to a central triazine core. Growth of the Newkome dendron has a more pronounced effect on the half-wave potential for the one-electron oxidation of the ferrocene residue than growth of the Fréchet dendron. All dendrimers show reversible or quasireversible voltammetric behavior at scan rates in the range 0.10-2.0 V s-1.     

Cobaltocenium-functionalized poly(propylene imine) dendrimers: redox and electromicrogravimetric studies and AFM imaging

The first four generations of cobaltocenium-functionalized, diaminobutane-based poly(propylene imine) dendrimers DAB-dend-Cb,(PFb)x (x = 4, 8, 16, and 32; Cb=[Co(eta5-C5H4CONH)(eta5-C5H5)] (1-4) have been synthesized and characterized. The redox activity of the cobaltocenium centers in 1-4 has been characterized by using cyclic voltammetry and the electrochemical quartz-crystal microbalance (EQCM). All of the dendrimers exhibit reversible redox chemistry associated with the cobaltocenium/cobaltocene redox couple. Upon reduction. the dendrimers exhibit a tendency to electrodeposit onto the electrode surface, which is more pronounced for the higher generations. Pt and glassy carbon electrodes could be modified with films derived from 1-4,exhibiting a well-defined and persistent electrochemical response. EQCM measurements show that the dendrimers adsorb, at open circuit, onto platinum surfaces at monolayer or submonolayer coverage. Cathodic potential scanning past -0.75 V at which the cobaltocenium sites are reduced, gave rise to the electrodeposition of multilayer equivalents of the dendrimers. The additional material gradually desorbs upon re-oxidation so that only a monolayer equivalent remains on the electrode surface. Changes in film morphology as a function of dendrimer generation and surface coverage were studied by using admittance measurements of the quartz-crystal resonator on the basis of its electrical equivalent circuit, especially in terms of its resistance parameter. In general, we find that films of the lower dendrimer generation 1 behave rigidly, whereas those of the higher generation 4 exhibit viscoelastic behavior with an intermediate behavior being exhibited by 2 and 3. Using tapping-mode atomic force microscopy (AFM). we have been able to obtain molecularly resolved images of dendrimer 4 adsorbed on a Pt(111) electrode.     

Voltammetric behavior of poly(amidoamine) dendrimers containing nitroxyl radical end groups

A series of fully functionalized poly(amidoamine) dendrimers with 4-carboxy-2,2,6,6-tetramethylpiperidin-1-yloxyl end groups were prepared. Cyclic voltammetric studies indicated that the pendant nitroxyl radical end groups are non-interacting electrochemically equivalent redox centers, which are oxidizable at the same potential. The results of controlled-potential electrolysis of the dendrimers showed that all the nitroxyl radical groups at the periphery of the dendrimers are accessible to the electrode surface for electron transfer.     

Investigation on critical aggregation concentration of carbosiloxane dendrimer in dilute solution probed by rhodamine B

We propose a novel method for probing aggregation of dendrimers by investigating the isomerization equilibrium between pink zwitterionic form (Z-form) and colorless lactonic form (L-form) of rhodamine B (RhB) molecules in dilute solution. Investigation using carbosiloxane dendrimers (CSiO-D) with different generations as the model dendrimer molecules showed that the equilibrium constant of isomerization of RhB increased dramatically at the critical aggregation concentration (CAC) of dendrimers. The redox potential differences between isomers of RhB indicated that aggregation of CSiO-D accelerated the isomerization of RhB and stabilized the L-form of RhB. The data on Gibbs energy and electrolytic conductivity provided further evidence for confirming the CAC of dendrimers in dilute solution and showed good agreement with our other experimental results. The proposed method is effective in estimating the CAC of dendrimers in dilute solution.     

A new series of dendrimers containing two ferrocenyl units and a Fréchet dendron around a triazine nucleus

The preparation and characterization of a new series of dendrimers containing two identical aminoferrocenyl units and a Fréchet dendron around a triazine nucleus is described. A sizable degree of electronic communication between the two redox active ferrocenyl residues was detected in dichloromethane solution containing tetrabutylammonium tetra(pentafluorophenyl)borate as the supporting electrolyte. Binding of glutarimide via hydrogen bonding interactions to all the dendrimers was easily detected by using (1)H NMR spectroscopic and voltammetric techniques.     

Hetero type III-B rotaxane dendrimers

Type III-B rotaxane dendrimer is a member of the rotaxane dendrimer family, but it is defined as dendritic polyrotaxane, branched through rings. Such rotaxane dendrimers have been synthesized through versatile copper-catalyzed azide-alkyne cycloaddition reactions. In order to incorporate more functionalities into type III-B rotaxane dendrimers, three different potential functionalities—azobenzene toward light switch, daisy chain toward higher degree of contraction, and extension and cyclobis (paraquat-p-phenylene) (CBPQT) toward redox chemistry—were combined into the original design of type III-B rotaxane dendrimers. Two azo-based G1/G2 and four new G1 and G2 hetero type III-B rotaxane dendrimers were synthesized and characterized by ¹H NMR spectroscopy and electrospray ionization mass spectrometry.     

Electron capture dissociation,electron detachment dissociation,and collision-induced dissociation of polyamidoamine (PAMAM) dendrimer ions with amino,amidoethanol, and sodium carboxylate surface groups

Here, we investigate the effect of the structure (generation) and nature of the surface groups of different polyamidoamine (PAMAM) dendrimers on electron-mediated dissociation, either electron capture dissociation (ECD) or electron detachment dissociation (EDD), and compare the fragmentation with that observed in collision-induced dissociation (CID). ECD and EDD of the PAMAM dendrimers resulted in simple mass spectra, which are straightforward to interpret, whereas CID produced complex mass spectra. The results show that electron-mediated dissociation (ECD and EDD) of PAMAM dendrimers does not depend on the nature of the surface group but tends to occur within the innermost generations. CID of the PAMAM dendrimers showed a strong dependence on the nature of the surface group and occurred mostly in the outer generation. The results demonstrate the potential utility of ECD and EDD as a tool for the structural analysis of PAMAM dendrimers.     

Properties of mixed alkanethiol-dendrimer layers and their applications in biosensing

We studied the properties of mixed alkanethiol-dendrimer layers on a gold support and their application in biosensing. We showed that properties of glucose sensor can be modified using a different ratio of 1-hexadecanethiol (HDT) and poly(amidoamine) dendrimer of first generation (G1). The cyclic voltammetry in the presence of the redox couple, Fe(CN)(6)(3-)/Fe(CN)(6)(4-), was used for estimating how effectively the layer blocks the redox probe's access to the electrode surface. A scanning electrochemical microscope (SECM) was used to image the resulting distribution of the organic compounds. We found that with increasing content of dendrimers, the integrity of the layers was improved.     

Ruthenium(II) dendrimers containing carbazole-based chromophores as branches

Three new luminescent and redox-active Ru(II) complexes containing novel dendritic polypyridine ligands have been synthesized, and their absorption spectra, luminescence properties (both at room temperature in fluid solution and at 77 K in rigid matrix), and redox behavior have been investigated. The dendritic ligands are made of 1,10-phenanthroline coordinating subunits and of carbazole groups as branching sites. The first and second generation species of this novel class of dendritic ligands (L1 and L2, respectively; see Figure 1 for their structural formulas) have been prepared and employed. The metal dendrimers investigated are [Ru(bpy)(2)(L1)](2+) (1; bpy = 2,2'-bipyridine), [Ru(bpy)(2)(L2)](2+) (2), and [Ru(L1)(3)](2+) (3; see Figure 2). For the sake of completeness and comparison purposes, also the absorption spectra, redox behavior, and luminescence properties of L1 and L2 have been studied, together with the properties of 3,6-di(tert-butyl)carbazole (L0) and [Ru(bpy)(2)(phen)](2+) (4, phen = 1,10-phenanthroline). The absorption spectra of the free dendritic ligands show features which can be assigned to the various subunits (i.e., carbazole and phenanthroline groups) and additional bands at lower energies (at lambda > 300 nm) which are assigned to carbazole-to-phenanthroline charge-transfer (CT) transitions. These latter bands are significantly red-shifted upon acid and/or zinc acetate addition. Both L1 and L2 exhibit relatively intense luminescence at room temperature in fluid solution (lifetimes in the nanosecond time scale, quantum yields of the order of 10(-2)-10(-1)) and at 77 K in rigid matrix (lifetimes in the millisecond time scale). Such a luminescence is assigned to CT states at room temperature and to phenanthroline-centered pi-pi triplet levels at 77 K. The room-temperature luminescence of L1 and L2 is totally quenched by acid or zinc acetate. The metal dendrimers exhibit the typical absorption and luminescence properties of Ru(II) polypyridine complexes. In particular, metal-to-ligand charge-transfer (MLCT) bands dominate the visible absorption spectra, and formally triplet MLCT levels govern the excited-state properties. Excitation spectroscopy evidences that all the light absorbed by the dendritic branches is transferred with unitary efficiency to the luminescent MLCT states in 1-3, showing that the new metal dendrimers can be regarded as efficient light-harvesting antenna systems. All the free ligands and metal dendrimers exhibit a rich redox behavior (except L2 and 3, whose redox behavior was not investigated because of solubility reasons), with clearly attributable reversible carbazole- and metal-centered oxidation and polypyridine-centered reduction processes. The electronic interaction between the carbazole redox-active sites of the dendritic ligands is affected by Ru(II) coordination.     

A dendrimer-based electron antenna: paired electron-transfer reactions in dendrimers with a 4,4'-bipyridine core and naphthalene peripheral groups

Paired electron transfers (ET) induced by the absorption of two photons by synthetic dendrimers are observed in first-, second-, and third-generation dendrimers comprised of a viologen-like core and an array of naphthalene peripheral groups. Flash photolysis and transient absorption techniques show that the yield of photoinduced double ET depends on laser intensity in the two largest dendrimers, NBV2(+2) and NBV3(+2). Their photochemical behavior thus requires an unusual multiphoton kinetic scheme. These dendrimers constitute the first synthetic models capable of multiple electron redox events deriving from a defined molecular architecture, thus mimicking natural light-collecting antenna systems.     

FeCp]+-induced hexafunctionalization of hexamethylbenzene with dendrons for the direct synthesis of redox-active iron-centered metallodendrimers

Phenol tri- and nonaallyl dendrons (3 and 7, respectively) were functionalized at the focal position to give the new triallyl dendrons 4 and 6 and the nonaallyl dendrons 11 and 13 that contain a iodoalkyl or a bromobenzyl termini. All these dendrons were used for the [FeCp]+-induced hexafunctionalization of hexamethylbenzene in [FeCp(eta6-C6Me6)][PF6] (1) under mild conditions in the presence of KOH. These reactions directly yielded the 18-allyl and 54-allyl dendrimers 9, 10, and 14 with a [FeCp(eta6-arene)]+ unit located at the dendrimer core. Cyclic voltammetry studies were recorded in THF and DMF with these metallodendrimers and compared with those of analogous dendrimers or complexes of smaller size that contain a [FeCp(eta6-arene)]+ unit at the core. The decreased rate of heterogeneous electron transfer when the dendritic size increases first disclosed by Diederich and Gross is confirmed. The variation of the redox potential of the Fe(II/I) redox system with increasing dendritic size is negligible even in a solvent of high dielectric constant such as DMF. This trend is attributed to fact that the involved "redox" orbital is buried on the metal center, well protected by the shell of alkyl chains (electron-reservoir nature), unlike in ferrocene. The chemical irreversibility increases in THF as the dendrimer size increases, due to more facile ligand substitution with THF at the 19-electron level when the chain bulk increases.     

New metallodendrimers containing an octakis(diphenylphosphino)-functionalized silsesquioxane core and ruthenium(II)-based chromophores

A new class of surface-modified dendrimers has been prepared by reactions of 8 equiv of the terpyridine-functionalized polyether monodendrons with a polyhedral oligomeric silsesquioxane (POSS) core. Subsequent reactions of these spherically shaped organic dendrimers with Ru(II)-based precursors afford photo- and redox-active metallodendrimers. These new dendrimers have been characterized using a combination of mass spectral analysis (MALDI-TOF/MS, ESI/MS, and FAB/MS), nuclear magnetic resonance (1H, 13C, 29Si, and 31P(1H) NMR), photophysical analyses (electronic absorption, emission, excited-state lifetime, and quantum yield) and electrochemical measurement (cyclic voltammetry). Specifically, 31P(1H) NMR is used to monitor the completion of reactions and the purity of dendrimers and metallodendrimers. These new metallodendrimers exhibit large extinction coefficients that coincide with the number of peripheral Ru(II)-based chromophores. With the use of (-CH2-Ph-tpy)RuII(bpy)2 type of chromophores, all metallodendrimers are found emissive at room temperature, with lifetimes in the range of 605-890 ns. Photophysical data also indicate similar steady-state emission maxima and single-exponential decay kinetics for all metallodendrimers, and the observed overall quantum yields of the G1, G2, and G3 metallodendrimers are found to be 14, 20, and 7 times higher than that of the monomeric model complex (CH3-Ph-tpy)Ru(bpy)2(PF6)2. Electrochemical studies reveal the presence of surface-confined species, in addition to the ligand-centered and metal-centered redox processes.     

Ferrocene encapsulated within symmetric dendrimers: a deeper understanding of dendritic effects on redox potential

Ferrocene has been encapsulated within a symmetric ether-amide dendritic shell and its redox potential monitored in a variety of solvents. The dendritic effect generated by the branched shell is different in different solvents. In less polar, non hydrogen bond donor solvents, attachment of the branched shell to ferrocene increases its E(1/2), indicating that oxidation to ferrocenium (charge buildup) becomes thermodynamically hindered by the dendrimer, a result explained by the dendrimer providing a less polar medium than that of the surrounding electrolyte solution. The effect of electrolyte concentration on redox potential was also investigated, and it was shown that the concentration of "innocent" electrolyte has a significant effect on the redox potential by increasing the overall polarity of the surrounding medium. Dendritic destabilization of charge buildup is in agreement with the majority of reported dendritic effects. A notable exception to this is provided by the asymmetric ferrocene dendrimers previously reported by Kaifer and co-workers, in which the branching facilitated oxidation, and it is proposed that in this case the dendritic effect is generated by a different mechanism. Interestingly, in methanol, the new symmetric ferrocene dendrimer exhibited almost no dendritic effect, a result explained by the ability of methanol to interact extensively with the branched shell, generating a more open superstructure. By comparison of all the new data with other reports, this study provides a key insight into the structure-activity relationships which control redox processes in dendrimers and also an insight into the electrochemical process itself.     

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.