A number of novel salts of dipolar cations containing dimethylamino electron donor and pyridinium electron acceptor groups have been prepared and studied by using various physical techniques including hyper-Rayleigh scattering and Stark spectroscopy. In addition to showing very large molecular static first hyperpolarizabilities, several of these salts exhibit pronounced bulk quadratic nonlinear optical effects. Related compounds in which the electron donor is a ruthenium(II) ammine centre have also been investigated, allowing interesting comparisons to be made between transition metal-containing and purely organic nonlinear optical chromophores.
A series of complex salts in which trans-bis[1,2-phenylenebis(dimethylarsine)]chlororuthenium(II) electron donor groups are connected to pyridyl or pyridinium electron acceptors has been prepared. These chromophores exhibit intense, visible metal-to-ligand charge-transfer (MLCT) absorptions and reversible Ru(III/II) (and also in some cases ligand-based) redox processes. Stark (electroabsorption) spectroscopic studies have been used to determine dipole moment changes for the MLCT excitations. Static first hyperpolarizabilities have been calculated according to the two-state model, allowing the derivation of structure-activity correlations for the molecular quadratic nonlinear optical responses.
A series of 2,2':4,4'':4',4'''-quaterpyridinium ligands bearing different N-substituents (methyl, Me; phenyl, Ph; 4-acetylphenyl, AcPh; 2-pyrimidyl, Pym; 3,5-bis-methoxycarbonyl-phenyl, MCP) have been prepared. These compounds feature powerful electron acceptor moieties and hence have been combined with different electron-rich metal centres to give complexes suitable for quadratic nonlinear optical properties. A series of dipolar ruthenium(II) ammine complex salts of the form [RuII(NH3)4LA][PF6]4 and octupolar complex salts of the form [MII(LA)3][PF6]8 (M = Ru(II)/Fe(II), LA = a 2,2':4,4'':4',4'''-quaterpyridinium ligand) have been prepared. These compounds exhibit multiple intense, low energy metal-to-ligand charge-transfer (MLCT) absorptions in the visible region and reversible M(III/II) redox processes. First hyperpolarizabilities have been determined directly by using hyper-Rayleigh scattering and indirectly via calculation from Stark (electroabsorption) spectroscopic data.
A series of novel metal-organic in-plane complexes trans- (formula available in paper) have been investigated using the hyper-Rayleigh scattering technique. Like other in-plane complexes reported recently they exhibit very large and tunable static first hyperpolarizabilities which are associated with intense visible metal-to-ligand charge-transfer excitations. Moreover, a good correlation was found between the hyperpolarizabilities and the electrochemical properties of the complexes. Furthermore, the effect of the oxidation state of the metal upon the molecular optical nonlinearity has been investigated. Chemical oxidation of the metal is proven to be an excellent tool to reversibly switch the molecular first hyperpolarizabilities of the trans- substituted ruthenium complexes.
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