Luminescent First-Row Transition Metal Complexes

Capturing dynamic ligand-to-metal charge transfer with a long-lived cationic intermediate for anionic redox

Reversible anionic redox reactions represent a transformational change for creating advanced high-energy-density positive-electrode materials for lithium-ion batteries. The activation mechanism of these reactions is frequently linked to ligand-to-metal charge transfer (LMCT) processes, which have not been fully validated experimentally due to the lack of suitable model materials. Here we show that the activation of anionic redox in cation-disordered rock-salt Li_1.17Ti_0.58Ni_0.25O₂ involves a long-lived intermediate Ni^3+/4+ species, which can fully evolve to Ni²⁺ during relaxation. Combining electrochemical analysis and spectroscopic techniques, we quantitatively identified that the reduction of this Ni^3+/4+ species goes through a dynamic LMCT process (Ni^3+/4+-O^2- → Ni²⁺-O^n-). Our findings provide experimental validation of previous theoretical hypotheses and help to rationalize several peculiarities associated with anionic redox, such as cationic-anionic redox inversion and voltage hysteresis. This work also provides additional guidance for designing high-capacity electrodes by screening appropriate cationic species for mediating LMCT.

Borylation in the Second Coordination Sphere of FeII Cyanido Complexes and Its Impact on Their Electronic Structures and Excited-State Dynamics

Second coordination sphere interactions of cyanido complexes with hydrogen-bonding solvents and Lewis acids are known to influence their electronic structures, whereby the non-labile attachment of B(C₆F₅)₃ resulted in several particularly interesting new compounds lately. Here, we investigate the effects of borylation on the properties of two Fe^II cyanido complexes in a systematic manner by comparing five different compounds and using a range of experimental techniques. Electrochemical measurements indicate that borylation entails a stabilization of the Fe^II-based t_2g-like orbitals by up to 1.65 eV, and this finding was confirmed by Mössbauer spectroscopy. This change in the electronic structure has a profound impact on the UV–vis absorption properties of the borylated complexes compared to the non-borylated ones, shifting their metal-to-ligand charge transfer (MLCT) absorption bands over a wide range. Ultrafast UV–vis transient absorption spectroscopy provides insight into how borylation affects the excited-state dynamics. The lowest metal-centered (MC) excited states become shorter-lived in the borylated complexes compared to their cyanido analogues by a factor of ∼10, possibly due to changes in outer-sphere reorganization energies associated with their decay to the electronic ground state as a result of B(C₆F₅)₃ attachment at the cyanido N lone pair.

Borylation in the second coordination sphere of two well-known Fe^II cyanido complexes leads to isocyanoborato complexes. The effects of borylation on their electronic structure and photophysical properties are thoroughly investigated with a range of experimental techniques.

Synthesis and Fluorescence Studies of Diborene Coinage Metal Complexes

Nine different coinage metal (Cu, Ag) π complexes of diborenes with various anionic diborene (aryl, heteroaryl) and metal substituents (Cl, Br, C₆F₅, C₂SiMe₃), stabilizing neutral donors (N-heterocyclic carbene = NHC, phosphine), configurations (cis/trans, acyclic/cyclic diborene), and charges (neutral, cationic) were synthesized and characterized by multinuclear NMR spectroscopy and X-ray crystallographic analyses. Their optical properties were investigated by UV-vis absorption and steady-state as well as time-resolved luminescence spectroscopy in solution and the solid state to gain insights into the excited-state behavior of this unusual class of photoactive compounds and to provide structure-property relationships. The structural and electronic modification of the (B═B)···M motif greatly influences not only the visible light absorption but also the photostability and quantum yields, which can reach high values of up to f = 0.42. The lifetimes are found in the nanosecond regime, providing estimated radiative rate constants over a wide range of k_r = 1.3-14 × 10⁷ s^-1, indicative of fluorescence. Intersystem crossing (ISC) is sufficiently slow for prompt emission from the S₁ state to be observed, while the spin-orbit coupling in the T₁ state is too weak for phosphorescence to occur at room temperature. ISC can be accelerated, however, by modifying diborene ligand substitution and the coinage metal center, hinting at the potential for exploiting the properties of long-lived triplet excited states of metal diborene complexes in the future.

Luminescent and Photofunctional Transition Metal Complexes: From Molecular Design to Diagnostic and Therapeutic Applications

There has been emerging interest in the exploitation of the photophysical and photochemical properties of transition metal complexes for diagnostic and therapeutic applications. In this Perspective, we highlight the major recent advances in the development of luminescent and photofunctional transition metal complexes, in particular, those of rhenium(I), ruthenium(II), osmium(II), iridium(III), and platinum(II), as bioimaging reagents and phototherapeutic agents, with a focus on the molecular design strategies that harness and modulate the interesting photophysical and photochemical behavior of the complexes. We also discuss the current challenges and future outlook of transition metal complexes for both fundamental research and clinical applications.

Shedding Light on the Oxidizing Properties of Spin-Flip Excited States in a CrIII Polypyridine Complex and Their Use in Photoredox Catalysis

The photoredox activity of well-known Ru^II complexes stems from metal-to-ligand charge transfer (MLCT) excited states, in which a ligand-based electron can initiate chemical reductions and a metal-centered hole can trigger oxidations. Cr^III polypyridines show similar photoredox properties, although they have fundamentally different electronic structures. Their photoactive excited state is of spin-flip nature, differing from the electronic ground state merely by a change of one electron spin, but with otherwise identical d-orbital occupancy. We find that the driving-force dependence for photoinduced electron transfer from 10 different donors to a spin-flip excited state of a Cr^III complex is very similar to that for a Ru^II polypyridine, and thereby validate the concept of estimating the redox potential of d³ spin-flip excited states in analogous manner as for the MLCT states of d⁶ compounds. Building on this insight, we use our Cr^III complex for photocatalytic reactions not previously explored with this compound class, including the aerobic bromination of methoxyaryls, oxygenation of 1,1,2,2-tetraphenylethylene, aerobic hydroxylation of arylboronic acids, and the vinylation of N-phenyl pyrrolidine. This work contributes to understanding the fundamental photochemical properties of first-row transition-metal complexes in comparison to well-explored precious-metal-based photocatalysts.

Phosphorescent Tris-cyclometalated Pt(IV) Complexes with Mesoionic N-Heterocyclic Carbene and 2-Arylpyridine Ligands

The synthesis, structure, photophysical properties, and electrochemistry of the first series of Pt(IV) tris-chelates bearing cyclometalated aryl-NHC ligands are reported. The complexes have the general formula [Pt(trz)₂(C∧N)]⁺, combining two units of the cyclometalated, mesoionic aryl-NHC ligand 4-butyl-3-methyl-1-phenyl-1H-1,2,3-triazol-5-ylidene (trz) with a cyclometalated 2-arylpyridine [C∧N = 2-(2,4-difluorophenyl)pyridine (dfppy), 2-phenylpyridine (ppy), 2-(p-tolyl)pyridine (tpy), 2-(2-thienyl)pyridine (thpy), 2-(9,9-dimethylfluoren-2-yl)pyridine (flpy)], and presenting a mer arrangement or metalated aryls. They exhibit a significant photostability under UV irradiation and long-lived phosphorescence in the blue to yellow color range, arising from ³LC excited states involving the C∧N ligands, with quantum yields of up to 0.34 in fluid solution and 0.77 in the rigid matrix at 298 K. The time-dependent density functional theory (TD-DFT) calculations reveal that nonemissive, deactivating excited states of ligand-to-metal charge-transfer (LMCT) character are pushed to high energies as a consequence of the strong σ-donating ability of the carbenic moieties, making the Pt(trz)₂ subunit an essential structural component that enables efficient emissions from the chromophoric C∧N ligands, with potential application for the development of different Pt(IV) emitters with tunable properties.

Complex-as-Ligand Strategy as a Tool for the Design of a Binuclear Nonsymmetrical Chromium(III) Assembly: Near-Infrared Double Emission and Intramolecular Energy Transfer

The chromium(III) polypyridyl complexes are appealing for their long-lived near-infrared (NIR) emission reaching the millisecond range and for the strong circularly polarized luminescence of their isolated enantiomers. However, harnessing those properties in functional polynuclear Cr^III devices remains mainly inaccessible because of the lack of synthetic methods for their design and functionalization. Even the preparation and investigation of most basic nonsymmetrical Cr^III dyads exhibiting directional intramolecular intermetallic energy transfer remain unexplored. Taking advantage of the inertness of heteroleptic chromium(III) polypyridyl building blocks, we herein adapt the "complex-as-ligand" strategy, largely used with precious 4d and 5d metals, for the preparation of a binuclear nonsymmetrical Cr^III complex (3d metal). The resulting [(phen)₂Cr(L)Cr(tpy)]⁶⁺ dyad shows dual long-lived NIR emission and a directional intermetallic energy transfer that is controlled by the specific arrangements of the different coordination spheres. This strategy opens a route for building predetermined polynuclear assemblies with this earth-abundant metal.

Manganese(I) Complex with Monodentate Arylisocyanide Ligands Shows Photodissociation Instead of Luminescence

Recently reported manganese(I) complexes with chelating arylisocyanide ligands exhibit luminescent metal-to-ligand charge-transfer (MLCT) excited states, similar to ruthenium(II) polypyridine complexes with the same d6 valence electron configuration used for many different applications in photophysics and photochemistry. However, chelating arylisocyanide ligands require substantial synthetic effort, and therefore it seemed attractive to explore the possibility of using more readily accessible monodentate arylisocyanides instead. Here, we synthesized the new Mn(I) complex [Mn(CNdippPhOMe2)6]PF6 with the known ligand CNdippPhOMe2 = 4-(3,5-dimethoxyphenyl)-2,6-diisopropylphenylisocyanide. This complex was investigated by NMR spectroscopy, single-crystal structure analysis, high-resolution electrospray ionization mass spectrometry (HR-ESI-MS) measurements, IR spectroscopy supported by density functional theory (DFT) calculations, cyclic voltammetry, and time-resolved as well as steady-state UV-vis absorption spectroscopy. The key finding is that the new Mn(I) complex is nonluminescent and instead undergoes arylisocyanide ligand loss during continuous visible laser irradiation into ligand-centered and charge-transfer absorption bands, presumably owed to the population of dissociative d-d excited states. Thus, it seems that chelating bi- or tridentate binding motifs are essential for obtaining emissive MLCT excited states in manganese(I) arylisocyanides. Our work contributes to understanding the basic properties of photoactive first-row transition metal complexes and could help advance the search for alternatives to precious metal-based luminophores, photocatalysts, and sensors.

Photochemistry and Photophysics of Charge-Transfer Excited States in Emissive d10/d0 Heterobimetallic Titanocene Tweezer Complexes

Transition-metal complexes that undergo ligand-to-metal charge transfer (LMCT) to d⁰ metals are of interest as possible photocatalysts due to the lack of deactivating d-d states. Herein, the synthesis and characterization of nine titanocene complexes of the formula Cp₂Ti(C₂Ar)₂·MX (where Ar = phenyl, dimethylaniline, or triphenylamine; and MX = CuCl, CuBr, or AgCl) are presented. Solid-state structural characterization demonstrates that MX coordinates to the alkyne tweezers and CuX coordination has a greater structural impact than AgCl. All complexes, including the parent complexes without coordinated MX, are brightly emissive at 77 K (emission max between 575 and 767 nm), with the coordination of MX redshifting the emission in all cases except for the coordination of AgCl into Cp₂Ti(C₂Ph)₂. TDDFT investigations suggest that emission is dominated by arylalkynyl-to-titanium ³LMCT in all cases except Cp₂Ti(C₂Ph)₂·CuBr, which is dominated by CuBr-to-Ti charge transfer. In room-temperature fluid solution, only Cp₂Ti(C₂Ph)₂ and Cp₂Ti(C₂Ph)₂·AgCl are emissive, albeit with photoluminescent quantum yields ≤2 × 10^-4. The parent complexes photodecompose in room-temperature solution with quantum yields, Φ_rxn, between 0.25 and 0.99. The coordination of MX decreases Φ_rxn by two to three orders of magnitude. There is a clear trend that Φ_rxn increases as the emission energy increases. This trend is consistent with a competition between energy-gap-law controlled nonradiative decay and thermally activated intersystem crossing between the ³LMCT state and the singlet transition state for decomposition.

Red Light-Based Dual Photoredox Strategy Resembling the Z-Scheme of Natural Photosynthesis

Photoredox catalysis typically relies on the use of single chromophores, whereas strategies, in which two different light absorbers are combined, are rare. In photosystems I and II of green plants, the two separate chromophores P₆₈₀ and P₇₀₀ both absorb light independently of one another, and then their excitation energy is combined in the so-called Z-scheme, to drive an overall reaction that is thermodynamically very demanding. Here, we adapt this concept to perform photoredox reactions on organic substrates with the combined energy input of two red photons instead of blue or UV light. Specifically, a Cu^I bis(α-diimine) complex in combination with in situ formed 9,10-dicyanoanthracenyl radical anion in the presence of excess diisopropylethylamine catalyzes ca. 50 dehalogenation and detosylation reactions. This dual photoredox approach seems useful because red light is less damaging and has a greater penetration depth than blue or UV radiation. UV-vis transient absorption spectroscopy reveals that the subtle change in solvent from acetonitrile to acetone induces a changeover in the reaction mechanism, involving either a dominant photoinduced electron transfer or a dominant triplet-triplet energy transfer pathway. Our study illustrates the mechanistic complexity in systems operating under multiphotonic excitation conditions, and it provides insights into how the competition between desirable and unwanted reaction steps can become more controllable.

Green-Light-Driven Fe(III)(btz)3 Photocatalysis in the Radical Cationic [4+2] Cycloaddition Reaction

Green-light-driven Fe^III(btz)₃ photocatalysis for the radical cationic [4+2] cycloaddition of terminal styrenes and nucleophilic dienes has been investigated. The Fe-MIC (mesoionic carbene) complex forms a ligand-to-metal charge-transfer transition state with relatively high excited-state reduction potentials that can selectively oxidize terminal styrene derivatives. Unique multisubstituted cyclohexenes and structurally complex biorelevant cyclohexenes were constructed, highlighting the usefulness of this mild and practical first-row transition metal complex system.

Ligand-Field Spectroscopy of Co(III) Complexes and the Development of a Spectrochemical Series for Low-Spin d6 Charge-Transfer Chromophores

A study of a series of six-coordinate Co(III) complexes has been carried out to quantify spectroscopic parameters for a range of ligands that are commonly employed to realize strong charge-transfer absorptions in low-spin, d⁶ systems. Identification of any three ligand-field transitions allows for the determination of the splitting parameter (10 Dq) as well as the Racah B and C parameters for a given compound. The data revealed a relatively small spread in the magnitude of 10 Dq, ranging from ca. 23 000 cm^-1 in the case of [Co(pyrro-bpy)₃]³⁺ (where pyrro-bpy is 4,4'-dipyrrolidinyl-2,2'-bipyridine) to ca. 26 000 cm^-1 for [Co(terpy)₂]³⁺ (where terpy is 2,2':6',2″-terpyridine). Significantly, trends across the series suggest that polypyridyl ligands behave as net π-donors when interacting with Co(III), in contrast to the net π-accepting character they exhibit when bound to second- and third-row metals. The influence of strong σ donation associated with carbene-based ligands was evident from the data acquired for [Co(B^MeImPy)₂]³⁺ (where B^MeImPy is 3,3'-(pyridine-2,6-diyl)bis(1-methyl-1H-3-imidazolium)), where a 10 Dq value of ca. 30 000 cm^-1 was determined. Spectroscopic data were also analyzed for [Fe(bpy)₃]²⁺ using the results on [Co(bpy)₃]³⁺ as a reference point. A value for 10 Dq of 21 000 cm^-1 was estimated, indicating a reduction in the ligand-field strength of ca. 3000 cm^-1 upon replacing Co(III) with Fe(II). We suggest that this approach of taking advantage of the blueshift of the charge-transfer feature in Co(III) complexes to reveal otherwise obscured ligand-field bands can be a useful tool for the development of new ligand systems to expand the photofunctionality of first-row transition-metal-based chromophores.

Decoding Key Transient Inter-Catalyst Interactions in a Reductive Metallaphotoredox-Catalyzed Allylation Reaction

Metallaphotoredox chemistry has recently witnessed a surge in interest within the field of synthetic organic chemistry through the use of abundant first-row transition metals combined with suitable photocatalysts. The intricate details arising from the combination of two (or more) catalytic components during the reaction and especially the inter-catalyst interactions remain poorly understood. As a representative example of a catalytic process featuring such intricacies, we here present a meticulous study of the mechanism of a cobalt-organophotoredox catalyzed allylation of aldehydes. Importantly, the commonly proposed elementary steps in reductive metallaphotoredox chemistry are more complex than previously assumed. After initial reductive quenching, a transient charge-transfer complex forms that interacts with both the transition-metal catalyst and the catalytic base. Surprisingly, the former interaction leads to deactivation due to induced charge recombination, while the latter promotes deprotonation of the electron donor, which is the crucial step to initiate productive catalysis but is often neglected. Due to the low efficiency of this latter process, the overall catalytic reaction is photon-limited and the cobalt catalyst remains in a dual resting state, awaiting photoinduced reduction. These new insights are of general importance to the synthetic community, as metallaphotoredox chemistry has become a powerful tool used in the formation of elusive compounds through carbon-carbon bond formations. Understanding the underlying aspects that determine the efficiency of such reactions provides a conceptually stronger reactivity paradigm to empower future approaches to synthetic challenges that rely on dual metallaphotoredox catalysis.

Luminescent Metal Complexes for Bioassays in the Near-Infrared (NIR) Region

Near-infrared (NIR, 700-1700 nm) luminescent imaging is an emerging bioimaging technology with low photon scattering, minimal autofluorescence, deep tissue penetration, and high spatiotemporal resolution that has shown fascinating promise for NIR imaging-guided theranostics. In recent progress, NIR luminescent metal complexes have attracted substantially increased research attention owing to their intrinsic merits, including small size, anti-photobleaching, long lifetime, and metal-centered NIR emission. In the past decade, scientists have contributed to the advancement of NIR metal complexes involving efforts to improve photophysical properties, biocompatibility, specificity, pharmacokinetics, in vivo visualization, and attempts to exploit new ligand platforms. Herein, we summarize recent progress and provide future perspectives for NIR metal complexes, including d-block transition metals and f-block lanthanides (Ln) as NIR optical molecular probes for bioassays.

How the Way a Naphthalimide Unit is Implemented Affects the Photophysical and -catalytic Properties of Cu(I) Photosensitizers

Driven by the great potential of solar energy conversion this study comprises the evaluation and comparison of two different design approaches for the improvement of copper based photosensitizers. In particular, the distinction between the effects of a covalently linked and a directly fused naphthalimide unit was assessed. For this purpose, the two heteroleptic Cu(I) complexes CuNIphen (NIphen = 5-(1,8-naphthalimide)-1,10-phenanthroline) and Cubiipo (biipo = 16H-benzo-[4′,5′]-isoquinolino-[2′,1′,:1,2]-imidazo-[4,5-f]-[1,10]-phenanthroline-16-one) were prepared and compared with the novel unsubstituted reference compound Cuphen (phen = 1,10-phenanthroline). Beside a comprehensive structural characterization, including two-dimensional nuclear magnetic resonance spectroscopy and X-ray analysis, a combination of electrochemistry, steady-state and time-resolved spectroscopy was used to determine the electrochemical and photophysical properties in detail. The nature of the excited states was further examined by (time-dependent) density functional theory (TD-DFT) calculations. It was found that CuNIphen exhibits a greatly enhanced absorption in the visible and a strong dependency of the excited state lifetimes on the chosen solvent. For example, the lifetime of CuNIphen extends from 0.37 µs in CH2Cl2 to 19.24 µs in MeCN, while it decreases from 128.39 to 2.6 µs in Cubiipo. Furthermore, CuNIphen has an exceptional photostability, allowing for an efficient and repetitive production of singlet oxygen with quantum yields of about 32%.

On the use of vibronic coherence to identify reaction coordinates for ultrafast excited-state dynamics of transition metal-based chromophores

The question of whether one can use information from quantum coherence as a means of identifying vibrational degrees of freedom that are active along an excited-state reaction coordinate is discussed. Specifically, we are exploring the notion of whether quantum oscillations observed in single-wavelength kinetics data exhibiting coherence dephasing times that are intermediate between that expected for either pure electronic or pure vibrational dephasing are vibronic in nature and therefore may be coupled to electronic state-to-state evolution. In the case of a previously published Fe(II) polypyridyl complex, coherences observed subsequent to ¹A₁ → ¹MLCT excitation were linked to large-amplitude motion of a portion of the ligand framework; dephasing times on the order of 200-300 fs suggested that these degrees of freedom could be associated with ultrafast (∼100 fs) conversion from the initially formed MLCT excited state to lower-energy, metal-centered ligand-field excited state(s) of the compound. Incorporation of an electronically benign but sterically restrictive Cu(I) ion into the superstructure designed to interfere with this motion yielded a compound exhibiting a ∼25-fold increase in the compound's MLCT lifetime, a result that was interpreted as confirmation of the initial hypothesis. However, new data acquired on a different chemical system - Cr(acac')₃ (where acac' represents various derivatives of acetylacetonate) - yielded results that call into question this same hypothesis. Coherences observed subsequent to ⁴A₂ → ⁴T₂ ligand-field excitation on a series of molecules implicated similar vibrational degrees of freedom across the series, but exhibited dephasing times ranging from 340 fs to 2.5 ps without any clear correlation to the dynamics of excited-state evolution in the system. Taken together, the results obtained on both of these chemical platforms suggest that while identification of coherences can indeed point to degrees of freedom that should be considered as candidate modes for defining reaction trajectories, our understanding of the factors that determine the interplay across coherences, dephasing times, and electronic and geometric structure is insufficient at the present time to view this parameter as a robust metric for differentiating active versus spectator modes for ultrafast dynamics.

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 (L^CNC), in which a central amido π-donor is flanked by two σ-donating N-heterocyclic carbene subunits, we obtained a robust homoleptic complex [Co(L^CNC)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(L^CNC)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(L^CNC)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.

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.

Heteroleptic mer-[Cr(N∩N∩N)(CN)3] complexes: synthetic challenge, structural characterization and photophysical properties

The substitution of three water molecules around trivalent chromium in CrBr₃·6H₂O with the tridentate 2,2′:6′,2′′-terpyridine (tpy), N,N′-dimethyl-N,N′-di(pyridine-2-yl)pyridine-2,6-diamine (ddpd) or 2,6-di(quinolin-8-yl)pyridine (dqp) ligands gives the heteroleptic mer-[Cr(L)Br₃] complexes. Stepwise treatments with Ag(CF₃SO₃) and KCN under microwave irradiations provide mer-[Cr(L)(CN)₃] in moderate yields. According to their X-ray crystal structures, the associated six-coordinate meridional [CrN₃C₃] chromophores increasingly deviate from a pseudo-octahedral arrangement according to L = ddpd ≈ dpq ≪ tpy; a trend in line with the replacement of six-membered with five-membered chelate rings around Cr^III. Room-temperature ligand-centered UV-excitation at 18 170 cm⁻¹ (λ_exc = 350 nm), followed by energy transfer and intersystem crossing eventually yield microsecond metal-centered Cr(²E → ⁴A₂) phosphorescence in the red to near infrared domain 13 150–12 650 cm⁻¹ (760 ≤ λ_em ≤ 790 nm). Decreasing the temperature to liquid nitrogen (77 K) extends the emission lifetimes to reach the millisecond regime with a record of 4.02 ms for mer-[Cr(dqp)(CN)₃] in frozen acetonitrile.

The heteroleptic mer-[Cr(L)(CN)₃] (L = tpy, ddpd, dqp) complexes with their C_2v-symmetrical [CrC₃N₃] luminescent chromophores represent the missing links between pseudo-octahedral [CrN₆] and [CrC₆] units found in their well-known homoleptic parents.

Spin-flip luminescence

In molecular photochemistry, charge-transfer emission is well understood and widely exploited. In contrast, luminescent metal-centered transitions only came into focus in recent years. This gave rise to strongly phosphorescent Cr^III complexes with a d³ electronic configuration featuring luminescent metal-centered excited states which are characterized by the flip of a single spin. These so-called spin-flip emitters possess unique properties and require different design strategies than traditional charge-transfer phosphors. In this review, we give a brief introduction to ligand field theory as a framework to understand this phenomenon and outline prerequisites for efficient spin-flip emission including ligand field strength, symmetry, intersystem crossing and common deactivation pathways using Cr^III complexes as instructive examples. The recent progress and associated challenges of tuning the energies of emissive excited states and of emerging applications of the unique photophysical properties of spin-flip emitters are discussed. Finally, we summarize the current state-of-the-art and challenges of spin-flip emitters beyond Cr^III with d², d³, d⁴ and d⁸ electronic configuration, where we mainly cover pseudooctahedral molecular complexes of V, Mo, W, Mn, Re and Ni, and highlight possible future research opportunities.

Phototherapeutic anticancer strategies with first-row transition metal complexes: a critical review

Photodynamic therapy (PDT) and photoactivated chemotherapy (PACT) are therapeutic techniques based on a photosensitizer (PS) and light. These techniques allow the spatial and temporal control of the activation of drugs with light. Transition metal complexes are attractive compounds as photoactivatable prodrugs since their excited states can be appropriately designed by subtle modifications of the ligands, the metal centre, or the oxidation state. However, most metal-based PSs contain heavy metals such as Ru, Os, Ir, Pt or Au, which are expensive and non-earth-abundant, contrary to first-row transition metals. In this context, the exploration of the photochemical properties of complexes based on first-row transition metals appears to be extremely promising. This did encourage several groups to develop promising PSs based on these metals. This review presents up-to-date state-of-the-art information on first-row-transition metal complexes, from titanium to zinc in regard to their application as PSs for phototherapeutic applications.

High Triplet Energy Iridium(III) Isocyanoborato Complex for Photochemical Upconversion, Photoredox and Energy Transfer Catalysis

Cyclometalated Ir(III) complexes are often chosen as catalysts for challenging photoredox and triplet-triplet-energy-transfer (TTET) catalyzed reactions, and they are of interest for upconversion into the ultraviolet spectral range. However, the triplet energies of commonly employed Ir(III) photosensitizers are typically limited to values around 2.5-2.75 eV. Here, we report on a new Ir(III) luminophore, with an unusually high triplet energy near 3.0 eV owing to the modification of a previously reported Ir(III) complex with isocyanoborato ligands. Compared to a nonborylated cyanido precursor complex, the introduction of B(C₆F₅)₃ units in the second coordination sphere results in substantially improved photophysical properties, in particular a high luminescence quantum yield (0.87) and a long excited-state lifetime (13.0 μs), in addition to the high triplet energy. These favorable properties (including good long-term photostability) facilitate exceptionally challenging organic triplet photoreactions and (sensitized) triplet-triplet annihilation upconversion to a fluorescent singlet excited state beyond 4 eV, unusually deep in the ultraviolet region. The new Ir(III) complex photocatalyzes a sigmatropic shift and [2 + 2] cycloaddition reactions that are unattainable with common transition metal-based photosensitizers. In the presence of a sacrificial electron donor, it furthermore is applicable to demanding photoreductions, including dehalogenations, detosylations, and the degradation of a lignin model substrate. Our study demonstrates how rational ligand design of transition-metal complexes (including underexplored second coordination sphere effects) can be used to enhance their photophysical properties and thereby broaden their application potential in solar energy conversion and synthetic photochemistry.

Visible-emitting Cu(i) complexes with N-functionalized benzotriazole-based ligands

Bidentate benzotriazole-based N-ligands are suited for the preparation of luminescent heteroleptic copper(i) complexes with noticeable emissions related to 3MLCT transitions.

Bulky ligands protect molecular ruby from oxygen quenching

Steric protection strongly reduces phosphorescence quenching of excited molecular rubies by oxygen. The most bulky ligand enables photoluminescence quantum yields up to 5.1% and lifetimes up to 518 µs in air-saturated acetonitrile.

Unravelling the benefits of transition-metal-co-doping in lanthanide upconversion nanoparticles

In this review we provide an overview of the current knowledge on lanthanide upconversion materials co-doped with transition metals. We focus on how the co-dopants affect the host lattice and the energy transfer processes.

Synthesis, structure, magnetic properties and thermal behaviour of Ba–MII (MII = Mn, Co, Cu, and Zn) allylmalonates

A series of Ba-MII complexes with allylmalonic acid anions [BaMII(Amal)2(H2O)3]n (MII = Mn, Co, Cu, and Zn) were synthesized. The magnetic measurements revealed slow magnetic relaxation in non-zero field (HDC = 1500 Oe) for CoII ions.

Luminescent chromium(0) and manganese(I) complexes

In this Frontier article, recently discovered chromium(0) and manganese(i) complexes emitting from metal-to-ligand charge transfer (MLCT) excited states are highlighted. Chelating isocyanide ligands give access to this new class of 3d⁶ emitters with MLCT lifetimes in (or close to) the nanosecond regime in solution at room temperature. Although the so far achievable luminescence quantum yields in these open-shell complexes are yet comparatively low, the photophysical properties of the new chromium(0) and manganese(i) isocyanides are reminiscent of those of well-known ruthenium(ii) polypyridines. Our findings provide insight into how undesired nonradiative MLCT deactivation in 3d⁶ complexes can be counteracted, and they seem therefore relevant for the further development of new luminescent first-row transition metal complexes based on iron(ii) and cobalt(iii) in addition to chromium(0) and manganese(i).

In this Frontier article, recently discovered chromium(0) and manganese(i) complexes emitting from metal-to-ligand charge transfer (MLCT) excited states are highlighted.

The diverse functions of isocyanides in phosphorescent metal complexes

In this Perspective, we highlight many examples of photoluminescent metal complexes supported by isocyanides, with an emphasis on recent developments including several from our own group. Work in this field has shown that the isocyanide can play important structural roles, both as a terminal ligand and as a bridging ligand for polynuclear structures, and can influence the excited-state character and excited-state dynamics. In addition, there are many examples of isocyanide-supported complexes where the isocyanide serves as a chromophoric ligand, meaning the low-energy excited states that are important in the photochemistry are partially or completely localized on the isocyanide. Finally, an emerging trend in the design of luminescent compounds is to use the isocyanide as an electrophilic precursor, converted to an acyclic carbene by nucleophilic addition which imparts certain photophysical advantages. This Perspective aims to show the diverse roles played by isocyanides in the design of luminescent compounds, showcasing the recent developments that have led to a substantial growth in fundamental knowledge, function, and applications related to photoluminescence.