Paola Ceroni

Dendrimers based on photoactive metal complexes. Recent advances

Abstract Recent advances in the field of photoactive dendrimers containing metal complexes are reviewed. Dendrimers with [Ru(bpy)3]2+ as a core exhibit the characteristic [Ru(bpy)3]2+-type luminescence that can be (i) protected from external quenchers by the dendrimer branches and (ii) sensitized by chromophoric groups contained in the periphery of the dendrimer (antenna effect). Several examples of dendrimers fully based on transition metal complexes (i.e., containing a metal at each branching point of the dendrimer structure) have been investigated with the purpose of light harvesting. Dendrimers containing one or more free base and metal porphyrin units have been investigated for light harvesting and for a variety of other purposes. Scattered examples of other types of photoactive dendrimers are also reviewed.

Energy Up‐Conversion by Low‐Power Excitation: New Applications of an Old Concept

Energy up-conversion is the process by which two absorbed photons are converted in one photon of higher energy. Such a process can be conveniently performed by low-power excitation through sensitized triplet-triplet annihilation and it is now an emerging technique with possible applications in different fields, including photovoltaic devices and bioimaging.

Fluorescent guests hosted in fluorescent dendrimers

Abstract We have investigated the formation of host–guest complexes between dendrimers of the poly(propylene amine) family functionalized with dansyl units at the periphery (hosts) with dye molecules (guests). Each dendrimer nD , where the generation number n goes from 1 to 5, comprises 2n+1 (i.e. 64 for 5D ) dansyl functions in the periphery and 2n+1−2 (i.e. 62 for 5D ) tertiary amine units in the interior. The most thoroughly investigated systems were those with eosin as a guest. The results obtained show that: (i) the nD dendrimers dissolved in dichloromethane solution extract eosin from aqueous solutions; (ii) the maximum number of eosin molecules hosted in the dendrimers increases with increasing dendrimer generation, up to a maximum of 12 for the 5D dendrimer; (iii) the fluorescence of the peripheral dansyl units of the dendrimers is completely quenched via energy transfer by the hosted eosin molecules; (iv) the fluorescence of the hosted eosin molecules is partially quenched; (v) the eosin molecules can occupy two different sites (or two families of substantially different sites) in the interior of the dendritic structure; (vi) excitation of eosin hosted in the dendrimers causes sensitization of the dioxygen emission via eosin triplet excited state. The behavior of fluorescein and rose bengal is qualitatively similar to that of eosin, whereas naphthofluorescein is not extracted. The maximum number of dye molecules extracted by the 4D dendrimer is 25 for rose bengal and ca. 1 for fluorescein, showing that the formation of host–guest species is related to the electronic properties rather than to the size of the dye molecules.

Turn-on Phosphorescence by Metal Coordination to a Multivalent Terpyridine Ligand: A New Paradigm for Luminescent Sensors

A hexathiobenzene molecule carrying six terpyridine (tpy) units at the periphery has been designed to couple the aggregation induced phosphorescence, displayed by the core in the solid state, to the metal binding properties of the tpy units. Upon Mg(2+) complexation in THF solution, phosphorescence of the hexathiobenzene core is turned on. Metal ion coordination yields the formation of a supramolecular polymer which hinders intramolecular rotations and motions of the core chromophore, thus favoring radiative deactivation of the luminescent excited state. Upon excitation of the [Mg(tpy)2](2+) units of the polymeric structure, sensitization of the core phosphorescence takes place with >90% efficiency. The light-harvesting polymeric antenna can be disassembled upon fluoride ion addition, thereby switching off luminescence and offering a new tool for fluoride ion sensing. This unique system can, thus, serve as cation or anion sensor.

Photoinduced reversible switching of porosity in molecular crystals based on star-shaped azobenzene tetramers

The development of solid materials that can be reversibly interconverted by light between forms with different physico-chemical properties is of great interest for separation, catalysis, optoelectronics, holography, mechanical actuation and solar energy conversion. Here, we describe a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their E-configuration, the porosity of which can be tuned by changing the peripheral substituents on the molecule. Efficient E→Z photoisomerization of the azobenzene units takes place in the solid state and converts the crystals into a non-porous amorphous melt phase. Crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrate that the photoisomerization enables reversible on/off switching of optical properties such as birefringence as well as the capture of CO2 from the gas phase. The linear design, structural versatility and synthetic accessibility make this new family of materials potentially interesting for technological applications.

Synthesis of a covalent monolayer sheet by photochemical anthracene dimerization at the air/water interface and its mechanical characterization by AFM indentation

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Synthesis of two-dimensional analogues of copolymers by site-to-site transmetalation of organometallic monolayer sheets

Monolayer sheets have gained attention due to the unique properties derived from their two-dimensional structure. One of the key challenges in sheet modification/synthesis is to exchange integral parts while keeping them intact. We describe site-to-site transmetalation of Zn(2+) in the netpoints of cm(2)-sized, metal-organic sheets by Fe(2+), Co(2+), and Pb(2+). This novel transformation was done both randomly and at predetermined patterns defined by photolithography to create monolayer sheets composed of different netpoints. All transmetalated sheets are mechanically strong enough to be spanned over 20 × 20 μm(2) sized holes. Density functional theory calculations provide both a model for the molecular structure of an Fe(2+)-based sheet and first insights into how transmetalation proceeds. Such transmetalated sheets with random and patterned netpoints can be considered as two-dimensional analogues of linear copolymers. Their nanoscale synthesis presents an advance in monolayer/polymer chemistry with applications in fields such as surface coating, molecular electronics, device fabrication, imaging, and sensing.

ENHANCED ACCEPTOR CHARACTER IN FULLERENE DERIVATIVES. SYNTHESIS AND ELECTROCHEMICAL PROPERTIES OF FULLEROPYRROLIDINIUM SALTS

A family of N-methylpyrrolidinium fullerene iodide salts, possessing one (or two) solubilizing chain(s), has been synthesized. As evidenced by CV and steady-state voltammetry experiments, carried out in tetrahydrofuran (THF) solutions, these species show enhanced electron-accepting properties with respect to both the parent fulleropyrrolidines and C60. CV measurements performed at −60 °C and scan rates of 50 V/s, made possible by the use of ultramicroelectrodes, have allowed the observation, for the first time in fullerene derivatives, of six fullerene-centered reductions. Bulk electrolysis of these novel compounds, performed at the stage of the first reduction potential, generates a stable zwitterion, with both anion and cation located on the fullerene derivative.

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.

Photoinduced Electron Transfer in a Tris(2,2′-bipyridine)-C60-ruthenium(II) Dyad: Evidence of Charge Recombination to a Fullerene Excited State

A novel fulleropyrrolidine covalently linked to a tris(2,2′-bipyridine)ruthenium(II) complex is presented (see picture). While electrochemical investigations suggest the absence of any ground-state interaction between ruthenium and fullerene chromophores, photoinduced optical absorption studies clearly show that electron transfer occurs to afford the Ru3+−C60 pair with characteristic fullerene radical anion band at λmax = 1040 nm.