ARTICLE DOI: 10.1002/zaac.200900303 Synthesis and Characterization of Extended Bis(terpyridine)ruthenium Amino Acids Katja Heinze,* [a] Klaus Hempel, [b] and Aaron Breivogel [a] Keywords: Amino Acids; Charge transfer; Donor-acceptor systems; Ruthenium; Terpyridine Abstract. (Oligopyridine)ruthenium(II) complexes have been widely used in dye sensitized solar cells and other sophisticated optical devi- ces due to their outstanding photophysical properties and their chemi- cal stability. Herein, we describe the longitudinal extension of our pre- viously reported bis(terpyridine)ruthenium(II) amino acid [Ru(tpy– NH 2 )(tpy–COOH)] 2+ (tpy = 4'-substituted 2,2':6',2''-terpyridine) by in- Introduction Artificial systems mimicking parts of the tremendously com- plex natural photosystem [1], e.g. light harvesting, energy trans- fer, charge separation, and electron transfer have attracted con- siderable interest in the past years because of potential applications in energy conversion devices such as dye sensitized solar cells [2, 3]. (Oligopyridine)ruthenium(II) complexes are es- pecially attractive candidates as they combine tunable photo- physical properties with high chemical stability and synthetic flexibility [4–11]. Complexes based on the [Ru(bpy) 3 ] 2+ motif (bpy = 2,2'-bipyridine) are advantageous because of the excellent photophysical properties (long lived excited charge transfer state) [9]. However, from a synthetic viewpoint [Ru(tpy) 2 ] 2+ type com- plexes (tpy = 2,2':6',2''-terpyridine) are preferable as they are achiral and incorporation into multinuclear systems is feasible avoiding the formation of diastereomers [5–8]. Several success- ful concepts to improve the excited state properties of [Ru(tpy) 2 ] 2+ type complexes have been described in the litera- ture, one being a push-pull substitution pattern at the two terpyri- dine ligands tpy–X / tpy–Y [12–15]. Moreover, the substituents X, Y at the two tpy ligands are positioned in a well-defined relative orientation e.g. forming an exact 180° X–Ru–Y angle. This is quite difficult to achieve with bidentate X–bpy and Y–bpy ligands * Prof. Dr. K. Heinze Fax: +49-6131-3927277 E-Mail: katja.heinze@uni-mainz.de [a] Institute of Inorganic and Analytical Chemistry Johannes Gutenberg-University of Mainz Duesbergweg 10–14 55128 Mainz, Germany [b] Department of Inorganic Chemistry Ruprecht Karls-University of Heidelberg Im Neuenheimer Feld 270 69120 Heidelberg, Germany Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/zaac.200900303 or from the author. Z. Anorg. Allg. Chem. 2009, 635, 2541–2549 © 2009 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim 2541 sertion of para-phenylene spacers –C 6 H 4 – between the terpyridine and the functional groups. The influence of the para-phenylene spacer on the absorption and emission properties is investigated using UV/Vis absorption and emission spectroscopy and is discussed within a quali- tative molecular orbital picture. coordinated to ruthenium(II). However, a well-defined orienta- tion is essential for efficient vectorial electron or energy transfer. Peptides are natural vectorial systems and ruthenium(II)-con- taining artificial amino acids A–E have been designed for in- corporation into peptides (Scheme 1) [16–20]. The placement of a ruthenium complex fragment at the side chain of an amino acid (B–E) [17–20] results in poorly defined relative orienta- tions and prevents an effective electronic interaction with other functional units. Recently, we have introduced the ruthe- nium(II) amino acid A, in which the metal is placed between the functional groups [16]. Multifunctional systems based on A show an interaction between appended functional groups and the [Ru(tpy) 2 ] 2+ moiety. For example, ferrocenyl appended [Ru(tpy) 2 ] 2+ amides show photoinduced electron transfer (PET) from the ferrocenyl substituent to the ruthenium moiety upon irradiation into the MLCT band of the [Ru(tpy) 2 ] 2+ unit [16]. Chromophore appended bis(terpyridine)ruthenium am- ides display energy transfer upon excitation of the chromo- phore (e.g. coumarin) at higher energy [21]. In rod-like multi- nuclear systems composed of derivatives of A with well- defined relative orientations an electronic communication be- tween the subunits might be expected. “Expanded oligo-l-leucines” F (Scheme 2) were synthesized by Ueyama by a stepwise N-terminal elongation, including al- ternate complexation and coupling reactions, i.e. the [Ru(X– C 6 H 4 –tpy)(Y–C 6 H 4 –tpy)] 2+ subunit was not synthesized and in- corporated as such into the oligopeptide [22]. The [Ru(tpy– C 6 H 4 –)(tpy–C 6 H 4 –)] 2+ units appear electronically isolated from each other as all complexes F display a single redox wave at 1.26 V versus SCE for the Ru II /Ru III couples and a constant energy of the 1 MLCT absorption maximum (λ max = 494 nm in CH 3 CN). This finding might be due to the presence of leucine spacers and/or the presence of para-phenylene spacers be- tween the ruthenium moieties (Scheme 2). Colbran described a dinuclear complex [(tpy)Ru(tpy–C 6 H 4 – NHCO–C 6 H 4 –tpy)Ru(tpy)] 4+ G with the bridging ligand tpy–