Photochemistry and Photophysics of Coordination Compounds

156 S. Campagna et al.
[Ru(bpy)3] 2+ parent compound, because the dendrimer branches protect the
Ru–bpy-based core from dioxygen quenching [247].
More recently, dendrimers based on {Ru(phen)3} 2+ or {Ru(bpy)2(phen)} 2+
(phen = 1,10-phenanthroline) cores with appended carbazole chromophoric
units have been investigated (see, e.g., 33) [248]. In these compounds, energy
transfer from the peripheral carbazole units to the metal-based core occurs
with ca. 100% efficiency, with sensitization of the 3 MLCT luminescence at ca.
630 nm.
The {Ru(bpy)3} 2+ core was used to build a first-generation dendrimer
containing 12 coumarin 450 units in the periphery (34). Inacetonitrilesolution,
excitation of the coumarin 450 chromophores resulted in luminescence
emission at 625 nm, typical of the 3 MLCT excited state of the {Ru(bpy)3} 2+
core, with only a minor residual fluorescence of the initially excited chromophores.
An antenna effect is thus operative, with an estimated intramolecular
energy transfer efficiency close to unity [249]. Using the same dendrimer
and the [Ru(dmb)3] 2+ (dmb = 4,4 ′ -methyl-2,2 ′ -bipyridine) complex, which
served as a model for the “naked” dendrimer core, it was possible to investigate
the shielding effect of the dendritic structure on the intermolecular energy
and electron transfer processes [250]. Bimolecular quenching constants
were measured in acetonitrile solution for dioxygen, 9-methylanthracene
(MA), phenothiazine (PTZ), and methyl viologen (MV 2+ ), using three dif-