Encapsulation of luminescent homoleptic [Ru(dpp)3](2+)-type chromophores within an amphiphilic dendritic environment.

A new series of homoleptic metallodendrimers has been synthesized through ruthenium-metal complexation by dendritically modified bathophenanthroline ligands. The presence of hydrophilic oligo(ethylene glycol) groups on the surface of the monodisperse metal complexes enabled the solubilization of all of the fractal species in a wide range of solvents, including water. The specific properties of all of these compounds have been systematically investigated by using photophysical techniques as a function of the generation number. Accordingly, the encapsulation of the highly luminescent [Ru(dpp)(3)](2+)-type (dpp=4,7-diphenyl-1,10-phenanthroline) core unit within a dendritic microenvironment creates a powerful means to shield the center from dioxygen quenching. This shielding effect, as exerted on the phosphorescent ruthenium-derived center, is reflected by enhanced emission intensities and extended excited-state lifetimes that are close to the highest values reported so far, even in an air-equilibrated aqueous medium. Interestingly, when inspecting the largest dendritic assembly, that is, the third-generation assembly, significant drops in emission quantum yields and lifetimes are observed. This anomalous behavior has been attributed to the folding of the branches towards the luminescent core.

Photoswitchable metal coordinating tweezers operated by light-harvesting dendrimers.

A dendrimer bearing two cyclam units linked by an azobenzene moiety, and luminescent naphthalene units at the periphery performs three different functions (light-harvesting, photoisomerization and coordination of metal ions) which can cooperate or interfere depending on the nature of the metal ion. It is thus an example of light controlled molecular tweezers in which Zn(II) coordination allows 100% efficient photosensitization of azobenzene switching, while Cu(II) shuts down azobenzene isomerization.

A molecular clip throws new light on the complexes formed by a family of cyclam-cored dendrimers with Zn(II) ions. Efficient energy transfer in the heteroleptic complexes.

Complexation of Zn(II) ions by cyclam cored dendrimers appended with four (G0), eight (G1) and 16 naphthyl chromophores (G2) at the periphery have been investigated in CH3CN-CH2Cl2 1 : 1 (v/v) solution by absorption and emission, ESI-mass and ¹H NMR spectroscopy. The results obtained can be interpreted by the formation of complexes of 2 : 1 dendrimer to metal stoichiometry, at low metal ion concentration, and 1 : 1 complexes upon further addition of Zn(II) ions, for all the dendrimer generations. Upon addition of a molecular clip C²â» consisting of two anthracene sidewalls bridged by a benzene group with two sulfate substituents in the para positions, heteroleptic complexes of general formula [GnZnC] are formed. Interestingly, in these complexes, a very efficient quenching (practically 100%) of the dendrimer naphthyl luminescence and sensitization (ca. 90%) of the clip anthracene emission take place. The complex [G2ZnC] exhibits a very high molar absorption coefficient in the UV spectral region owing to the 16 naphthyl chromophores of the dendrimer and the two anthracene units of the clip (ε = 1.7 × 105 M⁻¹ cm⁻¹ at 263 nm). Furthermore, the excitation energy absorbed by the naphthyl chromophores is efficiently funneled to the two anthracene units of the clip, which emits in the blue spectral region.

Metal ion driven formation of a light-harvesting antenna investigated by sensitized luminescence and fluorescence anisotropy.

Zn(ii) complexation drives the formation of a light-harvesting antenna constituted by two multicomponent luminescent ligands: a cyclam-cored dendrimer decorated at the periphery with 16 naphthyl units and an anthracene-based molecular clip.

Cyclam-cored dendrimers appended with four dendrons of two different types: intradendrimer energy transfer.

We have synthesized two cyclam-cored dendrimers appended with dendrons of two different types by proper protection/deprotection of the cyclam unit. The resulting dendrimers contain six naphthyl and two dansyl units (N6 D2) or two dansyl and six naphthyl units (N2 D6) at the periphery. Their photophysical properties have been compared to those of a dendrimer containing 8 dansyl units (D8) and a previously investigated dendrimer containing 8 naphthyl units (N8). The absorption spectra are those expected on the basis of the number of chromophores, demonstrating that no ground state interaction takes place. The emission spectra of N2 D6 and N6 D2 show naphthalene localized and naphthalene excimer emission similar to those observed in the case of N8, together with a much stronger dansyl emission with maximum at 525 nm. Addition of CF(3)SO(3)H to dendrimer solutions in CH(3)CN/CH(2)Cl(2) 1:1 (v/v) leads to protonation of the aliphatic amine units of the cyclam core at first and then of the aromatic amine of each dansyl chromophores. Cyclam can be diprotonated and this affects dansyl absorption and, most significantly, emission bands by a charge perturbation effect. Each dansyl unit is independently protonated in both dendrimers. The most interesting photophysical feature of these heterofunctionalized cyclam-cored dendrimers is the occurrence of an intradendrimer photoinduced energy transfer from naphthyl to dansyl chromophores of two different dendrons (interdendron mechanism). The efficiency of this process is 50 % for N6 D2 and it can be increased up to 75 % upon protonation of the cyclam core and formation of N6 D2(2H(+)). This arises from the fact that protonation of the amine units of the cyclam prevents formation of exciplexes upon naphthyl excitation, thus shutting down one of the deactivation processes of the fluorescent naphthyl excited state.

A light-harvesting antenna resulting from the self-assembly of five luminescent components: a dendrimer, two clips, and two lanthanide ions.

We have investigated the self-assembly of three luminescent species in CH(3)CN/CH(2)Cl(2), namely: 1) a polylysin dendrimer (D) composed of 21 aliphatic amide units and 24 green luminescent dansyl chromophores at the periphery, 2) a molecular clip (C) with two blue luminescent anthracene sidewalls and a benzene bridging unit that bears two sulfate groups in the para position, and 3) a near infrared (NIR)-emitting Nd(3+) ion. For purposes of comparison, analogous systems have also been investigated in which Gd(3+) replaced Nd(3+). The dendrimer and the clip can bind Nd(3+) ions with formation of [D.2Nd(3+)] and [C.Nd(3+)] complexes, in which energy transfer from dansyl and, respectively, anthracene to Nd(3+) ion takes place with 65 and 8% efficiency, in air-equilibrated solution. In the case of [C.Nd(3+)], the energy-transfer efficiency is quenched by dioxygen, thereby showing that the energy donor is the lowest triplet excited state of anthracene. In [D.2Nd(3+)] the intrinsic emission efficiency of Nd(3+) is much higher (ca. 5 times) than in [C.Nd(3+)] because of a better protection of the excited lanthanide ion towards nonradiative deactivation caused by interaction with solvent molecules. By mixing solutions of D, Nd(3+), and C with proper concentrations, a supramolecular structure with five components of three different species, [D.2Nd(3+).2C], is formed. The excitation light absorbed by the clips is transferred with 100% efficiency to the dansyl units of the dendrimer and then to the Nd(3+) ions with 65% efficiency either in the presence or absence of dioxygen. These results show that the [D.2Nd(3+).2C] complex is able to efficiently harvest UV light by the 24 dansyl units of the dendrimer and the four anthracene chromophores of the two clips, and efficiently transfer it to the encapsulated Nd(3+) ions that emit in the NIR spectral region.

Internal dynamics and energy transfer in dansylated POPAM dendrimers and their eosin complexes.

Internal dynamics of dansylated poly(propyleneamine) dendrimers (POPAM, G1-G4) in solution and excitation energy transfer from dansyls to eosin in POPAM-eosin complexes have been studied by time-resolved fluorescence spectroscopy and molecular dynamics (MD) simulations. Combining the results from fluorescence anisotropy and the MD simulation studies suggests three time domains for the internal dynamics of the G3 and G4 generations, about 60 ps for motions of the outer-sphere dansyls, 500-1000 ps for restricted motions of back-folded dansyls, and 1500-2600 ps for the overall rotation. For the smaller generations, the contribution from the restricted motions was not entirely evident. Eosin binding hinders fast rotation of the dansyl fragments in the largest G4 dendrimer, but the motion of back-folded dansyls is more hindered in the pure dendrimer. Both fluorescence anisotropy and MD results for the G4 dendrimer support the "soft" dendrimer picture with almost free mobility and substantial back-folding of the dansyls of the dendrimers in solution. Analysis of time-dependent spectral shifts of fluorescence reveals 20-30 ps excited-state solvation relaxation around a single dansyl of a dendrimer. Dendrimer-independent excitation energy transfer from 4 to 8 ps from dansyls to eosins in POPAM-eosin complexes G2-G4 was observed.

Tweezering the core of dendrimers: medium effect on the kinetic and thermodynamic properties.

We have investigated the complex formation between dendritic guests and a molecular tweezer host by NMR, absorption, and emission spectroscopy as well as electrochemical techniques. The dendrimers are constituted by an electron-acceptor 4,4'-bipyridinium core appended with one (DnB(2+)) or two (Dn(2)B(2+)) polyaryl-ether dendrons. Tweezer T comprises a naphthalene and four benzene components bridged by four methylene groups. Medium effects on molecular recognition phenomena are discussed and provide insight into the conformation of dendrimers: change in solvent polarity from pure CH(2)Cl(2) to CH(2)Cl(2)/CH(3)CN mixtures and addition of tetrabutylammonium hexafluorophosphate (NBu(4)PF(6), up to 0.15 M), the supporting electrolyte used in the electrochemical measurements, have been investigated. The association constants measured in different media show the following trend: (i) they decrease upon increasing polarity of the solvent, as expected for host-guest complexes stabilized by electron donor-acceptor interactions; (ii) no effect of generation and number of dendrons (one for the DnB(2+) family and two for the Dn(2)B(2+) family) appended to the core is observed in higher polarity media; and (iii) in a low-polarity solvent, like CH(2)Cl(2), the stability of the inclusion complexes is higher for DnB(2+) dendrimers than for Dn(2)B(2+) ones, while within each dendrimer family it increases by decreasing dendron generation, and upon addition of NBu(4)PF(6). The last result has been ascribed to a partial dendron unfolding. Kinetic investigations performed in lower polarity media evidence that the rate constants of complex formation are slower for symmetric Dn(2)B(2+) dendrimers than for the nonsymmetric DnB(2+) ones, and that within the Dn(2)B(2+) family, they decrease by increasing dendron generation. The dependence of the rate constants for the formation and dissociation of the complexes upon addition of NBu(4)PF(6) has also been investigated and discussed.

Conformational flexibility of tetralactam macrocycles and their intermolecular hydrogen-bonding patterns in the solid state.

Despite their rigid scaffold, tetralactam macrocycles (TLMs) display a remarkable degree of conformational flexibility, as revealed by analysis of the corresponding X-ray crystal structures. This flexibility is not limited to the rotatability of the TLM amide groups but also applies to the m-xylene rings, and it thus has a great impact on the overall shape of the macrocycle cavity. The conformational properties of the TLMs give rise to a broad variety of intermolecular hydrogen-bonding patterns, including infinite ladders, an interesting catemer motif, and short C-HO=C hydrogen bonds. These results are in accord with previous theoretical calculations, support a structural model proposed earlier for an interpretation of scanning tunneling microscopy images, and substantially contribute to the understanding of the adaptability of macrocyclic scaffolds, which is crucial for guest binding or templated syntheses with TLMs.

Adducts between dansylated poly(propylene amine) dendrimers and anthracene clips mediated by Zn(II) ions: highly efficient photoinduced energy transfer.

A family of poly(propylene amine) dendrimers, decorated at their periphery with 4, 16, and 32 dansyl units and a molecular clip, composed of two anthracene sidewalls and a disulfate benzene bridging unit, show intense UV absorption and strong fluorescence in the visible region when in a CH(3)CN/CH(2)Cl(2) (1:1, v/v) solvent mixture. Both these classes of compounds are good ligands for Zn(II) ions, as demonstrated by the changes in the absorption and fluorescence spectra upon addition of metal ions. These coordinating properties have been exploited in the self-assembly of complex structures in which the interaction between a dansylated dendrimer and anthracene-functionalized clips is mediated by Zn(II) ions. The self-assembly process is reversible and the number of metal ions and molecular clips associated with each dendrimer increases with the generation number. In these adducts, an energy transfer process from the anthracene to yield the fluorescent excited state of dansyl takes place with almost unitary efficiency.

Photoswitchable dendritic hosts: a dendrimer with peripheral azobenzene groups.

We have studied the adducts formed by eosin (E) with a fourth generation dendrimer (D) that comprises 30 tertiary amine units in the interior and 32 naphthyl and 32 trans azobenzene units in the periphery. We have found that: (i) the all trans dendrimer D(32t) can be converted by irradiation with 365 nm light (Phi=0.12) into species containing, as an average, 4 trans and 28 cis azobenzene units, D(4t28c), that at 313 K undergoes a D(4t28c) --> D(32t) thermal back reaction (k = 7.0 x 10(-5) s(-1)); (ii) D(32t) and D(4t28c) extract 8 and, respectively, 6 eosin molecules from water at pH 7, yielding the species D(32t) subset 8E and D(4t28c) subset 6E; (iii) eosin uptake is significantly faster for D(32t) than for D(4t28c); (iv) irradiation at 365 nm of the D(32t) subset 8E species at 298 K leads to the release of two eosin molecules with formation of a photostable D(15t17c) subset 6E species (Phi = 0.15) that is also obtained from the back thermal reaction of D(4t28c) subset6E at 313 K (k = 2.7 x 10(-5) s(-1)); (v) thermal release of E from D(32t) subset 6E is much faster than from D(4t28c) subset 6E; and (vi) excitation of E in the adducts sensitizes the cis --> trans (but not the trans --> cis) isomerization. The results obtained show that the isomerization of the 32 peripheral azobenzene units controls to some extent the hosting capacity of the dendrimer and, viceversa, eosin molecules hosted in the dendrimer affect the isomerization process of its azobenzene units.

Converting self-assembled gold nanoparticle/dendrimer nanodroplets into horseshoe-like nanostructures by thermal annealing.

A simple and effective nonlithographic method to produce a novel organization of noble metal nanoparticles into horseshoe-like nanostructures via self-assembly is described. The adsorption of Au nanoparticles stabilized with the dendrimer 1,2,3,4,5,6-hexakis[(3',5'-bis(benzyloxy)benzyl)sulfanylmethyl]benzene (S(6)G(1)) on hydrophilic surfaces (native oxide-terminated Si(111)) resulted in the formation of spatially correlated droplet aggregates. Annealing of Au/S(6)G(1) in thin films caused amalgamated droplets to form arrays of horseshoe-like nanostructures with an average size of approximately 250 nm and an average height of 13 nm. The mobility and the manner in which the semicapped Au nanoparticles are distributed on the hydrophilic substrate are believed to be the promoters that control the growth of the nucleation to create the horseshoe-like structures. Atomic force microscopy (AFM) measurements demonstrated the changes in height and size of the nanoparticles before and after the annealing process. Oxygen plasma etching was used to remove the S(6)G(1) dendrimer to reveal the orientation of the Au nanocrystals in the nanostructure matrix.

First generation TREN dendrimers functionalized with naphthyl and/or dansyl units. Ground and excited state electronic interactions and protonation effects.

We report the photophysical properties (absorption and emission spectra, quantum yield, and lifetime) of five dendrimers of first generation based on a TREN (tris(2-aminoethyl)amine) skeleton functionalized at the periphery with naphthyl and/or 5-dimethylamino-1-naphthalenesulfonamide (hereafter called dansyl) chromophores. Each dendrimer comprises one tertiary amine unit in the core and three branches carrying a sulfonimido unit at the periphery, each one substituted by two identical or different moieties. In particular, TD6 and TN6 contain dansyl (D) or naphthyl (N) units, respectively, while TD3B3, TN3B3 and TN3D3 contain dansyl, naphthyl or benzyl (B) units at the periphery. The spectroscopic behaviour of these dendrimers has been investigated in acetonitrile solution and compared with that of reference compounds. For all dendrimers the absorption bands are red shifted compared to those of monomeric naphthyl and dansyl reference compounds. Moreover, the intense naphthyl and dansyl fluorescence is greatly quenched because of strong interactions between the two aromatic moieties linked by a sulfonimido unit. Protonation of the amine units of the dendrimers by addition of CF(3)SO(3)H (triflic) acid causes a decrease in intensity of the luminescence and a change in the shape of the emission bands. The shapes of the titration curves depend on the dendrimer, but in any case the effect of acid can be fully reversed by successive addition of base (tributylamine). The obtained results reveal that among the intradendrimer interactions the most important one is that taking place (via mesomeric interaction) between the various chromophores and a pair of sulfonimido groups.

Mechanisms for fluorescence depolarization in dendrimers.

We have investigated the fluorescence properties of dendrimers (Gn is the dendrimer generation number) containing four different luminophores, namely terphenyl (T), dansyl (D), stilbenyl (S), and eosin (E). In the case of T, the dendrimers contain a single p-terphenyl fluorescent unit as a core with appended sulfonimide branches of different size and n-octyl chains. In the cases of D and S, multiple fluorescent units are appended in the periphery of poly(propylene amine) dendritic structures. In the case of E, the investigated luminophore is noncovalently linked to the dendritic scaffold, but is encapsulated in cavities of a low luminescent dendrimer. Depending on the photophysical properties of the fluorescent units and the structures of the dendrimers, different mechanisms of fluorescence depolarization have been observed: (i) global rotation for GnT dendrimers; (ii) global rotation and local motions of the dansyl units at the periphery of GnD dendrimers; (iii) energy migration among stylbenyl units in G2S; and (iv) restricted motion when E is encapsulated inside a dendrimer, coupled to energy migration if the dendrimer hosts more than one eosin molecule.

Fluorescent perylene diimide rotaxanes: spectroscopic signatures of wheel-chromophore interactions.

[2]- and [3]-rotaxanes with a tetraphenoxy perylene diimide core were synthesized. Hydrogen bonding between the wheel and the imide changes the optical properties of the perylene chromophore: the absorption and fluorescence spectra are red-shifted. The decay times of the rotaxanes are shorter in comparison with that of the axle. Single molecule fluorescence measurements reveal relatively narrow distributions of emission maxima and decay times. The averages are in agreement with ensemble measurements. The observed red shifts make the perylene diimide a suitable chromophore for sensing the position of the wheel on the axle.

Photoisomerization of azobenzene derivatives in nanostructured silica.

A series of derivatized azobenzene molecules are synthesized such that one of the phenyl groups can be chemically bonded to mesostructured silica and the other, derivatized with dendrons, is free to undergo large-amplitude light-driven motion. The silica frameworks on which the motion takes place are either 150 nm thick films containing ordered hexagonal arrays of tubes (inner diameter about 2 nm) containing the bonded azobenzenes, or particles (about 500 nm in diameter) containing the same ordered arrays of functionalized tubes. The photoisomerization yields and the rate constants for the thermal cis to trans back-reaction of the azobenzenes in the tubes are measured and compared to those of the molecules in solution. The rate constants decrease with increasing size of the dendrons. Fluorescence spectra of the cis and trans isomers in the pores show that the photoisomerization in the nanostructured materials is selectively driven by specific wavelengths of light and is reversible.

A cyclam core dendrimer containing dansyl and oligoethylene glycol chains in the branches: protonation and metal coordination.

We have synthesized a dendrimer (1) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core, appended with four benzyl substituents that carry, in the 3- and 5-positions, a dansyl amide derivative (of type 2), in which the amide hydrogen is replaced by a benzyl unit that carries an oligoethylene glycol chain in the 3- and 5-positions. All together, the dendrimer contains 16 potentially luminescent moieties (eight dansyl- and eight dimethoxybenzene-type units) and three distinct types of multivalent sites that, in principle, can be protonated or coordinated to metal ions (the cyclam nitrogen atoms, the amine moieties of the eight dansyl units, and the 16 oligoethylene glycol chains). We have studied the absorption and luminescence properties of 1, 2, and 3 in acetonitrile and the changes taking place upon titration with acid and a variety of divalent (Co2+, Ni2+, Cu2+, Zn2+), and trivalent (Nd3+, Eu3+, Gd3+) metal ions as triflate and/or nitrate salts. The results obtained show that: 1) double protonation of the cyclam ring takes place before protonation of the dansyl units; 2) the oligoethylene glycol chains do not interfere with protonation of the cyclam core and the dansyl units in the ground state, but affect the luminescence of the protonated dansyl units; 3) the first equivalent of metal ion is coordinated by the cyclam core; 4) the interaction of the resulting cyclam complex with the appended dansyl units depends on the nature of the metal ion; 5) coordination of metal ions by the dansyl units follows at high metal-ion concentrations; 6) the effect of the metal ion depends on the nature of the counterion. This example demonstrates that dendrimers may exhibit complete functionality resulting from the integration of the specific properties of their component units.