Ru Monoimines with Extended Excited-State Lifetimes and Geometrical Modulation of Photoinduced Mixed-Valence Interactions.

The photophysical properties of monodentate-imine ruthenium complexes do not usually fulfil the requirements for supramolecular solar energy conversion schemes. Their short excited-state lifetimes, like the 5.2 ps metal-to-ligand charge transfer (MLCT) lifetime of [Ru(py)4Cl(L)]+ with L = pz (pyrazine), preclude bimolecular or long-range photoinduced energy or electron transfer reactions. Here, we explore two strategies to extend the excited-state lifetime, based on the chemical modification of the distal N atom of pyrazine. On one hand, we used L = pzH+, where protonation stabilized MLCT states, rendering thermal population of MC states less favorable. On the other hand, we prepared a symmetric bimetallic arrangement in which L = {(µ-pz)Ru(py)4Cl} to enable hole delocalization via photoinduced mixed-valence interactions. A lifetime extension of 2 orders of magnitude is accomplished, with charge transfer excited states living 580 ps and 1.6 ns, respectively, reaching compatibility with bimolecular or long-range photoinduced reactivity. These results are similar to those obtained with Ru pentaammine analogues, suggesting that the strategy employed is of general applicability. In this context, the photoinduced mixed-valence properties of the charge transfer excited states are analyzed and compared with those of different analogues of the Creutz-Taube ion, demonstrating a geometrical modulation of the photoinduced mixed-valence properties.

ATP-Independent and Cell-Free Biosynthesis of ß-Hydroxy Acids Using Vinyl Esters as Smart Substrates.

In vitro biosynthetic pathways that condense and reduce molecules through coenzyme A (CoASH) activation demand energy and redox power in the form of ATP and NAD(P)H, respectively. These coenzymes must be orthogonally recycled by ancillary reactions that consume chemicals, electricity, or light, impacting the atom economy and/or the energy consumption of the biosystem. In this work, we have exploited vinyl esters as dual acyl and electron donor substrates to synthesize ß-hydroxy acids through a non-decarboxylating Claisen condensation, reduction and hydrolysis stepwise cascade, including a NADH recycling step, catalyzed by a total of 4 enzymes. Herein, the chemical energy to activate the acyl group with CoASH and the redox power for the reduction are embedded into the vinyl esters. Upon optimization, this self-sustaining cascade reached a titer of (S)-3-hydroxy butyrate of 24 mM without requiring ATP and simultaneously recycling CoASH and NADH. This work illustrates the potential of in vitro biocatalysis to transform simple molecules into multi-functional ones.

The Excited-State Creutz-Taube Ion.

The excited-state version of the Creutz-Taube ion was prepared via visible light excitation of [(NH3 )5 RuII (µ-pz)RuII (NH3 )5 ]4+ . The resulting excited state is a mixed valence {RuIII-δ (µ-pz⋅- )RuII+δ } transient species, which was characterized using femtosecond transient absorption spectroscopy with vis-NIR detection. Very intense photoinduced intervalence charge transfers were observed at 7500 cm-1 , revealing an excited-state electronic coupling element HDA =3750 cm-1 . DFT calculations confirm a strongly delocalized excited state. A notable consequence of strong electron delocalization is the nanosecond excited state lifetime, which was exploited in a proof-of-concept intermolecular electron transfer. The excited-state Creutz-Taube ion is established as a reference, and demonstrates that electron delocalization in the excited state can be leveraged for artificial photosynthesis or other photocatalytic schemes based on electron transfer chemistry.

Flavin-mediated photoactivation of Pt(IV) anticancer complexes: computational insights on the catalytic mechanism.

The mechanism for the photocatalytic activation of Pt(IV) anticancer prodrugs by riboflavin in the presence of NADH has been investigated by DFT. In the first step of the reaction, the oxidation kinetics of NADH to afford the catalytically active riboflavin hydroquinone is dramatically favoured by generation of the flavin triplet excited state. In the triplet, formation of a π-π stacked adduct promotes the hydride transfer from NADH to riboflavin with an almost barrierless pathway (2.7 kcal mol-1). In the singlet channel, conversely, the process is endergonic and requires overcoming a higher activation energy (19.2 kcal mol-1). In the second half of the reaction, the reduction of the studied Pt(IV) complexes by riboflavin hydroquinone occurs via an inner sphere mechanism, displaying free energy barriers smaller than 10 kcal mol-1. Pt reduction by bioreductants such as NADH and ascorbate involve instead less stabilized transition states (22.2-38.3 kcal mol-1), suggesting that riboflavin hydroquinone is an efficient reducing agent for Pt(IV) derivatives in biological settings.

Does geometry matter? Effect of the ligand position in bimetallic ruthenium polypyridine siblings.

In this work, we present the preparation of a complex [(tpy)(bpy)Ru(µ-CN)Ru(py)4(OH2)](PF6)3 (tpy = 2,2',6',2''-terpyridine; bpy = 2,2'-bipyridine; py = pyridine) that combines a ruthenium chromophore linked to another ruthenium ion that bears a labile position trans to the bridge. Substitution in this position is very attractive, as it allows us to place a quencher trans to the chromophore maximizing the separation between them. This complex allowed us to prepare a family of cyanide-bridged ruthenium polypyridines of general formula [Ru(tpy)(bpy)(µ-CN)Ru(py)4(L)]2/3+ (L = Cl-, NCS-, 4-dimethylaminopyridine or acetonitrile) and compare them with the related complexes [Ru(tpy)(bpy)(µ-CN)Ru(bpy)2(L)]2/3+ where the L ligand lies cis to the bridge. The mixed-valence form of these complexes shows evidence of strong coupling between the ruthenium ions and enhanced delocalization as the redox potential of the {Ru(py)4L} fragment increases. (TD)DFT calculations reproduce very well the experimental spectra of these complexes and indicate that when L = acetonitrile, the hole in the mixed-valence complex is almost equally distributed between both ruthenium ions. For L = DMAP and NCS- the π orbitals of the ligands are mixed with dπ orbitals of the Ru ions, resulting in partial delocalization of the charge on the ligands. The latter result illustrates that the trans configuration of these complexes is well-suited to extend the interaction beyond the bridged ruthenium ions.

A Hole Delocalization Strategy: Photoinduced Mixed-Valence MLCT States Featuring Extended Lifetimes.

Bimetallic trans-[RuII(tpm)(bpy)(µNC)RuII(L)4(CN)]2+, where bpy is 2,2'-bipyridine, tpm is tris(1-pyrazolyl)methane and L = 4-methoxypyridine (MeOpy) or pyridine (py), was examined using ultrafast vis-NIR transient absorption spectroscopy. Of great relevance are the longest-lived excited states in the form of strongly coupled photoinduced mixed-valence systems, which exhibit intense photoinduced absorptions in the NIR and are freely tunable by the judicious choice of the coordination spheres of the metallic ions. Using the latter strategy, we succeeded in tailoring the excited state lifetimes of bimetallic complexes and, in turn, achieving significantly longer values relative to related monometallic complexes. Notable is the success in extending the lifetimes, when considering the higher density of vibrational states, as they are expected to facilitate nonradiative ground-state recovery.

Trapping intermediate MLCT states in low-symmetry {Ru(bpy)} complexes.

The picosecond excited state dynamics of [Ru(tpm)(bpy)(NCS)]+ (RubNCS+ ) and [Ru(tpm)(bpy)(CN)]+ (RubCN+ ) (tpm = tris(1-pyrazolyl)methane, bpy = 2,2'-bipyridine) have been analyzed by means of transient absorption measurements and spectroelectrochemistry. Emissive 3MLCTs with (GS)HOMO(h+)-(GS)LUMO(e-) configurations are the lowest triplet excited states regardless of whether 387 or 505 nm photoexcitation is used. 387 nm photoexcitation yields, after a few picoseconds, the emissive 3MLCTs. In contrast, 505 nm photoexcitation populates an intermediate excited state that we assign as a 3MLCT state, in which the hole sits in a metal-centered orbital of different symmetry, prior to its conversion to the emissive 3MLCTs. The disparities in terms of electronic configuration between the intermediate and the emissive 3MLCTs have two important consequences. On one hand, both states feature very different fingerprint absorptions in transient absorption measurements. On the other hand, the reconfiguration is impeded by a kinetic barrier. As such, the conversion is followed spectroscopically and kinetically on the 300 ps timescale.

Exploring the localized to delocalized transition in non-symmetric bimetallic ruthenium polypyridines.

In this work, we report the evolution of the properties of the inter-valence charge transfer (IVCT) transition in a family of cyanide-bridged ruthenium polypyridines of general formula [RuII(tpy)(bpy)(µ-CN)RuIII(bpy)2(L)]3/4+ (tpy = 2,2',6',2''-terpyridine; bpy = 2,2'-bipyridine; L = Cl-, NCS-, 4-dimethylaminopyridine or acetonitrile). In these complexes, the redox potential difference between both ruthenium centers (ΔE) is systematically modified. A decrease in ΔE causes a red shift of the energy and an intensity enhancement of the observed IVCT transitions. For L = acetonitrile, the IVCT band becomes narrower and asymmetrical, and shows very little dependence on the nature of the solvent, suggesting a delocalized configuration, although a non-symmetrical one. Also, additional electronic transitions of low energy are clearly resolved in this complex. The observed variation in the properties of the IVCT transitions can be understood on the basis of DFT calculations, that point to increasing mixing between the dπ orbitals of both Ru ions.

Spectroscopic signatures of ligand field states in {Ru(II)(imine)} complexes.

Ligand field (LF) states have been present in discussions on the photophysics and photochemistry of ruthenium-iminic chromophores for decades, although there is very little documented direct evidence of them. We studied the picosecond transient absorption (TA) spectroscopy of four {Ru(II)(imine)} complexes that respond to the formula trans-[Ru(L)4(X)2], where L is either pyridine (py) or 4-methoxypyridine (MeOpy) and X is either cyanide or thiocyanate. Dicyano compounds behave as most ruthenium polypyridines and their LF states remain silent. In contrast, in the dithiocyanate complexes we found clear spectroscopic evidence of the participation of LF states in the MLCT decay pathway. These states are of donor and acceptor character simultaneously and this is manifested in the presence of MLCT and LMCT transient absorption bands of similar energy. Spectroelectrochemical techniques supported the interpretation of the absorption features of MLCT states, and DFT methods helped to assign their spectroscopic signatures and provided strong evidence on the nature of LF states.

Influence of the electronic configuration in the properties of d6-d5 mixed-valence complexes.

We report here the spectroscopic properties of four very closely related mixed-valence cyanide-bridged bimetallic complexes, trans-[Ru(T)(bpy)(µ-NC)Ru(L)4(CN)](3+) (T = tris(1-pyrazolyl)methane (tpm, a) or 2,2';6',2″-terpyridine, (tpy, b), and L = pyridine (py, 1) or 4-methoxypyridine (MeOpy, 2)). In acetonitrile all the complexes present intervalence charge transfer (IVCT) transitions in the NIR region, but their intensities are widely different, with the intensity of the transition observed for 1a-b(3+) around four times larger than that observed for 2a-b(3+). This contrasting behavior can be traced to the different nature of the dπ acceptor orbitals involved in these transitions, as confirmed by (TD)DFT calculations. The spectroscopy of 1a-b(3+) provides evidence that the spin density is delocalized over the two ruthenium ions, such as a narrowing of the IVCT bands that results in the resolution of the expected three bands, and a weak solvent dependence of the energy of these transitions. The spectroscopy of 2a-b(3+) instead indicates that the spin density is localized on one ruthenium ion. The IVCT in these systems is particularly weak due to the configuration of the Ru(III), where the vacant orbital is perpendicular to the cyanide bridge.

Class III delocalization in a cyanide-bridged trimetallic mixed-valence complex.

The NIR and IR spectroscopic properties of the cyanide-bridged complex, trans-[Ru(dmap)4 {(µ-CN)Ru(py)4 Cl}2 ](3+) (py=pyridine, dmap=4-dimethylaminopyridine) provide strong evidence that this trimetallic ion behaves as a Class III mixed-valence species, the first example reported of a cyanide-bridged system. This has been accomplished by tuning the energy of the fragments in the trimetallic complex to compensate for the intrinsic asymmetry of the cyanide bridge. Moreover, (TD)DFT calculations accurately predict the spectra of the trans-[Ru(dmap)4 {(µ-CN)Ru(py)4 Cl}2 ](3+) ion and confirms its delocalized nature.

Communication between remote moieties in linear Ru-Ru-Ru trimetallic cyanide-bridged complexes.

In this article, we report the structural, spectroscopic, and electrochemical properties of the cyanide-bridged complex salts trans-[(NC)Ru(II)(L)4(µ-CN)Ru(II)(py)4Cl]PF6 and trans-[Ru(II)(L)4{(µ-CN)Ru(II)(py)4Cl}2](PF6)2 (L = pyridine or 4-methoxypyridine). The mixed-valence forms of these compounds show a variety of metal-to-metal charge-transfer bands, including one arising from charge transfer between the remote ruthenium units. The latter is more intense when L = 4-methoxypyridine and points to the role of the bridging ruthenium unit in promoting mixing between the dπ orbitals of the terminal fragments.