A rotaxane is described in which a macrocycle moves reversibly between two hydrogen-bonding stations after a nanosecond laser pulse. Observation of transient changes in the optical absorption spectrum after photoexcitation allows direct quantitative monitoring of the submolecular translational process. The rate of shuttling was determined and the influence of the surrounding medium was studied: At room temperature in acetonitrile, the photoinduced movement of the macrocycle to the second station takes about 1 microsecond and, after charge recombination (about 100 microseconds), the macrocycle shuttles back to its original position. The process is reversible and cyclable and has properties characteristic of an energy-driven piston.

A 1,3-dipolar cycloaddition reaction of azomethine ylides to C 60 has been used to prepare a fulleropyrrolidine covalently linked to a substituted tris(2,2'bipyridine)ruthenium(ii) chromophore. Electrochemical studies revealed a single one-electron reversible oxidation of the ruthenium center and ten one-electron reversible reductions, five of them occurring at the C 60 core and five at the bipyridine (bpy) ligands. Steady-state fluorescence and time-resolved flash-photolytic investigations of dyad 6 are reported in solvents of different polarity. The emission in toluene/ CH 2 Cl 2 , CH 2 Cl 2 , and CH 3 CN was substantially quenched, relative to model complex 8, suggesting intramolecular quenching of the ruthenium MLCT excited state. Picosecond-resolved photolysis of 6 showed light-induced formation of the photoexcited ruthenium center, which undergoes rapid intramolecular electron transfer. Nanosecondresolved photolysis revealed a chargeseparated state (t 1/2 210 ns in CH 2 Cl 2 and t 1/2 100 ns in CH 3 CN) that decays to the ground state by regeneration of the ruthenium MLCT excited state in CH 2 Cl 2 or through the formation of the C 60 triplet excited state in CH 3 CN.

The electrochemical behavior of the family of mononuclear Re(I) complexes, [Re I (CO) 3 LX] n+ , with L ) 2,2′-bipyridine (bpy), 1,10-phenanthroline (phen) and X ) Cl, CN (n ) 0); ACN (n ) 1) and that of the metal-metal-bonded dimeric complexes [(L)(CO) 3 Re-Re(CO) 3 (L)] (L ) bpy, phen) has been studied in various aprotic solvents using cyclic voltammetry (CV), chronoamperometry, and spectroelectrochemistry. The wide cathodic potential window investigated has permitted the observation of the largest number of redox processes so far obtained for these species. A detailed description of the kinetics of both oxidation and reduction processes of such species is given. The electrochemistry of the complexes [Re(CO) 3 (L)CN] (L ) bpy, phen) is reported here for the first time. The electrochemical behavior of the monomeric species is greatly influenced by the nature of the monodentate ligand X. The concentration dependence of the cyclic voltammetric reductive behavior has evidenced the occurrence of bimolecular homogeneous processes. Such processes include homogeneous electron transfers and the competitive formation of metal-metal or µ-Cl bridged dimeric species. The influence of the solvent and that due to the presence or absence of Clions in solution is discussed. Digital simulation of the cyclic voltammetric curves is extensively used both for confirming the reaction mechanisms and for the evaluation of the relevant thermodynamic (E 1/2 ) and kinetic parameters. Linear correlation between spectroscopic and electrochemical data for the whole family of monomeric and dimeric complexes has been found.

The electrochemical behavior of a family of polynuclear ruthenium(II) bipyridine-type complexes with 2,3-bis(2-pyridyl)pyrazine (2,3-dpp) and 2,5-bis(2-pyridyl)pyrazine (2,5-dpp) as bridging ligands has been investigated in highly purified N,N-dimethylformamide solution. The compounds studied contain two, three, four, and six metal centers and have general formula [Ru n (bpy) n +2(2,X-dpp) n - 1]2 n +, where n = 2, 3, 4, or 6, X = 3 or 5, and bpy is 2,2‘-bipyridine. The wide cathodic potential window explored (up to ca. −3.1 V vs SCE) has allowed us to observe the most extensive ligand-centered redox series so far reported, comprising up to 26 reversible reduction processes for the hexanuclear complex. The redox standard potentials for overlapping processes in multielectron waves have been obtained from the analysis of the voltammetric curves and their digital simulation. The localization of each redox process and the mutual interactions of the redox centers have been elucidated through the analysis and comparison of the redox series of the various compounds. For the dinuclear species, the assignment of the redox sites has been confirmed by semiempirical molecular orbital calculations (ZINDO) and spectroelectrochemical experiments. Calculations also substantiate the important role played by bridging ligands in mediating the interactions between equivalent redox sites. Finally, it has been shown that the size of the supporting electrolyte cation has an influence on the processes occurring at the extreme cathodic region.