Highly Coupled Dyads Based on Phthalocyanine−Ruthenium(II) Tris(bipyridine) Complexes. Synthesis and Photoinduced Processes

A new series of multicomponent ZnPc−Ru(bpy)₃ systems, 1a−c, consisting of a zinc-phthalocyanine linked through conjugated and/or nonconjugated connections to a ruthenium(II) tris(bipyridine) complex, has been synthesized. The ruthenium complexes 1a−c were prepared from phthalocyanines 2a−c, bearing a 4-substituted-2,2‘-bipyridine ligand by treatment with [Ru(bpy)₂Cl₂]·2H₂O. Different synthetic strategies have been devised to prepare the corresponding dyad precursors (2a−c). Compound 2a, for example, with an ethenyl bridge, was synthesized by statistical condensation of 4-tert-butylphthalonitrile and 5-[(E)-2-(3,4-dicyanophenyl)ethenyl]-2,2‘-bipyridine (3) in the presence of zinc chloride. Compounds 2b and 2c, having, respectively, an amide or an ethynyl bridge, were prepared following a different synthetic approach. The method involves the coupling of an appropriate 5-substituted-2,2‘-bipyridine to an unsymmetrical phthalocyanine suitably functionalized with an amino (4) or an ethynyl group (5). The photophysical properties of the dyads that are ZnPc−Ru(bpy)₃ 1a−c and related model compounds have been determined by a variety of steady-state (i.e., fluorescence) and time-resolved methods (i.e., fluorescence and transient absorption). Clearly, intramolecular electronic interactions between the two subunits dominate the photophysical events following the initial excitation of either chromophore. These intramolecular interactions lead, in the case of photoexcited ZnPc, to faster intersystem crossing kinetics compared to a ZnPc reference, while photoexcited [Ru(bpy)₃]²⁺ undergoes a rapid and efficient transduction of triplet excited-state energy to the Pc.