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 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-NH 2)(EtOOC-tpy)][PF6]2, [Ru(ddpd){(MeOOC) 3-tpy}][PF6]2, and [Ru(ddpd-NH 2){(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). The synergy of push-pull substitution (R1-R3) and enlarged ligand bite angles lowers the energy of metal-to-ligand charge transfer (MLCT) states and increases the energy of metal-centered (MC) states in isomerically pure, directional, and multifunctional heteroleptic bis(tridentate) polypyridine complexes of RuII (see figure). These combined effects promote long-lived low-energy emission in fluid solution at RT (λmax up to 788 nm; Φ up to 1.1 %; τ up to 841 ns) approaching the values of the ubiquitous [Ru(bpy)3]2+ complex (bpy=2,2'-bipyridine). Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.