Two new fluorescent macrocyclic structures bearing two naphthalene (Np) units at both ends of a cyclic polyaminic chain were investigated with potentiometric, fluorescence (steady-state and time-resolved) and laser flash photolysis techniques. The fluorescence emission studies show the presence of an excimer species whose formation depends on the protonation state of the polyamine chains implying the existence of a bending movement (occurring in both the ground and in the first singlet excited state), which allows the two naphthalene units to approach and interact. For comparison purposes, one bis-chromophoric compound containing a rigid chain (piperazine unit) was also investigated. Its emission spectra shows a unique band decaying single exponentially thus showing that no excimer is formed. With the two new ligands, excimer formation occurs in all situations even at very acidic pH values when the protonation of the polyamine bridges is extensive. Coexistence of ground-state dimers with dynamic excimers was established based on steady-state and time-resolved fluorescence data. The energetics of excimer formation and dissociation were determined in ethanol and water. Different methods of decay analysis (independent decay deconvolution, global analysis and excimer deconvolution with monomer) were used to extract the kinetic (rate constants for excimer formation, dissociation, and decay) and thermodynamic parameters. In ethanol and acidified ethanol:water mixtures, an additional short decay time was found to exist and assigned to a dimer, whose presence is assumed to be responsible by the decrease in activation energy for excimer formation in this solvent. The results are globally discussed in terms of the small architectural differences that can induce significant changes in the photophysical behavior of the three studied compounds.

comprehensive investigation on the energetics and dynamics of a new fluorescent sensor constituted by a tripodal polyamine receptor containing three naphthalene fluorophores, compound L, is reported. The influence of external factors such as the solvent, hydrogen ion concentration, and temperature in the photophysics of the sensor is discussed. The temperature dependence of monomer/excimer interconversion of L revealed an average percentage relative sensitivity of 4.5%/degreesC thus portending its use as a temperature sensor. The activation energy for excimer formation (E-1 = 12 kJ mol(-1)) and dissociation (E-1 = 57 kJ mol(-1)), entropy change (DeltaS = -128 J K-1 mol(-1)), and the binding energy of the excimer (DeltaH = 45 kJ mol(-1)) were obtained in water at acidic pH values and ethanol (E-1 = 15 kJ mol(-1), E-1 = 40 kJ mol(-1), DeltaS = -61 J K-1 mol(-1), and DeltaH = 25 kJ mol(-1)). The dependence of the kinetic and thermodynamic parameters on the dielectric constant of the medium and on the degree of protonation of the polyamine chain was interpreted in terms of the excimer destabilization provoked by the electrostatic repulsion between the positively charged chains.

The synthesis of a new oxaaza macrocyclic ligand, L, derived from O-1,O-7-bis(2-formylphenyl)-1,4,7-trioxaheptane and tren containing an amine terminal pendant arm, and its metal complexation with alkaline earth (M = Ca2+, Sr2+, Ba2+), transition (M = Co2+, Ni2+, Cu2+, Zn2+, Cd2+), post-transition (M = Pb2+), and Y3+ and lanthanide (M = La3+, Er3+) metal ions are reported. Crystal structures of [H2L](ClO4)(2).3H(2)O, [PbL](ClO4)(2), and [ZnLCl](ClO4).H2O are also reported. In the [PbL] complex, the metal ion is located inside the macrocyclic cavity coordinated by all N4O3 donor atoms while, in the [ZnLCI] complex, the metal ion is encapsulated only by the nitrogen atoms present in the ligand. pi-pi interactions in the [H2L](ClO4)(2).3H(2)O and [PbL](ClO4)(2) structures are observed. Protonation and Zn2+, Cd2+, and Cu2+ complexation were studied by means of potentiometric, UV-vis, and fluorescent emission measurements. The 10-fold fluorescence emission increase observed in the pH range 7-9 in the presence of Zn2+ leads to L as a good sensor for this biological metal in water solution.

The fluorescence emission properties of a series of chemosensors containing a polyamine receptor bearing an anthracene signaling unit were studied. The fluorescence emission intensity is dependent on the protonation degree of the receptor, the fully protonated form exhibiting the highest emission intensity. By removing protons from the nitrogens a quenching effect can be observed, due to an electron-transfer from the amine to the excited fluorophore. The rate constant of the quenching process is exponentially dependent on the distance of the nitrogen from which the electron is transferred (beta = 0.6Angstrom(-1)). The ability of the chemosensors for signaling anions was tested through the model anion hexacyanocobaltate(III). The temperature dependence of the association constants shows that at least for this compound, the change in solvation entropy is probably the controlling parameter to account for the binding. (C) 2003 Elsevier Science B.V. All rights reserved.

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.

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.

Reaction of Re-2{mu-O2CC(CH3)(3)}(4)Cl-2 with [(CO)(5)M-CN]Na (M = Cr, Mo, W) leads to tetranuclear complexes of formula Re-2{mu-O2CC(CH3)(3)}(4)[-NC-M(CO)(5)](2) (M = Cr, Mo, W). These complexes were characterized by H-1-, C-13-, and Mo-95-NMR, IR and Raman spectroscopy, elemental analysis and examined by cyclic voltammetry. The applied methods show the donor capabilities of the [(CO)(5)M-CN](-) ligands which shift electron density towards the Re centers weakening the Re-Re quadruple bond. The Re-Re bond lengths and the v(Re-Re) force constants are estimated based on the FT-IR and Raman examinations. Photochemical examinations and TG/MS experiments have also been conducted. The latter methods shows that the product complexes decompose around 100 degrees C, by first loosing their carbonyl substituents; as do the Cr, Mo, W precursor compounds. The dirhenium tetrapivalate unit decomposes only at higher temperatures in a distinct second step.

Reacting transition metal complexes in low oxidation states, containing one or two cyanide ligands, with methyltrioxorhenium(VII) leads to bridged mixed metal compounds in good yields. The Re(VII) core is then surrounded by five or six ligands, respectively. The strength of these CN bridges and thus the stability of the newly generated bimetallic compound strongly depends on the donor strength of the ligands surrounding of the Cr/Mo/W or Fe moiety. The stability of the mixed metal molecules is reflected in the temperature dependent behavior of their O-17-NMR spectra, in their IR (Re=O) stretching frequencies and force constants, as well as several other spectroscopic data. UV-vis absorption spectra show the appearance of charge transfer bands. In the case of the mixed Mo/Re complexes the Mo-95-NMR spectroscopy is also a helpful tool to examine the donor capability of the Mo moiety. The described compounds also show photosensitivity. (C) 1998 Elsevier Science S.A. All rights reserved.