Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex.

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

A Nationwide, Prospective Study of Tracheal Intubation in Critically Ill Adults in Spain: Management, Associated Complications, and Outcomes.

OBJECTIVES: Our aims were to explore current intubation practices in Spanish ICUs to determine the incidence and risk factors of peri-intubation complications (primary outcome measure: major adverse events), the rate and factors associated with first-pass success, and their impact on mortality as well as the changes of the intubation procedure observed in the COVID-19 pandemic. DESIGN: Prospective, observational, and cohort study. SETTING: Forty-three Spanish ICU. PATIENTS: A total of 1837 critically ill adult patients undergoing tracheal intubation. The enrollment period was six months (selected by each center from April 16, 2019, to October 31, 2020). INTERVENTIONS: None. MEASUREMENT AND MAIN RESULTS: At least one major adverse peri-intubation event occurred in 40.4 % of the patients (973 major adverse events were registered) the most frequent being hemodynamic instability (26.5%) and severe hypoxemia (20.3%). The multivariate analysis identified seven variables independently associated with a major adverse event whereas the use of neuromuscular blocking agents (NMBAs) was associated with reduced odds of major adverse events. Intubation on the first attempt was achieved in 70.8% of the patients. The use of videolaryngoscopy at the first attempt was the only protective factor (odds ratio 0.43; 95% CI, 0.28-0.66; p < 0.001) for first-attempt intubation failure. During the COVID-19 pandemic, the use of videolaryngoscopy and NMBAs increased significantly. The occurrence of a major peri-intubation event was an independent risk factor for 28-day mortality. Cardiovascular collapse also posed a serious threat, constituting an independent predictor of death. CONCLUSIONS: A major adverse event occurred in up to 40% of the adults intubated in the ICU. Peri-intubation hemodynamic instability but not severe hypoxemia was identified as an independent predictor of death. The use of NMBAs was a protective factor for major adverse events, whereas the use of videolaringoscopy increases the first-pass success rate of intubation. Intubation practices changed during the COVID-19 pandemic.

Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)-Er(III) Assembly.

Pseudo-octahedral CrIIIN6 chromophores hold a unique appeal for low-energy sensitization of NIR lanthanide luminescence due to their exceptionally long-lived spin-flip excited states. This allure persists despite the obstacles and complexities involved in integrating both elements into a metallosupramolecular assembly. In this work, we have designed a structurally optimized heteroleptic CrIII building block capable of binding rare earths. Following a complex-as-ligand synthetic strategy, two heterometallic supramolecular assemblies, in which three peripherical CrIII sensitizers coordinated through a molecular wire to a central ErIII or YIII, have been prepared. Upon excitation of the CrIII spin-flip states, the downshifted Er(4I13/2 → 4I15/2) emission at 1550 nm was induced through intramolecular energy transfer. Time-resolved experiments at room temperature reveal a CrIII → ErIII energy transfer of 62-73% efficiencies with rate constants of about 8.5 × 105 s-1 despite the long donor-acceptor distance (circa 14 Å). This efficient directional intermetallic energy transfer can be rationalized using the Dexter formalism, which is promoted by a rigid linear electron-rich alkyne bridge that acts as a molecular wire connecting the CrIII and ErIII ions.

Reductive Photocatalytic Proton-Coupled Electron Transfer by a Zirconium-based Molecular Platform.

Reductive proton-coupled electron transfer (PCET) has important energetic implications in numerous synthetic and natural redox processes. The development of catalytic systems that can mediate such transformations has become an attractive target, especially when light is used to generate the reactive species towards solar-to-chemicals conversion. However, such approach becomes challenged by kinetic competition with H2 evolution. Here we describe the excited state reactivity of a molecular Zr-based platform under visible light irradiation for the efficient reduction of multiple bonds. Mechanistic investigations shine light on a charge separation process that colocalizes an excited electron and an acidic proton to promote selective PCET. We further leveraged this reactivity for the photocatalytic reduction of a variety of organic substrates. Our results demonstrate the promise of this molecular platform to design strong photocatalytic PCET mediators for reductive transformations. More broadly, we also show the potential relevance of PCET mechanisms in the (photo)redox chemistry of Zr-based molecular materials.

Enlightening the Alkali Ion Role in the Photomagnetic Effect of FeCo Prussian Blue Analogues.

FeCo Prussian blue analogues of general formula AxCoy[Fe(CN)6]z are responsive, non-stoichiometric materials whose magnetic and optical properties can be reversibly switched by light irradiation. However, elucidating the critical influence of the inserted alkali ion, A+, on the material's properties remains complicated due to their complex local structure. Here, by investigating soluble A ⊂ [Fe4-Co4] cyanido cubes (A = K, Rb, and Cs), both accurate structural and electronic information could be obtained. First, X-ray diffraction analyses reveal distinct interactions between the inserted A+ ions and the {Fe4-Co4} box, which impacts the structural distortion in the cubic framework. These distortions vanish, and a displacement of the small K+ ion from a corner toward the center is observed, as a cobalt corner CoIIHS is oxidized to CoIIILS. Second, cyclic voltammetry experiments performed at variable temperatures show distinct splitting of the CoIIHS ⇔ CoIIILS peak potentials for the different A+ cations, which can be qualitatively linked to different thermodynamic (standard potentials) and kinetic (energy barriers) parameters associated with the structural reorganization accompanying this redox-coupled spin state change. Moreover, for the first time, photomagnetism was investigated in frozen solution to avoid effects of intermolecular interactions. The results show that the metastable state is stabilized following the trend K > Rb > Cs. The outcome of these studies suggests that the interaction of the inserted alkali ions with the cyanide cage and the structural changes accompanying the electron transfer impact the stability of the photoinduced state and the relaxation temperature: the smaller the cation, the higher the structural reorganization and the associated energy barrier, and the more stable the metastable state.

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 CrIII 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 CrIII 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 CrIII 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.

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

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

Bright Long-Lived Circularly Polarized Luminescence in Chiral Chromium(III) Complexes.

A series of highly emissive inert and chiral CrIII complexes displaying dual circularly polarized luminescence (CPL) within the NIR region have been prepared and characterized. The helical [Cr(dqpR)2 ]3+ (dqp=2,6-di(quinolin-8-yl)pyridine; R=OCH3 , Br or C≡CH) complexes were synthesized as racemic mixtures and resolved into their respective PP and MM enantiomers by chiral stationary phase HPLC. The corresponding enantiomers show large glum ≈0.2 and high quantum yield of up to 17 %, which afford important CPL brightness of up to 170 m-1 cm-1 , a key point for applications as chiral luminescent probes. Moreover, the long-lived CP-NIR emission provided by these chromophores (ms range) in aqueous solution opens the way toward the quantification of chiral targets in biological systems with time-gated detection. Thus, such chiral chromophores based on earth abundant and inert 3d metals open new perspectives in the field of CPL and represent an alternative to precious 4d, 5d and to labile 4f metal-based complexes.

Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures.

The CrIIIN6 chromophores are particularly appealing for low-energy sensitization via energy transfer processes since they show extremely long excited state lifetimes reaching the millisecond range in the technologically crucial near-infrared domain. However, their properties were barely harnessed in multimetallic structures because of the lack of both monitoring methods and accessible synthetic pathways. We herein report a remedy to monitor and control the formation of CrIII-containing assemblies in solution via the design of a CrIIIN6 inert "complex-as-ligand" that can be included into polymetallic architectures. As a proof of concept, these CrN6 building blocks were reacted in solution with ZnII or FeII to give extended trinuclear linear Cr-M-Cr assemblies, the structure of which could be addressed by NMR spectroscopy despite the presence of two slowly relaxing CrIII paramagnetic centers. In addition to long CrIII excited state lifetimes and weak sensitivity to oxygen quenching, these polymetallic assemblies display controlled CrIII to MII energy transfers, which pave the way for use of the "complex-as-ligand" strategy for introducing photophysically active CrIII probes into light-converting polymetallic devices.

Electron Transfer in the Cs⊂{Mn4 Fe4 } Cubic Switch: A Soluble Molecular Model of the MnFe Prussian-Blue Analogues.

A mixed-valence {MnII 3 MnIII FeII 2 FeIII 2 } cyanide-bridged molecular cube hosting a caesium cation, Cs⊂{Mn4 Fe4 }, was synthesized and structurally characterized by X-ray diffraction. Cyclic-voltammetry measurements show that its electronic state can be switched between five different redox states, which results in a remarkable electrochromic effect. Magnetic measurements on fresh samples point to the occurrence of a spin-state change near room temperature, which could be ascribed to a metal-to-metal electron transfer converting the {FeII -CN-MnIII } pair into a {FeIII -CN-MnII } pair. This feature was only previously observed in the polymeric MnFe Prussian-blue analogues (PBAs). Moreover, this novel switchable molecule proved to be soluble and stable in organic solvents, paving the way for its integration into advanced materials.

Chiral Molecular Ruby [Cr(dqp)2]3+ with Long-Lived Circularly Polarized Luminescence.

The chiral resolution of a kinetically inert molecular ruby [Cr(dqp)2]3+ (1, dqp = 2,6-di(quinolin-8-yl)pyridine) displaying strong dual light emission at room temperature has been achieved. The wrapped arrangement of the six-membered dqp chelating ligands around the Cr(III) provided nonplanar helical conformations leading to the diastereoselective assembly of chiral bis-tridentate monometallic Cr(III)-helix. The PP-(+)-[Cr(dqp)2]3+ and MM-(-)-[Cr(dqp)2]3+ enantiomers could be separated and isolated by using cation-exchange chromatography and subsequent salt-metathesis with KPF6. X-ray crystallographic analysis based on Flack parameters assigned the absolute configurations of the two enantiomers. Circularly polarized luminescence (CPL) spectra showed two polarized emission bands within the NIR region corresponding to the characteristic metal-centered spin-flip Cr(2E → 4A2) and Cr(2T1 → 4A2) transitions with exceptionally high dissymmetry factors, |glum|, of 0.2 and 0.1, respectively, which are comparable to those reported for rare-earth chiral complexes. Photophysical properties also revealed an extremely long excited-state lifetime of 1.2 ms and a high quantum yield of 5.2% at room temperature in water. These properties make [Cr(dqp)2]3+ an ideal sensitizer for the preparation of enantiopure luminescent supramolecular energy-converting devices and also open up the possibility of using chiral Cr(III) chromophores for the construction of NIR-CPL materials and polarized photonic devices based on earth-abundant metals.

Lanthanide Tetrazolate Complexes Combining Single-Molecule Magnet and Luminescence Properties: The Effect of the Replacement of Tetrazolate N3 by ß-Diketonate Ligands on the Anisotropy Energy Barrier.

Three new sets of mononuclear Ln(III) complexes of general formulas [LnL3 ]⋅CH3 OH [Ln(III) =Yb (1), Er (2), Dy (3), Gd (4), and Eu (5)], [LnL2 (tmh)(CH3 OH)]⋅n H2 O⋅m CH3 OH [Ln(III) =Yb (1 b), Er (2 b), Dy (3 b), Gd (4 b)], and [LnL2 (tta)(CH3 OH)]⋅CH3 OH [Ln(III) =Yb (1 c), Er (2 c), Dy (3 c), Gd (4 c)] were prepared by the reaction of Ln(CF3 SO3 )⋅n H2 O salts with the tridentate ligand 2-(tetrazol-5-yl)-1,10-phenanthroline (HL) and, for the last two sets, additionally with the ß-diketonate ligands 2,2,6,6-tetramethylheptanoate (tmh) and 2-thenoyltrifluoroacetonate (tta), respectively. In the [LnL3 ]⋅CH3 OH complexes the Ln(III) ions are coordinated to three phenanthroline tetrazolate ligands with an LnN9 coordination sphere. Dynamic ac magnetic measurements on 1-3 reveal that these complexes only exhibit single-molecule magnet (SMM) behavior when an external dc magnetic field is applied, with Ueff values of 11.7 K (1), 16.0 K (2), and 20.2 K (3). When the tridentate phenanthroline tetrazolate ligand is replaced by one molecule of methanol and the ß-diketonate ligand tmh (1 b-3 b) or tta (1 c-3 c), a significant increase in Ueff occurs and, in the case of the Dy(III) complexes 3 b and 3 c, out-of-phase χ'' signals below 15 and 10 K, respectively, are observed in zero dc magnetic field. CASSCF+RASSI ab initio calculations performed on the Dy(III) complexes support the experimental results. Thus, for 3 the ground Kramers' doublet is far from being axial and the first excited state is found to be very close in energy to the ground state, so the relaxation barrier in this case is almost negligible. Conversely, for 3 b and 3 c, the ground Kramers' doublet is axial with a small quantum tunneling of the magnetization, and the energy difference between the ground and first Kramers' doublets is much higher, which allows these compounds to behave as SMMs at zero field. Moreover, these calculations support the larger Ueff observed for 3 b compared to 3 c. Additionally, the solid-state photophysical properties of 1, 2, 4, and 5 show that the phenanthroline tetrazolate ligand can act as an effective antenna to sensitize the characteristic Yb(III) , Er(III) , and Eu(III) emissions through an energy-transfer process.

Tuning the circularly polarized luminescence in homoleptic and heteroleptic chiral CrIII complexes.

A series of highly emissive inert and chiral CrIII complexes displaying positive and negative circularly polarized luminescence (CPL) within the near-infrared (NIR) region at room temperature have been prepared and characterized to decipher the effect of ligand substitution on the photophysical properties, more specifically on the chiroptical properties. The helical homoleptic [Cr(dqp-R)2]3+ (dqp = 2,6-di(quinolin-8-yl)pyridine; R = Ph, ≡-Ph, DMA, ≡-DMA (DMA = N,N-dimethylaniline)) and heteroleptic [Cr(dqp)(L)]3+ (L = 4-methoxy-2,6-di(quinolin-8-yl)pyridine (dqp-OMe) or L = N 2,N 6-dimethyl-N 2,N 6-di(pyridin-2-yl)pyridine-2,6-diamine (ddpd)) molecular rubies were synthesized as racemic mixtures and then resolved and isolated into their respective pure PP and MM enantiomeric forms by chiral stationary phase HPLC. The corresponding enantiomers show two opposite polarized emission bands within the 700-780 nm range corresponding to the characteristic metal-centered Cr(2E'→4A2) and Cr(2T1 '→4A2) transitions with large g lum ranging from 0.14 to 0.20 for the former transition. In summary, this study reports the rational use of different ligands on CrIII and their effect on the chiroptical properties of the complexes.

Magnetic circularly polarized luminescence from spin-flip transitions in a molecular ruby.

Magnetic circularly polarized luminescence (MCPL), i.e. the possibility of generating circularly polarized luminescence in the presence of a magnetic field in achiral or racemic compounds, is a technique of rising interest. Here we show that the far-red spin-flip (SF) transitions of a molecular Cr(iii) complex give intense MCD (magnetic circular dichroism) and in particular MCPL (g MCPL up to 6.3 × 10-3 T-1) even at magnetic fields as low as 0.4 T. Cr(iii) doublet states and SF emission are nowadays the object of many investigations, as they may open the way to several applications. Due to their nature, such transitions can be conveniently addressed by MCPL, which strongly depends on the zero field splitting and Zeeman splitting of the involved states. Despite the complexity of the nature of such states and the related photophysics, the obtained MCPL data can be rationalized consistently with the information recovered with more established techniques, such as HFEPR (high-frequency and -field electron paramagnetic resonance). We anticipate that emissive molecular Cr(iii) species may be useful in magneto-optical devices, such as magnetic CP-OLEDs.

Taming 2,2'-biimidazole ligands in trivalent chromium complexes.

Complete or partial replacement of well-known five-membered chelating 2,2'-bipyridine (bipy) or 1,10-phenanthroline (phen) ligands with analogous didentate 2,2'-biimidazole (H2biim) provides novel perspectives for exploiting the latter pH-tuneable bridging unit for connecting inert trivalent chromium with cationic partners. The most simple homoleptic complex [Cr(H2biim)3]3+ and its stepwise deprotonated analogues are only poorly soluble in most solvents and their characterization is limited to some solid-state structures, in which the pseudo-octahedral [CrN6] units are found to be intermolecularly connected via peripheral N-H⋯X hydrogen bonds. Moreover, the associated high-energy stretching N-H vibrations drastically quench the targeted near infrared (NIR) CrIII-based phosphorescence, which makes these homoleptic building blocks incompatible with the design of molecular-based luminescent assemblies. Restricting the number of bound 2,2'-biimidazole ligands to a single unit in the challenging heteroleptic [Cr(phen)2(Hxbiim)](1+x)+ (x = 2-0) complexes overcomes the latter limitations and allows (i) the synthesis and characterization of these [CrN6] chromophores in the solid state and in solution, (ii) the stepwise and controlled deprotonation of the bound 2,2'-biimidazole ligand and (iii) the implementation of Cr-centered phosphorescence with energies, lifetimes and quantum yields adapted for using the latter chromophores as sensitizers in promising 'complex-as-ligand' strategies.

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 CrIII. Room-temperature ligand-centered UV-excitation at 18 170 cm-1 (λexc = 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 ≤ λem ≤ 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.

Molecular light-upconversion: we have had a problem! When excited state absorption (ESA) overcomes energy transfer upconversion (ETU) in Cr(III)/Er(III) complexes.

Nine-coordinate [ErN9] or [ErN3O6] chromophores found in triple helical [Er(L)3]3+ complexes (L corresponds to 2,2',6',2''-terpyridine (tpy), 2,6-(bisbenzimidazol-2-yl)pyridine (bzimpy), 2,6-diethylcarboxypyridine (dpa-ester) or 2,6-diethylcarboxamidopyridine (dpa-diamide) derivatives), [Er(dpa)3]3- (dpa is the 2,6-dipicolinate dianion) and [GaErGa(bpb-bzimpy)3]9+ (bpb-bzimpy is 2,6-bis((pyridin-2-benzimidazol-5-yl)methyl-(benzimidazol-2-yl))pyridine) exhibit NIR (excitation at 801 nm) into visible (emission at 542 nm) linear light upconversion processes in acetonitrile at room temperature. The associated quantum yields 5.5(6) × 10-11 ≤ φuptot(ESA) ≤ 1.7(2) × 10-9 appear to be 1-3 orders of magnitude larger than those predicted by the accepted single-center excited-state absorption mechanism (ESA). Switching to the alternative energy transfer upconversion mechanism (ETU), which operates in multi-centers [CrErCr(bpb-bzimpy)3]9+, leads to an improved quantum yield of φuptot(ETU) = 5.8(6) × 10-8, but also to an even larger discrepancy by 4-6 orders of magnitude when compared with theoretical models. All photophysical studies point to Er(4I13/2) as being the only available 'long-lived' (1.8 ≤ τ ≤ 6.3 µs) and emissive excited state, which works as an intermediate relay for absorbing the second photon, but with an unexpected large cross-section for an intrashell 4f → 4f electronic transition. With this in mind, the ETU mechanism, thought to optimize upconversion via intermetallic Cr → Er communication in [CrErCr(bpb-bzimpy)3]9+, is indeed not crucial and the boosted associated upconversion quantum yield is indebted to the dominant contribution of the single-center erbium ESA process. This curious phenomenon is responsible for the successful implementation of light upconversion in molecular coordination complexes under reasonable light power intensities, which paves the way for applications in medicine and biology. Its origin could be linked with the presence of metal-ligand bonding.

Monitoring spin-crossover phenomena via Re(I) luminescence in hybrid Fe(II) silica coated nanoparticles.

Bare (1) and silica coated (1@SiO2) spin crossover (SCO) nanoparticles based on the polymer {[Fe(NH2Trz)3](BF4)2}n have been prepared following a water-in-oil synthetic procedure. For 1, the critical temperatures of the spin transition are TC↓ = 214.6 K and TC↑ = 220.9 K. For 1@SiO2, the abruptness of the transition is enhanced and the critical temperatures are centred at room temperature (TC↓ = 292.1 K and TC↑ = 296.3 K). An inert Re(I) complex of formula [Re(phen)(CO)3(PETES)](PF6) (phen = 1, 10-phenanthroline; PETES = 2(4-pyridylethyl)triethoxysilane) (Re) was also synthesized yielding intense green emission centred at λem = 560 nm. The grafting of this complex on the silica shell of 1@SiO2 led to a bifunctional SCO-luminescence composite (1@SiO2/Re) whose luminescence properties were tuned by the spin state switching. Temperature-variable photophysical studies showed that luminescence and spin transition were synchronized through a radiative (trivial) energy transfer mechanism between the Re(I) and the Fe(II)-LS (LS, Low Spin) centres.

Field-induced single ion magnet behaviour of discrete and one-dimensional complexes containing [bis(1-methylimidazol-2-yl)ketone]-cobalt(II) building units.

We describe herein the first examples of six-coordinate CoII single-ion magnets (SIMs) based on the ß-diimine Mebik ligand [Mebik = bis(1-methylimidazol-2-yl)ketone]: two mononuclear [CoII(Rbik)2L2] complexes and one mixed-valence {CoIII2CoII}n chain of formulas [CoII(Mebik)(H2O)(dmso)(µ-NC)2CoIII2(µ-2,5-dpp)(CN)6]n·1.4nH2O (3) [L = NCS (1), NCSe (2) and 2,5-dpp = 2,5-bis(2-pyridyl)pyrazine (3)]. Two bidentate Mebik molecules plus two monodentate N-coordinated pseudohalide groups in cis positions build somewhat distorted octahedral surroundings around the high-spin cobalt(II) ions in 1 and 2. The diamagnetic [CoIII2(µ-2,5-dpp)(CN)8]2- metalloligand coordinates the paramagnetic [CoII(Mebik)(H2O)(dmso)]2+ complex cations in a bis-monodentate fashion to afford neutral zigzag heterobimetallic chains in 3. Ab initio calculations, and cryomagnetic dc (2.0-300 K) and ac (2.0-12 K) measurements as well as EPR spectroscopy for 1-3 show the existence of magnetically isolated high-spin cobalt(II) ions with D values of 59.84-89.90 (1), 66.32-93.90 (2) and 70.40-127.20 cm-1 (3) and field-induced slow relaxation of the magnetization, being thus new examples of SIMs with transversal magnetic anisotropy. The analysis of their relaxation dynamics reveals that the relaxation of the magnetization occurs by the Raman (with values of the n parameter covering the range 6.0-6.8) and direct spin-phonon processes.

Beyond Chiral Organic (p-Block) Chromophores for Circularly Polarized Luminescence: The Success of d-Block and f-Block Chiral Complexes.

Chiral molecules are essential for the development of advanced technological applications in spintronic and photonic. The best systems should produce large circularly polarized luminescence (CPL) as estimated by their dissymmetry factor (g lum), which can reach the maximum values of -2 ≤ g lum ≤ 2 when either pure right- or left-handed polarized light is emitted after standard excitation. For matching this requirement, theoretical considerations indicate that optical transitions with large magnetic and weak electric transition dipole moments represent the holy grail of CPL. Because of their detrimental strong and allowed electric dipole transitions, popular chiral emissive organic molecules display generally moderate dissymmetry factors (10-5 ≤ g lum ≤ 10-3). However, recent efforts in this field show that g lum can be significantly enhanced when the chiral organic activators are part of chiral supramolecular assemblies or of liquid crystalline materials. At the other extreme, chiral EuIII- and SmIII-based complexes, which possess intra-shell parity-forbidden electric but allowed magnetic dipole transitions, have yielded the largest dissymmetry factor reported so far with g lum ~ 1.38. Consequently, 4f-based metal complexes with strong CPL are currently the best candidates for potential technological applications. They however suffer from the need for highly pure samples and from considerable production costs. In this context, chiral earth-abundant and cheap d-block metal complexes benefit from a renewed interest according that their CPL signal can be optimized despite the larger covalency displayed by d-block cations compared with 4f-block analogs. This essay thus aims at providing a minimum overview of the theoretical aspects rationalizing circularly polarized luminescence and their exploitation for the design of chiral emissive metal complexes with strong CPL. Beyond the corroboration that f-f transitions are ideal candidates for generating large dissymmetry factors, a special attention is focused on the recent attempts to use chiral CrIII-based complexes that reach values of g lum up to 0.2. This could pave the way for replacing high-cost rare earths with cheap transition metals for CPL applications.