Excited State Dynamics of Two New Ru(II) Cyclometallated Dyes: Relation to Cells for Solar Energy Conversion and Comparison to Conventional Systems

Abstract

The preparation, characterization, and photophysical properties of a series of ruthenium(II) complexes possessing the cyclometallating deprotonated 2-phenyl pyridine ligand, phpy–, together with dppn (benzo[i]dipyrido[3,2-a:2′,3′-c]phenazine), a ligand with an extended π-system, are reported. Related complexes have been used as efficient dyes in dye-sensitized solar cells (DSSCs), and the Ru→dppn metal-to-ligand charge transfer (MLCT) absorption of the new complexes [Ru(phpy)(bpy)(dppn)]+ (4) and [Ru(phpy)(dppn)2]+ (5) is red-shifted relative to the Ru→bpy MLCT peak in [Ru(phpy)(bpy)2]+ (3). These new compounds are compared to conventional complexes where phpy– is replaced by 2,2′-bipyridine (bpy), including [Ru(bpy)3]2+, [Ru(bpy)2(dppn)]2+ (1), and [Ru(bpy)(dppn)2]2+ (2). Unlike 1 and 2, with long-lived dppn-centered 3ππ* excited states (τ ∼ 20 μs), the corresponding cyclometallated complexes 4 and 5 exhibit weakly emissive Ru→dppn 3MLCT states with transient absorption lifetimes of 25 and 45 ps, respectively, which are significantly shorter than that of 3, ∼9 ns. Although it is desirable to shift the absorption of ruthenium dyes used in DSSCs to lower energies, it is evident from this work, that for cyclometallated phpy– complexes, lowering the energy of the 3MLCT state below that of 3 results in significant shortening of the excited state lifetime. The fast excited state decay, together with the lower energy of the 1MLCT state, may result in lower charge injection efficiencies from these types of complexes.