Polyaryl ether dendrimer with a 4-phenylacetyl-5-pyrazolone-based terbium(III) complex as core: synthesis and photophysical properties

A series of novel dendritic beta-diketone ligands, 1-phenyl-3-[G-n]-4-phenylacetyl-5-pyrazolone (n = 0-3, G stands for polyaryl ether), were synthesized by introducing Fréchet-type dendritic branches. The corresponding Tb3+-cored dendritic complexes were characterized by X-ray crystallography, elemental analysis, ESI mass spectra, and FT-IR spectra. These dendritic complexes, prepared from aqueous solution, exhibit high stability. Interestingly, the study of photophysical properties shows that the luminescence quantum yields of the dendritic Tb-complexes increase from 0.1 to 2.26% with an increase of the dendritic generation from 0 to 3. Importantly, an "energy-reservoir effect" was observed in the dendritic system using the method based on the resonance energy transfer from these complexes to rhodamine 6G. With the increase of the dendritic generation, the metal-centered luminescence quantum yield was almost the same, and the energy transfer (phi(transfer)) from the ligand to Tb(3+) increased. Further measurements of the triplet state and oxygen quenching of these dendritic complexes verify that this enhancement of the energy transfer (phi(transfer)) is attributed to both an "antenna effect" and a "shell effect".     

树枝形聚合物修饰双8-羟基喹啉衍生物的合成及稳态光物理性质研究

设计合成了1 3代芳醚骨架树枝形聚合物修饰的双8 羟基喹啉衍生物.对这些化合物在不同溶剂中的荧光光谱研究表明,随着代数的增加,目标树枝形聚合物的荧光量子产率增大,树枝形聚合物对核心发色团具有一定的隔离作用,并且目标分子内可以发生从骨架向核心发色团的能量传递.     

A tetraphenylmethane based dendritic tolan-anthracene dyad: synthesis and energy transfer properties

In order to synthesize a peripherally rigid dendritic donor-acceptor dyad for energy transfer studies, a tritolan dendron based on a tetraphenylmethane scaffold was prepared from New Fuchsin. The dendron showed a small degree of homoconjugation but a large hypochromic effect. Coupling of two such dendrons with an anthracene core led to a dendritic tolan-anthracene dyad whose steady state photophysical studies (UV, PL, PLE) showed vectorial transfer of excitation energy from the surface tolan units to the anthracene core.     

Optical and photophysical properties of light-harvesting phenylacetylene monodendrons based on unsymmetrical branching

The optical and photophysical properties of phenylacetylene dendritic macromolecules based on unsymmetrical branching are investigated using steady-state and time-dependent spectroscopy. Monodendrons, up to the fourth generation, are characterized with and without a fluorescent perylene trap at the core. The higher generation monodendrons without the perylene trap exhibit high molar extinction coefficients (>10(5) M(-1) cm(-1)) and high fluorescence quantum yields (65-81%). When a perylene trap is placed at the core, then the monodendrons typically exhibit high energy transfer quantum yields (approximately 90%), as well as subpicosecond time scale excited-state dynamics, as evidenced by ultrafast pump-probe measurements. The photophysical properties of the unsymmetrical monodendrons are compared to those of phenylacetylene monodendrons with symmetrical branching, which have been described recently. The high fluorescence quantum yields and large energy transfer quantum efficiencies exhibited by the unsymmetrical monodendrons suggest they have potential for applications in molecular-based photonics devices.     

Control of electron transport using redox‐active core dendrimers

We are constructing a model system to elucidate the molecular structure‐property relationships for attenuation of electron transfer (e.g. electron encapsulation). This information is relevant in bio‐electron transfer schemes and in emerging molecular electronics schemes such as storage of information using individual molecules. Our system consists of an inorganic cluster surrounded by dendritic ligands which act as an organic coating. Although the electrochemical and photophysical properties of a variety of metal clusters have been established, very little has been described on the chemistry on metal clusters.     

Dendritic phthalocyanines: synthesis,photophysical properties,and aggregation behavior

This mini-account describes our recent effort to exploit dendritic phthalocyanines, focusing on their photophysical properties and aggregation behavior in water and in microheterogeneous media. Two series of dendritic phthalocyanines have been prepared. The ones with terminal ester functionalities are non-aggregated in common organic solvents, exhibiting an intramolecular singlet-singlet energy transfer from the excited aryl-containing dendrons to the phthalocyanine core. The other series contain terminal carboxylate groups of which the aggregation tendency in water decreases as the size of the dendron increases. The lower-generation analogues are susceptible to surfactants, in particular the cationic n-hexadecyltrimethylammonium bromide (CTAB), and poly(ethylene oxide) (PEO), which are very effective to disrupt the molecular aggregation of phthalocyanines. The interactions have been monitored by absorption and fluorescence spectroscopy together with laser light scattering. The photophysical properties of the dendrimer/PEO complexes have also been studied by transient spectroscopy. To cite this article: D.K.P. Ng, C. R. Chimie 6 (2003).     

60]Fullerene: a versatile photoactive core for dendrimer chemistry

Owing to their special photophysical properties, fullerene derivatives are good candidates to demonstrate dendritic effects. In particular, the triplet lifetimes of a C(60) core can be used to evaluate its degree of isolation from external contacts. On the other hand, the fullerene core can act as a terminal energy receptor in dendrimer-based light-harvesting systems. When a fullerodendrimer is further functionalized with a suitable electron donor, it may exhibit the essential features of a multicomponent artificial photosynthetic system in which photoinduced energy transfer from the antenna to the C(60) core is followed by electron transfer.     

The synthesis and study of fluorescent PAMAM-based dendritic molecules

New water soluble and fluorescent PAMAM-based dendritic molecules based on the naphthalimide derivative 7H-benz[de]benzimidazo[2,1-a]isoquinoline-7-one as the fluorescent unit, have been prepared and their photophysical properties studied in aqueous solution over a wide pH range. The dendrons are all strongly fluorescent through an intramolecular charge transfer (ICT) excited state, but this can be modulated by photoinduced electron transfer (PeT) processes, which increases with higher PAMAM dendron generation.     

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.     

Electrostatic complexation and photoinduced electron transfer between Zn-cytochrome c and [olyanionic fullerene dendrimers

Two dendritic fullerene (DF) monoadducts, 2 and 3, which can carry up to 9 and 18 negative charges, respectively, were examined with respect to electrostatic complexation with Cytochrome c (Cytc). To facilitate comprehensive photophysical investigations, the zinc analogue of Cytc (ZnCytc) was prepared according to a novel, modified procedure. The association of ZnCytc and DF, and consequential photoinduced electron transfer within ZnCytc-DF from the photoexcited protein to the fullerene, was proven by fluorescence spectroscopy and transient absorption spectroscopy. These findings were also supported by circular dichroism as well as by extensive molecular dynamics (MD) simulations.     

Asymmetric annellation effects—II

The comparison of the U.V. absorption spectra of acenes, 1:2–3:4-dibenzacenes and tetrabenzacenes shows a strong asymmetric annellation effect. This is explained on the basic assumption that an aromatic sextet or benzenoid ring can transfer only two electrons to another ring. Three benzenoid rings can thus produce an induced aromatic sextet in an included ring of the type of the central ring in triphenylene.

The synthesis of tetrabenzotetracene is described.     

Jacketed homopolymer with bipolar dendritic side groups and its applications in electroluminescent devices

A dendritic monomer with bipolar side groups containing dendritic carbazole and oxadiazole structures was synthesized by a convergent strategy. The homopolymer was synthesized through a conventional radical polymerization. The number‐average molecular weight determined by gel permeation chromatography was 40,000 g/mol. Its 5% weight loss temperature was 358 °C. Its photophysical properties were studied in solution and in film. The photoluminescent emission peak of the film was at 408 nm, which had a blue shift of 9 nm compared with that of the tetrahydrofuran solution. And there was an energy transfer from oxadiazole to carbazole. The highest occupied molecular orbital (HOMO) and lower unoccupied molecular orbital (LUMO) levels calculated from cyclic voltammetry data were ‐5.55 and ‐2.52 eV, respectively, and the band gap was 3.03 eV, which suggested that the polymer had both hole‐ and electron‐transporting capabilities. The efficiencies of the single‐layer device based on this homopolymer were much higher than those of the same‐generation homopolymer without the oxadiazole moiety. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2011     

Light harvesting and energy transfer within coumarin‐labeled polymers

Linear copolymers that have pendant coumarin‐2 and coumarin‐343 chromophores were prepared as analogues to previously synthesized light‐harvesting dendrimers. The chromophore ratios within these polymers were maintained similar to those of the various generation dendrimers to investigate the effect of polymer architecture on the energy‐transfer efficiency between the coumarin‐2 donors and coumarin‐343 acceptors. Both physical and photophysical properties of these polymers were analyzed and compared to those of the analogous dendrimers. Energy‐transfer efficiencies were relatively high in the polymers; however, deleterious excimer formation between the coumarin‐343 chromophores diminished the quantum yield of fluorescence of the polymers when compared to the analogous dendrimers. Overall, it was found that the ultimate performance of the dendritic light‐harvesting antennae was superior to that of the polymeric analogues, but the polymers were more practical in terms of synthetic accessibility. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1366–1373, 2001     

Rodlike macromolecules through spatial overlapping of thiophene dendrons

Two novel dendritic macromonomers 7 and 8 functionalized with electroactive conjugated thiophene oligomers were synthesized by stepwise cross‐coupling reactions and the introduction of a vinyl group at the focal point. Both macromonomers were polymerized into dendronized polymers 9 and 10 by using a radical polymerization method. The photophysical and redox behaviors of dendronized polymers 9 and 10 are significantly different from those of the corresponding macromonomers. This difference may result from the spatial overlapping of thiophene dendrons through π–π interactions when the dendrons are connected to a polymer backbone. The dendronized polymers can organize into large‐area two‐dimensional sheets with a thickness of 4.8 nm. Polymer 9 , which has all‐dendritic thiophene side chains, exhibited enhanced conductivity by partial doping with iodine or nitrosonium tetrafluoroborate (NOBF₄). The novel amphiphilic dendronized polymer 15 was synthesized by the atom‐transfer radical polymerization of macromonomer 7 from a poly(ethylene glycol) (PEG) macroinitiator and was found to have a self‐organized structure in water.     

Valence isomerization in dendrimers by photo-induced electron transfer and energy transfer from the dendrimer backbone to the core

A series of poly(aryl ether) dendrimers with a norbornadiene (NBD) group attaching to the core (Gn-NBD), generations 1–4, were synthesized and characterized, and their photophysical and photochemical properties were examined. The fluorescence of the dendrimer backbone is quenched by the norbornadiene group as a result of the electron transfer and energy transfer from the dendrimer backbone to the norbornadiene group in Gn-NBD. Selective excitation of the dendrimer backbone results in an isomerization of the norbornadiene group to the quadricyclane (QC) group. The intramolecular electron transfer and energy transfer efficiencies are ca. 0.93, 0.73, 0.54, 0.30 in dichloromethane, and ca. 0.90, 0.70, 0.55, 0.34 in tetrahydrofuran for generations 1–4, respectively, with the rate constant ca. 10¹⁰ s⁻¹. The light-harvesting ability of these dendritic molecules is demonstrated by the enhanced valence isomerization rate of NBD to QC with increasing generation.     

Photophysical and self‐assembly behavior of poly(amidoamine) dendrons with chromophore as scaffold: The effect of dendritic architecture

Two series of amphiphiles composed of hydrophilic poly(amidoamine) dendrons (from the first to the third generation) as the shell and hydrophobic aromatic chromophores (3,6‐di(maleimidyl)‐9‐phenyl carbazole and 9‐(4′‐maleimidyl phenyl)‐3‐maleimidyl carbazole) as the central scaffold were synthesized. The effect of dendritic architecture on the photophysical properties and the self‐assembly behavior of these amphiphiles were studied by UV–vis absorption spectroscopy, fluorescence spectroscopy, and transmission electron microscopy (TEM) measurements. Both the generation of dendritic shell and the location of dendrons at the chromophoric scaffold had great effect on the photophysical properties of these amphiphiles. In addition, different spherical aggregates were formed from these amphiphiles in the aqueous solution at different concentrations. Because of the combined effects of steric hindrance and architecture of dendritic shells, the amphiphiles from G2 dendron with central chromophore self‐organized into ordered aggregates more readily than that from G1 and G3. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 4584–4593, 2008     

Emission frequency modulation by electronic confinement effect: Congo Red incorporated within a dendritic structure

We have investigated the photophysical properties of an organic dye (Congo Red) incorporated within the internal cavities of a dendrimer (type polypropylenimine of fifth-generation modified with a dense shell of amino acids). In this paper we show that the luminescence properties of Congo Red encapsulated into the "dendritic box" can be modulated by the electronic confinement effect. The emission frequencies of this organic dye incorporated within the dendritic structure can be red shifted with respect to their emission in solution, and the magnitude of this shifting can be modulated under appropriate experimental conditions.     

PAMAM修饰的六苯并蔻衍生物的合成及其光物理研究

设计合成了外围以0.5、1.5、2.5代聚酰胺-胺(PAMAM)树枝形聚合物修饰的六苯并蔻衍生物HBC-C₆-PAMAM-D_m(m=0.5, 1.5, 2.5), 化合物结构通过了¹HNMR、¹³CNMR、IR和MS的表征.测定了HBC-C₆-PAMAM-D_m在四氢呋喃、乙腈和二氯甲烷中的吸收和荧光发射光谱, 三种溶剂中不同代数的HBC-C₆-PAMAM-D_m均以聚集态形式存在,外围树枝形聚合物骨架与核心HBC发色团的振动耦合作用随代数增加而增强,导致HBC振动能级不均化,引起吸收和荧光光谱发生变化.     

From Graftable Biphotonic Chromophores to Water‐Soluble Organic Nanodots for Biophotonics: The Importance of Environmental Effects

The photophysical and two‐photon absorption (TPA) properties of biphotonic chromophores with one or two phenol pendant units were studied and compared with that of a model biphotonic quadrupolar chromophore. A water‐soluble dendritic structure was then synthesized by using the pendant moieties as starting points for the construction of dendritic branches. We show that the polarity of the environment significantly modulates both the fluorescence and the TPA responses of the different chromophoric derivatives. This extends to more subtle effects that involve phenol pendant moieties that were found to act as discrete solvating units and to modify both the photophysics and the TPA response of the chromophore. This demonstrates the high sensitivity of the TPA response of quadrupolar derivatives to minute alterations in the environment. Moreover, the dendritic branches were found to behave as a peculiar cybotactic environment that was able to tune the fluorescence and TPA response of the inner chromophore by creating a polar environment. This reveals a new direction for exploiting such effects by playing on the dendritic architecture (e.g., the nature and shape of the building blocks, the geometry and position of the chromophore) to modulate the TPA responses.     

Dendritic encapsulation of active core molecules

Specific properties of several series of dendrimers have been systematically investigated as a function of the generation number. The encapsulation of a redox-active unit, namely a bis(phenanthroline) copper(I) complex, has been evidenced by the attenuation of the electron transfer rate with increasing molecular size. On the other hand, photophysical studies of dendrimers with a fullerene core have shown that the shielding effect of the dendritic shell has a dramatic effect on the lifetime of the first triplet excited state of the core unit. Actually, the fullerene is a very sensitive probe and lifetime measurements in different solvents can be used to evaluate the degree of isolation of the central C₆₀ moiety from external contacts. Finally, the inclusion abilities of dendrophanes with a cyclotriveratrylene (CTV) core for fullerenes have shown that the dendritic architecture is not only able to isolate a central functional core but can also modulate its binding properties by means of the size and the nature of the surrounding dendrons. To cite this article: J.-F. Nierengarten, C.R. Chimie 6 (2003).