Veronica Vicinelli

A Copper(I) Bis‐phenanthroline Complex Buried in Fullerene‐Functionalized Dendritic Black Boxes

Virtual inaccessibility to external contact was revealed by electrochemical investigations for a bis(1,10-phenanthroline)copper(I) core embedded in dendrimers with up to 16 peripheral fullerene units (shown schematically). With increasing numbers of fullerene units, less and less light is available to the core, and the small quantity of light energy that reaches the central Cu(I) complex is returned to the external fullerenes by energy transfer-the central core is buried in a dendritic black box.

Dendrimers with a cyclam core. Absorption spectra, multiple luminescence, and effect of protonation

Abstract We have synthesized two dendrimers ( 4 and 5 ) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam) core appended with four dimethoxybenzene and eight naphthyl units ( 4 ) and 12 dimethoxybenzene and 16 naphthyl units ( 5 ). The absorption and luminescence spectra of these compounds and the changes taking place upon protonation of their cyclam core have been investigated. In acetonitrile-dichloromethane 1:1 v/v solution they exhibit three types of emission bands, assigned to naphthyl localized excited states ( λ max =337 nm), naphthyl excimers ( λ max ca 390 nm), and naphthyl-amine exciplexes ( λ max =480 nm). The tetraamine cyclam core undergoes only two protonation reactions, whose constants have been obtained by fitting the spectral changes. Protonation not only prevents exciplex formation for electronic reasons, but also causes strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers.

A Dendritic Antenna for Near-Infrared Emission of Nd3+ Ions

An interior of 18 amide groups and a periphery functionalized with 24 dansyl groups forms a light-harvesting dendrimer which features intense absorption bands in the near-UV spectral region and a strong fluorescence band in the visible region. Upon encapsulation of Nd(3+) ions, the fluorescence of the dansyl groups is quenched and an intense sensitized near-infrared emission of Nd(3+) is observed. The associated energy transfer is shown in the cartoon.

Dendrimers with a 4,4′-bipyridinium core and electron-donor branches. Electrochemical and spectroscopic properties

The synthesis, characterization, and electrochemical and spectroscopic properties of a new family of dendrimers comprising a 4,4′-bipyridinium core and 1,3-dimethyleneoxybenzene-type dendrons are described. The electrochemical results show that there is no significant inhibition of electron transfer involving the dendrimer core by the dendrons. The strong fluorescence of the 1,3-dimethyleneoxybenzene units of the branches is completely quenched as a result of charge-transfer interactions with the core.

Synthesis and electronic properties of donor-linked fullerenes towards photochemical molecular devices

a ´´ ´ Abstract We report our recent progress in the synthesis and the study of donor-linked fullerenes. The synthesis of C polyadducts 60 with well defined three-dimensional structure provides versatile building blocks for the preparation of porphyrin-fullerene derivatives with unusual stereochemical and electronic properties. Phenanthroline ligands substituted with fullerene- functionalized dendritic wedges and their copper(I) complexes have been prepared. Electrochemical and photophysical investigations have shown that the bis(phenanthroline)copper(I) central core is somehow buried in a dendritic black box. Finally, fullerene-oligophenylenevinylene hybrids have been synthesised in order to provide a molecular approach to photovoltaic conversion, and to prevent the morphology problems resulting from the phase separation in donor-acceptor composites.

Electronic properties of oligophenylenevinylene and oligophenyleneethynylene arrays constructed on the upper rim of a calix[4]arene core

Heck and Sonogashira cross-coupling reactions have been used for the functionalization of the upper rim of a tetraiodinated calix[4]arene. In this way, oligophenylenevinylene (OPV) and oligophenyleneethynylene (OPE) arrays have been constructed on the calix[4]arene core to produce covalent assemblies of four π-conjugated chromophores. Electrochemical properties have been investigated by cyclic voltammetry in different solvent/electrolyte systems. Both OPV and OPE calixarenes show the simultaneous reduction of three out of four units, followed, at more negative potential, by the reduction of the fourth unit. This behaviour can be rationalized in view of a partial deconjugation in one OPV or OPE unit. On the other hand, these calixarenes show only oxidation processes corresponding to the exchange of one electron, thus demonstrating the lack of electrochemical equivalency of the OPV or OPE units, in contrast to the reduction behaviour. Electronic absorption and emission spectra have been recorded in solvents of different polarity (toluene, dichloromethane, and benzonitrile). The absorption spectra of the OPV calixarene do not match to the sum of the component units as a consequence of the partial deconjugation of the OPV arms in the calixarene structure. On the contrary, fluorescence spectra, quantum yields and excited state lifetimes of the OPV calixarene are nearly identical to those of the corresponding model compound. The trend of the absorption and emission spectra for the OPE calixarene is reversed relative to that of the OPV counterpart. Absorption spectra are well-matched with those of four trimeric OPE fragments but emission spectra exhibit, besides the typical OPE monomeric fluorescence, a broader and longer-lived emission feature on the low-energy spectral side, attributable to excimer-type interactions. Therefore the arrangement of OPV or OPE units within the same calix-4-arene skeleton brings about different effects on the electronic and luminescence properties of the multicomponent system.

Cyclam-based dendrimers as ligands for lanthanide ions

We have investigated the complexation of lanthanide ions (Nd3+, Eu3+, Gd3+, Tb3+, Dy3+) with three cyclam-based ligands (cyclam = 1,4,8,11-tetraazacyclotetradecane), namely 1,4,8,11-tetrakis(naphthylmethyl)cyclam (1), and two dendrimers consisting of a cyclam core appended with four dimethoxybenzene and eight naphthyl units (2) and twelve dimethoxybenzene and sixteen naphthyl units (3). In the free ligands the fluorescence of the naphthyl units is strongly quenched by exciplex formation with the cyclam nitrogens. Complexation with the metal ions prevents exciplex formation and revives the intense naphthyl fluorescence. Fluorescence and NMR titration experiments have revealed the formation of complexes with different metal/ligand stoichiometries in the case of 1, 2 and 3. Surprisingly, the large dendrimer 3 gives rise to a stable [M(3)3]3+ species. Energy transfer from the lowest singlet and triplet excited states of the peripheral naphthyl units to the lower lying excited states of Nd3+, Eu3+, Tb3+, Dy3+ coordinated to the cyclam core does not take place.

Photochemical and photophysical properties of a poly(propylene amine) dendrimer functionalised with E-stilbene units

A second generation poly(propylene amine) dendrimer functionalized at the periphery with eight E-stilbene and eight 4-tert-butylbenzenesulfonyl units has been prepared. The absorption spectrum, fluorescence spectrum and decay, E<==>Z photoisomerization, and photocyclization of the Z-isomer of the stilbene units have been investigated in air equilibrated acetonitrile solutions. For comparison purposes, a reference compound of the peripheral dendrimer units, namely 4-tert-butyl-N-propyl-N-(4-styryl-benzyl)-benzenesulfonamide, has also been studied. The quantum yield of the E-->Z photoisomerization reaction (0.30) and the fluorescence quantum yield of the E isomer (0.014) are substantially smaller for the units appended to the dendrimer compared to those of the reference compound (0.50 and 0.046, respectively). The presence of a red tail and the biexponential decay of the emission band of the dendrimer indicate formation of excimers between the stilbene units appended at the poly(propylene amine) dendritic structure. Under the experimental conditions used (lambda(exc)= 313 nm), a Z/E photostationary state (around 9 : 1 for both reference compound and dendrimer ) is reached in the time scale of minutes. On continuing irradiation, other photoreactions take place in the time scale of hours: the stilbene moiety of compound undergoes photocyclization to phenanthrene (quantum yield 0.015), whereas in dendrimer photocyclization to phenanthrene is accompanied by other processes, including a photoreaction involving the internal amine groups.

Simple and Dendritic Cyclam Derivatives. Photophysical Properties, Effect of Protonation and Zn2+ Coordination, Preliminary Screening as Inhibitors of Tumour Cell Growth

We have synthesized two novel dendrimers (BG1 and BG2) consisting of a 1,4,8,11-tetraazacyclotetradecane (cyclam, 1) core with appended four dimethoxybenzene and eight benzyl units (BG1) and twelve dimethoxybenzene and sixteen benzyl units (BG2). The absorption and luminescence spectra of these compounds and the changes taking place upon protonation and Zn2+ coordination of their cyclam core have been investigated in acetonitrile-dichloromethane 1:1 v/v solution. For comparison purposes, the absorption and luminescence spectra of 1,4,8,11-tetrabenzyl-cyclam (2), and dendrons BD1 and BD2, model compounds of the branches of BG1 and BG2 respectively, have also been studied. BD1, BD2, BG1, and BG2 exhibit the absorption and emission spectra of their 1,3-dimethoxybenzene unit, but in the two dendrimers the emission intensity is quenched by the cyclam amine groups and increases upon protonation and metal coordination. In order to test if these cyclam derivatives have an antitumour effect, we have studied their action on proliferation in the human neuroblastoma TS12 cell line. Screening experiments have shown that cell proliferation was (i) strongly reduced by the tetrabenzyl substituted cyclam 2, and (ii) unaffected by cyclam and the benzo dendrimers BG1 and BG2. Antitumour screening experiments have also been performed on the tetranaphthyl substituted cyclam 3 and the naphtho-dendrimer NG2, whose photophysical properties have been previously studied. Cell proliferation came out to be moderately reduced by 3, whereas dendrimer NG2 had no effect, similar to dendrimers BG1 and BG2.