Pt(II)-bis(quinolinyl) complexes for photocatalytic hydrogen production.

Three Pt(II)-bis(quinolinyl) complexes with varying electron densities were synthesized, structurally characterized and used for photocatalytic hydrogen production under different conditions. All the complexes were found to be active for hydrogen production giving a maximum turnover number (TON) of 1230 surpassing the conventionally used Pt-terpyridyl complexes.

Red Absorbing Cyclometalated Ir(III) Diimine Photosensitizers Competent for Hydrogen Photocatalysis.

Two new cyclometalated Ir(III) diimine complexes were used as photosensitizers for homogeneous hydrogen evolution reaction (HER). These complexes were characterized by electrochemistry, ultraviolet-visible absorption, time-resolved and steady-state photoluminescence spectroscopy as well as by theoretical methods. The metal-ligand-to-ligand charge transfer character of their lowest excited state was shown to be competent for efficient H2 photoproduction in the presence of [Co(dmgH)2(py)Cl] as the hydrogen evolution catalyst, triethanolamine as the sacrificial electron donor, and HBF4 as the proton source. Under optimized experimental conditions, both complexes displayed HER over a period of more than 90 h, with turnover numbers reaching up to 11,650, 10,600, and 174 molH2 molPS-1 under blue-, green-, and red-light irradiation, respectively. Both complexes showed higher stability and efficiency vs HER than most of the previously described systems of the same kind.

Active repair of a dinuclear photocatalyst for visible-light-driven hydrogen production.

The molecular apparatus behind biological photosynthesis retains its long-term functionality through enzymatic repair. However, bioinspired molecular devices designed for artificial photosynthesis, consisting of a photocentre, a bridging ligand and a catalytic centre, can become unstable and break down when their individual modules are structurally compromised, halting their overall functionality and operation. Here we report the active repair of such an artificial photosynthetic molecular device, leading to complete recovery of catalytic activity. We have identified the hydrogenation of the bridging ligand, which inhibits the light-driven electron transfer between the photocentre and catalytic centre, as the deactivation mechanism. As a means of repair, we used the light-driven generation of singlet oxygen, catalysed by the photocentre, to enable the oxidative dehydrogenation of the bridging unit, which leads to the restoration of photocatalytic hydrogen formation.

Dinuclear 2,4-di(pyridin-2-yl)-pyrimidine based ruthenium photosensitizers for hydrogen photo-evolution under red light.

In this study, we report two dinuclear Ru(II) complexes C1 and C2 and compare them to their mononuclear analogues Ref1 and Ref2. The dinuclear species exhibit a much stronger absorption, longer excited-state lifetimes and higher luminescence quantum yields than the mononuclear complexes. In addition, C1 and C2 are easier to reduce. An estimation of the driving forces for the electron transfer processes relevant to photocatalytic hydrogen evolution suggests that C1 and Ref2 possess similar activity as photosensitizer (PS). Yet, the improved photophysical properties of C1 make it a more promising candidate for hydrogen evolution. In hydrogen evolution experiments, C1 indeed exhibits increased activity as PS, however, the catalytic system loses its activity after only a few hours. C2 is less active than the mononuclear complexes despite its superior photophysical properties. This observation is attributed to a lack of driving force for the electron transfer towards the catalyst. Further studies of the dinuclear complex C1 show that it is indeed the PS, which decomposes under the catalytic conditions, presumably due to the electron transfer towards the catalyst being the rate-limiting step.

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.

N,N'-Bis[2,6-bis-(1-methyl-eth-yl)phen-yl]pyridine-4-carboximidamide toluene hemisolvate.

The title compound, C30H39N3·0.5C7H8, is a symmetrically N,N'-disubstituted aryl-amidine containing a 4-pyridyl substituent on the carbon atom of the N-C-N linkage and bulky 2,6-diiso-propyl-phenyl groups on the nitro-gen atoms. It crystallizes in the Z-anti configuration and its amidine C-N bonds present amine [1.368 (1) Å] and imine [1.286 (1) Å] features. Intra-molecular hydrogen bonds are present in the structure together with inter-molecular N-H⋯N and C-H⋯N inter-actions linking the mol-ecules in chains along the a- and c-axis directions.

In-Depth Study of the Electronic Properties of NIR-Emissive κ3N Terpyridine Rhenium(I) Dicarbonyl Complexes.

The structure-properties relationship in a series of carbonyl rhenium(I) complexes based on substituted terpyridine ligands of general formula [Re(κxN-Rtpy)(CO)yL]n+ is explored by both experimental and theoretical methods. In these compounds, the terpyridine ligands adopt both bidentate (κ2N) and terdentate (κ3N) coordination modes associated with three or two carbonyls, respectively. Conversion from the κ2N to the κ3N coordination mode leads to large changes in the absorption spectra and oxidation potentials due to destabilization of the HOMO level of each complex. The absorption profiles of the κ3N complexes cover the whole visible spectra with lower maxima around 700 nm, tailing out to 800 nm, while no emission is observed with Br- as the axial ligand L. When the axial ligand is modified from the native halide to pyridine or triphenylphosphine, the lowest absorption band is blue-shifted by 60 and 90 nm, respectively. These cationic complexes are near-infrared emitters with emission maxima between 840 and 950 nm for the pyridine compounds and 780-800 nm for the triphenylphosphine compounds.

Substituted 2,4-Di(pyridin-2-yl)pyrimidine-Based Ruthenium Photosensitizers for Hydrogen Photoevolution under Red Light.

The photocatalytic reduction of water to form hydrogen gas (H2) is a promising approach to collect, convert, and store solar energy. Typically, ruthenium tris(bipyridine) and its many derivatives are used as photosensitizers (PSs) in a variety of photocatalytic conditions. The bis(terpyridine) analogues, however, have only recently gained attention for this application because of their poor photophysical properties. Yet, by the introduction of electron-donating or -withdrawing groups on the terpyridine ligands, the photophysical and electrochemical properties can be considerably improved. In this study, we report a series of nonsymmetric 2,6-di(pyridin-2-yl)pyrimidine ligands with peripheral pyridine substituents in different positions and their corresponding ruthenium(II) complexes. The presence of the pyrimidine ring stabilizes the lowest unoccupied molecular orbital, leading to a red-shifted emission and prolonged excited-state lifetimes as well as higher luminescence quantum yields compared to analogous terpyridine complexes. Furthermore, all complexes are easier to reduce than the previously reported bis(terpyridine) complexes used as PSs. Interestingly, the pyridine substituent in the 4-pyrimidine position has a greater impact on both the photophysical and electrochemical properties. This correlation between the substitution pattern and properties of the complexes is further investigated by using time-dependent density functional theory. In hydrogen evolution experiments under blue- and red-light irradiation, all investigated complexes exhibit much higher activity compared to the previously reported ruthenium(II) bis(terpyridine) complexes, but none of the complexes are as stable as the literature compounds, presumably because of an additional decomposition pathway of the reduced PS competing with electron transfer from the reduced PS to the catalyst.

Controlling photocatalytic reduction of CO2 in Ru(II)/Re(I) dyads via linker oxidation state.

Electronic communication between the linked metal centers in Ru(ii)-Re(i) dyads is tuned using the oxidation state (S and SO2) of sulfur-bridged ligands. Higher catalytic activity is seen for the SO2-bridged dyad in the photocatalytic reduction of CO2.

Heteroleptic ruthenium bis-terpyridine complexes bearing a 4-(dimethylamino)phenyl donor and free coordination sites for hydrogen photo-evolution.

Motivated by the recent report of a heteroleptic ruthenium bis-terpyridine complex [Ru(toltpy)(bipytpy)](PF6)2 (toltpy: 4'-(tolyl)-2,2':6',2''-terpyridine; bipytpy: 4'-(4-bromophenyl)-4,4''':4'',4''''-di-pyridinyl-2,2':6',2''-terpyridine) capable of driving the photo-evolution of hydrogen with a constant rate of activity for 12 days [M. Rupp et al., Inorg. Chem., 2019, 58, 9127-9134], we investigated the impact of an internal electron donor on the photoactivity of three new ruthenium bis-terpyridine photosensitizers. We used 4'-(N,N-dimethylaminophenyl)-4,4''-di-tert-butyl-2,2':6',2''-terpyridine (Dtpy) as the donor ligand. These complexes also bear peripheral coordination sites at various positions on the terpyridine backbone, allowing for photosensitizer-catalyst interactions. Their performances in photocatalysis under blue and green light (450 and 525 nm, respectively) were measured, in the presence of triethanolamine, using a cobaloxime catalyst [Co(dmgH)2(H2O)2](BF4)2. Complexes C1[Ru(Dtpy)(pytpy)](PF6)2 (pytpy: 4'-(pyridin-4-yl)-2,2':6',2''-terpyridine) and C3[Ru(Dtpy)(bipytpy)](PF6)2 appeared to have a photostability similar to that of [Ru(toltpy)(bipytpy)](PF6)2 but with lower activity rates, while C2[Ru(Dtpy)(pz2bpy)](PF6)2 (pz2bpy: 2,6-di(pyrazin-2-yl)-4,4'-bipyridine) showed a better peak activity, however, followed by a progressive decay. After 24 hours, complexes C1, C2 and C3 had reached TONs of 18, 35 and 52 under blue light and 14, 20 and 47 under green light, respectively, and were still found to be active. Their photophysical and electronic properties are discussed to rationalize the photocatalytic trends.

Design and photophysical studies of iridium(iii)-cobalt(iii) dyads and their application for dihydrogen photo-evolution.

We report several new dyads constituted of cationic iridium(iii) photosensitizers and cobalt(iii) catalyst connected via free pendant pyridine on the photosensitizers. These dyads were studied by X-ray crystallography, electrochemistry, absorption and emission spectroscopy as well as theoretical calculations and were shown to efficiently produce H2 under visible light irradiation. In every case, the dyad outperformed the equivalent system without a pendant pyridine. The dependence between irradiation wavelength and photocatalytic performances was also studied, with H2 being evolved with turn-over numbers up to 295, 251, 188 and 78 molH2 molPS-1 under blue, green, yellow and red light, respectively.

Photocatalytic Hydrogen Evolution Driven by a Heteroleptic Ruthenium(II) Bis(terpyridine) Complex.

Since the initial report by Lehn et al. in 1979, ruthenium tris(bipyridine) ([Ru(bpy)3]2+) and its numerous derivatives were applied as photosensitizers (PSs) in a large panel of photocatalytic conditions while the bis(terpyridine) analogues were disregarded because of their low quantum yields and short excited-state lifetimes. In this study, we prepared a new terpyridine ligand, 4'-(4-bromophenyl)-4,4‴:4″,4‴'-dipyridinyl- 2,2':6',2″-terpyridine (Bipytpy) and used it to prepare the heteroleptic complex [Ru(Tolyltpy)(Bipytpy)](PF6)2 (1; Tolyltpy = 4'-tolyl-2,2':6',2'-terpyridine). Complex 1 exhibits enhanced photophysical properties with a higher quantum yield (7.4 × 10-4) and a longer excited-state lifetime (3.8 ns) compared to those of [Ru(Tolyltpy)2](PF6)2 (3 × 10-5 and 0.74 ns, respectively). These enhanced photophysical characteristics and the potential for PS-catalyst interaction through the peripheral pyridines led us to apply the complex for visible-light-driven hydrogen evolution. The photocatalytic system based on 1 as the PS, triethanolamine as a sacrificial donor, and cobaloxime as a catalyst exhibits sustained activity over more than 10 days under blue-light irradiation (light-emitting diode centered at 450 nm). A maximum turnover number of 764 was obtained after 12 days.

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.

Mimicking 2,2':6',2'':6'',2'''-quaterpyridine complexes for the light-driven hydrogen evolution reaction: synthesis, structural, thermal and physicochemical characterizations.

The synthetic difficulties associated with quaterpyridine (qtpy) complexes have limited their use in the formation of various metallosupramolecular architectures in spite of their diverse structural and physicochemical properties. Providing a new facile synthetic route to the synthesis of functionalised qtpy mimics, we herein report the synthesis of three novel -NH2 functionalized qtpy-like complexes 12-14 with the general formula M(C16H14N12)(NO3)2 (M = Co(ii), Ni(ii) and Cu(ii)) in high yield and purity. Characterization of these complexes has been done by single crystal X-ray diffraction (SCXRD), thermogravimetric analysis, UV-Vis, infrared, mass spectrometry and cyclic voltammetry. As indicated by SCXRD, in all the synthesized complexes, the metal ions show a strongly distorted octahedral coordination geometry and typical hydrogen bonding networks involving DAT groups. In addition, complexes 12-14 have been analyzed as potential photocatalysts for hydrogen evolution reaction (HER) displaying good turnover numbers (TONs). Hydrogen produced from these photocatalysts can serve as the possible alternative for fossil fuels. To the best of our knowledge, this is the only study showcasing -NH2 functionalized qtpy-like complexes of Co(ii), Ni(ii) and Cu(ii) and employing them as photocatalysts for HER. Thus, a single proposed strategy solves two purposes-one related to synthesis while second is related to our environment.

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.

Alkyl chain length effects on double-deck assembly at a liquid/solid interface.

Controlled double-deck packing is an appealing means to expand upon conventional 2D self-assembly which is critical in crystal engineering, yet it is rare and poorly understood. Herein, we report the first systematic study of double-deck assembly in a series of alkylated aminoquinone derivatives at the liquid-solid interface. The competition between the fraction of alkyl chains adsorbed on the surface and the optimal conformation of the alkyl chains near the head group leads to a stepwise structural transformation ranging from complete double-deck packing to complete monolayer packing. Alkyl chains on the bottom or top layer of the double-deck assemblies were selectively visualized by carefully tuning the scanning tunneling microscopy settings. A method to easily identify mirror image domains was discovered based on the coincidence of domain boundaries with a graphite main axis. The effect of molecular symmetry and metal complexation on the formation of the double-deck assembly was also explored. Based on 2D crystal engineering principles, this bottom-up double-deck assembly can potentially provide an essential toehold for constructing precise 3D hierarchical structures.

Probing the effect of ß-triketonates in visible and NIR emitting lanthanoid complexes.

An isomorphous series of lanthanoid complexes containing tribenzoylmethanide (tbm) and 1,10-phenanthroline (phen) ligands has been synthesised and structurally characterised. These complexes, formulated as [Ln(phen)(tbm)3] (Ln = Eu3+, Er3+ and Yb3+), were compared with analogous dibenzoylmethanide (dbm) [Ln(phen)(dbm)3] complexes to investigate the effect of changing ß-diketonate to ß-triketonate ligands on the photophysical properties of the complex. The photophysical properties for the Eu3+ complexes were similar for both systems, whereas a modest enhancement was observed for Yb3+ and Er3+ moving from the dbm to the tbm complexes. A detailed study of the NIR photophysical properties was achieved by adapting the integrating sphere method for the calculation of overall quantum yields in the solid state.

Modular Cavities: Induced Fit of Polar and Apolar Guests into Halogen-Based Receptors.

Neutral triangular macrocyclic compounds, [PdX2(4,7-phen)]3·(DMF)3·Et2O (X = Cl, Br; 4,7-phen = 4,7-phenanthroline; DMF = N, N'-dimethylformamide; Et2O = diethyl ether), were synthesized, and their molecular structures were characterized. Solution-state 1H NMR results suggested the formation of metal-ligand bonds, and single-crystal X-ray crystallography revealed clear triangular structures. A detailed examination of the structures indicated the formation of two kinds of cavities in the solid state, where a triangular unit works as a halogen-based receptor for polar and apolar solvents through weak hydrogen-bonding and dipole-dipole interaction.

Blue-Emissive Cobalt(III) Complexes and Their Use in the Photocatalytic Trifluoromethylation of Polycyclic Aromatic Hydrocarbons.

Room-temperature luminescent CoIII complexes (1 and 2) are presented that exhibit intense ligand-to-metal and ligand-to-ligand charge transfer absorption in the low-energy UV region (λabs ≈360-400 nm) and low-negative quasi-reversible reduction events (E1/2(red) =-0.58 V and -0.39 V vs. SCE for 1 and 2, respectively). The blue emission of 1 and 2 at RT is due to the large bite angles and strong σ-donation of the ligands, the combined effect of which helps to separate the emissive 3 LMCT (triplet ligand-to-metal charge transfer) and the non-emissive 3 MC (triplet metal-centered) states. 1 and 2 were found to be powerful photo-oxidants (ECoIII*/CoII =2.26 V and 2.75 V vs. SCE of 1 and 2, respectively) and were used as inexpensive photoredox catalysts for the regioselective mono(trifluoromethylation) of polycyclic aromatic hydrocarbons (PAHs) in good yields (ca. 40-58 %).

A Smorgasbord of 17 Cobalt Complexes Active for Photocatalytic Hydrogen Evolution.

Seventeen cobalt complexes-eleven dinuclear cobalt(II) complexes and three tetranuclear cobalt complexes (two mixed valent) of ditopic ligands, with varying N-donor aromatic bridging moieties and pendant pyridine side arms, as well as three mononuclear cobalt(II) complexes of Schiff base macrocyclic ligands-have been screened for photocatalytic hydrogen evolution reaction (HER) activity. All 17 complexes are active catalysts for the HER, in both DMF and aqueous solution, in tandem with the [Ru(bpy)3 ]2+ (bpy=2,2'-bipyridine) photosensitiser. All are benchmarked to the literature standard [CoIII (dmgH)2 (py)Cl] (dmg=dimethylglyoxime, py=pyridine) under identical conditions. Two families of dinuclear cobalt(II) complexes of bis-tetradentate ligands that provide a triazole bridging moiety and mononuclear cobalt(II) complexes of tetradentate Schiff base macrocycles were found to be the most active catalysts, outperforming [CoIII (dmgH)2 (py)Cl] by two- to three-fold. Within these two families, the use of shorter alkyl linkers between the N donors, and hence, smaller chelate ring sizes, was found to significantly enhance catalytic performance, whereas the variation of peripheral functional groups was found to have little effect. This last point will be convenient for subsequent surface immobilisation studies.