Mateusz Rebarz

An efficient Ru(II) -Rh(III) -Ru(II) polypyridyl photocatalyst for visible-light-driven hydrogen production in aqueous solution.

The development of multicomponent molecular systems for the photocatalytic reduction of water to hydrogen has experienced considerable growth since the end of the 1970s. Recently, with the aim of improving the efficiency of the catalysis, single-component photocatalysts have been developed in which the photosensitizer is chemically coupled to the hydrogen-evolving catalyst in the same molecule through a bridging ligand. Until now, none of these photocatalysts has operated efficiently in pure aqueous solution: a highly desirable medium for energy-conversion applications. Herein, we introduce a new ruthenium-rhodium polypyridyl complex as the first efficient homogeneous photocatalyst for H2 production in water with turnover numbers of several hundred. This study also demonstrates unambiguously that the catalytic performance of such systems linked through a nonconjugated bridge is significantly improved as compared to that of a mixture of the separate components.

[RhIII(dmbpy)2Cl2]+ as a Highly Efficient Catalyst for Visible‐Light‐Driven Hydrogen Production in Pure Water: Comparison with Other Rhodium Catalysts

We report a very efficient homogeneous system for the visible-light-driven hydrogen production in pure aqueous solution at room temperature. This comprises [Rh(III) (dmbpy)(2)Cl(2)]Cl (1) as catalyst, [Ru(bpy)(3)]Cl(2) (PS1) as photosensitizer, and ascorbate as sacrificial electron donor. Comparative studies in aqueous solutions also performed with other known rhodium catalysts, or with an iridium photosensitizer, show that 1) the PS1/1/ascorbate/ascorbic acid system is by far the most active rhodium-based homogeneous photocatalytic system for hydrogen production in a purely aqueous medium when compared to the previously reported rhodium catalysts, Na(3)[Rh(I) (dpm)(3)Cl] and [Rh(III)(bpy)Cp*(H(2)O)]SO(4) and 2) the system is less efficient when [Ir(III) (ppy)(2)(bpy)]Cl(PS2) is used as photosensitizer. Because catalyst 1 is the most efficient rhodium-based H(2)-evolving catalyst in water, the performance limits of this complex were further investigated by varying the PS1/1 ratio at pH 4.0. Under optimal conditions, the system gives up to 1010 turnovers versus the catalyst with an initial turnover frequency as high as 857 TON h(-1). Nanosecond transient absorption spectroscopy measurements show that the initial step of the photocatalytic H(2)-evolution mechanism is a reductive quenching of the PS1 excited state by ascorbate, leading to the reduced form of PS1, which is then able to reduce [Rh(III)(dmbpy)(2)Cl(2)](+) to [Rh(I)(dmbpy)(2)](+). This reduced species can react with protons to yield the hydride [Rh(III)(H)(dmbpy)(2)(H(2)O)](2+), which is the key intermediate for the H(2) production.

Deciphering the protonation and tautomeric equilibria of firefly oxyluciferin by molecular engineering and multivariate curve resolution

The mysterious flashes of light communicated by fireflies conceal a rich and exciting solution spectrochemistry that revolves around the chemiexcitation and photodecay of the fluorophore, oxyluciferin. A triple chemical equilibrium by double deprotonation and keto–enol tautomerism turns this simple molecule into an intricate case where the relative spectral contributions of six chemical species combine over a physiologically relevant pH range, rendering physical isolation and spectral characterization of most of the species unmanageable. To disentangle the individual spectral contributors, here we demonstrate the advantage of chemical oriented multivariate data analysis. We designed a set of specific oxyluciferin derivatives and applied a multivariate curve resolution-alternating least squares (MCR-ALS) procedure simultaneously to an extensive set of pH-dependent spectroscopic data for oxyluciferin and the target derivatives. The analysis provided, for the first time, the spectra of the pure individual components free of contributions from the other forms, their pH-dependent profiles and distributions, and the most accurate to date values for the three equilibrium constants.

Emission Properties of oxyluciferin and Its Derivatives in Water: Revealing the Nature of the Emissive Species in Firefly Bioluminescence

The first systematic steady-state and time-resolved emission study of firefly oxyluciferin (emitter in firefly bioluminescence) and its analogues in aqueous buffers provided the individual emission spectra of all chemical forms of the emitter and the excited-state equilibrium constants in strongly polar environment with strong hydrogen bonding potential. The results confirmed the earlier hypothesis that excited-state proton transfer from the enol group is favored over proton transfer from the phenol group. In water, the phenol-keto form is the strongest photoacid among the isomers and its conjugate base (phenolate-keto) has the lowest emission energy (634 nm). Furthermore, for the first time we observed green emission (525 nm) from a neutral phenol-keto isomer constrained to the keto form by cyclopropyl substitution. The order of emission energies indicates that in aqueous solution a second deprotonation at the phenol group after the enol group had dissociated (that is, deprotonation of the phenol-enolate) does not occur in the first excited state. The pH-dependent emission spectra and the time-resolved fluorescence parameters revealed that the keto-enol tautomerism reaction, which can occur in a nonpolar environment (toluene) in the presence of a base, is not favored in water.

Synthesis, Characterization, and Photocatalytic H2-Evolving Activity of a Family of [Co(N4Py)(X)]n+ Complexes in Aqueous Solution

A series of [Co(III)(N4Py)(X)](ClO4)n (X = Cl(-), Br(-), OH(-), N3(-), NCS(-)-κN, n = 2: X = OH2, NCMe, DMSO-κO, n = 3) complexes containing the tetrapyridyl N5 ligand N4Py (N4Py = 1,1-di(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine) has been prepared and fully characterized by infrared (IR), UV-visible, and NMR spectroscopies, high-resolution electrospray ionization mass spectrometry (HRESI-MS), elemental analysis, X-ray crystallography, and electrochemistry. The reduced Co(II) and Co(I) species of these complexes have been also generated by bulk electrolyses in MeCN and characterized by UV-visible and EPR spectroscopies. All tested complexes are catalysts for the photocatalytic production of H2 from water at pH 4.0 in the presence of ascorbic acid/ascorbate, using [Ru(bpy)3](2+) as a photosensitizer, and all display similar H2-evolving activities. Detailed mechanistic studies show that while the complexes retain the monodentate X ligand upon electrochemical reduction to Co(II) species in MeCN solution, in aqueous solution, upon reduction by ascorbate (photocatalytic conditions), [Co(II)(N4Py)(HA)](+) is formed in all cases and is the precursor to the Co(I) species which presumably reacts with a proton. These results are in accordance with the fact that the H2-evolving activity does not depend on the chemical nature of the monodentate ligand and differ from those previously reported for similar complexes. The catalytic activity of this series of complexes in terms of turnover number versus catalyst (TONCat) was also found to be dependent on the catalyst concentration, with the highest value of 230 TONCat at 5 × 10(-6) M. As revealed by nanosecond transient absorption spectroscopy measurements, the first electron-transfer steps of the photocatalytic mechanism involve a reductive quenching of the excited state of [Ru(bpy)3](2+) by ascorbate followed by an electron transfer from [Ru(II)(bpy)2(bpy(•-))](+) to the [Co(II)(N4Py)(HA)](+) catalyst. The reduced catalyst then enters into the H2-evolution cycle.

Cobalt(III) tetraaza-macrocyclic complexes as efficient catalyst for photoinduced hydrogen production in water: Theoretical investigation of the electronic structure of the reduced species and mechanistic insight.

We recently reported a very efficient homogeneous system for visible-light driven hydrogen production in water based on the cobalt(III) tetraaza-macrocyclic complex [Co(CR)Cl2](+) (1) (CR=2,12-dimethyl-3,7,11,17-tetra-azabicyclo(11.3.1)-heptadeca-1(17),2,11,13,15-pentaene) as a noble metal-free catalyst, with [Ru(II)(bpy)3](2+) (Ru) as photosensitizer and ascorbate/ascorbic acid (HA(-)/H2A) as a sacrificial electron donor and buffer (PhysChemChemPhys 2013, 15, 17544). This catalyst presents the particularity to achieve very high turnover numbers (TONs) (up to 1000) at pH 4.0 at a relative high concentration (0.1mM) generating a large amount of hydrogen and having a long term stability. A similar activity was observed for the aquo derivative [Co(III)(CR)(H2O)2](3+) (2) due to substitution of chloro ligands by water molecule in water. In this work, the geometry and electronic structures of 2 and its analog [Zn(II)(CR)Cl](+) (3) derivative containing the redox innocent Zn(II) metal ion have been investigated by DFT calculations under various oxidation states. We also further studied the photocatalytic activity of this system and evaluated the influence of varying the relative concentration of the different components on the H2-evolving activity. Turnover numbers versus catalyst (TONCat) were found to be dependent on the catalyst concentration with the highest value of 1130 obtained at 0.05 mM. Interestingly, the analogous nickel derivative, [Ni(II)(CR)Cl2] (4), when tested under the same experimental conditions was found to be fully inactive for H2 production. Nanosecond transient absorption spectroscopy measurements have revealed that the first electron-transfer steps of the photocatalytic H2-evolution mechanism with the Ru/cobalt tetraaza/HA(-)/H2A system involve a reductive quenching of the excited state of the photosensitizer by ascorbate (kq=2.5×10(7) M(-1) s(-1)) followed by an electron transfer from the reduced photosensitizer to the catalyst (ket=1.4×10(9) M(-1) s(-1)). The reduced catalyst can then enter into the cycle of hydrogen evolution.

Revisited photophysics and photochemistry of a Ru-TAP complex using chloride ions and a calix[6]crypturea

The effects of the nonprotonated and protonated calix[6]crypturea 1/1(•)H(+) on the PF6(-) and Cl(-) salts of a luminescent Ru-TAP complex (TAP = 1,4,5,8-tetraazaphenanthrene) were investigated. Thus, the phototriggered basic properties of this complex were examined with 1(•)H(+) in acetonitrile (MeCN) and butyronitrile (BuCN). The Ru excited complex was shown to be able to extract a proton from the protonated calixarene, accompanied by a luminescence quenching in both solvents. However, in BuCN, the Cl(-) salt of the complex exhibited a surprising behavior in the presence of 1/1(•)H(+). Although an emission decrease was observed with the protonated calixarene, an emission increase was evidenced in the presence of nonprotonated 1. As the Cl(-) ions were shown to inhibit the luminescence of the complex in BuCN, this luminescence increase by nonprotonated 1 was attributed to the protection effect of 1 by encapsulation of the Cl(-) anions into the tris-urea binding site. The study of the luminescence lifetimes of the Ru-TAP complex in BuCN as a function of temperature for the PF6(-) and Cl(-) salts in the absence and presence of 1 led to the following conclusions. In BuCN, in contrast to MeCN, in addition to ion pairing, because of the poor solvation of the ions, the luminescent metal-to-ligand charge transfer ((3)MLCT) state could reach two metal-centered ((3)MC) states, one of which is in equilibrium with the (3)MLCT state during the emission lifetime. The reaction of Cl(-) with this latter (3)MC state would be responsible for the luminescence quenching, in agreement with the formation of photosubstitution products.

Photoaddition of two guanine bases to single Ru-TAP complexes. Computational studies and ultrafast spectroscopies to elucidate the pH dependence of primary processes.

The covalent photoadduct (PA) between [Ru(TAP)3](2+) (TAP = 1,4,5,8-tetraazaphenanthrene) and guanosine monophosphate (GMP) opened the way to interesting photobiological applications. In this context, the PAs capability upon illumination to give rise to the addition of a second guanine base is especially interesting. The origins of these intriguing properties are for the first time thoroughly investigated by an experimental and theoretical approach. The PAs spectroscopic and redox data combined with TDDFT results corroborated with resonance Raman data show that the properties of this PA (pKa around 7) depend on the solution pH. Theoretical results indicate that the acid form PA.H(+) when excited should relax to MLCT (metal-to-ligand charge transfer) excited states, in contrast to the basic form PA whose excited state should have LLCT/ILCT (ligand-to-ligand charge transfer/intra ligand charge transfer) characteristics. Ultrafast excitation of PA.H(+) at pH 5.9 produces continuous dynamic processes in a few hundred picoseconds involving coupled proton-electron transfers responsible for luminescence quenching. Long-lived species of a few microseconds capable of reacting with GMP are produced at that pH, in agreement with the formation of covalent addition of a second GMP to PA, as shown by mass spectrometry results. In contrast, at pH 8 (mainly nonprotonated PA), other ultrafast transient species are detected and no GMP biadduct is formed in the presence of GMP. This pH dependence of photoreaction can be rationalized with the different nature of the excited states, thus at pH 8, unreactive LLCT/ILCT states and at pH 5.9 reactive MLCT states.