Intramolecular electron (eT) and energy transfer (ET) have shown to occur in a covalently linked donor-acceptor (CLDA) system consisting of a naphthalene donor covalently linked through a polyamine chain connector to an anthracene acceptor; the connector has been chosen in order to switch ON or OFF the energy flux as a function of its protonation state as well as by co-ordination to Zn2+. The largest energy transfer efficiency (eta = 0.61) occurs for the fully protonated form (pH < 2), while at pH > 9 (eT) from the lone pairs of the nitrogens to the excited fluorophore takes place, leading to complete quenching of the emission. On the other hand at neutral and basic pH values. coordination of Zn2+ prevents the eT quenching allowing the ET process to occur. (C) 2002 Elsevier Science B.V. All rights reserved.

In this work a new water-soluble long-lifetime chemosensor, containing a polyamine unit connected to a complexed Ru(II) metal center, is described. Its crystal structure has been characterized by X-ray analysis. The polyamine macrocyclic unit is capable of anchoring cationic or anionic substrates, according to its protonation state. Examples of electron transfer involving the ruthenium complex core and the bound substrate are presented. The photocatalytic ability of such a system is illustrated by the oxidation of iodide to iodine promoted by light absorption at 436 nm.

The reaction of the cyanide anion [M(CO)(5)CN](-) (M = Cr or Mo) with metallocenes of Groups 4 and 6 produced polynuclear complexes of the type [CpCp 'M(CO){-NC-M ' (CO)(5)}]BF4 (M = M0, W; M ' = Mo, Cr, Cp '= Cp, Ind), Cp2TiCl{-NC-Mo(CO)(5)} and Cp2Ti{-NC-Mo(CO)(5)}(2). These complexes were characterised by H-1-, C-13- and Mo-95-NMR, IR and UV-vis spectroscopies, elemental analysis and examined by cyclic voltammetry. These methods show that the [M(CO)(5)CN]- ligands shift the electron density towards the metallocene centres. The complex [Cp2W(CO){-NC-Mo(CO)(5)}](+) is additionally examined by single crystal X-ray structure determination. The Density Functional Theory (DFT) calculations with the ADF program were performed on selected compounds to understand the nature of the redox processes taking place. Compared with a nitrile, the coordination of a [M(CO)-,CN]- fragment to the metallocene moiety does not significantly change the geometrical features. but leads to the stabilisation of the HOMO of the latter. with all the oxidation processes occurring in the pentacarbonyl moiety of the binuclear species. Time-dependent DFT calculations were used to identify the band appearing in the visible spectrum of Cp2TiCl{-NC-Mo(CO)(5)} as a Mo to Ti charge transfer. (C) 2001 Elsevier Science BN. All rights reserved.

The synthesis of the macrocyclic ligand 4,4 '-(2,5,8, 11,14-pentaaza[15])-2,2 ' -bipyridylophane (L3), which contains a pentaamine chain linking the 4,4 ' -positions of a 2,2 ' -dipyridine moiety, is reported. Protonation and Zn(II) complexation by L3 and by macrocycle L2, containing the same pentaamine chain connecting the 6,6 ' -positions of 2,2 ' -dipyridine, were studied by means of potentiometric, UV-vis, and fluorescent emission measurements. While in L2 all the nitrogen donor atoms are convergent inside the macrocyclic cavity, in L3 the heteroaromatic nitrogen atoms are located outside. Both ligands form mono- and dinuclear Zn(II) complexes in aqueous solution. In the mononuclear Zn(II) complexes with L2, the metal is coordinated inside the macrocyclic cavity, bound to the heteroaromatic nitrogen donors and three amine groups of the aliphatic chain. As shown by the crystal structure of the [ZnL2](2+) complex, the two benzylic nitrogens are not coordinated and facile protonation of the complex takes place at slightly acidic pH values. Considering the mononuclear [ZnL3](2+) complex, the metal is encapsulated inside the cavity, not coordinated by the dipyridine unit. Protonation of the complex occurs on the aliphatic polyamine chain and gives rise to translocation of the metal outside the cavity, bound to the, heteroaromatic nitrogens.

Coordination/detachment of a pendent functionality in the Zn(ii) complex with a macrocyclic ligand L gives rise to on/off switching of exciplex emission, defining an elementary molecular machine whose movements are driven by both pH and light.