Herein, we thoroughly investigate the (photo)physical properties and the geometry of the first reported Coulombic dyad, composed of dicationic tris(1,10-phenanthroline)ruthenium(II) and tetraanionic 1,3,6,8-pyrenetetrasulfonate. The mixed dynamic and static quenching behavior of the photoexcited ruthenium complex by the pyrene derivative in water was investigated by time-resolved (laser flash photolysis as well as time-correlated single photon counting) and steady-state emission spectroscopy, highlighting their differences and the importance of accurate models to determine the association constant. Temperature-dependent measurements revealed a decrease in dynamic and static quenching efficiency at elevated temperatures and provided the enthalpy and entropy of ion-pair formation in water. The time constant for intra-ion-pair Dexter energy transfer from the triplet excited state of the metal complex to the organic chromophore was determined to be similar to 87 ps using femtosecond transient absorption spectroscopy. Structural insights were obtained from single-crystal X-ray crystallography and molecular dynamics simulations. The simulations revealed a high persistence of the ion-pair in the ground-state and a dynamic yet geometrically well-defined association. A structural similarity was found between solution and solid-state arrangements, indicating pi-interactions besides Coulombic interactions between the ions. These interactions enable sufficient orbital overlap, rationalizing the observed efficient Dexter energy transfer, which is as fast as in many covalently linked donor-acceptor systems. These in-depth investigations provide a comprehensive picture of structure-property relationships in Coulombic dyads, offering valuable insights for their future design and applications.

The successful application of transition metal based photoactive complexes in lighting, imaging, sensing, and photocatalysis is usually based on the triplet metal-to-ligand charge transfer (3MLCT) excited state of precious metal complexes with 4d6 and 5d6 valence electron configuration. These photocatalysts exhibit excited state lifetimes exceeding hundreds of nanoseconds. Simple transition metal complexes with 3d6 valence electron configuration containing abundant metals exhibit lifetimes of less than 1-2 nanoseconds and they require multistep ligand syntheses mitigating large-scale implementation. We report that a commercially available bis(imidazolium) pyridine pro-ligand [H2pbmi]2+ and a manganese(II) salt give access to the tetracarbene manganese(I) complex [Mn(pbmi)2]+. This purple colored complex phosphoresces at room temperature in fluid solution. Its 3MLCT excited state lifetime of 190 ns exceeds those of simple 3MLCT photoactive 3d6 complexes by more than one to two orders of magnitude. In combination with its reversible ground state MnII/I redox chemistry, this translates to a 3MLCT excited state capable of reducing an organic substrate by bimolecular quenching. The combination of manganese(I) with rigid carbene and pyridine chelate ligands expands key strategies for photoactive 3d6 metal complexes of earth-abundant metals with 3MLCT lifetimes rivalling those of precious metals and providing a conceptual starting point for a sustainable photochemistry.

We recently reported a molecular design for carbonylpyridine molybdenum(0) complexes that unlocks long-lived luminescent and photoactive charge-transfer states. Here, we translate this strategy to chromium(0), and tungsten(0) and report three fully characterized tricarbonyl metal(0) complexes featuring a tripodal ligand with a remote n-butyl substituent in the backbone. All complexes show phosphorescence in the red to near-infrared spectral region from metal-to-ligand charge-transfer excited states. Surprisingly, the alkyl chain significantly affects excited state relaxation: lifetimes are shortened in solution but extended in the solid state by one order of magnitude compared to the molybdenum(0) complex with a methyl substituent. Temperature-dependent luminescence and NMR spectroscopy in combination with quantum chemical calculations reveal the reasons for these disparate effects. The n-butyl substituent distorts the metal coordination geometry. The resulting structural flexibility flattens the potential energy surfaces in solution, which lowers the barrier for the population of distorted metal-centered states and facilitates nonradiative relaxation. In the solid state, the rigidified alkyl chain separates neighboring molecules, which reduces self-quenching. Our study sheds light on the relationship between structure and excited state relaxation to inform the development of photoactive complexes based on earth-abundant metals.

Herein, we report a new approach for the light-mediated generation of sulfamoyl radicals using sulfur dioxide as key building block and the direct application of these radicals in the synthesis of sulfonamides. In the presence of different photoredox catalysts, sulfamoyl radicals can be generated directly from SO2 or the SO2 surrogate DABSO (1,4-diazabicyclo[2.2.2]octane & sdot;bis (sulfur dioxide) adduct) and N-aminopyridinium salts as nitrogen radical precursors. Trapping of the in situ generated sulfamoyl radicals with selected electron-rich olefins affords different sulfonamides in up to 86% yield in a three-component procedure. This transformation provides a complementary approach for the in situ generation of sulfamoyl radicals as synthetic intermediates for the assembly of the sulfonamide functionality, a privileged motif in active pharmaceutical ingredients.

The use of stable photosensitizers made on a multigram‐scale from cheap and available starting materials offers an extraordinary opportunity to translate small‐scale proof‐of‐concept photoreactions to widespread applications in non‐specialized laboratories, thereby promoting the implementation of light‐driven syntheses. Herein, we report an easy, multigram‐scale preparation of the exceptionally photo‐ and redoxstable chromium(III) complex [Cr(tpe)2]3+ (tpe=1,1,1‐tris(pyrid‐2‐yl)ethane) at room temperature. This photocatalyst was successfully employed in acetonitrile and in the “green” solvent water as demonstrated in visible‐light induced Schenck‐Ene reactions, Newman‐Kwart rearrangements, radical cation Diels‐Alder cycloadditions, oxidative decarboxylation and trifluoromethylation reactions with excellent substrate conversions and high turnover numbers. Gram‐scale photocatalytic reactions were conducted using a simple round‐bottom flask as reaction vessel and irradiation with energy‐efficient blue LEDs or even with sunlight. The stable photocatalyst was successfully recycled in up to 90–92% yields after the photoreactions.

Activation of halido gold-(I) precatalysts Au-(L)-X to a cationic species [Au-(L)]+ with a vacant coordination site for substrate binding typically requires abstraction of the halide X-. The Fischer-type carbene gold-(I) precatalysts 2-4 feature redox-active dimethylanilinyl, 2-furyl, and ferrocenyl substituents without (2a-4a) and with (2b-4b), a dangling dimethylamino substituent. After single-electron oxidation, 2-4 catalyze the cyclization of N(2-propyn-1-yl)-benzamide to 2-phenyl-5-vinylidene-2-oxazoline without the presence of a halide scavenger. While all dimethylanilinyl and 2-furyl substituted precatalysts likely form catalytically active nanoparticles after oxidation, the ferrocenyl substituted gold-(I) complexes 4a and 4b operate in a homogeneous fashion. The dimethylamino substituted ferrocenyl precatalyst 4b is the most active one. The formation of the catalytically active molecular species after oxidation was probed by stopped-flow experiments, quantitative EPR spectroscopy, and quantum chemical calculations to arrive at a consistent mechanistic picture that involves one-electron oxidation of the ferrocene, valence isomerization to a gold-(II) species and anion coordination to the gold-(II) center. This oxidation/isomerization/coordination activation mechanism is fundamentally different from the typical activation of gold-(I) precatalysts by halide abstraction and opens new avenues in gold catalysis beyond gold-(I) and gold-(III) catalyses.

The interest in chromium(III) complexes has been renewed over the past decade for the design of efficient earth-abundant phosphorescent red-to-near-infrared spin-flip emitters and photocatalysts with long excited state lifetimes. In this context, we report the energy tuning of spin-flip excited states based on heteroleptic bis(tridentate) polypyridine chromium(III) complexes [3X,Y]3+, namely, [3NMe,CH2]3+, [3NMe,S]3+ and [3CH2,S]3+ with the tridentate ligands LX and LY [X/Y = NMe, N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine; X/Y = CH2, 2,6-bis(2-pyridylmethyl)pyridine and X/Y = S, 2,6-bis(pyridine-2-ylthio)pyridine]. The heteroleptic complexes [3X,Y]3+ are obtained via a novel synthetic approach toward the required intermediate labile triflato complexes Cr(LX)(OTf)3 (2X) from the respective chlorido precursors CrCl3(LX) (1X) using trimethylsilyl trifluoromethanesulfonate. Spin-flip energies were experimentally detected by vis/near-infrared absorption and emission spectroscopy as well as computationally derived by multireference calculations. Together with the known homoleptic molecular ruby complexes, the three resulting series of luminescent complexes [3X,X]3+/[3X,Y]3+/[3Y,Y]3+ allow delineation of an additive nephelauxetic effect of the ligands with chromium(III) ions and thus prediction of spin-flip emission energies of derived molecular rubies.

Tris(cyclometalated) chromium(III) complexes fac-Cr(C∧X)3 with dimethylamine (X = NMe2, fac-1) and diphenylphosphane (X = PPh2, fac-2) donors were fully characterized by single crystal X-ray structure analyses, cyclic voltammetry, and absorption/emission spectroscopy. Both complexes are luminescent in the near-infrared spectral region in the solid state or in frozen solution. The phosphane complex fac-2 is hemilabile and forms a weakly emissive, five-coordinate complex with a dangling PPh2 donor in fluid solution. Spectroscopic assignments are aided by quantum chemical calculations at the DFT and CASSCF/NEVPT2 levels of theory. fac-1 and fac-2 complement the cyclometalated pyridine complex fac-Cr(ppy)3 (ppy– = deprotonated 2-phenylpyridine) and allow establishment of a nephelauxetic series of aryl, pyridine, phosphane, and amine donors.

Recently the interest in the challenge of matching or surpassing the exceptional properties of well-established ruthenium, iridium or rare earth photoactive complexes by employing 3d transition metals with tailor-made ligands to tame the capricious nature of base metals has grown dramatically. Recent advances in the development of Molecular Rubies - defined as photoactive, octahedral chromium(III) complexes with tridentate strong-field ligands forming six-membered chelate rings - are discussed in this chapter. Starting with the prototypical complex [Cr(ddpd)2](3+) reported in 2015, this complex class has evolved to compete with photoactive complexes of the precious and rare earth metals, such as [Ru(bpy)3](2+), fac-Ir(ppy) 3 or Eu(fod) 3 in terms of photoluminescence lifetime, quantum yield and photochemical stability. This review discusses key design aspects to optimize the photophysical and -chemical properties of Molecular Rubies and summarizes latest optical and photochemical applications.

Six-coordinate chromium(iii) complexes with high spin-flip (SF) photoluminescence quantum yields and lifetimes (molecular rubies) have attracted huge interest in the past years due to their applicability in sensing, photocatalysis or circularly polarised emission. However, clearcut design rules for high quantum yields and lifetimes are still lacking due to the multidimensional problem of the non-radiative decay of the SF states. Based on an isostructural series of complexes differing in the ligand backbone, we disentangle decisive structural and electronic features for SF excited state energies and non-radiative decays promoted by spin-orbit coupling, Jahn-Teller distortions and (thermally activated) multiphonon relaxation. This analysis goes beyond the classical increasing of the ligand field strength or the metal-ligand covalency to reduce non-radiative decay or to tune the SF energy. The results underscore the utility of the combination of near-infrared absorption, variable temperature emission and fs-transient absorption spectroscopy as well as photolysis and high-level quantum chemical calculations to obtain a comprehensive picture of the excited dynamics on ultrafast and long timescales.

Mixed N-heterocyclic carbene (NHC) / pyridyl iron(II) complexes have attracted a great deal of attention recently because of their potential as photocatalysts and light sensitizers made from Earth-abundant elements. The most decisive challenge for their successful implementation is the lifetime of the lowest triplet metal-to-ligand charge transfer state ( 3 MLCT), which typically decays via a triplet metal-centered ( 3 MC) state back to the ground state. We reveal by variable-temperature ultrafast transient absorption spectroscopy that the tripodal iron(II) bis(pyridine) complex isomers trans - and cis -[Fe(pdmi) 2 ] 2+ with four NHC donors show 3 MLCT→ 3 MC population transfers with very different barriers and rationalize this by computational means. While trans -[Fe(pdmi) 2 ] 2+ possesses an unobservable activation barrier, the cis isomer exhibits a barrier of 492 cm −1 , which leads to a nanosecond 3 MLCT lifetime at 77 K. The kinetic and quantum chemical data were analyzed in the context of semi-classical Marcus theory revealing a high reorganization energy and small electronic coupling between the two triplet states. This highlights the importance of detailed structural control and kinetic knowledge for the rational design of photosensitizers from first row transition metals such as iron.

Highly reducing or oxidizing photocatalysts are a fundamental challenge in the field of inorganic and organic photochemistry. Only a few transition metal complexes with earth-abundant metal ions have so far advanced to excited state oxidants, including chromium, iron and cobalt. All these photocatalysts require high energy light for excitation and their oxidizing power has not been fully exploited due to significant energy dissipation before reaching the photoactive state. Herein we demonstrate that the complex [Mn(dgpy)2]4+ based on earth-abundant manganese can be excited with low-energy NIR light (850 nm, 1.46 eV) to yield a luminescent mixed 2LMCT/2MC excited state (1435 nm, 0.86 eV) with a lifetime of 1.6 ns. The dissipated energy amounts to 0.60 eV. In spite of this energy loss, *[Mn(dgpy)2]4+ with its excited state redox potential Ered* of 1.80 V vs SCE outcompetes the strongest reported precious metal photooxidant (iridium(III)). *[Mn(dgpy)2]4+ oxidizes naphthalene (Eox 1.31 1.54 V vs. SCE) to its radical cation giving the manganese(III) complex [Mn(dgpy)2]3+ in a clean outer-sphere electron transfer process. Unexpectedly, mesitylene, toluene, benzene and nitriles with even extremely high oxidation potentials up to Eox = 2.4 V provoke the [Mn(dgpy)2]4+/3+ reduction under photolysis. A higher energy short-lived 4LMCT excited state with a lifetime of 0.78 ps is made responsible for these demanding oxidations, which proceed by static rather than dynamic quenching. This dual excited state reactivity from 2LMCT/2MC and 4LMCT states is linked to the 4LMCT 2LMCT/2MC intersystem crossing process. These unique findings demonstrate how the design of manganese complexes (i) expands the absorption cross section to 400 850 nm, (ii) increases the 2LMCT/2MC state lifetime to the nanosecond range allowing luminescence and classical dynamic photoredox processes and (iii) enables non-classical static quenching of an extremely oxidizing 4LMCT excited state by the solvent. This conceptually novel approach of static quenching by the solvent minimizes free energy losses, harnesses the full photooxidizing power and thus allows even oxidation of nitriles and benzene using earth-abundant elements and low-energy light.

Photoactive chromium(III) complexes saw a conceptual breakthrough with the discovery of the prototypical molecular ruby mer-[Cr(ddpd)2]3+ (ddpd = N,N'-dimethyl-N,N'-dipyridin-2-ylpyridine-2,6-diamine), which shows intense long-lived near-infrared (NIR) phosphorescence from metal-centered spin-flip states. In contrast to the numerous studies on chromium(III) photophysics, only 10 luminescent molybdenum(III) complexes have been reported so far. Here, we present the synthesis and characterization of mer-MoX3(ddpd) (1, X = Cl; 2, X = Br) and cisfac-[Mo(ddpd)2]3+ (cisfac-[3]3+), an isomeric heavy homologue of the prototypical molecular ruby. For cisfac-[3]3+, we found strong zero-field splitting using magnetic susceptibility measurements and electron paramagnetic resonance spectroscopy. Electronic spectra covering the spin-forbidden transitions show that the spin-flip states in mer-1, mer-2, and cisfac-[3]3+ are much lower in energy than those in comparable chromium(III) compounds. While all three complexes show weak spin-flip phosphorescence in NIR-II, the emission of cisfac-[3]3+ peaking at 1550 nm is particularly low in energy. Femtosecond transient absorption spectroscopy reveals a short excited-state lifetime of 1.4 ns, 6 orders of magnitude shorter than that of mer-[Cr(ddpd)2]3+. Using density functional theory and ab initio multireference calculations, we break down the reasons for this disparity and derive principles for the design of future stable photoactive molybdenum(III) complexes.

The photophysics and photochemistry of 3d transition metal complexes strongly differ from their heavier 4d and 5d homologs. The distinct excited state dynamics are a direct consequence of the weaker ligand field splitting and the smaller spin-orbit coupling in 3d transition metal complexes. The often very fast non-radiative relaxation of photoexcited 3d transition metal complexes to the ground state has prevented a widespread use of these much more abundant transition metals in photophysical and photochemical applications so far. Recent exciting advancements in ligand design, synthesis, ultrafast spectroscopy, computational chemistry and understanding of excited state dynamics as well as the awareness for a more sustainable photochemistry led to a paradigm change in the reputation and emerging importance of 3d transition metals. The introduction chapter contrasts the photophysical background of 3d transition metal complexes with that of their heavier homologs. This is followed by overviews of the excited state reactivity of 3d transition metal complexes, namely luminescence, bi- and unimolecular reactivity. Luminescence from excited states is divided into spin-flip and charge transfer luminescence. The charge transfer excited states and the resulting luminescence can be of ligand-to-metal, metal-to-ligand, ligand-to-ligand charge transfer and charge transfer to solvent character. Bimolecular reactivity covers photoinduced electron and energy transfer reactions. In solution, the lifetimes of the reacting excited states play a particularly important and challenging role. Dissociative unimolecular reactivity comprises dissociation of ligands such as CO and NO, CO 2 dissociation from carboxylato complexes, N 2 dissociation from azido complexes, M C bond homolysis and other M X dissociations as well as photoisomerizations. These unimolecular reactions can occur on ultrafast timescales.

Luminescent complexes of earth-abundant first-row transition metals are of renewed, broad interest due to their spectroscopic and photochemical properties as well as emerging applications. New strong-field polypyridine ligands have led to six-coordinate 3d3 chromium(III) complexes with intense spin-flip luminescence in solution at room temperature. The ground and emissive states both arise from the (t2)3 electron configuration involving the dπ levels (O point group symmetry labels). Pseudoctahedral 3d8 nickel(II) complexes with such strong ligands are a priori also promising candidates for spin-flip luminescence. In contrast, the relevant electron configurations involve the dσ orbitals and (e)2 configurations. We have prepared the known nickel(II) complexes [Ni(terpy)2]2+, [Ni(phen)3]2+, and [Ni(ddpd)2]2+ as well as the novel complexes [Ni(dgpy)2]2+ and [Ni(tpe)2]2+ forming a series with increasing ligand field strengths (terpy = 2,2':6',2″-terpyridine; phen = 1,10-phenanthroline; ddpd = N,N'-dimethyl-N,N'-dipyridine-2-ylpyridine-2,6-diamine; dgpy = 2,6-diguanidylpyridine; tpe = 1,1,1-tris(pyrid-2-yl)ethane). The lowest-energy singlet and triplet excited states of these nickel(II) complexes are analyzed based on absorption spectra using ligand field theory and CASSCF-NEVPT2 calculations for vertical transition energies and a model based on coupled potential energy surfaces, leading to calculated absorption spectra in good agreement with the experimental data. No photoluminescence signal was observed in the wavelength ranges identified through the analyses of the absorption spectra. The models provide insight into key differences between the nickel(II) complexes and their strongly luminescent chromium(III) analogues.

Photoactive complexes with earth-abundant metals have attracted increasing interest in the recent years fueled by the promise of sustainable photochemistry. However, sophisticated ligands with complicated syntheses are oftentimes required to enable photoactivity with nonprecious metals. Here, we combine a cheap metal with simple ligands to easily access a photoactive complex. Specifically, we synthesize the molybdenum(0) carbonyl complex Mo(CO)3(tpe) featuring the tripodal ligand 1,1,1-tris(pyrid-2-yl)ethane (tpe) in two steps with a high overall yield. The complex shows intense deep-red phosphorescence with excited state lifetimes of several hundred nanoseconds. Time-resolved infrared spectroscopy and laser flash photolysis reveal a triplet metal-to-ligand charge-transfer (3MLCT) state as the lowest excited state. Temperature-dependent luminescence complemented by density functional theory (DFT) calculations suggest thermal deactivation of the 3MLCT state via higher lying metal-centered states in analogy to the well-known photophysics of [Ru(bpy)3]2+. Importantly, we found that the title compound is very photostable due to the lack of labilized Mo-CO bonds (as caused by trans-coordinated CO) in the facial configuration of the ligands. Finally, we show the versatility of the molybdenum(0) complex in two applications: (1) green-to-blue photon upconversion via a triplet-triplet annihilation mechanism and (2) photoredox catalysis for a green-light-driven dehalogenation reaction. Overall, our results establish tripodal carbonyl complexes as a promising design strategy to access stable photoactive complexes of nonprecious metals avoiding tedious multistep syntheses.

Great progress has been achieved on phosphorescent or photoactive complexes of the Earth-abundant transition metals, while examples for phosphorescent heavy main group element complexes are rare, in particular for group 14 complexes in the oxidation state +II. The known compounds often show only weak phosphorescence with fast non-radiative deactivation. The underlying photophysical processes and the nature of the phosphorescent electronic states have remained essentially unexplored. The present combined photophysical and theoretical study on tin(ii) and lead(ii) complexes E(bpep) with the dianionic tridentate ligand bpep2- (E = Sn, Pb; H2bpep = 2-[1,1-bis(1H-pyrrol-2-yl)ethyl]pyridine) provides unprecedented insight in the excited state energy landscape of tetrel(ii) complexes. The tin complex shows green intraligand charge transfer (ILCT) phosphorescence both in solution and in the solid state. In spite of its larger heavy-atom effect, the lead complex only shows very weak red phosphorescence from a strongly distorted ligand-to-metal charge transfer (LMCT) state at low temperatures in the solid state. Detailed (TD-)DFT calculations explain these observations and delineate the major path of non-radiative deactivation via distorted LMCT states. These novel insights provide rational design principles for tetrel(ii) complexes with long-lived phosphorescence.

Molecular entities with doublet or triplet ground states find increasing interest as potential molecular quantum bits (qubits). Complexes with higher multiplicity might even function as qudits and serve to encode further quantum bits. Vanadium(II) ions in octahedral ligand fields with quartet ground states and small zero-field splittings qualify as qubits with optical read out thanks to potentially luminescent spin-flip states. We identified two V2+ complexes [V(ddpd)(2)](2+) with the strong field ligand N,N'-dimethyl-N,N'-dipyridine-2-yl-pyridine-2,6-diamine (ddpd) in two isomeric forms (cis-fac and mer) as suitable candidates. The energy gaps between the two lowest Kramers doublets amount to 0.2 and 0.5 cm(-1) allowing pulsed EPR experiments at conventional Q-band frequencies (35 GHz). Both isomers possess spin-lattice relaxation times T-1 of around 300 mu s and a phase memory time T-M of around 1 mu s at 5 K. Furthermore, the mer isomer displays slow magnetic relaxation in an applied field of 400 mT. While the vanadium(III) complexes [V(ddpd)(2)](3+) are emissive in the near-IR-II region, the [V(ddpd)(2)](2+) complexes are non-luminescent due to metal-to-ligand charge transfer admixture to the spin-flip states.

The chromium(III) complex [Cr(ddpd)2 ][BF4 ]3 shows two spin-flip emission bands in the near-infrared spectral region. These bands shift bathochromically by -14.1 and -7.7 cm-1 kbar-1 under hydrostatic pressure (Angew. Chem. Int. Ed. 2018, 57, 11069). The present study elucidates the structural changes of the chromium(III) cations under pressure using density functional theory with periodic boundary conditions and the resulting effects on the excited state energies using high-level CASSCF-NEVPT2 calculations. The differences of the bands in pressure sensitivity are traced back to a different orbital occupation of the intraconfigurational excited states.

In order to expand and exploit the useful properties of d6-iron(II) and d5-iron(III) complexes in potential magnetic, photophysical, or magnetooptical applications, crucial ligand-controlled parameters are the ligand field strength in a given coordination mode and the availability of suitable metal and ligand frontier orbitals for charge-transfer processes. The push-pull ligand 2,6-diguanidylpyridine (dgpy) features low-energy π* orbitals at the pyridine site and strongly electron-donating guanidinyl donors combined with the ability to form six-membered chelate rings for optimal metal-ligand orbital overlap. The electronic ground states of the pseudo-octahedral d6- and d5-complexes mer-[Fe(dgpy)2]2+, cis-fac-[Fe(dgpy)2]2+, and mer-[Fe(dgpy)2]3+ as well as their charge-transfer (CT) and metal-centered (MC) excited states are probed by variable temperature UV/vis absorption, NMR, EPR, and Mössbauer spectroscopy, magnetic susceptibility measurements at variable temperature as well as quantum chemical calculations.