Water-Soluble Tris(cyclometalated) Iridium(III) Complexes for Aqueous Electron and Energy Transfer Photochemistry.

Cyclometalated iridium(III) complexes are frequently employed in organic light emitting diodes, and they are popular photocatalysts for solar energy conversion and synthetic organic chemistry. They luminesce from redox-active excited states that can have high triplet energies and long lifetimes, making them well suited for energy transfer and photoredox catalysis. Homoleptic tris(cyclometalated) iridium(III) complexes are typically very hydrophobic and do not dissolve well in polar solvents, somewhat limiting their application scope. We developed a family of water-soluble sulfonate-decorated variants with tailored redox potentials and excited-state energies to address several key challenges in aqueous photochemistry.First, we aimed at combining enzyme with photoredox catalysis to synthesize enantioenriched products in a cyclic reaction network. Since the employed biocatalyst operates best in aqueous solution, a water-soluble photocatalyst was needed. A new tris(cyclometalated) iridium(III) complex provided enough reducing power for the photochemical reduction of imines to racemic mixtures of amines and furthermore was compatible with monoamine oxidase (MAO-N-9), which deracemized this mixture through a kinetic resolution of the racemic amine via oxidation to the corresponding imine. This process led to the accumulation of the unreactive amine enantiomer over time. In subsequent studies, we discovered that the same iridium(III) complex photoionizes under intense irradiation to give hydrated electrons as a result of consecutive two-photon excitation. With visible light as energy input, hydrated electrons become available in a catalytic fashion, thereby allowing the comparatively mild reduction of substrates that would typically only be reactive under harsher conditions. Finally, we became interested in photochemical upconversion in aqueous solution, for which it was desirable to obtain water-soluble iridium(III) compounds with very high triplet excited-state energies. This goal was achieved through improved ligand design and ultimately enabled sensitized triplet-triplet annihilation upconversion unusually far into the ultraviolet spectral range.Studies of photoredox catalysis, energy transfer catalysis, and photochemical upconversion typically rely on the use of organic solvents. Water could potentially be an attractive alternative in many cases, but photocatalyst development lags somewhat behind for aqueous solution compared to organic solvent. The purpose of this Account is to provide an overview of the breadth of new research perspectives that emerged from the development of water-soluble fac-[Ir(ppy)]3 complexes (ppy = 2-phenylpyridine) with sulfonated ligands. We hope to inspire the use of some of these or related coordination compounds in aqueous photochemistry and to stimulate further conceptual developments at the interfaces of coordination chemistry, photophysics, biocatalysis, and sustainable chemistry.

Photocatalytic Regeneration of a Nicotinamide Adenine Nucleotide Mimic with Water-Soluble Iridium(III) Complexes.

Nicotinamide adenine nucleotide (NADH) is involved in many biologically relevant redox reactions, and the photochemical regeneration of its oxidized form (NAD+) under physiological conditions is of interest for combined photo- and biocatalysis. Here, we demonstrate that tri-anionic, water-soluble variants of typically very lipophilic iridium(III) complexes can photo-catalyze the reduction of an NAD+ mimic in a comparatively efficient manner. In combination with a well-known rhodium co-catalyst to facilitate regioselective reactions, these iridium(III) photo-reductants outcompete the commonly used [Ru(bpy)3]2+ (bpy = 2,2'-bipyridine) photosensitizer in water by up to 1 order of magnitude in turnover frequency. This improved reactivity is attributable to the strong excited-state electron donor properties and the good chemical robustness of the tri-anionic iridium(III) sensitizers, combined with their favorable Coulombic interaction with the di-cationic rhodium co-catalyst. Our findings seem relevant in the greater context of photobiocatalysis, for which access to strong, efficient, and robust photoreductants with good water solubility can be essential.

Molecular Design Principles to Elongate the Metal-to-Ligand Charge Transfer Excited-State Lifetimes of Square-Planar Nickel(II) Complexes.

Square-planar NiII complexes and their electronically excited states play key roles in cross-coupling catalysis and could offer new opportunities to complement well-known isoelectronic PtII luminophores. Metal-to-ligand charge transfer (MLCT) excited states and their deactivation pathways are particularly relevant in these contexts. We sought to extend the lifetimes of 3MLCT states in square-planar NiII complexes by creating coordination environments that seemed particularly well adapted to the 3d8 valence electron configuration. Using a rigid tridentate chelate ligand, in which a central cyclometalated phenyl unit is flanked by two coordinating N-heterocyclic carbenes, along with a monodentate isocyanide ligand, a very strong ligand field is created. Bulky substituents at the isocyanide backbone furthermore protect the NiII center from nucleophilic attack in the axial directions. UV-Vis transient absorption spectroscopies reveal that upon excitation into 1MLCT absorption bands and ultrafast intersystem crossing to the 3MLCT excited state, the latter relaxes onward into a metal-centered triplet state (3MC). A torsional motion of the tridentate ligand and a NiII-carbon bond elongation facilitate 3MLCT relaxation to the 3MC state. The 3MLCT lifetime gets longer with increasing ligand field strength and improved steric protection, thereby revealing clear design guidelines for square-planar NiII complexes with enhanced photophysical properties. The longest 3MLCT lifetime reached in solution at room temperature is 48 ps, which is longer by a factor of 5-10 compared to previously investigated square-planar NiII complexes. Our study contributes to making first-row transition metal complexes with partially filled d-orbitals more amenable to applications in photophysics and photochemistry.

Cobalt(III) Carbene Complex with an Electronic Excited-State Structure Similar to Cyclometalated Iridium(III) Compounds.

Many organometallic iridium(III) complexes have photoactive excited states with mixed metal-to-ligand and intraligand charge transfer (MLCT/ILCT) character, which form the basis for numerous applications in photophysics and photochemistry. Cobalt(III) complexes with analogous MLCT excited-state properties seem to be unknown yet, despite the fact that iridium(III) and cobalt(III) can adopt identical low-spin d6 valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (LCNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(LCNC)2](PF6), featuring a photoactive excited state with substantial MLCT character. Compared to the vast majority of isoelectronic iron(II) complexes, the MLCT state of [Co(LCNC)2](PF6) is long-lived because it does not deactivate as efficiently into lower-lying metal-centered excited states; furthermore, it engages directly in photoinduced electron transfer reactions. The comparison with [Fe(LCNC)2](PF6), as well as structural, electrochemical, and UV-vis transient absorption studies, provides insight into new ligand design principles for first-row transition-metal complexes with photophysical and photochemical properties reminiscent of those known from the platinum group metals.

Oxidase-Type C-H/C-H Coupling Using an Isoquinoline-Derived Organic Photocatalyst.

Oxidase-type oxidation is an attractive strategy in organic synthesis due to the use of O2 as the terminal oxidant. Organic photocatalysis can effect metal-free oxidase chemistry. Nevertheless, current methods are limited in reaction scope, possibly due to the lack of suitable photocatalysts. Here we report an isoquinoline-derived diaryl ketone-type photocatalyst, which has much enhanced absorption of blue and visible light compared to conventional diaryl ketones. This photocatalyst enables dehydrogenative cross-coupling of heteroarenes with unactivated and activated alkanes as well as aldehydes using air as the oxidant. A wide range of heterocycles with various functional groups are suitable substrates. Transient absorption and excited-state quenching experiments point to an unconventional mechanism that involves an excited state "self-quenching" process to generate the N-radical cation form of the sensitizer, which subsequently abstracts a hydrogen atom from the alkane substrate to yield a reactive alkyl radical.

A Near-Infrared-II Emissive Chromium(III) Complex.

The combination of π-donating amido with π-accepting pyridine coordination units in a tridentate chelate ligand causes a strong nephelauxetic effect in a homoleptic CrIII complex, which shifts its luminescence to the NIR-II spectral range. Previously explored CrIII polypyridine complexes typically emit between 727 and 778 nm (in the red to NIR-I spectral region), and ligand design strategies have so far concentrated on optimizing the ligand field strength. The present work takes a fundamentally different approach and focusses on increasing metal-ligand bond covalence to shift the ruby-like 2 E emission of CrIII to 1067 nm at 77 K.

UV Light Generation and Challenging Photoreactions Enabled by Upconversion in Water.

Sensitized triplet-triplet annihilation (sTTA) is the most promising mechanism for pooling the energy of two visible photons, but its applications in solution were so far limited to organic solvents, with a current maximum of the excited-singlet state energy of 3.6 eV. By combining tailor-made iridium complexes with naphthalenes, we demonstrate blue-light driven upconversion in water with unprecedented singlet-state energies approaching 4 eV. The annihilators have outstanding excited-state reactivities enabling challenging photoreductions driven by sTTA. Specifically, we found that an aryl-bromide bond activation can be achieved with blue photons, and we obtained full conversion for the very energy-demanding decomposition of a persistent ammonium compound as typical water pollutant, not only with a cw laser but also with an LED light source. These results provide the first proof-of-concept for the usage of low-power light sources for challenging reactions employing blue-to-UV upconversion in water and pave the way for the further development of sustainable light-harvesting applications.

Picosecond reactions of excited radical ion super-reductants.

Classical photochemistry requires nanosecond excited-state lifetimes for diffusion-controlled reactions. Excited radicals with picosecond lifetimes have been implied by numerous photoredox studies, and controversy has arisen as to whether they can actually be catalytically active. We provide direct evidence for the elusive pre-association between radical ions and substrate molecules, enabling photoinduced electron transfer beyond the diffusion limit. A strategy based on two distinct light absorbers, mimicking the natural photosystems I and II, is used to generate excited radicals, unleashing extreme reduction power and activating C(sp2)-Cl and C(sp2)-F bonds. Our findings provide a long-sought mechanistic understanding for many previous synthetically-oriented works and permit more rational future photoredox reaction development. The newly developed excitation strategy pushes the current limits of reactions based on multi-photon excitation and very short-lived but highly redox active species.

Isoacridone dyes with parallel reactivity from both singlet and triplet excited states for biphotonic catalysis and upconversion.

Metal-based photosensitizers commonly undergo quantitative intersystem crossing into photoactive triplet excited states. In contrast, organic photosensitizers often feature weak spin-orbit coupling and low intersystem crossing efficiencies, leading to photoactive singlet excited states. By modifying the well-known acridinium dyes, we obtained a new family of organic photocatalysts, the isoacridones, in which both singlet- and triplet-excited states are simultaneously photoactive. These new isoacridone dyes are synthetically readily accessible and show intersystem crossing efficiencies of up to 52%, forming microsecond-lived triplet excited states (T1), storing approximately 1.9 eV of energy. Their photoactive singlet excited states (S1) populated in parallel have only nanosecond lifetimes, but store ∼0.4 eV more energy and act as strong oxidants. Consequently, the new isoacridone dyes are well suited for applications requiring parallel triplet-triplet energy transfer and photoinduced electron transfer elementary steps, which have become increasingly important in modern photocatalysis. In proof-of-principle experiments, the isoacridone dyes were employed for Birch-type arene reductions and C-C couplings via sensitization-initiated electron transfer, substituting the commonly used iridium or ruthenium based photocatalysts. Further, in combination with a pyrene-based annihilator, sensitized triplet-triplet annihilation upconversion was achieved in an all-organic system, where the upconversion quantum yield correlated with the intersystem crossing quantum yield of the photosensitizer. This work seems relevant in the greater contexts of developing new applications that utilize biphotonic photophysical and photochemical behavior within metal-free systems.

Zinc(II) Complexes with Triplet Charge-Transfer Excited States Enabling Energy-Transfer Catalysis, Photoinduced Electron Transfer, and Upconversion.

Many CuI complexes have luminescent triplet charge-transfer excited states with diverse applications in photophysics and photochemistry, but for isoelectronic ZnII compounds, this behavior is much less common, and they typically only show ligand-based fluorescence from singlet π-π* states. We report two closely related tetrahedral ZnII compounds, in which intersystem crossing occurs with appreciable quantum yields and leads to the population of triplet excited states with intraligand charge-transfer (ILCT) character. In addition to showing fluorescence from their initially excited 1ILCT states, these new compounds therefore undergo triplet-triplet energy transfer (TTET) from their 3ILCT states and consequently can act as sensitizers for photo-isomerization reactions and triplet-triplet annihilation upconversion from the blue to the ultraviolet spectral range. The photoactive 3ILCT state furthermore facilitates photoinduced electron transfer. Collectively, our findings demonstrate that mononuclear ZnII compounds with photophysical and photochemical properties reminiscent of well-known CuI complexes are accessible with suitable ligands and that they are potentially amenable to many different applications. Our insights seem relevant in the greater context of obtaining photoactive compounds based on abundant transition metals, complementing well-known precious-metal-based luminophores and photosensitizers.

Photo-triggered hydrogen atom transfer from an iridium hydride complex to unactivated olefins.

Many photoactive metal complexes can act as electron donors or acceptors upon photoexcitation, but hydrogen atom transfer (HAT) reactivity is rare. We discovered that a typical representative of a widely used class of iridium hydride complexes acts as an H-atom donor to unactivated olefins upon irradiation at 470 nm in the presence of tertiary alkyl amines as sacrificial electron and proton sources. The catalytic hydrogenation of simple olefins served as a test ground to establish this new photo-reactivity of iridium hydrides. Substrates that are very difficult to activate by photoinduced electron transfer were readily hydrogenated, and structure-reactivity relationships established with 12 different olefins are in line with typical HAT reactivity, reflecting the relative stabilities of radical intermediates formed by HAT. Radical clock, H/D isotope labeling, and transient absorption experiments provide further mechanistic insight and corroborate the interpretation of the overall reactivity in terms of photo-triggered hydrogen atom transfer (photo-HAT). The catalytically active species is identified as an Ir(ii) hydride with an IrII-H bond dissociation free energy around 44 kcal mol-1, which is formed after reductive 3MLCT excited-state quenching of the corresponding Ir(iii) hydride, i.e. the actual HAT step occurs on the ground-state potential energy surface. The photo-HAT reactivity presented here represents a conceptually novel approach to photocatalysis with metal complexes, which is fundamentally different from the many prior studies relying on photoinduced electron transfer.