Two RuII Linkage Isomers with Distinctly Different Charge Transfer Photophysics.

The ligand PHEHAT (PHEHAT = 1,10-phenanthrolino[5,6-b]1,4,5,8,9,12-hexaazatriphenylene) presents a structural asymmetry that has a dramatic influence on the photophysical properties depending on the chelation site of the metal ion in the linkage isomers. While [RuII(phen)2HATPHE]2+ behaves classically, like [RuII(bpy)3]2+, [RuII(phen)2PHEHAT]2+ exhibits an unusual behavior. It appears that this complex has two 3MLCT bright states, the lower one being weakly emissive or nonemissive depending on the solvent and temperature. Different photophysical techniques involving a wide range of various temperatures and timescales are essential to analyze this difference. A full photophysical scheme is proposed based on experimental data and density functional theory calculations. While previous studies focused on high temperatures and longer timescale emission, we explore the complexes at very low temperatures and very short times in order to obtain a more complete picture of the intriguing photophysical behavior of these complexes.

A nano-junction of self-assembled mixed-metal-centre molecular wires on transparent conductive oxides.

The fabrication of stable, highly conductive molecular nano-junctions is one of the main research goals in the field of molecular electronics. In this paper we report on the self-assembly and functional characterisation of highly conductive molecular wires, based on mixed-metal polynuclear complexes, at the surface of a transparent conductive oxide. The adopted synthetic approach involves metal-coordination reactions on oxide surfaces, pre-functionalised with a monolayer of terpyridine moieties that are used as anchoring sites for the integration of ditopic, redox-active ruthenium-bisterpyridine molecules through iron(ii) centres. By the stepwise iteration of the iron-coordination reaction, molecular wires of the desired length can be prepared, which alternate iron and ruthenium centres in the wire backbone. The stepwise assembly of the wires at the transparent conductive oxide surface was characterised by means of UV-Vis spectroscopy and, at the nanoscale, by means of ToF-SIMS measurements. The electrical characteristics of the wires were obtained by the liquid-metal eutectic-gain nano-junction technique, with results that show good electron transport capabilities along the wires. The demonstrated feasibility of the integration of these metal-polypyridinic, redox-active, conductive wires at the surface of a transparent and conductive oxide, and the evidence for good electrical conduction indicates prospective applications in the field of nanoscale molecular optoelectronics.

Ultrafast charge transfer excited state dynamics in trifluoromethyl-substituted iridium(iii) complexes.

Time-resolved spectroscopy was exploited to gain new insights into the nature and dynamics of charge transfer excited states of bis-cyclometalated Ir(iii) complexes. We showed that its dynamics is strongly influenced by the nature of the diimine ligand due to the existence of a ligand-ligand charge transfer process in the picosecond timescale. All the results are supported by DFT/TD-DFT calculations and spectroelectrochemistry.

Unusual Photooxidation of S-Bonded Mercaptopyridine in a Mixed Ligand Ruthenium(II) Complex with Terpyridine and Bipyridine Ligands.

An unusual photooxidation of a coordinated 4-mercaptopyridine ( SpyH) ligand in the [Ru(Hmctpy)(dmbpy)(κ S-SpyH)]2+complex (Hmctpy = 4'-carboxy-2,2';6',2″-terpyridine, dmbpy = 4,4'-dimethyl-2,2'-bipyridine) takes place under visible and UV irradiation, in aerated acetonitrile. The [Ru(mctpy)(dmbpy)(κ S-SO2py)] sulfinato product has been characterized by a variety of methods, including X-ray diffraction which supports the presence of the Ru-κ S-SpyH isomer in the starting complex. The photooxidation of the 4-mercaptopyridine ligand enhances the back-bonding interactions in the complex by means of the strongly acceptor 4-pyridinesulfinato-SO2py species, increasing the redox potential of the Ru(III)/Ru(II) couple significantly from 1.23 to 1.62 V. It also led to pronounced changes in the electronic and NMR spectra of the complexes, corroborated by DFT and ZINDO-S calculations. A possible mechanism based on referenced data of photooxidation has been proposed, which involves the formation of a reactive oxygen species and intermediate endoperoxide species, yielding a very stable Ru-sulfinato product. This novel species exhibits stronger luminescence (Φ f = 0.004) than the starting complex under UV excitation.

Converging Energy Transfer in Polynuclear Ru(II) Multiterpyridine Complexes: Significant Enhancement of Luminescent Properties.

Ruthenium-based complexes are widely used as photocatalysts, as photosensitizers, or as building blocks for supramolecular assemblies. In the field of solar energy conversion, building light harvesting antenna is of prime interest. Nevertheless, collecting light is mandatory but not sufficient; once collected and transferred, the exciton has to be long-lived enough to be transferred to a catalytic site. If Ru(II) terpyridine complexes are prime building blocks for structural reasons, the short lifetime of their excited state prevents their use as a harvesting center in light antennae. In this paper, we present new polynuclear assemblies, based on Ru(II)-terpyridine units where delocalization of the excited state is combined with an antenna effect. As a consequence, complexes C1-C3 display long-lived excited states compared to [Ru(tpy)2]2+, making them promising efficient antenna building blocks to be connected to a final acceptor or a catalytic center.

Trifluoromethyl-Substituted Iridium(III) Complexes: From Photophysics to Photooxidation of a Biological Target.

Photodynamic therapeutic agents are of key interest in developing new strategies to develop more specific and efficient anticancer treatments. In comparison to classical chemotherapeutic agents, the activity of photodynamic therapeutic compounds can be finely controlled thanks to the light triggering of their photoreactivity. The development of type I photosensitizing agents, which do not rely on the production of ROS, is highly desirable. In this context, we developed new iridium(III) complexes which are able to photoreact with biomolecules; namely, our Ir(III) complexes can oxidize guanine residues under visible light irradiation. We report the synthesis and extensive photophysical characterization of four new Ir(III) complexes, [Ir(ppyCF3)2(N^N)]+ [ppyCF3 = 2-(3,5-bis(trifluoromethyl)phenyl)pyridine) and N^N = 2,2'-dipyridyl (bpy); 2-(pyridin-2-yl)pyrazine (pzpy); 2,2'-bipyrazine (bpz); 1,4,5,8-tetraazaphenanthrene (TAP)]. In addition to an extensive experimental and theoretical study of the photophysics of these complexes, we characterize their photoreactivity toward model redox-active targets and the relevant biological target, the guanine base. We demonstrate that photoinduced electron transfer takes place between the excited Ir(III) complex and guanine which leads to the formation of stable photoproducts, indicating that the targeted guanine is irreversibly damaged. These results pave the way to the elaboration of new type I photosensitizers for targeting cancerous cells.

Photo-Induced Assembly of a Luminescent Tetraruthenium Square.

Self-assembly is a powerful synthetic tool that has led to the development of one-, two- and three-dimensional architectures. From MOFs to molecular flasks, self-assembled materials have proven to be of great interest to the scientific community. Here we describe a strategy for the construction and de-construction of a supramolecular structure through unprecedented photo-induced assembly and dis-assembly. The combination of two approaches, a [n×1]-directional bonding strategy and a ligand photo-dissociation strategy, allows the photo-induced assembly of a polypyridyl RuII precursor into a discrete molecular square. Diffusion-ordered NMR spectroscopy confirmed the synthesis of a higher volume species, while the identity of the species was established by high-resolution mass spectrometry and single-crystal X-ray diffraction studies. The self-assembled square is not obtained by classical thermal techniques in similar conditions, but is obtained only by light-irradiation. The tetraruthenium square has an excited-state lifetime (135 ns), 40 times that of its mononuclear precursor and its luminescence quantum yield (1.0 %) is three orders of magnitude higher. These remarkable luminescence properties are closely related to the relatively rigid square structure of the tetraruthenium assembly, as suggested by slow radiationless decay and transient absorption spectroscopy. The results described herein are a rare example of photo-induced assembly and dis-assembly processes, and can open the way to a new avenue in supramolecular chemistry, leading to the preparation of structurally organized supermolecules by photochemical techniques.

Going against the flow: Os(ii)-to-Ru(ii) energy transfer in rod-like polypyridyl chromophore.

The judicious design of a unique mixed-metal Ru-Os molecular rod led to the first photoinduced energy transfer from osmium-to-ruthenium in a polypyridyl complex. The absorbed light is directed from the osmium metal center to the peripheral ruthenium moieties, where only the low energy luminescence from the heteroleptic ruthenium phen-hpp complex was observed.

Visible and Near-IR Emissions from k2 N- and k3 N-Terpyridine Rhenium(I) Assemblies Obtained by an [n×1] Head-to-Tail Bonding Strategy.

An [n×1] head-to-tail bonding strategy has been used for the synthesis of ReI metallacycles. From a k3 N-dicarbonyl precursor, a single discrete [4×1] square assembly was isolated and characterized, whereas a k2 N-tricarbonyl precursor led to two major species, a square and a [3×1] triangular assembly. Solid-state X-ray diffraction study has confirmed the high angular distortion (71° to 96°) of the k2 N precursors. The electrochemical reversibility of the triangular (5) and square (6, 7) assemblies is increased with respect to that of their precursors. Photophysical investigation has confirmed pronounced red-shifts in the emissions of the k3 N-dicarbonyl species 4 (935 nm) and 7 (980 nm), as confirmed by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations.

A heptanuclear light-harvesting metal-based antenna dendrimer with six Ru(ii)-based chromophores directly powering a single Os(ii)-based energy trap.

The novel dendritic-shaped, heptanuclear compound 1 - the first mixed Os-Ru dendrimer with tris-chelating bridging ligands - has been synthesized and its photophysics studied by fs pump-probe transient spectroscopy. In 1, all light energy absorbed by six identical Ru(ii) chromophores is funnelled to the luminescent central Os(ii) core, with a time constant of 11 ps, by Dexter energy transfer.

Ruthenium bistridentate complexes with non-symmetrical hexahydro-pyrimidopyrimidine ligands: a structural and theoretical investigation of their optical and electrochemical properties.

Six ruthenium complexes were synthesized based on three non-symmetrical tridentate ligands bearing the strongly electron-donating group 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-α]pyrimidine (hpp), bpyG (bpyG = 2,2'-bipyridyl-6-hpp), phenG (phenG = 2-hpp-1,10-phenanthroline) and QpyG (QpyG = 2-hpp-6-quinolylpyridyl). The fac-/mer-conformation of the homoleptic species has a dramatic effect on the optical properties, where the fac-isomer absorption is red-shifted by 150 nm, thus reaching the near-IR at approximately 850 nm. Owing to the interesting structural effect on the optical properties, density functional theory (DFT) and time-dependent DFT calculations have been implemented to enlighten the experimental data and prove that exciton coupling is at the origin of the observed shift. The electronic properties have been investigated and, as corroborated by electrochemical data, the presence of the hpp ligand strongly affects the oxidation potential of the ruthenium metal ion, which allows facile fine-tuning of the electronic properties. The luminescence properties of all the compounds have also been investigated (λmax emission = 781-817 nm) and the complexes have longer excited-state lifetimes at room temperature than the parent bis(2,2':6',2''-terpyridine)ruthenium(ii) by 10 to 30 times.

A family of Ru(II) complexes built on a novel sexipyridine building block: synthesis, photophysical properties and the rare structural characterization of a triruthenium species.

A new tris-2'',4'',6''-(2,2'-bipyridin-4-yl)-1'',3'',5''-triazine ligand and its family of ruthenium coordination complexes are described along with their characterization by electrochemical and photophysical methods as well as a rare single crystal X-ray analysis of a triruthenium polypyridine complex.

Synthesis of discrete Re(I) di- and tricarbonyl assemblies using a [4 × 1] directional bonding strategy.

Discrete self-assembly of two Re(I) squares was achieved by a simple and efficient strategy where the complexes, [Re(4-pytpy-κ(2)N)(CO)3Br] and [Re(4-pytpy-κ(3)N)(CO)2Br], act as their own ligands. The photophysical and electrochemical properties of the assemblies and their precursors are described along with solid-state X-ray diffraction studies.