Carlo Bruno

Three State Redox-Active Molecular Shuttle That Switches in Solution and on a Surface

Although the desirability of developing synthetic molecular machine systems that can function on surfaces is widely recognized, to date the only well-characterized examples of electrochemically switchable rotaxane-based molecular shuttles which can do so are based on the tetracationic viologen macrocycle pioneered by Stoddart. Here, we report on a [2]rotaxane which features succinamide and naphthalene diimide hydrogen-bonding stations for a benzylic amide macrocycle that can shuttle and switch its net position both in solution and in a monolayer. Three oxidation states of the naphthalene diimide unit can be accessed electrochemically in solution, each one with a different binding affinity for the macrocycle and, hence, corresponding to a different distribution of the rings between the two stations in the molecular shuttle. Cyclic voltammetry experiments show the switching to be both reversible and cyclable and allow quantification of the translational isomer ratios (thermodynamics) and shuttling dynamics (kinetics) for their interconversion in each state. Overall, the binding affinity of the naphthalene diimide station can be changed by 6 orders of magnitude over the three states. Unlike previous electrochemically active amide-based molecular shuttles, the reduction potential of the naphthalene diimide unit is sufficiently positive (-0.68 V) for the process to be compatible with operation in self-assembled monolayers on gold. Incorporating pyridine units into the macrocycle allowed attachment of the shuttles to an acid-terminated self-assembled monolayer of alkane thiols on gold. The molecular shuttle monolayers were characterized by X-ray photoelectron spectroscopy and their electrochemical behavior probed by electrochemical impedance spectroscopy and double-potential step chronoamperometry, which demonstrated that the redox-switched shuttling was maintained in this environment, occurring on the millisecond time scale.

Growth of p- and n-Dopable Films from Electrochemically Generated C60 Cations

The formidable electron-acceptor properties of C60 contrast with its difficult oxidations. Only recently it has become possible to achieve reversibility of more than one electrochemical anodic process versus the six reversible cathodic reductions. Here we exploit the reactivity of electrochemical oxidations of pure C60 to grow a film of high thermal and mechanical stability on the anode. The new material differs remarkably from its precursor since it conducts both electrons and holes. Its growth and properties are consistently characterized by a host of techniques that include atomic force microscopy (AFM), Raman and infrared spectroscopies, X-ray-photoelectron spectroscopy (XPS), secondary-ion mass spectrometry (SIMS), scanning electron microscopy and energy-dispersive X-ray analysis (SEM-EDX), matrix-assisted laser desorption/ionization (MALDI), and a variety of electrochemical measurements.

Synthesis, photophysical, electrochemical, and electrochemiluminescent properties of 5,15-bis(9-anthracenyl)porphyrin derivatives

Novel 5,15-bis(9-anthracenyl)porphyrin derivatives (, ) were synthesized by stepwise Suzuki-type coupling reactions using anthracenyl-boronates bearing various electronically active moieties. Absorption spectra of these porphyrin conjugates reveal some degree of delocalisation with the directly linked chromophores, particularly in the case of anthracenyl-porphyrin bearing dimethylanilino moieties at the two extremities. Fluorescence and 77 K phosphorescence properties indicate that the excitation energy is invariably funnelled to the lowest singlet and triplet states of the porphyrin chromophore. The latter levels have been probed also by transient absorption spectroscopy, showing the typical triplet features detected in meso-substituted porphyrins. Extensive electrochemical studies have been performed to unravel the electronic properties of the newly synthesized porphyrins. Low-temperature cyclic voltammetry investigations showed that the anthracenyl-porphyrins are capable of undergoing as many as four electron transfer processes. In particular, by means of UV-Vis-NIR spectroelectrochemical measurements, a NIR-centred intramolecular photoinduced intervalence charge transfer (IV-CT) from a neutral N,N-dimethylanilino moiety to the N,N-dimethylanilino radical cation has been observed for the doubly-oxidised porphyrin (2+). The molecules also showed unexpected electrogenerated chemiluminescence properties, which revealed to be largely controlled by the electronic characteristics of the peripheral anthracenyl substituents. The structural and the electronic properties of these complexes have been also characterised by DFT calculations, as well as by X-ray crystallographic analyses.