Controlling the Photophysical Properties of a Series of Isostructural d Complexes Based on Cr, Mn, and Fe.

Development of first-row transition metal complexes with similar luminescence and photoredox properties as widely used Ru polypyridines is attractive because metals from the first transition series are comparatively abundant and inexpensive. The weaker ligand field experienced by the valence d-electrons of first-row transition metals challenges the installation of the same types of metal-to-ligand charge transfer (MLCT) excited states as in precious metal complexes, due to rapid population of energetically lower-lying metal-centered (MC) states. In a family of isostructural tris(diisocyanide) complexes of the 3d metals Cr, Mn, and Fe, the increasing effective nuclear charge and ligand field strength allow us to control the energetic order between the MLCT and MC states, whereas pyrene decoration of the isocyanide ligand framework provides control over intraligand (IL) states.

First-Row d 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 3d metal photosensitizer yielding an upconversion performance competitive with precious metal-based analogues.

Photoredox-active Cr(0) luminophores featuring photophysical properties competitive with Ru(II) and Os(II) complexes.

Coordination complexes of precious metals with the d valence electron configuration such as Ru(II), Os(II) and Ir(III) are used for lighting applications, solar energy conversion and photocatalysis. Until now, d complexes made from abundant first-row transition metals with competitive photophysical and photochemical properties have been elusive. While previous research efforts focused mostly on Fe(II), we disclose that isoelectronic Cr(0) gives access to higher photoluminescence quantum yields and excited-state lifetimes when compared with any other first-row d metal complex reported so far.

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 d valence electron configurations due to their close chemical relationship. Using a rigid tridentate chelate ligand (L), 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)](PF), featuring a photoactive excited state with substantial MLCT character.

Magnetic-Field Universality of the Kondo Effect Revealed by Thermocurrent Spectroscopy.

Probing the universal low-temperature magnetic-field scaling of Kondo-correlated quantum dots via electrical conductance has proved to be experimentally challenging. Here, we show how to probe this in nonlinear thermocurrent spectroscopy applied to a molecular quantum dot in the Kondo regime. Our results demonstrate that the bias-dependent thermocurrent is a sensitive probe of universal Kondo physics, directly measures the splitting of the Kondo resonance in a magnetic field, and opens up possibilities for investigating nanosystems far from thermal and electrical equilibrium.

The surprising effects of sulfur: achieving long excited-state lifetimes in heteroleptic copper(i) emitters.

A series of heteroleptic [Cu(N^N)(P^P)][PF] complexes is reported in which N^N is a di(methylsulfanyl)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeS)phen) or di(methoxy)-1,10-phenanthroline (2,9-, 3,8- or 4,7-(MeO)phen) and P^P is bis(2-(diphenylphosphano)phenyl)ether (POP) or 4,5-bis(diphenylphosphano)-9,9-dimethylxanthene (xantphos). The effects of the different substituents are investigated through structural, electrochemical and photophysical studies and by using DFT and TD-DFT calculations. Introducing methylsulfanyl groups in the 2,9-, 3,8- or 4,7-positions of the phen domain alters the composition of the frontier molecular orbitals of the [Cu(N^N)(P^P)] complexes significantly, so that ligand-centred (LC) transitions become photophysically relevant with respect to metal-to-ligand charge transfer (MLCT).

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 3d 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.

Luminescent First-Row Transition Metal Complexes.

Precious and rare elements have traditionally dominated inorganic photophysics and photochemistry, but now we are witnessing a paradigm shift toward cheaper and more abundant metals. Even though emissive complexes based on selected first-row transition metals have long been known, recent conceptual breakthroughs revealed that a much broader range of elements in different oxidation states are useable for this purpose. Coordination compounds of V, Cr, Mn, Fe, Co, Ni, and Cu now show electronically excited states with unexpected reactivity and photoluminescence behavior.

Controlling the Entropy of a Single-Molecule Junction.

Single molecules are nanoscale thermodynamic systems with few degrees of freedom. Thus, the knowledge of their entropy can reveal the presence of microscopic electron transfer dynamics that are difficult to observe otherwise. Here, we apply thermocurrent spectroscopy to directly measure the entropy of a single free radical molecule in a magnetic field.

Pyrene-Decoration of a Chromium(0) Tris(diisocyanide) Enhances Excited State Delocalization: A Strategy to Improve the Photoluminescence of 3d Metal Complexes.

There is a long-standing interest in iron(II) complexes that emit from metal-to-ligand charge transfer (MLCT) excited states, analogous to ruthenium(II) polypyridines. The 3d electrons of iron(II) are exposed to a relatively weak ligand field, rendering nonradiative relaxation of MLCT states via metal-centered excited states undesirably efficient. For isoelectronic chromium(0), chelating diisocyanide ligands recently provided access to very weak MLCT emission in solution at room temperature.

Cooperative Co-Activation of Water and Hypochlorite by a Non-Heme Diiron(III) Complex.

Aqueous solutions of the iron(III) complex of ,,'-tris(2-pyridylmethyl)ethylenediamine-'-acetate (tpena) react with hypochlorite (ClO) to produce the reactive high-valent [Fe(O)(tpena)]. Under catalytic conditions, in bicarbonate-buffered media (pH 8) with a set ionic strength (10 mM NaCl), kinetic analysis shows that two equivalents of [Fe(O)(tpena)] per one ClO are produced, with benign chloride ions the only byproduct. An unprecedented supramolecular activation of ClO by {(HCO)⊂[(tpena)Fe(μ-O)Fe(Htpena)]} is proposed.

Engineering the Oxidative Potency of Non-Heme Iron(IV) Oxo Complexes in Water for C-H Oxidation by a Donor and Variation of the Second Coordination Sphere.

A series of iron(IV) oxo complexes, which differ in the donor (CHpy or CHCOO) to the oxo group, three with hemilabile pendant donor/second coordination sphere base/acid arms (pyH/py or ROH), have been prepared in water at pH 2 and 7. The ν values of 832 ± 2 cm indicate similar Fe═O bond strengths; however, different reactivities toward C-H substrates in water are observed. HAT occurs at rates that differ by 1 order of magnitude with nonclassical KIEs ( = 30-66) consistent with hydrogen atom tunneling.

Photosynthesis of a Dihydroimidazopyridine Chelate Shines Light on the Reactions of a Photoactivated Iron(III) Complex with O.

The high-spin ( = /) meridional diastereoisomer of [Fe(tpena)] (tpena = -tris(2-pyridylmethyl)ethylendiamine--acetate), [Fe(tpena)], undergoes photolytic CO release to produce an iron(II) intermediate of a radical dihydroimidazopyridine ligand (). The structure of this unprecedented transient iron(II)() complex is supported by UV-vis and Mössbauer spectroscopies, DFT calculations, as well as the X-ray structural characterization of an μ-oxo iron(III) complex of the oxidized derivative of , namely, [FeO(Cl)()](ClO)(MeCN) ( = 2-(2-(bis(pyridin-2-ylmethyl)amino)ethyl)-2,3-dihydro-1-imidazo[1,5-]pyridin-4-ium). [FeO(Cl)()] is obtained only in the absence of O.

Donor Influence on O-O Bond Lability in Iron(III) Hydroperoxo Complexes: Oxidation Catalysis and Ligand Transformation.

The Fe/Fe redox potentials for [Fe(tpen)], [Fe(tpena)], and [Fe(tpenO)] (-R-,''-tris(2-pyridylmethyl)ethane-1,2-diamine, where R = CHCHN, CHCOO, CHCHO, respectively) span 470 mV with the oxidation potentials following the order [Fe(tpenO)] (MeOH) < [Fe(tpena)] (MeCN) < [Fe(tpen)] (MeCN). In their +3 oxidation states the complexes react with 1 equiv of HO to give the purple [Fe(OOH)(HL)] ( = 2 for L = tpena, tpenO; = 3 for L = tpen). A pyridine arm is decoordinated in these complexes, furnishing a second coordination sphere base which is protonated at ambient pH.

Photoinduced O-Dependent Stepwise Oxidative Deglycination of a Nonheme Iron(III) Complex.

The iron(III) complex [Fe(tpena)] (tpena = N, N, N'-tris(2-pyridylmethyl)ethylendiamine- N'-acetate) undergoes irreversible O-dependent N-demethylcarboxylation to afford [Fe(SBPy3)(MeCN)] (SBPy3 = N, N-bis(2-pyridylmethyl)amine- N-ethyl-2-pyridine-2-aldimine), when irradiated with near-UV light. The loss of a mass equivalent to the glycyl group in a process involving consecutive C-C and C-N cleavages is documented by the measurement of the sequential production of CO and formaldehyde, respectively. Time-resolved UV-vis absorption, Mössbauer, EPR, and Raman spectroscopy have allowed the spectroscopic characterization of two iron-based intermediates along the pathway.

Novel conformationally constrained 2'-C-methylribonucleosides: synthesis and incorporation into oligonucleotides.

Synthesis of two novel conformationally constrained bicyclic ribonucleoside phosphoramidites bearing a 2'-C-methyl substituent has been accomplished. These phosphoramidites were used to incorporate the corresponding 2'-C-methyl nucleotides into oligonucleotides and to study their effects on duplex thermal stability. Whereas the C2'-O4'-linked LNA-type derivative induced severe destabilization of duplexes formed with complementary DNA and RNA, the C3'-O4'-linked derivative induced RNA-selective hybridization with increased affinity relative to that of the unmodified DNA-based probe.

Directing a Non-Heme Iron(III)-Hydroperoxide Species on a Trifurcated Reactivity Pathway.

The reactivity of [Fe (tpena)] (tpena=N,N,N'-tris(2-pyridylmethyl)ethylenediamine-N'-acetate) as a catalyst for oxidation reactions depends on its ratio to the terminal oxidant H O and presence or absence of sacrificial substrates. The outcome can be switched between: 1) catalysed H O disproportionation, 2) selective catalytic oxidation of methanol or benzyl alcohol to the corresponding aldehyde, or 3) oxidative decomposition of the tpena ligand. A common mechanism is proposed involving homolytic O-O cleavage in the detected transient purple low-spin (S=1/2 ) [(tpenaH)Fe O-OH] .

Noncovalent Halogen Bonding as a Mechanism for Gas-Phase Clustering.

Gas-phase clustering of nonionizable iodylbenzene (PhIO) is attributed to supramolecular halogen bonding. Electrospray ionization results in the formation of ions of proton-charged and preferably sodium-charged clusters assignable to [H(PhIO) ], n = 1-7; [Na(PhIO) ], n = 1-6; [Na(PhIO) ], n = 7-20; [HNa(PhIO) ], n = 6-19; [HNa(PhIO) ], n = 15-30; and [Na(PhIO) ], n = 14-30. The largest cluster detected has a supramolecular mass of 7147 Da.

Reduction of hypervalent iodine by coordination to iron(iii) and the crystal structures of PhIO and PhIO.

The iodine L-edge X-ray Absorption Near Edge Structure (XANES) of organic and inorganic iodine compounds with formal iodine oxidation states ranging from -1 to +7 shows edge energies spanning from 4560.8 eV to 4572.5 eV.

A coordinatively flexible hexadentate ligand gives structurally isomeric complexes M2(L)X3 (M = Cu, Zn; X = Br, Cl).

Polypyridyl multidentate ligands based on ethylenediamine backbones are important metal-binding agents with applications in biomimetics and homogeneous catalysis. The seemingly hexadentate tpena ligand [systematic name: N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetate] reacts with zinc chloride and zinc bromide to form trichlorido[μ-N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetato]dizinc(II), [Zn2(C22H24N5O2)Cl3], and tribromido[μ-N,N,N'-tris(pyridin-2-ylmethyl)ethylenediamine-N'-acetato]dizinc(II), [Zn2Br3(C22H24N5O2)]. One Zn(II) ion shows the anticipated N5O coordination in an irregular six-coordinate site and is linked by an anti carboxylate bridge to a tetrahedral ZnX3 (X = Cl or Br) unit.

Halogen-Bonding-Assisted Iodosylbenzene Activation by a Homogenous Iron Catalyst.

The iron(III) complex of hexadentate N,N,N'-tris(2-pyridylmethyl)ethylendiamine-N'-acetate (tpena(-) ) is a more effective homogenous catalyst for selective sulfoxidation and epoxidation with insoluble iodosylbenzene, [PhIO]n , compared with soluble methyl-morpholine-N-oxide (NMO). We propose that two molecules of [Fe(tpena)](2+) cooperate to solubilize PhIO, extracting two equivalents to form the halogen-bonded dimeric {[Fe(tpena)OIPh]2}(4+). The closest intradimeric I⋅⋅⋅O distance, 2.