Structural Characterization and Lifetimes of Triple-Stranded Helical Coinage Metal Complexes: Synthesis, Spectroscopy and Quantum Chemical Calculations

This work reports on a series of polynuclear complexes containing a trinuclear Cu, Ag, or Au core in combination with the fac-isomer of the metalloligand [Ru(pypzH)3 ](PF6 )2 (pypzH=3-(pyridin-2-yl)pyrazole). These (in case of the Ag and Au containing species) newly synthesized compounds of the general formula [{Ru(pypz)3 }2 M3 ](PF6 ) (2: M=Cu; 3: M=Ag; 4: M=Au) contain triple-stranded helical structures in which two ruthenium moieties are connected by three N-M-N (M=Cu, Ag, Au) bridges. In order to obtain a detailed description of the structure both in the electronic ground and excited states, extensive spectroscopic and quantum chemical calculations are applied. The equilateral coinage metal core triangle in the electronic ground state of 2-4 is distorted in the triplet state. Furthermore, the analyses offer a detailed description of electronic excitations. By using time-resolved IR spectroscopy from the microsecond down to the nanosecond regime, both the vibrational spectra and the lifetime of the lowest lying electronically excited triplet state can be determined. The lifetimes of these almost only non-radiative triplet states of 2-4 show an unusual effect in a way that the Au-containing complex 4 has a lifetime which is by more than a factor of five longer than in case of the Cu complex 2. Thus, the coinage metals have a significant effect on the electronically excited state, which is localized on a pypz ligand coordinated to the Ru atom indicating an unusual cooperative effect between two moieties of the complex.

Blue light-induced low mechanical stability of ruthenium-based coordination bonds: an AFM-based single-molecule force spectroscopy study

A HA–Ru^II complex was conjugated to a hyaluronan polymer through amide bonds. In AFM experiments using the “multi-fishhook” approach, the cantilever tip made contact with the polymeric molecule, resulting in stretching, indicated by sawtooth-like force-extension curves.

Long-lived MLCT states for Ru(ii) complexes of ferrocene-appended 2,2'-bipyridines

In this study, we present two ruthenium(ii) diimine complexes appended with ferrocene which show metal to ligand charge transfer 3MLCT emission lifetimes around 630 ns. We also present a similar complex with two ferrocene units which has decreased emission. These complexes have been studied by electrochemical, electronic absorption, and Raman, resonance Raman and transient resonance Raman means, coupled with density functional theoretical approaches. For these systems, the optical spectra are dominated by a low energy ruthenium(ii) MLCT transition; which can be modulated by the presence of pendant ferrocene units and the extent of conjugation of the ferrocenyl bipyridine backbone. Tuning of the lowest energy transition in terms of intensity (4 to 18 × 10-3 M-1 cm-1) and energy (535 to 563 nm) was achieved by these means.

[Cr(ttpy)2]3+ as a multi-electron reservoir for photoinduced charge accumulation

[Cr(ttpy)2]3+ (ttpy = 4'-(4-methylphenyl)-2,2':6,2''-terpyridine) exhibits rich electrochemical and photophysical properties. Cyclic voltammetry performed in CH3CN shows in the cathodic part the presence of three one-electron reversible systems at -0.47, -0.85 and -1.35 V vs. Ag/AgNO3 10-2 M. These systems are attributed to the reduction of the terpyridine ligands with a partial delocalization of the charge on the tolyl for the last reduction event. The three different reduced species were generated by exhaustive electrolysis and characterized by EPR and UV-visible spectroscopy; DFT calculations were performed to locate the spin density of the electrons added during the reduction. Visible light irradiation of [Cr(ttpy)2]3+ induces the population of a luminescent metal-centered excited state with a lifetime of 270 ns in deoxygenated CH3CN. This excited state can be quenched by an electron transfer process with triphenylphosphine (PPh3) or triethanolamine (TEOA). Using TEOA as a sacrificial electron donor, the doubly reduced species (i.e.[Cr(ttpy)2] +) can be generated under continuous irradiation. In the presence of [Ru(bpy)3]2+ as an additional photosensitizer, the photoreduction of [Cr(ttpy)2]3+ towards [Cr(ttpy)2]+ is accelerated. The trinuclear [{RuII(bpy)2(bpy-O-tpy)}2CrIII]7+ complex ([Ru2Cr]7+) in which a CrIII-bis-terpyridine centre is covalently linked to two RuII-tris-bipyridine moieties by oxo bridges has been synthesised. Its electrochemical, photophysical and photochemical properties were investigated in deoxygenated CH3CN. Cyclic voltammetry indicates only a poor electronic communication between the different subunits, whereas luminescence experiments show a strong quenching of the RuII* excited state by an intramolecular process. Continuous irradiation of [Ru2Cr]7+ under visible conditions in the presence of TEOA leads to [Ru2Cr]4+ where three electrons are stored on the [Cr(ttpy)] subunit.

Unusual Role of Excited State Mixing in the Enhancement of Photoinduced Ligand Exchange in Ru(II) Complexes

Four Ru(II) complexes were prepared bearing two new tetradentate ligands, cyTPA and 1-isocyTPQA, which feature a piperidine ring that provides a structurally rigid backbone and facilitates the installation of other donors as the fourth chelating arm, while avoiding the formation of stereoisomers. The photophysical properties and photochemistry of [Ru(cyTPA)(CH3CN)2]2+ (1), [Ru(1-isocyTPQA)(CH3CN)2]2+ (2), [Ru(cyTPA)(py)2]2+ (3), and [Ru(1-isocyTPQA)(py)2]2+ (4) were compared. The quantum yield for the CH3CN/H2O ligand exchange of 2 was measured to be Φ400 = 0.033(3), 5-fold greater than that of 1, Φ400 = 0.0066(3). The quantum yields for the py/H2O ligand exchange of 3 and 4 were lower, 0.0012(1) and 0.0013(1), respectively. DFT and related calculations show the presence of a highly mixed 3MLCT/3ππ* excited state as the lowest triplet state in 2, whereas the lowest energy triplet states in 1, 3, and 4 were calculated to be 3LF in nature. The mixed 3MLCT/3ππ* excited state places significant spin density on the quinoline moiety of the 1-isocyTPQA ligand positioned trans to the photolabile CH3CN ligand in 2, suggesting the presence of a trans-type influence in the excited state that enhances ligand exchange. Ultrafast spectroscopy was used to probe the excited states of 1-4, which confirmed that the mixed 3MLCT/3ππ* excited state in 2 promotes ligand dissociation, representing a new manner to effect photoinduced ligand exchange. The findings from this work can be used to design improved complexes for applications that require efficient ligand dissociation, as well as for those that require minimal deactivation of the 3MLCT state through low-lying metal-centered states.

Repairable photoactive polymer systems via metal–terpyridine-based self-assembly

The photocatalytic activity of polymeric systems ceased upon disassembly and was restored upon assembly through metal ion–ligand interaction.

Photoinduced Charge Separation within Metallo-supramolecular Wires Built around a [Ru(bpy)3](2+)-Bisterpyridine Linear Entity

A [Ru(bpy)3](2+)-like complex (L1) bearing two free terpyridine groups at the 5 and 5' positions of the same bipyridine, linked by the rigid and linear 2,5-dimethyl phenylene bridges has been synthesized to open access to two classes of linear molecular wires with photosensitive properties: a bimetallic coordination polymer and an inorganic triad. In this Research Article, we report on the synthesis and characterization of the resulting [{Ru(II_)Fe(II)}n](4n+) alternated bimetallic polymer and the [Co(III_)Ru(II_)Fe(II)](7+) triad based on the building block L1. The [{Ru(II_)Fe(II)}n](4n+) polymer is fully characterized in solution. Cyclic voltammetry and emission lifetime measurements show that the bridging ligand allows interaction between the metal centers in the excited state despite the lack of interactions in the ground state. Under visible irradiation, the polymer can be fully oxidized in the presence of a sacrificial electron acceptor in solution. Thin robust films of the polymer are easily obtained on ITO by a simple electrochemical procedure based on an electroreduction adsorption process. The ITO/[{Ru(II_)Fe(II)}n](4n+)-modified electrode behaves as a photocathode under irradiation in the presence of ArN2(+). The magnitude of the photocurrent is dependent on the film thickness, probably limited by the diffusion of charge in thicker film. On the other hand L1 is also used to construct a well-ordered triad in association with Co(III) and Fe(II) metallic centers as electron acceptor and donor, respectively. The metallic triad is anchored on ITO or on a SiO2 wafer, starting from a terpyridine phosphonate modified surface. AFM images prove the presence of the triad in a linear upward orientation. Irradiation of the ITO/[Co(III_)Ru(II_)Fe(II)](7+) modified surface in the presence of triethanolamine in CH3CN induces the generation of an anodic photocurrent of around 30 μA.cm(-2). The photocurrent density generated by the ITO/[Co(III_)Ru(II_)Fe(II)](7+) electrode, appears to be more stable than in the case of ITO/[{Ru(II_)Fe(II)}n](4n+) because of the presence of the anchoring group. Moreover, this photocurrent magnitude represents an enhancement of 30% compared to our previous triad ( Dalton Trans. 2014 , 43 , 12156 - 12159 ), proving the advantage of a linear and rigid spacer for the construction of such molecular assemblies with photoinduced charge transfer abilities.

Comparison of inverse and regular 2-pyridyl-1,2,3-triazole "click" complexes: structures, stability, electrochemical, and photophysical properties

Two inverse 2-pyridyl-1,2,3-triazole "click" ligands, 2-(4-phenyl-1H-1,2,3-triazol-1-yl)pyridine and 2-(4-benzyl-1H-1,2,3-triazol-1-yl)pyridine, and their palladium(II), platinum(II), rhenium(I), and ruthenium(II) complexes have been synthesized in good to excellent yields. The properties of these inverse "click" complexes have been compared to the isomeric regular compounds using a variety of techniques. X-ray crystallographic analysis shows that the regular and inverse complexes are structurally very similar. However, the chemical and physical properties of the isomers are quite different. Ligand exchange studies and density functional theory (DFT) calculations indicate that metal complexes of the regular 2-(1-R-1H-1,2,3-triazol-4-yl)pyridine (R = phenyl, benzyl) ligands are more stable than those formed with the inverse 2-(4-R-1H-1,2,3-triazol-1-yl)pyridine (R = phenyl, benzyl) "click" chelators. Additionally, the bis-2,2'-bipyridine (bpy) ruthenium(II) complexes of the "click" chelators have been shown to have short excited state lifetimes, which in the inverse triazole case, resulted in ejection of the 2-pyridyl-1,2,3-triazole ligand from the complex. Under identical conditions, the isomeric regular 2-pyridyl-1,2,3-triazole ruthenium(II) bpy complexes are photochemically inert. The absorption spectra of the inverse rhenium(I) and platinum(II) complexes are red-shifted compared to the regular compounds. It is shown that conjugation between the substituent group R and triazolyl unit has a negligible effect on the photophysical properties of the complexes. The inverse rhenium(I) complexes have large Stokes shifts, long metal-to-ligand charge transfer (MLCT) excited state lifetimes, and respectable quantum yields which are relatively solvent insensitive.

Alcohol-soluble bis(tpy)thiophenes: new building units for constitutional dynamic conjugated polyelectrolytes

Constitutional dynamics and spectral properties of conjugated ionic unimers and their metallo-supramolecular dynamers with Zn(ii) and Fe(ii) ion couplers are presented together with new knowledge on the MLCT band of bis(tpy)Fe(ii) units.