Understanding the Excited State Behavior of Cyclometalated Bis(tridentate)ruthenium(II) Complexes: A Combined Experimental and Theoretical Study

The synthesis and characterization of the donor-acceptor substituted cydometalated ruthenium(II) polypyridine complex isomers [Ru(dpb-NHCOMe)(tpyCO OEt)](PF6) 1 (PF6) and [Ru(dpb-COOEt)(tpyNHCOMe)] (PF6) 2(PE6) (dpbH = 1,3-dipyridin-2-ylbenzene, tpy = 2,2';6,2"-terpyridine) with inverted functional group pattern are described. A combination of resonance Raman spectroscopic and computational techniques shows that all intense visible range absorption bands arise from mixed Ru -> tpy/Ru -> dpb metal-to-ligand charge transfer (MLCT) excitations. 2(PF6) is weakly phosphorescent at room temperature in fluid solution and strongly emissive at 77 K in solid butyronitrile matrix, which is typical for ruthenium(II) polypyridine complexes. Density functional theory calculations revealed that the weak emission of 2(PF6) arises from a (MLCT)-M-3 state that is efficiently thermally depopulated via metal-centered ((MC)-M-3) excited states. The activation barrier for the deactivation process was estimated experimentally from variable-temperature emission spectroscopic measurements as 11 kJ mol(-1). In contrast, 1(PF6) is nonemissive at room temperature in fluid solution and at 77 K in solid butyronitrile matrix. Examination of the electronic excited states of 1(PF6) revealed a ligand-to-ligand charge-transfer ((LL)-L-3'CT) as lowest-energy triplet state due to the very strong push-pull effect across the metal center. Because of the orthogonality of the participating ligands, emission from the (LL)-L-3'CT is symmetry-forbidden. Hence, in this type of complex a stronger push-pull effect does not increase the phosphorescence quantum yields but completely quenches the emission.