文摘
The synthesis and characterization of the donor鈥揳cceptor substituted cyclometalated ruthenium(II) polypyridine complex isomers [Ru(dpb-NHCOMe)(tpy-COOEt)](PF6) 1(PF6) and [Ru(dpb-COOEt)(tpy-NHCOMe)](PF6) 2(PF6) (dpbH = 1,3-dipyridin-2-ylbenzene, tpy = 2,2鈥?6,2鈥?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 鈫?tpy/Ru 鈫?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 3MLCT state that is efficiently thermally depopulated via metal-centered (3MC) excited states. The activation barrier for the deactivation process was estimated experimentally from variable-temperature emission spectroscopic measurements as 11 kJ mol鈥?. 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 (3LL鈥睠T) as lowest-energy triplet state due to the very strong push鈥損ull effect across the metal center. Because of the orthogonality of the participating ligands, emission from the 3LL鈥睠T is symmetry-forbidden. Hence, in this type of complex a stronger push鈥損ull effect does not increase the phosphorescence quantum yields but completely quenches the emission.
