Effects of Sequence, Connectivity, and Counter Ions in New Amide-Linked Ru(tpy)₂–Re(bpy) Chromophores on Redox Chemistry and Photophysics

Abstract

New cationic metallo ligands L1-L3 based on bis(terpyridine) ruthenium(II) complexes decorated with differently substituted 2,2'-bipyridines attached via amide groups (5-NHCO-bpy, 4-CONH-bpy, 5-CONH-bpy) were prepared. Coordination of Re(I)Cl(CO)(3) fragments to the bpy unit gives the corresponding bimetallic Ru~Re complexes 1-3. Hydrogen bonds of the bridging amide groups to [PF(6)](-) counterions or to water molecules are observed both in the solid state and in solution. The impact of the amide orientation, the connecting site, and the coordination of counterions on redox and photophysical properties is explored. Both the metallo ligands L1-L3 and the bimetallic complexes 1-3 are emissive at room temperature in fluid solution. The emission originates from (3)MLCT(Ru) states in all cases. Accordingly, the first oxidation of L1-L3 and 1-3 to [L1](+)-[L3](+) and [1](+)-[3](+) is assigned to the Ru(II/III) couple, while the first reduction to [L1](-)-[L3](-) and [1](-)-[3](-) occurs at the tpy-CO ligand as shown by UV/vis, IR, and EPR spectroscopy of the chemically generated radicals. Under rapid freezing conditions, radicals [2](-) and [3](-) are stabilized as different valence isomers with the odd electron localized at the [bpy-CO](•) bridging unit instead of the [tpy-CO](•). Furthermore, in radical [3](-) this valence equilibrium is shifted from [bpy-CO](•) to [tpy-CO](•) by coordination of [PF(6)](-) counterions to the bridging amide unit and back by replacing the [PF(6)](-) counterion with [BPh(4)](-). Photoinduced electron transfer (λ(exc) = 500 nm) to L1-L3 and to 1-3 is successful using triethanolamine (TEOA) as a reducing agent. Photocatalytic reduction of CO(2) by TEOA and 1-3 is hampered by the wrong site of electron localization in the one-electron reduced species [1](-)-[3](-).

Long‐Lived, Emissive Excited States in Direct and Amide‐Linked Thienyl‐Substituted RuII Complexes

“…Although Δ-and Λ-isomers of these heteroleptic complexes are possible, no efforts were made to isolate the isomers. 1 H and 13 C NMR spectra are consistent with symmetrical species in solution, confirm-ing the expected C 2 symmetry of these ML 2 L′-type complexes. [37] Solid State Characterization X-ray quality crystals of heteroleptic complexes 1-4 were grown from concentrated CH 2 Cl 2 or CH 3 CN solutions layered with Et 2 O, allowing for characterization of these complexes in the solid state.…”

“…The target complexes were isolated as hexafluorophosphate salts by addition of a saturated aqueous NH 4 PF 6 solution resulting in formation of precipitates that were orange or red in color. These precipitates were purified by column chromatography with CH 3 CN/H 2 O/KNO 3(aq) as eluent and the final products were characterized using 1 H and 13 C NMR spectroscopy, high resolution mass spectrometry and various ground and excited state spectroscopic methods. Although Δ-and Λ-isomers of these heteroleptic complexes are possible, no efforts were made to isolate the isomers.…”

“…[9][10][11][12] In addition, significant efforts to increase the rate of forward electron transfer (or decrease the rate of back electron transfer) by varying the substituents and connectivity to the central photosensitizer in these types of systems have also been made. [4,[13][14][15] One general strategy to extend the lifetime of these CS states is to lengthen the bridge or linker between the ruthenium core and the peripheral electron-donating/accepting moieties thereby decreasing the rate of back electron transfer. [16][17][18][19][20] Ruthenium based dyads and triads have been investigated with both saturated and π-conjugated linkers between the core and the peripheral substituents, in an effort to mediate electron transfer.…”

“…In general, the use of synthetically accessible amide linkers has also been previously reported in a number of other systems with applications in photochemical energy conversion. [13,[30][31][32][33][34][35][36] In this work, we report the preparation and characterization of a series of new Ru II complexes bearing thienyl groups appended by direct C-C, secondary amide and tertiary amide linkages (Scheme 1), and the corresponding ligands. The photophysical properties of these complexes are correlated in an effort to understand electronic contributions from the amide linkage.…”

A importância do estado excitado 3MLCT de compostos de Ru(II), Re(I) e Ir(III) no desenvolvimento de fotossensores, OLEDS e fotorredução de CO2

“…Buscando evitar o processo de difusão, usa-se a abordagem supramolecular para unir covalentemente um fotossensibilizador redox, como um composto polipiridínico de Ru(II), a um catalisador, como um composto de Re(I). 178,[225][226][227][228][229] Desta maneira, aproveita-se a intensa absorção de luz visível característica de um com as propriedades catalíticas do outro, Esquema 3. Esta abordagem deve levar em conta fatores como, por exemplo, o comprimento das cadeias carbônicas e insaturações das estruturas do ligante-ponte.…”