Molecular triads composed of ferrocene,C60, and nitroaromatic entities: electrochemical,computational, and photochemical investigations

Synthesis and physicochemical characterization of a series of molecular triads composed of ferrocene, C(60), and nitroaromatic entities are reported. Electrochemical studies revealed multiple redox processes involving all three redox active ferrocene, C(60), and nitrobenzene entities. Up to eight redox couples within the accessible potential window of o-dichlorobenzene containing 0.1 M (TBA)ClO(4) are observed. A comparison between the measured redox potentials with those of the starting compounds revealed absence of any significant electronic interactions between the different redox entities. The geometric and electronic structure of the triads are elucidated by using ab initio B3LYP/3-21G methods. In the energy-optimized structures, as predicted by electrochemical studies, the first HOMO orbitals are found to be located on the ferrocene entity, while the first LUMO orbitals are mainly on the C(60) entity. The coefficients of the subsequent LUMO orbitals track the observed site of electrochemical reductions of the triads. The photochemical events of the triads are probed by both steady-state and time-resolved techniques. The steady-state emission intensities of the triads and the starting dyad, 2-(ferrocenyl)fulleropyrrolidine, are found to be completely quenched compared to fulleropyrrolidine bearing no redox active substituents. The subpicosecond and nanosecond transient absorption spectral studies revealed efficient charge separation (and rapid charge recombination) in the triads, and this has been attributed to the close spacing of the redox entities of the triad to one another.     

Mimicking photosynthetic antenna-reaction-center complexes with a (boron dipyrromethene)3-porphyrin-C60 pentad

A highly efficient functional mimic of the photosynthetic antenna-reaction-center complexes has been designed and synthesized. The model contains a zinc(II) porphyrin (ZnP) core, which is connected to three boron dipyrromethene (BDP) units by click chemistry, and to a C(60) moiety using the Prato procedure. The compound has been characterized using various spectroscopic methods. The intramolecular photoinduced processes of this pentad have also been studied in detail with steady-state and time-resolved absorption and emission spectroscopic methods, both in polar benzonitrile and nonpolar toluene. The BDP units serve as the antennae, which upon excitation undergo singlet-singlet energy transfer to the porphyrin core. This is then followed by an efficient electron transfer to the C(60) moiety, resulting in the formation of the singlet charge-separated state (BDP)(3)-ZnP(·+) -C(60)(·-) , which has a lifetime of 476 and 1000 ps in benzonitrile and toluene, respectively. Interestingly, a slow charge-recombination process (k(CR)(t)=2.6×10(6) s(-1)) and a long-lived triplet charge-separated state (τ(CS)(T)=385 ns) were detected in polar benzonitrile by nanosecond transient measurements.     

Synthesis, electrochemistry, and interactions with beta-cyclodextrin of dendrimers containing a single ferrocene subunit located "off-center"

Two series of dendrimers containing a single ferrocene unit located in the focal point of these macromolecules have been synthesized and characterized. The first series of dendrimers has considerable lipophilic character, with tert-butyl ester groups located in their peripheral regions. In contrast, the second series of dendrimers was obtained by the hydrolysis of these peripheral ester groups, yielding water-soluble dendrimers with carboxylic acid groups in their surfaces. The electrochemical properties of these macromolecules were strongly affected by the dendritic groups attached to the ferrocene subunits. Host-guest interactions between the water-soluble dendrimers and the well-known receptor beta-cyclodextrin were also investigated. The dendritic groups were found to hamper markedly the formation of inclusion complexes between the cyclodextrin receptor and the dendrimer's ferrocene unit.     

Polarity effects on the photophysics of dendrimers with an oligophenylenevinylene core and peripheral fullerene units

Highly soluble dendritic branches with fullerene subunits at the periphery and a carboxylic acid function at the focal point have been prepared by a convergent approach. They have been attached to an oligophenylenevinylene (OPV) core bearing two alcohol functions to yield dendrimers with two, four or eight peripheral C60 groups. Their photophysical properties have been systematically investigated in solvents of increasing polarity; that is, toluene, dichloromethane, and benzonitrile. Ultrafast OPV-->C60 singlet energy transfer takes place for the whole series of dendrimers, whatever the solvent. Electron transfer from the fullerene singlet is thermodynamically allowed in CH2Cl2 and benzonitrile, but not in apolar toluene. For a given solvent, the extent of electron transfer, signaled by the quenching of the fullerene fluorescence, is not the same along the series, despite the fact that identical electron transfer partners are present. By increasing the dendrimer size, electron transfer is progressively more difficult due to isolation of the central OPV core by the dendritic branches, which hampers solvent induced stabilization of charge separated couples. Compact structures of the hydrophobic dendrimers are favored in solvents of higher polarity. These structural effects are also able to rationalize the unexpected trends in singlet oxygen sensitization yields.     

A mixed fullerene-ferrocene thermotropic liquid crystal: synthesis, liquid-crystalline properties, supramolecular organization and photoinduced electron transfer

Grafting of a ferrocene-containing liquid-crystalline malonate derivative to C60 led to the mixed fullerene-ferrocene material 1 which gave rise to a smectic A phase. Cholesterol was used as liquid-crystalline promoter. X-ray diffraction experiments and volumetric measurements indicated that 1 is organized in double layered structures. The corresponding supramolecular organization within the mesomorphic lamellar phase is characterized by a microsegregation of the different units (ferrocene, fullerene, and cholesterol) in distinct sublayers. In such a smectic A phase, C60 imposes the arrangement of the other molecular moieties. Photophysical studies revealed that electron transfer occurs from the donor ferrocene to the electron accepting fullerene. The formation of a long-lived radical pair, with lifetimes of the order of several hundred nanoseconds, was confirmed by time-resolved spectrometry, especially in the near infrared region, in which the radical anion of the fullerene moiety displays its characteristic fingerprint absorption.     

Pico- and nano-second laser flash photolysis study on photoinduced charge separation in oligothiophene-C60 dyad molecules

Photoinduced charge separation (CS) and charge recombination (CR) processes of octathiophene-C₆₀ and dodecathiophene-C₆₀ dyad molecules (8T-C₆₀ and 12T-C₆₀, respectively) have been investigated by time-resolved absorption spectroscopy in the visible and near-IR regions. In toluene, ¹8T*-C₆₀ and ¹12T*-C₆₀ showed energy transfer to ¹C*-moiety predominantly, while 60 contribution of CS was small. In various polar solvents, on the other hand, CS states were predominantly formed from both singlet-excited oligothiophene and ¹C6*0-moiety because of lower CS level in polar environments. The CR process generating both the triplet state of oligothiophene and the ground state was confirmed in anisole and anisole/toluene mixture within a few nanoseconds. In more polar solvents (dielectric constant (∈_s) > 7), CS states showed two components decay: Slow decay component showed lifetime in the hundred nanosecond-region, while fast component decayed within a few nanoseconds. For the mechanism of the long-living CS state in polar solvents (∈_s > 7), equilibrium between the CS state and the triplet state was proposed. Furthermore, effects of length of oligothiophene on the CS and CR processes were discussed on the basis of the free energy changes.     

Multiple photosynthetic reaction centres composed of supramolecular assemblies of zinc porphyrin dendrimers with a fullerene acceptor

Multiple photosynthetic reaction centres have successfully been constructed using supramolecular complexes of zinc porphyrin dendrimers [D(ZnP)(n): n = 4, 8, 16] with fulleropyrrolidine bearing a pyridine ligand (C(60)py). Efficient energy migration occurs completely between the ZnP units of dendrimers prior to the electron transfer with increasing the generation of dendrimers to attain an extremely long charge-separation lifetime.     

混合三聚反应及结构的研究 II: 单核及双核铁混合三聚反应的研究

通过甲酰基二茂铁与取代水杨醛和氨反应, 直接合成了含一个二茂铁的混合三聚产物, N,N'-二[2-羟基-5-取代苄烯]二茂铁甲二胺. 茂环上碳原子的化学位移^1^3C NMR与苯环上R取代基的Hammett常数之间存在着很好的线性关系. 循环伏安法测出的E1/2也与hammett常数之间存在着很好的线性关系. 通过混合三聚反应, 也合成了另一类含两个二茂铁化合物, N,N'-二[二茂铁甲烯]-2-羟基-5-特丁基苯甲二胺. ^1H及^1^3C NMR表明, 分子内由于氢键和立体效应的影响, 导致两个茂环上的化学环境不等性.     

Dendritic liquid-crystalline fullerene-ferrocene dyads

First- and second-generation ferrocene-based dendrimers, fullerene and a second-generation liquid-crystalline poly(arylester) dendrimer carrying four cyanobiphenyl units were assembled to elaborate polyfunctional materials displaying mesomorphic and electronic properties. The targeted compounds gave rise to enantiotropic smectic A phases and organized into bilayer structures within the smectic layers. Cyclic voltammetry investigations revealed oxidation and reduction processes in agreement with the presence of both ferrocene and fullerene units. Finally, strong quenching of the fluorescence was obtained for the fullerene-ferrocene dyads suggesting efficient elecron transfer from the ferrocene-based dendrimer to fullerene.     

Hydrogen bonding interfaces in fullerene*TTF ensembles

Novel thermodynamically stable supramolecular donor-acceptor dyads have been synthesized. In particular, we assembled successfully C(60), as an electron acceptor, with the strong electron donor TTF through a complementary guanidinium-carboxylate ion pair. Two strong and well-oriented hydrogen bonds, in combination with ionic interactions, ensure the formation of stable donor-acceptor dyads. The molecular architecture has been fine-tuned by using chemical spacers of different lengths (i.e., phenyl versus biphenyl) and functional groups (i.e., ester versus amide), thus providing meaningful incentives to differentiate between through-bond and through-space electron-transfer scenarios. In electrochemical studies, both the donor and acceptor character of the TTF and C(60) units, respectively, have been clearly identified. Steady-state and time-resolved emission studies, however, show a solvent-dependent fluorescence quenching in C(60)*TTF dyads as well as the formation of the C(60)(*)(-)*TTF(*)(+) radical ion pairs, for which we determined lifetimes that are in the range of hundred of nanoseconds to microseconds. The complex network that connects C(60) with TTF in the dyads and the flexible nature of the spacer result in through-space electron-transfer processes. This first example of electron transfer in C(60)-based dyads, connected by strong hydrogen bonds, demonstrates that this approach can add outstanding benefits to the construction of artificial photosynthetic systems that bear a closer resemblance to the natural one.     

Siloxane/silane‐crosslinked systems from supercritical carbon dioxide: II. Pendant phenyl poly(carbosilane/siloxane)s

New silicone‐containing polymers with crosslinkable units have been synthesized by hydrosilation polymerization in both toluene and supercritical carbon dioxide (70°C, 3000 psi) catalyzed by platinum‐divinyltetramethyldisiloxane (Pt‐DVTMS). It was found that high molecular polymers were obtained in both toluene and supercritical carbon dioxide. The polymers were characterized by FTIR, NMR, GPC, TGA, and DSC. The molecular weights of these polymers ranged from 9000 to 39,000. With further hydrolysis and thermal curing, the molecular weight can be increased significantly. Comparison of the properties between reactions in toluene versus supercritical carbon dioxide indicated that the green solvent is a usable alternative for hydrosilation polymerization. The new polymers synthesized in either toluene or supercritical carbon dioxide are thermally stable, ranged from 350 to 488°C. Copyright © 2008 John Wiley & Sons, Ltd.     

Construction of regulated nanospace around a porphyrin core

A series of 1,3,5-phenylene-based rigid dendritic porphyrins were synthesized by Suzuki coupling between a porphyrin core and dendron units. The intramolecular energy transfer was studied by absorption and fluorescence spectroscopies. The encapsulation of the porphyrin core within the 1,3,5-phenylene dendron units was found to provide highly efficient energy transfer from the dendron units to the porphyrin core. The dendritic wedge structure affected the energy transfer efficiency. The 1,3,5-phenylene-based rigid dendron units act as highly efficient light-harvesting antennae. These dendritic porphyrins have also been examined as C(60) hosts and substrate-selective oxidation catalysts. The attachment of the second generation of 1,3,5-phenylene-based dendron units with the porphyrin core enabled a stable inclusion of C(60) in toluene. Furthermore, the size and shape of the nanospace in the rigid dendritic porphyrins were found to affect the selectivity of substrates in the catalytic olefin oxidations.     

Charge transfer events in semiconducting single-wall carbon nanotubes

Electron-donating ferrocene units have been attached to SWNTs, with different degrees of functionalization. By means of a complementary series of novel spectroscopic techniques (i.e., steady-state and time-resolved), we have documented that mutual interactions between semiconducting SWNT and the covalently attached electron donor (i.e., ferrocene) lead, in the event of photoexcitation, to the formation of radical ion pairs. In the accordingly formed radical ion pairs, oxidation of ferrocene and reduction of SWNT were confirmed by spectroelectrochemistry. It is, however, shown that only a few semiconducting SWNTs [i.e., (9,4), (8,6), (8,7), and (9,7)] are susceptible to photoinduced electron transfer processes. These results are of relevant importance for the development of SWNT-based photovoltaics.     

Synthesis and characterization of a ferrocene-linked bis-fullerene[60] dumbbell

A new [60]fullerene dumbbell consisting of two fulleropyrrolidines connected to a central ferrocene unit by amide linkages has been prepared and fully characterized by elemental analysis, (1)H NMR, UV/Vis, fluorescence and mass spectrometry. The electrochemical properties as determined by cyclic voltammetry show ground state electronic communication between the ferrocene and the fullerene units. In addition, the preparaton of a ferrocene building block for an alternative linking approach is presented.     

Donor/Acceptor fulleropyrrolidine triads

New C(60)-based triads, constituted by a fulleropyrrolidine moiety and two different electroactive units [donor 1-donor 2 (10, 15a,b), or donor 1-acceptor (17, 21)], have been synthesized by 1,3-dipolar cycloaddition reaction of azomethyne ylides to C(60) and further acylation reaction on the pyrrolidine nitrogen. The electrochemical study reveals some electronic interaction between the redox-active chromophores. Triads bearing tetrathiafulvalene (TTF) and ferrocene (Fc) (10) or pi-extended TTFs and Fc (15a,b) show reduction potentials for the C(60) moiety which are cathodically shifted in comparison to the parent C(60). In contrast, triads endowed with Fc and anthraquinone (AQ) (17) or Fc and tetracyanoanthraquinodimethane (TCAQ) (21) present reduction potentials for the C(60) moiety similar to C(60). Fluorescence experiments and time-resolved transient absorption spectroscopy reveal intramolecular electron transfer (ET) processes from the stronger electron donor (i.e., TTF or extended TTF) to the fullerene singlet excited state, rather than from the poorer ferrocene donor in 10, 15a,b. No evidence for a subsequent ET from ferrocene to TTF(*)(+) or pi-extended TTF(*)(+) was observed.     

Electrochemical studies of porphyrin-appended dendrimers

The electrochemical properties of porphyrin-appended dendrimers containing 2-, 4-, 8-, 16-, 32- and 64-porphyrin macrocycles in their free-base and zinc(II) forms have been investigated. Both series gave diffusional based voltammetric responses in dichloromethane. There was minimal effect of dendrimer generation on the redox potentials. Multiple pi-cation and anion radicals as well as dications and dianions were formed on the surface of the dendrimers on oxidation or reduction as appropriate, with each cyclic voltammetric wave representing electron transfer to or from multiple non-interacting porphyrin sites. Electrostatic interactions in the higher generation dendrimers result in kinetic effects being observed for the highly charged species generated when each porphyrin unit is doubly or triply oxidised. The number of electrons transferred on reduction or oxidation of the dendrimers was evaluated using steady-state microelectrode voltammetry. For the lower generations of species a good correlation was observed between numbers of electrons transferred and number of porphyrin entities per molecule; for the dendrimers containing 32 and 64 units, however, slight negative deviations were observed, possibly due to electrostatic interactions as the porphyrins become closer packed.     

Synthesis of Binaphthol Based Poly(arylene) Dendrimers and Their Use as Enantioselective Fluorescent Scensors

New optically active dendrimers (1-3) containing rigid and cross-conjugated units have been synthesized and characterized. UV and fluorescence spectroscopic studies demonstrate that the energy harvested by the periphery of the dendrimers can be efficiently transferred to the core. The fluorescence of the dendrimers can be quenched by amino alcohols (4-6) both efficiently and enantioselectively. The energy migration and light harvesting effects of the dendrimers make the higher generation dendrimers more sensitive fluorescent quencher than the lower ones.     

Single-component and mixed ferrocene-terminated alkyl monolayers covalently bound to Si(111) surfaces

Self-assembled ferrocene monolayers covalently bound to monocrystalline Si(111) surfaces have been prepared from the attachment of an amine-substituted ferrocene derivative to a pre-assembled acid-terminated alkyl monolayer using carbodiimide coupling. This derivatization strategy yielded nanometer-scale clean, densely packed monolayers, with the ferrocene units being more than 20 A from the semiconductor surface. The amount of immobilized electroactive units could be varied in the range 2 x 10(-11) to approximately 3.5 x 10(-10) mol cm(-2) by diluting the ferrocene-terminated chains by inert n-decyl chains. The highest coverage obtained for the single-component monolayer corresponded to 0.25-0.27 bound ferrocene per surface silicon atom. The electrochemical characteristics of the mixed n-decyl/ferrocene-terminated monolayers were found to not depend significantly on the surface coverage of ferrocene units. The reversible one-electron wave of the ferrocene/ferrocenium couple was observed at E degrees ' = 0.50 +/- 0.01 V vs SCE, and the rate constant of electron transfer kapp was about 50 s(-1).     

Selective sensors for Zn2+ cations based on new green fluorescent poly(amidoamine) dendrimers peripherally modified with 1,8-naphthalimides

The paper reports on the spectral photophysical characteristics of two new fluorescent PAMAM dendrimers of zero and second generation decoreted with 1,8-naphthalimide units, designed for ionic detection. The dendrimers were studied by (1)H NMR, (13)C NMR, FT-IR spectroscopy and elemental analysis. Their ability to detect ions has been evaluated in acetonitrile by monitoring the quenching of the fluoresence intensity. Different ions have been tested: Zn(2+), Co(2+), Ni(2+), Cu(2+) and Fe(3+) for the purpose. The results have shown clearly that only Zn(2+) could be efficiently detected using the dendrimer of second generation. In addition, it has been shown that for both dendrimers in a acetonitrile-water solution, the fluoresence intensity is pH dependant, hence could find application as a detector of harmful pH changes in the environment.     

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.