Diimine Photosensitizers Research Articles

Bifurcation of Excited-State Population Leads to Anti-Kasha Luminescence in a Disulfide-Decorated Organometallic Rhenium Photosensitizer.

We report a rhenium diimine photosensitizer equipped with a peripheral disulfide unit on one of the bipyridine ligands, [Re(CO)3(bpy)(S-Sbpy4,4)]+ (1+, bpy = 2,2'-bipyridine, S-Sbpy4,4 = [1,2]dithiino[3,4-c:6,5-c']dipyridine), showing anti-Kasha luminescence. Steady-state and ultrafast time-resolved spectroscopies complemented by nonadiabatic dynamics simulations are used to disclose its excited-state dynamics. The calculations show that after intersystem crossing the complex evolves to two different triplet minima: a (S-Sbpy4,4)-ligand-centered excited state (3LC) lying at lower energy and a metal-to-(bpy)-ligand charge transfer (3MLCT) state at higher energy, with relative yields of 90% and 10%, respectively. The 3LC state involves local excitation of the disulfide group into the antibonding σ* orbital, leading to significant elongation of the S-S bond. Intriguingly, it is the higher-lying 3MLCT state, which is assigned to display luminescence with a lifetime of 270 ns: a signature of anti-Kasha behavior. This assignment is consistent with an energy barrier ≥ 0.6 eV or negligible electronic coupling, preventing reaction toward the 3LC state after the population is trapped in the 3MLCT state. This study represents a striking example on how elusive excited-state dynamics of transition-metal photosensitizers can be deciphered by synergistic experiments and state-of-the-art calculations. Disulfide functionalization lays the foundation of a new design strategy toward harnessing excess energy in a system for possible bimolecular electron or energy transfer reactivity.

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.

Photocatalytic activity of hybrid P450 Enzymes

The Cytochromes P450 are heme-thiolate enzymes that carry out selective oxyfunctionalization of a wide range of substrates using molecular dioxygen and two reducing equivalents provided by redox partner enzymes. Alternative approaches to activate P450 enzymes have received recent attention in an attempt to circumvent the use of redox partners and cofactors. In particular, the use of light-harvesting units has proven useful in triggering P450 activity upon visible light-excitation. Our laboratory has taken advantage of the inorganic Ru(II)-diimine photosensitizers and their unique excited properties to harness P450 synthetic potential. Their covalent attachment to non-native single cysteine residues has enabled rapid electron injection into the heme domain of several P450 enzymes leading to high photocatalytic activity and coupling efficiency. Recently, we also developed a bimolecular system to activate P450 with peroxygenase activity. The crystal structure of the most efficient hybrid enzyme reveals that the photosensitizer is ideally positioned to deliver electrons to the heme active site utilizing the natural electron transfer pathway. A combination of rational and directed evolution approaches are currently employed to optimize the biocatalyst photocatalytic activity and expand the scope of the light-driven P450 activity. In addition, the tunability and derivatization of the inorganic complexes have promoted the formation of dimeric structures and heterogeneous biocatalysts and provided insights into the rate-limiting steps in the light-driven hybrid P450 enzyme.

Surface immobilized copper(I) diimine photosensitizers as molecular probes for elucidating the effects of confinement at interfaces for solar energy conversion.

Heteroleptic copper(i) bis(phenanthroline) complexes with surface anchoring carboxylate groups have been synthesized and immobilized on nanoporous metal oxide substrates. The species investigated are responsive to the external environment and this work provides a new strategy to control charge transfer processes for efficient solar energy conversion.

Coupling efficiency in light-driven hybrid P450BM3 and CYP119 enzymes

The light-driven hybrid P450 enzyme approach utilizing the photochemical properties of a covalently attached Ru(II)-diimine photosensitizer was extended to the archaeal Sulfolobus acidocaldarius CYP119 enzyme leading to high photocatalytic activity in the hydroxylation of the chromogenic substrate, 11-nitrophenoxyundecanoic acid. The determined kcat was greater than those reported with various natural redox partners. In addition, the sacrificial electron donor, diethyldithiocarbamate, used in the photocatalytic reaction is shown to play a dual role. It acts as an efficient quencher of the Ru(II) excited state leading to a highly reducing species necessary to inject electrons into the heme. It is also known for its antioxidant properties and is shown herein to be a useful probe to determine coupling efficiency in the light-driven hybrid enzymes.

Selective Light-Driven Chemoenzymatic Trifluoromethylation/Hydroxylation of Substituted Arenes

The merging of photoredox trifluoromethylation with hybrid P450 BM3 variants has enabled the selective light-driven functionalization of several arenes. This approach capitalizes on the unique photochemical properties of the Ru(II)-diimine photosensitizer to initiate single electron transfer events. Under photoredox conditions, a CF3 radical promoted by the d6 metal complex can add to arenes. In the hybrid P450 BM3 enzymes, the covalently attached Ru(II)-diimine photosensitizer provides the necessary electrons to perform, upon visible light activation, P450 oxyfunctionalizations on the trifluoromethylated substrates.

Selective C–H bond functionalization with light-driven P450 biocatalysts

The unique photochemical properties of Ru(II)-diimine photosensitizers have enabled light-induced electron transfers in hybrid P450 heme domain enzymes. Rapid quenching of the excited state by soluble molecules generates either a highly oxidative or reductive species depending on the nature of the quencher. Under flash quench oxidative conditions, the heme cofactor of the P450 BM3 enzyme is oxidized to a high-valent ferryl species. Meanwhile, a photogenerated reductive species is able to deliver the necessary electrons to P450 heme active sites and sustain photocatalytic activity in the selective hydroxylation of substrate C–H bonds. Les remarquables propriétés photochimiques des photosensibilisateurs à base de ruthénium(II) et de bipyridine ont rendu possible des transferts d'électrons induits par la lumière dans les domaines héminiques hybrides de l‘enzyme P450 BM3. La désactivation rapide de leurs états excités par des composés solubles génère, soit un oxydant, soit un réducteur, selon la nature du composé. En conditions oxydantes, le cofacteur hème est oxydé en une espèce ferryle. Un composé très reducteur peut aussi être photogénéré ; il est alors en mesure de délivrer les électrons nécessaires au cytochrome P450 et de maintenir l'activité photocatalytique dans l'hydroxylation sélective des liaisons C–H de nombreux composés organiques.

Ru(II)-diimine functionalized metalloproteins: From electron transfer studies to light-driven biocatalysis

The unique photochemical properties of Ru(II)-diimine complexes have helped initiate a series of seminal electron transfer studies in metalloenzymes. It has thus been possible to experimentally determine rate constants for long-range electron transfers. These studies have laid the foundation for the investigation of reactive intermediates in heme proteins and for the design of light-activated biocatalysts. Various metalloenzymes such as hydrogenase, carbon monoxide dehydrogenase, nitrogenase, laccase and cytochrome P450 BM3 have been functionalized with Ru(II)-diimine complexes. Upon visible light-excitation, these photosensitized metalloproteins are capable of sustaining photocatalytic activity to reduce small molecules such as protons, acetylene, hydrogen cyanide and carbon monoxide or activate molecular dioxygen to produce hydroxylated products. The Ru(II)-diimine photosensitizers are hence able to deliver multiple electrons to metalloenzymes buried active sites, circumventing the need for the natural redox partners. In this review, we will highlight the key achievements of the light-driven biocatalysts, which stem from the extensive electron transfer investigations. This article is part of a Special Issue entitled Biodesign for Bioenergetics — the design and engineering of electronic transfer cofactors, proteins and protein networks, edited by Ronald L. Koder and J.L. Ross Anderson.

Photoinduced electron transfer in rhenium(I)-oligotriarylamine molecules.

Two molecular triads with an oligotriarylamine multielectron donor were synthesized and investigated with a view to obtaining charge-separated states in which the oligotriarylamine is oxidized 2-fold. Such photoinduced accumulation of multiple redox equivalents is of interest for artificial photosynthesis. The first triad was comprised of the oligotriarylamine and two rhenium(I) tricarbonyl diimine photosensitizers each of which can potentially accept one electron. In the second triad the oligotriarylamine was connected to anthraquinone, in principle an acceptor of two electrons, via a rhenium(I) tricarbonyl diimine unit. With nanosecond transient absorption spectroscopy (using an ordinary pump-probe technique) no evidence for the generation of 2-fold oxidized oligotriarylamine or 2-fold reduced anthraquinone was found. The key factors limiting the photochemistry of the new triads to simple charge separation of one electron and one hole are discussed, and the insights gained from this study are useful for further research in the area of charge accumulation in purely molecular (nanoparticle-free) systems. An important problem of the rhenium-based systems considered here is the short wavelength required for photoexcitation. In the second triad, photogenerated anthraquinone monoanion is protonated by organic acids, and the resulting semiquinone species leads to an increase in lifetime of the charge-separated state by about an order of magnitude. This shows that the proton-coupled electron transfer chemistry of quinones could be beneficial for photoinduced charge accumulation.

Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d(6) metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium(II) and osmium(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ~750 ns in dichloromethane to ~3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ~50 to ~2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET.

A series of hybrid P450 BM3 enzymes with different catalytic activity in the light-initiated hydroxylation of lauric acid

We have developed a series of hybrid P450 BM3 enzymes to perform the light-activated hydroxylation of lauric acid. These enzymes contain a Ru(II)-diimine photosensitizer covalently attached to single cysteine residues of mutant P450 BM3 heme domains. The library of hybrid enzymes includes four non-native single cysteine mutants (K97C, Q397C, Q109C and L407C). In addition, mutations around the heme active site, F87A and I401P, were inserted in the Q397C mutant. Two heteroleptic Ru(II) complexes, Ru(bpy)2phenA (1) and Ru(phen)2phenA (2) (bpy=bipyridine, phen=1,10-phenanthroline, and phenA=5-acetamido-1,10-phenanthroline), are used as photosensitizers. Upon visible light irradiation, the hybrid enzymes display various total turnover numbers in the hydroxylation of lauric acid, up to 140 for the L407C-1 mutant, a 16-fold increase compared to the F87A/Q397C-1 mutant. CO binding studies confirm the ability of the photogenerated Ru(I) compound to reduce the fraction of ferric high spin species present in the mutants upon substrate binding.

Light-initiated hydroxylation of lauric acid using hybrid P450 BM3 enzymes

We have developed hybrid P450 BM3 enzymes consisting of a Ru(II)-diimine photosensitizer covalently attached to non-native single cysteine residues of P450 BM3 heme domain mutants. These enzymes are capable, upon light activation, of selectively hydroxylating lauric acid with 40 times higher total turnover numbers compared to the peroxide shunt.

Variation of charge transfer kinetics in structurally closely related dyads with rhenium photosensitizers

The kinetics for photoinduced charge separation and thermal charge-recombination processes in three donor–bridge–acceptor molecules with rhenium(I) tricarbonyl diimine photosensitizers were investigated. Time resolved luminescence and transient absorption spectroscopies reveal that in addition to driving force effects, differences in bridge-mediated electronic donor–acceptor coupling among the three dyads play important roles. Notably, it is seen that charge separation depends strongly on whether initial photoexcitation involves promotion of an electron towards or away from a phenothiazine electron donor. Thermal charge recombination rates are found to differ by a factor of 2 between two isomeric dyads due to an electronic coupling effect.

Properties of excited states and one-electron reduced forms of Ruthenium(II)-diimine photosensitizers

In this paper we compare the spectral, acid-base, electrochemical, and kinetic properties of the excited states and one-electron reduced forms of homo- and heteroleptic Ru(II)-diimine photosensitizers that contain 2,2’-bipyridine, 2,2’-bipyrazine, and 2,2’-bipyrimidine ligands. These species, represented as *(RuIIIL.-)2+ and (RuIIL.-)+, respectively, are orbital analogues in that the intra- or intermolecularly transferred electron resides on the same uniquely reduced ligand. The variations in the properties of these species, resulting from changes in the coordinating ligands, permit the fine-tuning of photochemical systems.

Energy Gap Dependence of the Efficiency of Charge Separation upon the Sacrificial Reductive Quenching of the Excited States of Ru(II)-Diimine Photosensitizers in Aqueous Solution

The yield of oxidized and reduced products released into the bulk solution upon electron−transfer quenching is the critical parameter governing the efficacy of a redox photosensitizer. The cage esc...