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 (MLCT)-M-1 absorption and the (MLCT)-M-3 emission to lower energy, enhance the emission quantum yield, and to prolong the (MLCT)-M-3 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 (MLCT)-M-1 and (MLCT)-M-3 states. The extended bite angle of the ddpd ligand increases the ligand field splitting and pushes the deactivating (MC)-M-3 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 ((MLCT)-M-3 and (MC)-M-3) as well as by time-dependent density functional calculations (excited singlet states).