"Chemistry-on-the-complex": functional RuII polypyridyl-type sensitizers as divergent building blocks

Ruthenium polypyridyl type complexes are potent photoactive compounds, and have found - among others - a broad range of important applications in the fields of biomedical diagnosis and phototherapy, energy conversion schemes such as dye-sensitized solar cells (DSSCs) and molecular assemblies for tailored photo-initiated processes. In this regard, the linkage of RuII polypyridyl-type complexes with specific functional moieties is highly desirable to enhance their inherent photophysical properties, e.g., with a targeting function to achieve cell selectivity, or with a dye or redox-active subunits for energy- and electron-transfer. However, the classical approach of performing ligand syntheses first and the formation of Ru complexes in the last steps imposes synthetic limitations with regard to tolerating functional groups or moieties as well as requiring lengthy convergent routes. Alternatively, the diversification of Ru complexes after coordination (termed "chemistry-on-the-complex") provides an elegant complementary approach. In addition to the Click chemistry concept, the rapidly developing synthesis and purification methodologies permit the preparation of Ru conjugates via amidation, alkylation and cross-coupling reactions. In this regard, recent developments in chromatography shifted the limits of purification, e.g., by using new commercialized surface-modified silica gels and automated instrumentation. This review provides detailed insights into applying the "chemistry-on-the-complex" concept, which is believed to stimulate the modular preparation of unpreceded molecular assemblies as well as functional materials based on Ru-based building blocks, including combinatorial approaches.

Fast, through-bond mediated energy transfer from Ir(iii) to Ru(ii) in di- and tetranuclear heterometallic assemblies: elucidation of a two-step Ir → Ir → Ru energy transfer process

Energy transfer processes triggered by light excitation (lambda(exc) = 278 nm) have been investigated in a heterometallic complex, Ir-Ru, and in a tetranuclear assembly, ((F4))(2)-Ir-Ru, containing iridium(III) and ruthenium(II) centres {Ir represents the unit [(ppy)(2)Ir(bpy)](+), Ru is [Ru(bpy)(3)](2+) and Ir(F4) is [Ir(F(2)ppy)(2)(4-bpy-C(6)H(4)-)](+) (ppy = 2-phenylpyridyl, bpy = 2,2'-bipyridyl)}. In the dinuclear species, the two metallic components are linked by a biphenylene bridge connected between the 4-positions of the bpy of Ir and one of the three bpy's of Ru. The tetrametallic species comprises the dimeric unit appended with Ir(F4) units at the 4'-positions of the ppy ligands. For both the dinuclear and tetranuclear complexes, the metal centers are held at fixed distances by the interposed phenylene bridges. We have obtained the optical (absorption and emission) properties for the mononuclear species Ir(F4), Ir and Ru and for the polymetallic species Ir-Ru and ((IrF4))(2)-Ir-Ru. For Ir-Ru, the Ir --> Ru energy transfer step is exothermic by ca. 0.22 eV, based on the emission energies of the respective mononuclear components at 77 K. In turn, within ((F4))(2)-Ir-Ru, the excited state energy of Ir(F4) is ca. 0.26 eV higher than Ir. By using lambda(exc) = 278 nm, it is possible to predominantly excite the iridium-based units of both Ir-Ru and (Ir(F4))(2)-Ir-Ru. For both cases, energy transfer is found to be fast and efficient with k(en) > 2 x 10(8) s(-1), leading to detectable emission only from the Ru component both at room temperature and at 77 K. In particular, for ((F4))(2)-Ir-Ru, based on the evaluated intermetallic distances (e.g. 28.5 A for the shortest Ir(F4)-Ru distance) and the overlap integrals of donor and acceptor units, the observed energy transfer is too fast to be accounted for by through-space Förster transfer. In this species, energy transfer probably occurs in a two-step process, Ir(F4)-->Ir-->Ru, both steps involving a through-bond Dexter mechanism.

A cross-coupling strategy for the synthesis of dimetallic assemblies containing mixed bipyridine–terpyridine bridging ligands: luminescence and energy transfer properties

Tris-bidentate complexes of the form [Ir(N[symbol: see text]C)2(N[symbol: see text]N)]+ and [Ru(N[symbol: see text]N)3]2+ incorporating a boronic acid substituent (N[symbol: see text]C=2-phenylpyridyl, N[symbol: see text]N=a 2,2'-bipyridyl ligand), are cross-coupled with bromo-substituted bis-terpyridyl ruthenium and iridium complexes to generate heterometallic assemblies comprising bpy-phi(n)-tpy bridges; they display efficient energy transfer from Ir(III) to Ru(II).

Metal-Directed Synthesis and Photophysical Studies of Trinuclear V-Shaped and Pentanuclear X-Shaped Ruthenium and Osmium Metallorods and Metallostars Based upon 4′-(3,5-Dihydroxyphenyl)-2,2′:6′,2′′-terpyridine Divergent Units

A new series of V-shaped trinuclear metallorods and X-shaped pentanuclear metallostars has been prepared by the reaction of metal complexes bearing pendant phenolic functionalities with complexes containing electrophilic ligands. Specifically, {M(tpy)2} motifs (M=Ru or Os; tpy=2,2':6',2''-terpyridine) bearing one or two pendant 3,5-dihydroxyphenyl substituents at the 4-position of the central ring of the tpy have been reacted with the complexes [Ru(tpy)(Xtpy)]2+ (X=Cl or Br) to form new ether-linked species. The energy transfer from ruthenium to osmium in these complexes has been investigated in detail and the efficiency of transfer shown to be highly temperature dependent; the energy transfer is highly efficient at low temperature, whereas at room temperature nonradiative and nontransfer deactivation of the excited {Ru(tpy)2}* domains is most significant.