Book Description

The photophysical properties of two families of polypyridyl complexes of ruthenium were studied in both fluid solutions at 298 K and frozen glasses at 77 K. The absorption and emission spectra, excited state lifetimes and quantum yields have been obtained for poly-amidoamine based dendrimers modified with (4, 8, 16, 32 and 64) pendant-[Ru(tpy)2] +2 (tpy is 2,2':6',2 & Prime;-terpyridine) and [Ru(bpy)3]+2 (bpy is 2,2'-bipyridine) chromophores and for two 2,2 ':6',2 & Prime;-terpyridine bridging ligands, [ctpy-x-ctpy] and [ctpy-b-ctpy], with one, two or multiple Ru centers. In analogy to the behavior exhibited by the reference compounds ([Ru(tpy)2]+2 and [Ru(bpy) 3]+2), the bipyridine derivatives exhibit longer lifetimes and higher quantum yields when compared to the corresponding terpyridine complexes. In the case of dendrimers, generation-dependent changes were observed which are discussed in terms of the effects of solvent on the coupling between the emitting metal-to-ligand charge transfer state and the nearby metal-centered state. Solvent effects on the coupling were explored by comparing results in butyronitrile and dimethylacetamide with the latter being used as a mimic of the dendritic backbone. The results suggest that for the higher generations, the dendritic backbone acts as a solvent in affecting the photophysical behavior. For the family of chiral multimetallic compounds, the room temperature absorption and emission maxima of {[(( - )-ctpy)Ru]-( - )-[ctpy-b-ctpy]-[Ru(( - )-ctpy)]}(PF6)4 and (( - )-[ctpy-b-ctpy])-{[Ru(( - )-[ctpy-b-ctpy])](PF 6)2)n were shifted to lower energies and also exhibited significantly longer luminescence lifetimes when compared to the [Ru(( - )-ctpy)2](PF6)2, {[(( - )-ctpy)Ru]-( - )-[ctpy-x-ctpy]-[Ru(( - )- ctpy)]}(PF6)4 and (( - )-[ctpy-x-ctpy])-{[Ru(( - )-[ctpy-x-ctpy])](PF6) 2}n. The shifts in energy are discussed using a particle-in-a-box model. The luminescence lifetimes are discussed in terms of the accessibility of the metal-centered state. The Stern-Volmer quenching constants of the [Ru(bpy) 3]2+ pendant dendrimers were measured in the presence of three nitroaromatic quenchers (2,4,6-trinitrotoluene, 2,4-dinitrotoluene and nitrotoluene). The quenching efficiencies were calculated and found to peak for dend-16-Ru(bpy)3 corresponding to maximal chromophore accessibility. The quenching rates were dramatically enhanced by electrostatic attraction between the positively charged dendrimers and negatively charged quenchers (K3Fe(CN)6 and Na4Fe(CN)6). Investigation of the rotational anisotropy of second harmonic generation from dend-8-tpy/Fe2+ films deposited on a graphite substrate corroborated STM results showing the formation of highly ordered arrays.