Reversible or equilibrated energy-transfer between two chromophores with similar excited-state energies has also been used as a strategy for extending excited-state lifetimes of both Rubpy and Ruterpy analogues and has been the topic of several excellent recent reviews.141-143 Conjugates of a ruthenium-polypyridyl chromophore with an organic chromophore that has a triplet excited state similar in energy to the 3MLCT state can show reversible triplet-triplet energy transfer between the two chromophores, in essence leading to a ligand-centered triplet state (3LC) that is thermally equilibrated with the 3MLCT state.144 A particularly striking example of this effect is seen by comparing the luminescent lifetime of [Ru(bpy)3]2+ and compounds 7 and 8. Luminescent lifetimes at 298 K in deaerated MeCN increase from 0.920 |is
for [Ru(bpy)3]2+ to 23.7 |is and 148 |is for 7 and 8, respectively.145,146 Pyrene is by far the most organic chromophore used for such applications as its lowest triplet state is nearly isoenergetic with Rubpy 3MLCT state. Recently, Castellano and co-workers reported that the piperindinyl-naphthalimide (PNI) chormophore is also functional in this capacity with the complexes 9 and 10 exhibiting lifetimes on the order of 47 and 115 |is, respectively.147 This mechanism of transiently 'storing' the excited state as an organic triplet can greatly enhance the lifetime and therefore the utility of the inorganic chromophore, however, it should be noted that the equilibrium between the 3MLCT state and the 3LC state is easily perturbed by the solvent polarity and therefore the lifetimes are usually very sensitive to the solvent.
Compound 11 presents an interesting case in which the 3LC chromophore in equilibrium with the 3MLCT state is not so easily discerned. Chiorboli et. al. report a increase in excited state lifetime for compound 11 from 5 ns in MeCN to 1.3 |is in CH2Q2.148,149 In this case the organic chromophore is the tetraazapentacene backbone which is fused to two bipyridines to form the bridging tatpp ligand. Such intimate linking between the two chromophores typically leads to strong electronic coupl-ing,150 however in this case the electronic coupling between the tetra-azapentacene-like component and the bpy-like components is surprisingly weak. While unusual for most fused aromatic systems, this weak coupling between the bpy-like and phena-zine-like components is commonly observed in the complexes of closely related and well-studied dipyrido-[3,2-a:2',3'-c]-phenazine (dppz)151-156 and tetrapyrido-[3,2-a:2',3'-c:3",2"-h:2"',3"'-j]-phenazine (tpphz) ligands.157-159
Hanan and coworkers have applied the bichromophoric approach with considerable success to the Ruterpy complexes.103,160 Compound 12 shows excited state equilibrium between an anthracene 3LC state and a 'Ruterpy' 3MLCT state leading to room temperature luminescent lifetime of 1052 ns which is compared to 250 ps for [Ru(terpy)2]2+.102
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