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

Abstract

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