A heteroleptic iron(II) complex [Fe(dcpp)(ddpd)](2+) with a strongly electron-withdrawing ligand (dcpp, 2,6-bis(2-carboxypyridyl)pyridine) and a strongly electron-donating tridentate tripyridine ligand (ddpd, N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine) is reported. Both ligands form six-membered chelate rings with the iron center, inducing a strong ligand field. This results in a high-energy, high-spin state ((5) T2 , (t2g )(4) (eg *)(2) ) and a low-spin ground state ((1) A1 , (t2g )(6) (eg *)(0) ). The intermediate triplet spin state ((3) T1 , (t2g )(5) (eg *)(1) ) is suggested to be between these states on the basis of the rapid dynamics after photoexcitation. The low-energy π(*) orbitals of dcpp allow low-energy MLCT absorption plus additional low-energy LL'CT absorptions from ddpd to dcpp. The directional charge-transfer character is probed by electrochemical and optical analyses, Mößbauer spectroscopy, and EPR spectroscopy of the adjacent redox states [Fe(dcpp)(ddpd)](3+) and [Fe(dcpp)(ddpd)](+) , augmented by density functional calculations. The combined effect of push-pull substitution and the strong ligand field paves the way for long-lived charge-transfer states in iron(II) complexes.

The synthesis and photophysical properties of a ruthenium(II) complex bearing an electron-accepting 2,2';6',2''-terpyridine ligand and an electron-donating N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine (ddpd) ligand are presented. The heteroleptic complex is easily prepared isomerically pure and features intense low-energy metal-to-ligand charge-transfer (MLCT) absorption bands and intense room temperature (3)MLCT emission with a long (3)MLCT lifetime. The favorable photophysical properties are due to the strong ligand field imposed by the ddpd ligand.

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).

The push-pull-substituted bis(terpyridine)ruthenium(II) amino acid [Ru(4Ј-tpy-COOH)(4Ј-tpy-NH 2 )] 2+ ([5] 2+ ; tpy = 2,2Ј;6Ј,2ЈЈ-terpyridine) with carboxylic acid and amino substituents features exceptional chemical and photophysical properties. Its interaction with photons, electrons, and/or protons results in room-temperature phosphorescence, reversible oxidative and reductive redox chemistry, reversible acid/ base chemistry, proton-coupled electron transfer, photoinduced reductive and oxidative electron transfer, excited-state proton transfer and energy transfer reactions. These properties can be fine-tuned by variations of the bis(terpyridine) amino acid motif, namely extension of the π system and ex-[a] Institute www.eurjic.org MICROREVIEW curs. [1] The long excited-state lifetime (τ ≈ 1 μs) of the 3 MLCT state at room temperature in solution renders [Ru(bpy) 3 ] 2+ exceptionally suitable as a photoredox catalyst (Table 1). [38,45,46] The 3 MLCT state is emissive with a high phosphorescence quantum yield (Φ ≈ 10 %), which favors applications in light-emitting devices as luminescent sensors or as imaging agents. [46] The properties of [Ru(bpy) 3 ] 2+ can easily be tailored by modifications of the bpy ligand. However, the intrinsic Δ, Λ chirality of [Ru(bpy) 3 ] 2+ is a serious drawback when more than one bpy ligand is substituted or when more than one [Ru(bpy) 3 ] 2+ -type complexes are combined to form di-or oligonuclear complexes, because diastereomeric complexes (e.g. rac-Δ,Δ/Λ,Λ and meso-Δ,Λ) have to be separated or avoided by complicated synthetic procedures. [47][48][49] It is obvious that interaction with chiral molecules, such as DNA or proteins, will even modify the individual properties of Δ, Λ enantiomers, and any interaction with chiral biomolecules is complicated when using racemates, for example as anticancer drugs. [48d] Bis(tridentate) meridional coordination as in [Ru(tpy) 2 ] 2+ ([1] 2+ , Figure 2) avoids the formation of diastereomers even in the case of heteroleptic [Ru(tpy-R 1 )(tpy-R 2 )] 2+[50] and dinuclear complexes (tpy-R 1 , tpy-R 2 = 4Ј-substituted 2,2Ј;6Ј,2ЈЈ-terpyridine). [25,51,52] Furthermore, the stronger Aaron Breivogel received his diploma in chemistry at the Johannes Gutenberg University of Mainz, Germany, in 2009, and he is currently finishing his Ph.D. Thesis in the research group of Prof. Dr. Katja Heinze in inorganic chemistry. During his studies he spent one semester (2007/2008) at the University of Valencia, Spain, in the Department of Analytical Chemistry in the group of Prof. Dr. Miguel de la Guardia working on the quantitative determination of glycolic acid in cosmetics by online liquid chromatography and Fourier transform infrared spectroscopy. Currently he is working on the synthesis of bis(tridentate) complexes of ruthenium(II) and their applications in dye-sensitized solar cells and lightemitting electrochemical cells. He received a grant from the "International Research Training Group (IRTG) 1404 -Self-organized Materials for Optoelectronics" funded by t...

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