Photophysics of an Intramolecular Hydrogen‐Evolving Ru–Pd Photocatalyst

Photoinduced electron‐transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)₂Ru(tpphz)PdCl₂]²⁺ ( RuPd ; tbbpy=4,4′‐di‐tert‐butyl‐2,2′‐bipyridine, tpphz=tetrapyrido[3,2‐a:2′,3′c:3′′,2′′,‐h:2′′′,3′′′‐j]phenazine) have been studied by resonance Raman and ultrafast time‐resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)₂Ru(tpphz)]²⁺ subunit Ru with that of the supramolecular catalyst RuPd , the individual electron‐transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the molecular ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited‐state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine‐centered intraligand charge‐transfer state, which in RuPd precedes formation of the Pd‐reduced state. The photoreaction in the heterodinuclear supramolecular complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate‐determining steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge‐separated states.     

一个嫁接有羧酸羟基乙基酯的二吡啶吩嗪Ru(II)配合物的合成、与DNA的相互作用以及溶剂变色性质的研究

合成并表征了一个新的Ru(II)配合物[Ru(bpy)₂(hedppc)](ClO₄)₂ {bpy＝2,2'-联吡啶, hedppc＝二联吡啶[3,2-a: 2',3'-c]吩嗪-11-羧酸(2-羟乙基)酯}. 通过紫外-可见吸收光谱、与溴化乙锭竞争实验、粘度测量和DNA裂解实验研究了配合物与小牛胸腺DNA的相互作用性质. 结果表明配合物以插入模式与DNA键合,键合常数K_b＝(6.99±1.34)×10⁶ mol^－1•L (s＝2.03±0.04)与母体配合物[Ru(bpy)₂ (dppz)]^2＋相近,但光致发光和溶剂变色等光学性质与[Ru(bpy)₂ (dppz)]^2＋有明显的差别.     

Supramolecular Photoinduced Electron Transfer between a Redox‐Active Hexanuclear Metal–Organic Cylinder and an Encapsulated Ruthenium(II) Complex

By using redox‐active nickel(II) ions as the connect nodes, a hexanuclear metal–organic cylinder (Ni‐YL) was achieved through self‐assembly with a large cavity and an opening windows capable to accommodate guest molecules. The suitable cavity of Ni‐YL provides an opportunity to encapsulate the anionic ruthenium bipyridine derivative [Ru(dcbpy)₃] (dcbpy=2,2′‐bipyridine‐4,4′‐dicarboxylic acid) as the photosensitizer for light‐driven reactions. The host–guest behavior between Ni‐YL and [Ru(dcbpy)₃] was investigated by mass spectrometry, NMR spectroscopy, and computational studies, revealing an effective binding of the guest [Ru(dcbpy)₃] within the cavity of Ni‐YL. Optical experiments suggested a pseudo‐intramolecular photoinduced electron transfer (PET) process between the [Ru(dcbpy)₃] and the host Ni‐YL, leading to an efficient light‐driven hydrogen production based on this system. Control experiments with a mononuclear Ni complex as a reference photocatalyst and the inactive [Fe(dcbpy)₃] as an inhibitor for comparison were also performed to confirm such a supramolecular photocatalysis process.     

A Rigid Dinuclear Ruthenium(II) Complex as an Efficient Photoactive Agent for Bridging Two Guanine Bases of a Duplex or Quadruplex Oligonucleotide

The rigid dinuclear [(tap)₂Ru(tpac)Ru(tap)₂]⁴⁺ complex ( 1 ) (TAP=1,4,5,8‐tetraazaphenanthrene, TPAC=tetrapyridoacridine) is shown to be much more efficient than the mononuclear bis‐TAP complexes at photodamaging oligodeoxyribonucleotides (ODNs) containing guanine (G). This is particularly striking with the G‐rich telomeric sequence d(T₂AG₃)₄. Complex 1 , which interacts strongly with the ODNs as determined by surface plasmon resonance (SPR) and emission anisotropy experiments, gives rise under illumination to the formation of covalent adducts with the G units of the ODNs. The yield of photocrosslinking of the two strands of duplexes by 1 is the highest when the G bases of each strand are separated by three to four base pairs. This corresponds with each Ru(tap)₂ moiety of complex 1 forming an adduct with the G base. This separation distance of the G units of a duplex could be determined thanks to the rigidity of complex 1 . On the basis of results of gel electrophoresis, mass spectrometry, and molecular modelling, it is suggested that such photocrosslinking can also occur intramolecularly in the human telomeric quadruplex d(T₂AG₃)₄.     

Photophysics of a Ruthenium 4H‐Imidazole Panchromatic Dye in Interaction with Titanium Dioxide

The photophysics of bis(4,4′‐di‐tert‐butyl‐2,2′‐bipyridine‐κ²N,N′)[2‐(4‐carboxyphenyl)‐4,5‐bis(p‐tolylimino‐κN)imidazolato]ruthenium(II) hexafluorophosphate is investigated, both in solution and attached to a nanocrystalline TiO₂ film. The studied substitution pattern of the 4H‐imidazole ligand is observed to block a photoinduced structural reorganization pathway within the 4H‐imidazole ligand that has been previously investigated. Protonation at the 4H‐imidazole ring decreases the excited‐state lifetime in solution. When the unprotonated dye is anchored to TiO₂, photoinduced electron injection occurs from thermally nonrelaxed triplet metal‐to‐ligand charge transfer (³MLCT) states with a characteristic time constant of 0.5 ps and an injection efficiency of roughly 25 %. Electron injection from the subsequently populated thermalized ³MLCT state of the dye does not take place. The energy of this state seems to be lower than the conduction band edge of TiO₂.     

Enhanced Photostability of a Ruthenium(II) Polypyridyl Complex under Highly Oxidizing Aqueous Conditions by Its Partial Inclusion into a Cyclodextrin

The complex [Ru(bpy)₂L]²⁺, where bpy=2,2′‐bipyridine, L=4‐(phenylethynyl)‐2,2′‐bipyridine, was prepared in its racemic and resolved forms (Δ and Λ). The phenylethynyl unit on the bipyridine for the complex acts as a binding site for α‐cyclodextrin in water (1:1 complex, K=3390 L mol^?1) or β‐cyclodextrin (2:1 complex, K₁=887 L mol^?1, K₂=8070 L mol^?1). The presence of the cyclodextrin provides partial protection to the complex under light‐activated water oxidation conditions.     

Catalytic photooxidation of alcohols by an unsymmetrical tetra(pyridyl)pyrazine-bridged dinuclear Ru complex

The dinuclear complexes [(tpy)Ru(tppz)Ru(bpy)(L)](n+) (where L is Cl(-) or H(2)O, tpy and bpy are the terminal ligands 2,2':6',2'-terpyridine and 2,2'-bipyridine, and tppz is the bridging backbone 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were prepared and structurally and electronically characterized. The mononuclear complexes [(tpy)Ru(tppz)](2+) and [(tppz)Ru(bpy)(L)](m+) were also prepared and studied for comparison. The proton-coupled, multi-electron photooxidation reactivity of the aquo dinuclear species was shown through the photocatalytic dehydrogenation of a series of primary and secondary alcohols. Under simulated solar irradiation and in the presence of a sacrificial electron acceptor, the photoactivated chromophore-catalyst complex (in aqueous solutions at room temperature and ambient pressure conditions) can perform the visible-light-driven conversion of aliphatic and benzylic alcohols into the corresponding carbonyl products (i.e., aldehydes or ketones) with 100% product selectivity and several tens of turnover cycles, as probed by NMR spectroscopy and gas chromatography. Moreover, for aliphatic substrates, the activity of the photocatalyst was found to be highly selective toward secondary alcohols, with no significant product formed from primary alcohols. Comparison of the activity of this tppz-bridged complex with that of the analogue containing a back-to-back terpyridine bridge (tpy-tpy, i.e., 6',6'-bis(2-pyridyl)-2,2':4',4':2',2'-quaterpyridine) demonstrated that the latter is a superior photocatalyst toward the oxidation of alcohols. The much stronger electronic coupling with significant delocalization across the strongly electron-accepting tppz bridge facilitates charge trapping between the chromophore and catalyst centers and therefore is presumably responsible for the decreased catalytic performance.     

Ultrafast excited-state excitation dynamics in a quasi-two-dimensional light-harvesting antenna based on ruthenium(II) and palladium(II) chromophores

A detailed study on the excited-state-excitation migration taking place within the tetranuclear complex [{(tbbpy)(2)Ru(tmbi)}(2){Pd(allyl)}(2)](PF(6))(2) (tbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine and tmbi = 5,6,5',6'-tetramethyl-2,2'-bibenzimidazolate) is presented. The charge transfer is initiated by the photoexcitation into the lowest metal-to-ligand charge-transfer (MLCT) band of one of the peripheral ruthenium(II) chromophores and terminates on the central structurally complex Pd(2) (II)(allyl)(2) subunit. Thus, the system under investigation can be thought of as a functional model for the photosynthesis reaction center in plants. The kinetic steps involved in the overall process are inferred from femtosecond time-resolved transient-grating kinetics recorded at spectral positions within the regions of ground-state bleach and transient absorption. The kinetics features a complex non-exponential time behavior and can be fitted to a bi-exponential rise (tau(1)> or =200 fs, tau(2) approximately 1.5 ps) and a mono- or bi-exponential decay, depending on the experimental situation. The data leads to the formulation of a model for the intramolecular excitation-hopping ascribing intersystem crossing and subsequent cooling as the two fastest observed processes. Following these initial steps, charge transfer from the ruthenium to the central complex Pd(2)(allyl)(2) moiety is observed with a characteristic time constant of 50 ps. A 220-ps component that is observed in the ground-state recovery only is attributed to excitation equilibration between the two identical Pd(allyl) chromophores.     

一个新型钌(II)配合物对F－的双重光谱识别

采用紫外-可见光谱和荧光光谱滴定方法研究了钌(II)配合物[Ru(bpy)(H2iip)2](ClO4)2 [bpy＝2,2’-联吡啶, H2iip＝2-吲哚基-咪唑并[4,5-f][1,10]-邻菲罗啉]在DMSO溶液中对卤素离子的识别性质. 结果表明该配合物能比色和荧光双重光谱高选择性识别F－.     

Distance Dependence of Bidirectional Concerted Proton–Electron Transfer in Phenol‐Ru(2,2′‐bipyridine)32+ Dyads

Proton‐coupled electron transfer (PCET) was investigated in three covalent donor–bridge–acceptor molecules with different bridge lengths. Upon photoexcitation of their Ru(bpy)₃²⁺ (bpy=2,2′‐bipyridine) photosensitizer in acetonitrile, intramolecular long‐range electron transfer from a phenolic unit to Ru(bpy)₃²⁺ occurs in concert with release of the phenolic proton to pyrrolidine base. The kinetics of this bidirectional concerted proton–electron transfer (CPET) reaction were studied as a function of phenol–Ru(bpy)₃²⁺ distance by increasing the number of bridging p‐xylene units. A distance decay constant (β) of 0.67±0.23 Å^?1 was determined. The distance dependence of the rates for CPET is thus not significantly steeper than that for ordinary (i.e., not proton coupled) electron transfer across the same bridges, despite the concerted motion of oppositely charged particles into different directions. Long‐range bidirectional CPET is an important reaction in many proteins and plays a key role in photosynthesis; our results are relevant in the context of photoinduced separation of protons and electrons as a means of light‐to‐chemical energy conversion. This is the first determination of β for a bidirectional CPET reaction.     

Efficient DNA Condensation Induced by Ruthenium(II) Complexes of a Bipyridine‐Functionalized Molecular Clip Ligand

Complexes of the type [Ru(bxbg)₂(N‐N)]²⁺, where N‐N denotes 2,2′‐bipyridine (bpy) ( 1 ), 1,10‐phenanthroline (phen) ( 2 ), dipyrido[3,2‐d:2′,3‐f] quinoxaline (dpq) ( 3 ), and dipyrido[3,2‐a:2′,3′‐c]phenazine (dppz) ( 4 ), incorporating bis(o‐xylene)bipyridine‐glycoluril (bxbg) as an ancillary “molecular clip” ligand, have been synthesized and characterized. These ruthenium(II) complexes of bis(o‐xylene)bipyridine‐glycoluril self‐associate in water through specific molecular recognition processes to form polycationic arrays. These arrays containing electrostatic binders as well as intercalator ligands at micromolar doses rapidly condense free DNA into globular nanoparticles of various sizes. The DNA condensation induced by these complexes has been investigated by electrophoretic mobility assay, dynamic light scattering, and transmission electron microscopy. The cellular uptake of complex–DNA condensates and the low cytotoxicity of these complexes satisfy the requirements of a gene vector.