First-Row d6 Metal Complex Enables Photon Upconversion and Initiates Blue Light-Dependent Polymerization with Red Light

Photosensitizers for sensitized triplet-triplet annihilation upconversion (sTTA-UC) often rely on precious heavy metals, whereas coordination complexes based on abundant first-row transition metals are less common. This is mainly because long-lived triplet excited states are more difficult to obtain for 3d metals, particularly when the d-subshell is only partially filled. Here, we report the first example of sTTA-UC based on a 3d⁶ metal photosensitizer yielding an upconversion performance competitive with precious metal-based analogues. Using a newly developed Cr⁰ photosensitizer featuring equally good photophysical properties as an Os^II benchmark complex in combination with an acetylene-decorated anthracene annihilator, red-to-blue upconversion is achievable. The upconversion efficiency under optimized conditions is 1.8 %, and the excitation power density threshold to reach the strong annihilation limit is 5.9 W/cm². These performance factors, along with high photostability, permit the initiation of acrylamide polymerization by red light, based on radiative energy transfer between delayed annihilator fluorescence and a blue light absorbing photo-initiator. Our study provides the proof-of-concept for photon upconversion with elusive first-row analogues of widely employed precious d⁶ metal photosensitizers, and for their application in photochemical reactions triggered by excitation wavelengths close to near-infrared.     

Electronic states of π complexes of <Emphasis Type="Italic">d</Emphasis><Superscript>0</Superscript> early transition metals and energy transfer to unsaturated hydrocarbons

The orbital nature of excited states of organometallic π-complexes of early transition metals of the third-fifth periods with the d ⁰ electronic configuration was surveyed, and their photophysical characteristics were discussed. Examples of long-lived luminescence generated by ligand-to-metal charge transfer in liquid solutions at room temperature were thoroughly considered. Regularities of nonradiative triplet-triplet energy transfer from aromatic molecules and d ⁰ metallocene complexes of Group 4B transition metals to unsaturated hydrocarbons were examined.     

Chromium(0), Molybdenum(0), and Tungsten(0) Isocyanide Complexes as Luminophores and Photosensitizers with Long-Lived Excited States

Arylisocyanide complexes based on earth-abundant Group 6 d⁶ metals are interesting alternatives to photoactive complexes made from precious metals such as Ru^II, Re^I, Os^II, or Ir^III. Some of these complexes have long-lived ³MLCT excited states that exhibit luminescence with good quantum yields as well as nano- to microsecond lifetimes, and they are very strongly reducing. Recent studies have demonstrated that Cr⁰, Mo⁰, and W⁰ arylisocyanide complexes have great potential for applications in luminescent devices, photoredox catalysis, and dye-sensitized solar cells.     

Sensitized luminescence from lanthanides in d–f bimetallic complexes

This article reviews progress in the research of transition metal–lanthanide (d–f) bimetallic complexes. Through efficient energy transfer, sensitized luminescence of lanthanide ions from the visible range (Eu^III) to the near-infrared region (Nd^III, Yb^III, Er^III and Pr^III) is obtained in these bimetallic assembles. The d-block in d–f bimetallic complexes mainly contributes to the improvement of lanthanide emission efficiency and the extension of the excitation window for the lanthanide complexes. Examples are catalogued by various transition metals, such as Ru^II, Os^II (Fe^II), Pt^II (Au^I), Pd^II, Re^I, Cr^III, Co^III, Zn^II and Ir^III. The relevant synthetic procedures, crystal structures and photophysical properties of these d–f complexes are briefly described. Additionally, the molecular properties responsible for the performance of certain d–f systems, such as energy levels, nuclear distances and coordination environments, will be discussed.     

Light-driven electron transfer in a modular assembly of a ruthenium(II) polypyridine sensitiser and a manganese(II) terpyridine unit separated by a redox active linkage. DFT analysis

A series of ruthenium polypyridine-based complexes covalently bound to a terpyridine coordinating site for Mn^II ion coordination has been developed. A redox active unit separates the photoactive unit and the manganese complex. Introducing ester groups on the bipyridine skeleton allows modulation of redox properties of the chromophore. Intramolecular electron transfer from the Mn^II to the photogenerated Ru^III was studied by time-resolved transient absorption and EPR. Photophysical studies support the participation of the imidazole unit in the electron transfer process from the Mn(II) complex and Ru(III) in the case of ester containing chromophores. DFT calculations were performed and used to rationalize the photophysical behavior of the complexes, in particular the effect of coordination of the Mn^II ion to the terpyridine cavity as well as the influence of the electron withdrawing groups on the Ru chromophore.     

Quinoxaline sensitised lanthanide ion luminescence: Syntheses,spectroscopy and X-ray crystal structure of Na{1,4,7-tris[(N-diethyl)carbamoylmethyl]-1,4,7,10-tetraazacyclododecane-10-(2-methylquinoxaline)}I3 C7H8

The synthesis of a quinoxaline-appended aza-macrocyclic ligand together with corresponding Ln^III complexes are described. The luminescence properties of the complexes show that the quinoxaline unit sensitises both visible (Eu^III) and near-IR (Nd^III and Yb^III) emitting lanthanide ions. UV–Vis absorption and time-resolved photophysical studies together with X-ray structural data suggest that as well as contributing chromophorically, the quinoxaline moiety generally participates in the first coordination sphere of Ln^III. The luminescent pH response of the Eu^III complex is also reported. The form of the steady state spectrum changes profoundly in the pH range 2–12, implying a change in the coordination environment, which was confirmed with time-resolved lifetime measurements that suggest an increase in europium inner sphere hydration at acidic pH.     

Spin Crossover and Long-Lived Excited States in a Reduced Molecular Ruby

The chromium(III) complex [Cr^III(ddpd)₂]³⁺ (molecular ruby; ddpd=N,N′-dimethyl-N,N′-dipyridine-2-yl-pyridine-2,6-diamine) is reduced to the genuine chromium(II) complex [Cr^II(ddpd)₂]²⁺ with d⁴ electron configuration. This reduced molecular ruby represents one of the very few chromium(II) complexes showing spin crossover (SCO). The reversible SCO is gradual with T_1/2 around room temperature. The low-spin and high-spin chromium(II) isomers exhibit distinct spectroscopic and structural properties (UV/Vis/NIR, IR, EPR spectroscopies, single-crystal XRD). Excitation of [Cr^II(ddpd)₂]²⁺ with UV light at 20 and 290 K generates electronically excited states with microsecond lifetimes. This initial study on the unique reduced molecular ruby paves the way for thermally and photochemically switchable magnetic systems based on chromium complexes complementing the well-established iron(II) SCO systems.     

Electronic Structure and Magnetic Properties of a High-Spin MnIII Complex: [Mn(mesacac)3] (mesacac=1,3-Bis(2,4,6-trimethylphenyl)-propane-1,3-dionato)

Metal acetylacetonates of the general formula [M(acac)₃] (M^III=Cr, Mn, Fe, Co) are among the best investigated coordination compounds. Many of these first-row transition metal complexes are known to have unique electronic properties. Independently, photophysical research with different β-diketonate ligands pointed towards the possibility of a special effect of the 2,4,6-trimethylphenyl substituted acetylacetonate (mesacac) on the electron distribution between ligand and metal (MLCT). We therefore synthesized and fully characterized the previously unknown octahedral title complex. Its solid-state structure shows a Jahn-Teller elongation with two Mn−O bonds of 2.12/2.15 Å and four Mn−O bonds of 1.93 Å. Thermogravimetric data show a thermal stability up to 270 °C. High-resolution mass spectroscopy helped to identify the decomposition pathways. The electronic state and spin configuration of manganese were characterized with a focus on its magnetic properties by measurement of the magnetic susceptibility and triple-zeta density functional theory (DFT) calculations. The high-spin state of manganese was confirmed by the determination of an effective magnetic moment of 4.85 μ_B for the manganese center.     

Lanthanide(III) Complexes of Bis‐semicarbazone and Bis‐imine‐Substituted Phenanthroline Ligands: Solid‐State Structures,Photophysical Properties,and Anion Sensing

Phenanthroline‐based hexadentate ligands L¹ and L² bearing two achiral semicarbazone or two chiral imine moieties as well as the respective mononuclear complexes incorporating various lanthanide ions, such as La^III, Eu^III, Tb^III, Lu^III, and Y^III metal ions, were synthesized, and the crystal structures of [ML¹Cl₃] (M=La^III, Eu^III, Tb^III, Lu^III, or Y^III) complexes were determined. Solvent or water molecules act as coligands for the rare‐earth metals in addition to halide anions. The big Ln^III ion exhibits a coordination number (CN) of 10, whereas the corresponding Eu^III, Tb^III, Lu^III, and Y^III centers with smaller ionic radii show CN=9. Complexes of L², namely [ML²Cl₃] (M=Eu^III, Tb^III, Lu^III, or Y^III) ions could also be prepared. Only the complex of Eu^III showed red luminescence, whereas all the others were nonluminescent. The emission properties of the Eu derivative can be applied as a photophysical signal for sensing various anions. The addition of phosphate anions leads to a unique change in the luminescence behavior. As a case study, the quenching behavior of adenosine‐5′‐triphosphate (ATP) was investigated at physiological pH value in an aqueous solvent. A specificity of the sensor for ATP relative to adenosine‐5′‐diphosphate (ADP) and adenosine‐5′‐monophosphate (AMP) was found. ³¹P NMR spectroscopic studies revealed the formation of a [EuL²(ATP)] coordination species.     

Lanthanide luminescence efficiency in eight- and nine-coordinate complexes: Role of the radiative lifetime

The photophysical parameters for the sensitization of metal-centred luminescence are analyzed in two series of complexes with tridentate and hexadentate ligands having N_xO_y chelating units. In particular, the radiative lifetime τ_rad is experimentally estimated for 29 nine-coordinate Eu^III complexes and 10 eight- and nine-coordinate Yb^III complexes. The known dependence of τ_rad on refractive index is substantiated by comparing data for solid-state samples and solutions. Moreover, a clear dependence of τ_rad with the coordination environment is evidenced and in the case of Eu^III, a comparison between τ_rad and the nephelauxetic effect generated by the ligands is attempted. Altogether, this extensive analysis points to the importance of having a handle on τ_rad when designing ligands for highly luminescent lanthanide-containing molecular edifices. This, in turn, should stimulate initiating theoretical considerations to unravel a reliable relationship between τ_rad and the electronic structure of the ligands.     

Ambidenticity of a tridentate oxygen-nitrogen donor towards bivalent and trivalent transition metal ions

Summary N-salicylidene anthranilamide (H₂SAA) and its Cr^III, Mn^II, Fe^III, Co^II, Ni^II and Cu^II complexes were prepared and characterized by physicochemical and spectroscopic data. H₂SAA enolizes to give a dibasic ONO donor set in the divalent metal complexes. It also binds to the trivalent metal ions in a nonenolized form using a monobasic ONN donor set. Co^II is oxidized to Co^III during complexation. Octahedral geometries are proposed for Cr^III, Mn^II, Fe^III and Co^III complexes, while square planar geometries are suggested for the Ni^II and Cu^II complexes. Phenoxide bridging in the Cr^III and Fe^III complexes and enoxide bridging in the Ni^II and Cu^II complexes is proposed.     

Applications of several spectral techniques to characterize coordination compounds derived from 2,6-diacetylpyridine derivative

The coordination compounds of Cr^III, Mn^II and Co^II metal ions derived from quinquedentate 2,6-diacetylpyridine derivative have been synthesized and characterized by using the various physicochemical studies like stoichiometric, molar conductivity and magnetic, and spectral techniques like IR, NMR, mass, UV and EPR. The general stoichiometries of the complexes are found to be [Cr(H₂L)X] and [M(HL)X], where M = Mn(II) and Co(II); H₂L = dideprotonated ligand, HL = monodeprotonated ligand and X = NO₃⁻, Cl⁻ and OAc⁻. The studies reveal that the complexes possess monomeric compositions with six coordinated octahedral geometry (Cr^III and Mn^II complexes) and six coordinated tetragonal geometry (Co^II complexes).     

Homo- and hetero-nuclear chromium(III) complexes with natural ligands. Part 3. Chromium(III)/vanadium(IV) species isolated from the redox systems

Four new heteronuclear Cr^III/V^IV complexes have been isolated from the redox [Cr^III–V^v–L₁–L₂] systems (L₁ = glycine, glutaminic and nicotinic acids, L₂ = cysteine and glutathione). The complexes have been analysed by spectroscopic methods (diffuse reflectance u.v./vis., i.r.) and by FAB mass spectra. A significant bathochromic shift of the d–d Cr^III and V^IV transitions in heteronuclear complexes (d¹–d³) in comparison to the Cr^III homonuclear species (d³/d³) has been related to the interaction of two metal centres. Spectral analyses by the digital filter and band deconvolution methods are presented.     

Rational Improvement of Single-Molecule Magnets by Enforcing Ferromagnetic Interactions

The anisotropy barrier of polynuclear single-molecule magnets is expected to be higher with less tunneling the better stabilized the spin ground state is so that less M_S mixing in the ground state and with excited spin states occur. We have realized this experimentally in two structurally related heptanuclear SMMs: the triplesalen-based [Mn^III ₆ Cr^III]³⁺ and the triplesalalen-based *[Mn^III ₆ Cr^III]³⁺ . The ligand system triplesalen was developed to enforce ferromagnetic interactions by the spin-polarization mechanism. However, we found weak antiferromagnetic couplings, that we assigned to an inefficient spin-polarization by a heteroradialene formation. To prevent this heteroradialene formation, the triplesalalen ligand H₆talalen was designed. Here, we present the building block [(talalen )Mn^III₃]³⁺ and its application for the assembly of [{(talalen )Mn^III₃}₂{Cr^III(CN)₆}]³⁺ (= *[Mn^III ₆ Cr^III]³⁺ ). Both the trinuclear and heptanuclear complexes are SMMs. The comparison to the related triplesalen complex [(feld )Mn^III₃]³⁺ proves the absence of heteroradialene character and the enforcement of ferromagnetic Mn^III-Mn^III interactions in the (talalen )⁶⁻ complexes. This results in an increase of the barrier for spin reversal U_eff from 25 K in the triplesalen-based [Mn^III ₆ Cr^III]³⁺ SMMs to 37 K in the triplesalalen-based *[Mn^III ₆ Cr^III]³⁺ SMM proving the success of our concept. Based on this study, the next step in the rational improvement of our SMMs is discussed.     

Transition metal complexes of potassium dihydridobis-, hydridotris- and tetrakis-(thiophenolyl)borate anions

Summary The three new potential chelating ligands dihydridobis-, hydridotris- and tetrakis-(thiophenolyl)borate anions, and their chelates with first row transition metals have been synthesised. The divalent and trivalent metal ions form complexes in 12 and 13 (metal:ligand) ratios respectively. The number of ligands coordinated correspond to the number of anions replaced in the metal salis. The compounds were characterized by elemental analysis, i.r. spectra, magnetic susceptibility measurements and electronic spectral studies. The Cr^III and Fe^III complexes of dihydridobis- and hydridotris-(thiophenolyl)borates appear to be octahedral, and those of Cu^II are proposed to be square planar. Tetrahedral geometry is suggested for the Mn^II, Co^II and Ni^II complexes. The tetrakis-(thiophenolyl)borate yielded octahedral complexes with all the metal ions except for Cu^II which is square planar. The ligand field parameters 10Dq, B and have also been calculated wherever possible. The ligands may be placed in the vicinity of EDTA in the nephelauxetic series.     

Ytterbium(III) Porpholactones: β‐Lactonization of Porphyrin Ligands Enhances Sensitization Efficiency of Lanthanide Near‐Infrared Luminescence

The near‐infrared (NIR) luminescence efficiency of lanthanide complexes is largely dependent on the electronic and photophysical properties of antenna ligands. Although porphyrin ligands are efficient sensitizers of lanthanide NIR luminescence, non‐pyrrolic porphyrin analogues, which have unusual symmetry and electronic states, have been much less studied. In this work, we used porpholactones, a class of β‐pyrrolic‐modified porphyrins, as ligands and investigated the photophysical properties of lanthanide porpholactones Yb‐1 a – 5 a . Compared with Yb porphyrin complexes, the porpholactone complexes displayed remarkable enhancement of NIR emission (50–120 %). Estimating the triplet‐state levels of porphyrin and porpholactone in Gd complexes revealed that β‐lactonization of porphyrinic ligands lowers the ligand T₁ state and results in a narrow energy gap between this state and the lowest excited state of Yb³⁺. Transient absorption spectra showed that Yb^III porpholactone has a longer transient decay lifetime at the Soret band than the porphyrin analogue (30.8 versus 17.0 μs). Thus, the narrower energy gap and longer lifetime arising from β‐lactonization are assumed to enhance NIR emission of Yb porpholactones. To demonstrate the potential applications of Yb porpholactone, a water‐soluble Yb bioprobe was constructed by conjugating glucose to Yb ‐ 1 a . Interestingly, the NIR emission of this Yb porpholactone could be specifically switched on in the presence of glucose oxidase and then switched off by addition of glucose. This is the first demonstration that non‐pyrrolic porphyrin ligands enhance the sensitization efficiency of lanthanide luminescence and also display switchable NIR emission in the region of biological analytes (800–1400 nm).     

Photooxidizing Chromium Catalysts for Promoting Radical Cation Cycloadditions

The photooxidizing capabilities of selected Cr^III complexes for promoting radical cation cycloadditions are described. These complexes have sufficiently long‐lived excited states to oxidize electron‐rich alkenes, thereby initiating [4+2] processes. These metal species augment the spectrum of catalysts explored in photoredox systems, as they feature unique properties that can result in differential reactivity from the more commonly employed ruthenium or iridium catalysts.     

Ligational behaviour of biacetylmonoxime nicotinoyl hydrazone (H2BMNH) towards transition metal ions

Summary The synthesis and characterization of Mn^II, Co^II, Ni^II, Cu^II, Zn^II, Cd^II UO ₂ ²⁺ , Cr^III and Fe^III complexes of biacetylmonoxime nicotinoyl hydrazone (H₂BMNH) are reported. Elemental analysis, molar conductance, magnetic moment and spectral (i.r., visible and n.m.r.) measurements have been used to characterize the complexes. I.r. spectral data show that the ligand behaves in a bidentate and/or tridentate manner. An octahedral structure is proposed for the Mn^II, Ni^II, Cr^III and Fe^III complexes, while a square-planar structure is proposed for both Co^II and Cu^II complexes on the basis of magnetic and spectral measurements.     

Luminescence in colloidal Mn-doped semiconductor nanocrystals

Recent advances in nanocrystal doping chemistries have substantially broadened the variety of photophysical properties that can be observed in colloidal Mn²⁺-doped semiconductor nanocrystals. A brief overview is provided, focusing on Mn²⁺-doped II–VI semiconductor nanocrystals prepared by direct chemical synthesis and capped with coordinating surface ligands. These Mn²⁺-doped semiconductor nanocrystals are organized into three major groups according to the location of various Mn²⁺-related excited states relative to the energy gap of the host semiconductor nanocrystals. The positioning of these excited states gives rise to three distinct relaxation scenarios following photoexcitation. A brief outlook on future research directions is provided.     

Characterization and chemical reactivity of room-temperature-stable MnIII–alkylperoxo complexes

While alkylperoxomanganese(iii) (Mn^III–OOR) intermediates are proposed in the catalytic cycles of several manganese-dependent enzymes, their characterization has proven to be a challenge due to their inherent thermal instability. Fundamental understanding of the structural and electronic properties of these important intermediates is limited to a series of complexes with thiolate-containing N₄S⁻ ligands. These well-characterized complexes are metastable yet unreactive in the direct oxidation of organic substrates. Because the stability and reactivity of Mn^III–OOR complexes are likely to be highly dependent on their local coordination environment, we have generated two new Mn^III–OOR complexes using a new amide-containing N₅⁻ ligand. Using the 2-(bis((6-methylpyridin-2-yl)methyl)amino)-N-(quinolin-8-yl)acetamide (H^6Medpaq) ligand, we generated the [Mn^III(OO^tBu)(^6Medpaq)]OTf and [Mn^III(OOCm)(^6Medpaq)]OTf complexes through reaction of their Mn^II or Mn^III precursors with ^tBuOOH and CmOOH, respectively. Both of the new Mn^III–OOR complexes are stable at room-temperature (t_1/2 = 5 and 8 days, respectively, at 298 K in CH₃CN) and capable of reacting directly with phosphine substrates. The stability of these Mn^III–OOR adducts render them amenable for detailed characterization, including by X-ray crystallography for [Mn^III(OOCm)(^6Medpaq)]OTf. Thermal decomposition studies support a decay pathway of the Mn^III–OOR complexes by O–O bond homolysis. In contrast, direct reaction of [Mn^III(OOCm)(^6Medpaq)]⁺ with PPh₃ provided evidence of heterolytic cleavage of the O–O bond. These studies reveal that both the stability and chemical reactivity of Mn^III–OOR complexes can be tuned by the local coordination sphere.

A pair of room-temperature-stable Mn^III–alkylperoxo complexes were characterized and shown to oxidize PPh₃. Thermal decomposition studies provide evidence of both homolysis and heterolysis of the Mn^III–alkylperoxo O–O bond.