Infrared and photoluminescence spectroscopies have been used to investigate the local environment of the Eu3+ ions in luminescent sol−gel derived materialsdi-ureasilsbased on a hybrid framework represented by U(600). This host is composed of a siliceous backbone grafted, through urea cross-links, to both ends of polymer segments incorporating 8.5 oxyethylene repeat units. The active centers have been introduced as europium perchlorate, Eu(ClO4)3. Samples with compositions n = 232, 62, 23, 12, and 6 (where n denotes the ratio of (OCH2CH2) moieties per lanthanide ion) have been examined. The combination of the information retrieved from the analysis of characteristic bands of the FTIR spectrathe perchlorate and the Amide I/Amide II featureswith that obtained from the photoluminescence data demonstrates that at compositions n = 232 and 62 the anions are free, whereas the Eu3+ ions are complexed by the heteroatoms of the polyether chains. At higher salt concentration, the cations are bonded, not only to the ClO4 - ions, but also to the ether oxygen atoms of the organic segments and to the carbonyl oxygen atoms of the urea linkages. The dual behavior of U(600) with respect to cation coordination has been attributed to the presence in this nanohybrid of strong hydrogen-bonded urea−urea structures, which, at low salt content, cannot be disrupted, thus inhibiting the formation of Eu3+...OC(urea) contacts and promoting the interaction between the lanthanide ions and the (OCH2CH2) moieties. The present work substantiates the claim that the activation of the coordinating sites of the di-ureasil framework can be tuned by varying either the guest salt concentration at constant chain length or the length of the organic segments at constant salt concentration. This relevant property opens challenging new prospects in the fields of application of this class of hybrids.

Synthetic amino acids suitable for the assembly of small, redox-active metallopeptides are described. N α-((1,1-Dimethylethoxy)carbonyl)-N ε-(2-pyridylmethyl)-l-lysine (1), N α-acetyl-S-(2-pyridylmethyl)-l-cysteine (2), and N α-acetyl-S-(2-(2-pyridyl)ethyl)-l-cysteine (3) have been synthesized by alkylation of the N α-protected amino acids. Their [Ru(bipy)2]2+ complexes [(bipy)2Ru(BocLysCH2py)]2+ (4), [(bipy)2Ru(AcCysCH2py)]2+ (5), and [(bipy)2Ru(AcCys(CH2)2py)]2+ (6) have been prepared by reactions of the ligands with [Ru(bipy)2Cl2]. On the basis of 1H-NMR spectroscopy, 4−6 can be described best as trans-tetrapyridine complexes with the lysine amino N atom and the cysteine S atom occupying one of the apical positions. It was shown by luminescence spectroscopy that 4 can serve as a possible photoredox-active module for the construction of photochemically active peptides. The redox properties of the complexes are described with the aid of the Lever parameters. It was demonstrated that the amino acid ligands in 5 and 6 can be viewed as methionine units. Particularly interesting is the unique redox chemistry of 4. Upon metal oxidation, a two-electron ligand oxidation occurred, followed by fast hydrolytic cleavage of the lysine−methylpyridine N−C bond. The physical and chemical properties of the compounds are discussed in terms of future applications in biomimetic chemistry such as the activation of small molecules.