Excited State Tuning of Bis(tridentate) Ruthenium(II) Polypyridine Chromophores by Push-Pull Effects and Bite Angle Optimization: A Comprehensive Experimental and Theoretical Study.

The synergy of push-pull substitution and enlarged ligand bite angles has been used in functionalized heteroleptic bis(tridentate) polypyridine complexes of ruthenium(II) to shift the ¹MLCT absorption and the ³MLCT emission to lower energy, enhance the emission quantum yield, and to prolong the ³MLCT excited-state lifetime. In these complexes, that is, [Ru(ddpd)(EtOOC-tpy)][PF₆]₂, [Ru(ddpd-NH₂)(EtOOC-tpy)][PF₆]₂, [Ru(ddpd){(MeOOC)₃-tpy}][PF₆]₂, and [Ru(ddpd-NH₂){(EtOOC)₃-tpy}][PF₆]₂ the combination of the electron-accepting 2,2′;6′,2′′-terpyridine (tpy) ligand equipped with one or three COOR substituents with the electron-donating N, N′-dimethyl- N, N′-dipyridin-2-ylpyridine-2,6-diamine (ddpd) ligand decorated with none or one NH₂ group enforces spatially separated and orthogonal frontier orbitals with a small HOMO-LUMO gap resulting in low-energy ¹MLCT and ³MLCT states. The extended bite angle of the ddpd ligand increases the ligand field splitting and pushes the deactivating ³MC state to higher energy. The properties of the new isomerically pure mixed ligand complexes have been studied by using electrochemistry, UV/Vis absorption spectroscopy, static and time-resolved luminescence spectroscopy, and transient absorption spectroscopy. The experimental data were rationalized by using density functional calculations on differently charged species (charge n=0-4) and on triplet excited states (³MLCT and ³MC) as well as by time-dependent density functional calculations (excited singlet states). [ABSTRACT FROM AUTHOR]