Peter Belser

Molecular architecture in the field of photonic devices

Abstract We report the synthesis and photophysical properties of two different photonic devices. The first system describes dinuclear metal complexes with a rigid and linear bridging ligand (PAP) that contains an adamantane spacer. We discuss the correlation between the nature of the bridging ligand and the electrochemical as well as photophysical properties of the metal complexes. Two interesting observations can already be pointed out: (i) the lifetime of the intermediate electron-transfer product RuIII–PAP–OsII is very long (130 μs); and (ii) for the first time in a dinuclear Ru/Os system, the rate constant of energy transfer from the Ru(II) to the Os(II) unit is faster than the rate of the electron transfer from the Ru(II) to the Os(III) unit. The second system represents a photonic switch which is built up by two subunits, a rhenium complex as the active switching part and an anthracene moiety as detector. We discuss the synthesis, the reversibility of the switch and the energy transfer properties of the new system.

Synthesis of a novel rigid molecule family for the investigation of electron and energy transfer

Abstract Three new rigid bridging ligands for metal complexation (7=bmb, 8=bqb and 11=btb) were prepared from a rigid triptycene spacer connected to two bipyridine ligands using a Horner–Emmons type reaction. The triptycene spacer is substituted by methoxy groups in the case of bmb and in the case of bqb by a benzoquinone substituent. The corresponding metal complexes (ruthenium and/or osmium) were synthesised and the different luminescence behaviour was tested. They show great potential for the investigation of intramolecular electron and energy transfer reactions. The dinuclear metal complex RubqbOs is an interesting system in which the bridging ligand bqb acts as a redox switch, able to tune the conductivity for energy or electrons across the bridge.

Dinuclear RuII and/or OsII complexes of bis-bipyridine bridging ligands containing adamantane spacers: synthesis, luminescence properties, intercomponent energy and electron transfer processes

Abstract The ( E,E )-1,3-bis[2-(2,2′-bipyridine-5-yl)ethenyl]adamantane (BAB) and ( E,E )-3,3′-bis[2-(2,2′-bipyridine-5-yl)ethenyl]-1,1′-biadamantane (BAAB) bridging ligands made of two 2,2′-bipyridine groups (B) separated by spacers containing one and two adamantane (A) units have been synthesized. Their dinuclear complexes [(bpy) 2 Ru(BAB)Ru(bpy) 2 ] 4+ (Ru II .BAB.Ru II ), [(bpy) 2 Os-(BAB)Os(bpY) 2 ] 4+ (Os II .BAB.Os II ), [(bpy) 2 Ru(BAB)O9(bPY) 2 ] 4+ (ROSAB.Osn), [(bPY)2Ru(BAAB)-Ru(bpy)2] 4+ (Ru II .BAAB.Ru II ), and [(bpy) 2 Os(BAAB)Os(bpy) 2 ] 4+ (Os II .BAAB.Os II ) have been prepared as PF 6 − salts. In these novel compounds each Ru-based and Os-based unit displays its own absorption spectrum and electrochemical properties, regardless of the presence of a second metal-based unit. In the homodinuclear complexes also the luminescence properties of each unit are unaffected. In the mixed metal Ru II .BAB.Os II complex electronic energy transfer takes place from the Ru-based to the Os-based unit with rate constant 5.8 × 108 s −1 at room temperature, whereas at 77 K energy transfer takes place through two distinct processes with rate constants 1.4 × 108 S −1 and 2.0 × 107 s −1 , presumably because of the presence of two conformers (or families of conformers). Partial oxidation of the binuclear compounds Os II .BAB.Os II , Ru II .BAB.Os II , and Os II .BAAB.Os II by Ce IV in acetonitrile/water solutions leads to the mixed-valence Os II .BAB.Os III , Ru II .SAB.Os III , and Os II .BAAB.Os III species where the oxidized metal-based unit quenches by electron transfer the luminescent excited state of the unit that is not oxidized. At room temperature the rate constants for the excited state ∗ Os II .BAB.Os III → Os III .BAB.Os II , ∗ Os II .BAAB.Os III → Os III .BAAB.Os II processes are 4.0 × 109 s −1 and 8.8 × 108 si, respectively. For Ru II .BAB.Os III , the rate constants for the excited state ∗ Ru II .BAB.Os III → Ru III .BAB.Os II process is 2.8 × 109 s −1 and the rate constant for the back electron transfer process Ru III .BAB.Os II → Ru II .BAB.Os III is 4.0 × 107 s −1 .

Tailor made coordination compounds for photochemical purposes

Abstract An account of the strategies for the synthesis of coordination compounds with interesting photophysical and photochemical properties is given. Ru(II)-diimine complexes have been developed with various absorption and emission characteristics and for special chemical stability of the excited state species. The isomer problem, encountered in the synthesis of polynuclear complexes, which are candidates for Photochemical Molecular Devices (PMD), is attacked by using enantiomerically pure chiral building blocks or chiragen ligands, which form metal complexes with predetermined helical chirality. Cyclometallating ligands with Pt(II) and Pd(II) can be designed, so that the photochemically induced oxidative addition reactions lead to compounds of special interest, especially from the stereochemical point of view.

Luminescent Ruthenium Tripod Complexes: Properties in Solution and on Conductive Surfaces

Two luminescent ruthenium complexes containing tripod-type end groups linked through a rigid spacer to a phenanthroline derivative, able to confer an axial geometry to the complexes, are described. One of the compounds is functionalized with thioacetate groups in order to link the metal complex to metallic surfaces. The photophysical and electrochemical behavior of the complexes are studied in solution and on conductive substrates and, furthermore, self-assembled monolayers are investigated in a junction using gold and an indium gallium eutectic, as electrodes, and by time-resolved confocal microscopy. The results show that the complexes form very stable and well-ordered monolayers because of the tripod system, which can anchor the complex almost perpendicular to the surfaces.

Synthesis, metal complex formation, and switching properties of spiropyrans linked to chelating sites

Abstract The synthesis of 5-pinacolato-2,2′-bipyridine and its applicability in cross-coupling reactions is reported. The use of this framework in Suzuki type cross-coupling reactions, together with a recently published way to achieve indolization has been used to synthesize new spiropyran systems attached to two bipyridine moieties. The indolization method followed, is based on an ‘in situ’ hydrolysis/Fischer cyclization protocol reported by Buchwald and co-workers. The synthesis of a new phenanthroline based spirooxazine attached to a bipyridine moiety is also reported. One of the spiropyran system was used as a ligand to form a ruthenium metal complex. There photophysical properties were tested with respect to the application as sensitizer in functionalized, wire-type bridging ligands in heteronuclear metal complexes.

Host–Guest Interactions and Orientation of Dyes in the One-Dimensional Channels of Zeolite L

A combined experimental and modeling study of methylacridine (MeAcr(+)) dye-zeolite L composites unravels the microscopic origin of their functional properties. The anisotropic orientation of the cationic dye inside the ZL channel is unambiguously determined and understood. The most stable orientation of MeAcr(+), which features both its long and short molecular axes nearly perpendicular to the channel axis, is mainly determined by dye-ZL electrostatic interactions but also depends on the cosolvent water. In ZL, MeAcr(+) is not hydrogen bonded to water or ZL framework oxygens and is hydrophobically solvated by water molecules. These findings further support the hypothesis that the cosolvent can importantly influence properties of dye-zeolite composites. Of relevance for a deeper comprehension of the physical chemistry of these hybrids is the observation that trivial energy transfer processes (self-absorption) are often playing a significant role in the optical properties of the composites.

SYNTHESIS AND PHOTOPHYSICAL PROPERTIES OF CHIRAL, BINUCLEAR METAL COMPLEXES

Abstract The synthesis of isomerically pure multicentred species is an important task for preparative chemists. Only stereochemically well-defined structures of polynuclear metal complexes lead to photophysical results that are clearly interpretable. We have developed several strategies to synthesise such compounds that have a predetermined chirality (Δ- or Λ-helix) at each metal center. The approximate local symmetry of the described metal complexes is D 3 (octahedral, tris-bidentate). The methods are the following: (a) resolving a racemic building block followed by substitution of the two labile monodentate ligands by a bidentate diimine ligand under total retention of configuration. (b) Synthesis of an optically pure precursor complex in which two chelating bipyridine ligands are bridged and each contain an optically active pinene unit. The remaining coordination sites are then replaced by a bidentate diimine ligand. (c) Use of ligands that have sterically demanding substituents perpendicular to the molecular plane. The use of such ligands leads direct to optically active compounds. Examples of these methods will be given, while considering their photophysical application.

Rigid Rod-like Molecular Wires of Nanometric Dimension. Electronic Energy Transfer from a Naphthyl to an Anthracenyl Unit Connected by a 1,4-Pentaphenylene Spacer

Abstract We have synthesized the multicomponent species N (ph) 5 A , where a 1-naphthyl ( N ) and a 9-anthryl ( A ) units are connected by a 1,4-pentaphenylene rod-like spacer ( (ph) 5 ). The overall length of the compound is 3.1 nm and the center-to-center distance between the naphthyl and anthryl units is 2.6 nm. The absorption and emission spectra of N (ph) 5 A have been investigated in cyclohexane solution at 293 K and compared with those of model compounds of the component units. Emission spectra, fluorescence quantum yields and excited state lifetimes show that in N (ph) 5 A the fluorescence of the naphtyl and oligophenyl units is completely quenched by energy transfer to the fluorescent excited state of the anthryl unit. The mechanism of the energy transfer process is discussed.

Ligand-centered luminescence from a ruthenium(II) complex

Abstract The complex Ru( i -biq) 2+ 3 , where i -biq is 2,2,′-bis-isoquinoline, was synthesized and studied spectroscopically and electro-chemically. The strong luminescence exhibited at 77 K in rigid glasses is characterized as a triplet ligand-centered emission. It is the first example of ligand-centered luminescence from a Ru(II) complex.