This work emphasizes the role of metal complexation for controlling and tuning static/dynamic π-stacked organizations responsible for the programmed light emission of the bichromophoric pyrene-terimine ligand L2 upon photoexcitation. Its...

Abstract Ruthenium(II) polypyridyl complexes exhibit rich excited state reactivity from both photogenerated 3MLCT and higher energy thermally populated ligand-field (LF) excited states. In order to maximize the utility of the different reactivity of these two states, the energy gap between them must be tuned without sacrificing the favorable optical and photophysical properties of the complex. In probing design strategies aimed at enhancing the photostability of RuII complexes for nondissociative photochemical applications, we herein explore the use of Chugaev-type dicarbene ligands as strong σ-donors. Using a complementary combination of X-ray crystallography, electrochemistry, and optical and X-ray spectroscopies, we demonstrate how Chugaev-type dicarbene ligands destabilize LF states of photoactive RuII complexes without sacrificing the favorable optical and photophysical properties of the complex, and compare with π-accepting methylisocyanide ligands. Despite these design strategies aimed at suppressing dissociative excited-state ligand-field pathways, we further report an unexpected dechelative photoelimination of hydrazine from the RuII Chugaev-type dicarbene complexes that occurs directly from the 3MLCT excited state with quantum yields in the order of ca. 1–5%, and is sensitive to peripheral substitution of chromophoric bipyridine ligands. These findings contribute to growing understanding of molecular design principles for photoactive RuII complexes targeted at either dissociative/nondissociative photochemical applications and introduce a new potential scaffold and photorelease mechanism for photoactivated chemotherapy.

Chalcogen bonding (ChB) interactions involving group 16 elements are of growing interest due to their utility in supramolecular chemistry and catalysis. We present a detailed spectroscopic and theoretical investigation of ChB in a series of telluronium cation and Lewis base cocrystals, using 125Te and 31P solid-state NMR spectroscopy (SSNMR), single-crystal and powder X-ray diffraction, and Density Functional Theory (DFT)/Natural Localized Molecular Orbital (NLMO) calculations. The 125Te chemical shift tensor components correlate strongly with ChB strength and reveal a consistent increase in the delta 11 component and chemical shift anisotropy (CSA) span (Omega) with stronger interactions. This contrasts with prior observations in Te(II) and Se(II) systems, underscoring the unique behavior of Te(IV) cations with three sigma-holes. Octahedral distortion metrics (zeta, Delta, Sigma) are linked to CSA trends, supporting a model in which ChB-induced asymmetry drives tensor orientation. DFT calculations with spin-orbit relativistic corrections reproduce the experimental trend, while NLMO analysis attributes delta 11 shifts to specific bonding orbitals and their change in orientation. Our findings demonstrate the sensitivity of 125Te SSNMR to subtle structural and electronic variations in ChB systems and establish tensor analysis as a powerful probe of noncovalent interactions in chalcogen cocrystals.

The intramolecular π-polarization induced upon binding the bichromophoric ligand L1 to lanthanide ions produces intermolecular hetero-π-stacked pyrene-terimine organization in the resulting complexes as ascertained by ground state supramolecular aggregations leading to dimeric [L1Ln(hfac)3]2 (Ln = Y, Eu) units both in the solid state (X-ray crystal structure determination) and in dichloromethane solution (1H NMR titrations). Photoexcitation engenders additional polarization changes which further complicate/extend interaromatic aggregation processes via the formation of light-driven excimers and exciplexes. The various π-stacking intermolecular interactions operating in the [L1Y(hfac)3] host are probed by reaction with three competitive aromatics guests: electron-poor nitrobenzene, neutral benzene and electron-rich methoxybenzene.

Abstract A regio-, diastereo-, and enantioselective Pd-catalyzed allylation of the lithium salts of 6-membered cyclic allylboronates is reported. The system relies on the use of a commercially available C2-symmetric chiral bisphosphine ligand and gives access to borylated 1,5-dienes featuring vicinal 3,4-cis tertiary stereocenters. It delivers uniformly high enantiocontrol across the series, together with high regio- and diastereoselectivity for the majority of substrates. Furthermore, the method simplifies the synthesis of stereochemically complex tetrahydrofuran derivatives possessing three contiguous stereocenters after the standard oxidation and acidic treatment of the borylated products.

The connection of a dianionic 2,2’-biimidazolate (biim2−) bridging unit to cis-[Cr(N∩N)2]3+ (N∩N is a chelating didentate ligand) or cis-[Cr(N∩N∩N∩N)]3+ building blocks (N∩N∩N∩N is a chelating tetradentate ligand) produces heteroleptic pseudo-octahedral [CrN6]+ chromophores. Their reduced cationic charge is compatible with the subsequent complexation of trivalent lanthanides (Ln3+) to give d-f {[(N∩N)2Cr(biim)]nLn}(3+n)+ (n = 1–4), {[(N∩N)2Cr(biim)]Ln(Tp)2}2+ and {[(N∩N∩N∩N)Cr(biim)]Ln(Tp)2}2+ adducts (Tp is tri(1H-pyrazol-1-yl)-λ4-borate). Moving from polyaromatic N∩N (1,10 phenanthroline) to saturated N∩N∩N∩N polyamine (cyclam) receptors controls the photophysical properties and leads to tunable light conversion in the target heterometallic complexes when Eu(III) is exploited as the activator for downshifting and Er(III) as the activator for upconversion.

In an attempt to boost molecular-based excited-state absorption (ESA) via cross-relaxation (CR), the back-to-back ditridentate polyaromatic 2,2',6,6'-tetrakis(1-methyl-1H-benzo[d]imidazole-2-yl)-4,4'-bipyridine ligand (L4) was reacted with neutral [Ln(hfac)3] lanthanide cargoes (Ln = Y, Eu, and Er and H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) to give dinuclear erbium [(hfac)3LnL4Ln(hfac)3] ≡ [L4Ln2(hfac)6] adducts. Their crystal structures confirm the formation of dimeric molecular scaffolds made of two nine-coordinated trivalent [LnN3O6] chromophores separated by a 4,4'-bipyridine bridge with Ln···Ln distances within the nanometric range (Eu···Eu = 11.69 Å, Y···Y = 11.65 Å, and Er···Er = 12.12 Å). Thermodynamic studies in dichloromethane provide critical insights into the formation and stability of these adducts. Under near-infrared (NIR) excitation at 801 nm in solution, [L4Er2(hfac)6] exhibits ESA light upconversion with blueish-green emissions at 525 and 542 nm corresponding to Er(2H11/2,4S3/2 → 4I15/2) transitions. Thanks to the pertinent speciation in dichloromethane, we could extract a reliable upconversion quantum yield and brightness for the targeted dinuclear [L4Er2(hfac)6] adduct in solution. They largely overpass by 2 orders of magnitude those of the unsaturated mononuclear [L4Er(hfac)3] intermediate but remain comparable to data reported for related saturated monomeric adducts in the same conditions. No global beneficial cross-relaxation effect could thus be unambiguously identified.

A novel chiral chromium(III) molecular ruby [Cr(qpp)2]3+ (qpp = N-methyl-N-(pyridin-2-yl)-6-(quinolin-8-yl)pyridin-2-amine) has been synthesized, enantiomerically resolved, and fully characterized. The circularly polarized luminescence (CPL) spectra revealed two emission bands of opposite polarization in the near-infrared region (700-800 nm), corresponding to the metal-centered transitions Cr(2T(1) → 4A2) and Cr(2E(1) → 4A2). Notably, the dissymmetry factor glum reached 0.11 for the former transition, which is among the highest reported for chromium(III) systems. Comparison with structurally related homo- and heteroleptic chromium(III) complexes underscores the important role of the nephelauxetic effect in tuning CPL properties. Increased metal-ligand covalency, indicative of a stronger nephelauxetic effect, enhances orbital mixing and modifies the electronic character of the emissive states. These changes influence both electric and magnetic transition dipole moments, leading to noticeable variations in dissymmetry factor glum. Altogether, these observations highlight the potential of fine-tuning metal-ligand covalency as a rational strategy for optimizing the chiroptical properties of chromium(III) complexes, with promising implications for bioimaging, molecular probes, and circularly polarized optoelectronic devices.

The efficiency of intermetallic d-f energy transfers in molecular-based Fe(II)-Ln(III) dyads can be modulated by (i) the spin state of the [Fe(II)N6] unit, (ii) the intermetallic distance, and (iii) the choice of the trivalent lanthanide (Ln(III)). The programming of a monotonous and reliable change in Ln-based luminescence upon spin-crossover (SCO) processes is highly desired for easy and fast optical monitoring of Fe(II)-spin state, but Ln···Fe separations over the nanometer range are required to be compatible with detectable Ln-based light emission. Moving from a nonluminescent rigid macrobicyclic dinuclear [FeEu(L8)3]5+ triple-stranded helicate (Fe···Eu = 9.19 Å) toward the extended macrotetracyclic trinuclear [FeEuEu(L11)3]8+ analog (Fe···Euterminal = 17.66 Å) induces unprecedented optical reading of a [Fe(II)(Npyridine∩Nbenzimidazole)3]2+ spin-crossover platform upon modulation of the red emission of the terminal [EuN6O3] chromophore. The close-to-linear Eu(III)-based optical response to the high-spin Fe(II) mole fraction in solution contrasts with the less adapted wavy luminescence response reported pioneeringly for the hydrolysis-sensitive 2-(pyrimidin-2-yl)-1H-benzo[d]imidazole platform in [FeEu(L9)3]5+. The rational programming of variable optical detection modes (wavy response, linear increase, linear decrease) is discussed and can be deduced from the modeling of the coupled SCO-Fe(II)/Ln(III) centers developed in this work.

We report the metal-assisted synthesis of oligomeric nitrogen-rich DiL1 and PoL1n sensitizers from the monomer L1 via acyclic diene metathesis (ADMET) polymerization for the tuning of the photoluminescence properties of europium-containing complexes by ligand design. The binding of [Eu(hfac)3] cargoes (H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) to monomer L1 to give [L1Eu(hfac)3] during metathesis appeared to be essential (i) for protecting the Grubbs ruthenium catalyst to interact with the nitrogen atoms of L1 and (ii) to drive the Ru-catalyzed ADMET reaction toward dimeric {DiL1[Eu(hfac)3]2} and polymeric {PoL1n[Eu(hfac)3]n} assemblies. Compared with the monomeric tridentate L1-L4 ligands, significant electronic delocalization occurs through the π-conjugated benzimidazole-CH═CH-benzimidazole skeletons in the dimer DiL1 and polymers PoL1n, which results in a stepwise red-shift of the excited levels in these extended polyaromatic ligands. As a consequence, the sensitization of bound [Eu(hfac)3] units reveals a decrease of the photoluminescence quantum yields along the [L4Eu(hfac)3] > [L3Eu(hfac)3] > [L2Eu(hfac)3] ≈ [L1Eu(hfac)3] > {DiL1[Eu(hfac)3]2} > {PoL118[Eu(hfac)3]18} series due to unfavorable europium-to-ligand back energy transfers.

The efficient binding of receptors Lk (Lk = L5-L8) to [Eu(hfac)3] (H-hfac = 1,1,1,5,5,5-hexafluoropentane-2,4-dione) allows (i) to gather the basic unit-containing tridentate ligands Lk and the acidic unit-bearing bidentate hfac- co-ligands into a stable single molecular [LkEu(hfac)3] adduct working as a catalyst for Michael C─C bond formations, (ii) to enhance the acidity of the N-H units connected to the benzimidazole side arms of the bound ligands L5 and L7, and (iii) to reorganize their geometries into cis-cis conformation for activating Michael substrates 1 and 2 through H-bonding interactions on the second-sphere coordination to provide the target product 3 (up to 92% yield). A maximum enantiomeric excess of 21% could be achieved due to the long distance between the chiral sources and the catalytic sites in the europium complexes. Stepwise distortions from planarity of the terdentate Lk ligands in the yttrium complexes [LkY(hfac)3] (Lk = L2, L3, and L5-L10) can be tuned by addition of specific substituent into the benzimidazole side arms, thus realizing some control over their dual fluorescence and phosphorescence emission properties. All [LkEu(hfac)3] (Lk = L2, L3, and L5-L10) complexes display europium-centered photoluminescence properties induced by the antenna effect and modulated by the specific design of the light-harvesting aromatic ligand located in the first coordination sphere.

Efficient near-infrared (NIR) to visible (VIS) light upconversion should combine large absorption coefficients εNIR with very large quantum yields ϕUC so that the overall brightness BUC = εNIR·ϕUC is maximum. Relying on linear optics, several photons are collected by strongly absorbing dyes, stored on long-lived intermediate excited states and finally piled up using mechanisms of simple or double operator natures. The miniaturization to implement detectable linear light upconversion in a single molecule is challenging because of the existence of the thermal vibrational bath, which increases non-radiative relaxation and limits quantum yields to 10-9 ≤ ϕUC ≤ 10-6. An acceptable brightness thus requires the connection of a maximum of cationic cyanine dyes around trivalent lanthanide luminophores. Taking advantage of the thermodynamic benefit brought by strict self-assembly processes, three cationic IR-780 dyes could be arranged around a single Er(III) cation in the trinuclear [ZnErZn(L5)3]10+ triple-stranded helicate. NIR excitation at 801 nm in acetonitrile at room temperature induces light upconversion via the energy transfer upconversion (ETU) mechanism. The final green Er(2H11/2,4S3/2 → 4I15/2) emission with ϕUC = 3.6 × 10-8 shows a record brightness of BUC = 2.8 × 10-2 M-1 cm-1 (Pexc = 25 W cm-2) for a molecular-based upconversion process.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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 = N2,N6-dimethyl-N2,N6-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 glum 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.

A Ni-catalyzed enantioconvergent cross-coupling between beta-bromostyrenes and secondary Grignard reagents is reported. This C(sp2)-C(sp3) cross-coupling is applicable to a broad range of electrophilic and nucleophilic partners and affords the products in good to high levels of enantio-induction. Experimental mechanistic investigations revealed an unexpected binding mode of the chiral (P,N) ligand and support a radical rebound mechanism involving in-cage radicals. Kinetic experiments provide evidence for an off-cycle resting state featuring dinuclear species. Computational analyses are in line with this hypothesis and coherent with a catalytic cycle proceeding via a Ni(I)/Ni(III) manifold. They further suggest an enantio-determining radical capture event and shed light on the origin of the Dynamic Kinetic Resolution process.

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 & ctdot;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. Moving from homoleptic [Cr(biimidazole)3]3+ toward heteroleptic [Cr(phen)2(biimidazole)]3+ restores solubility, stability and pH-dependent near infrared phosphorescence at the molecular level.

Near-infrared (NIR) luminescence and photoacoustic (PA) imaging have attracted increasing attention for the real-time monitoring of biological samples due to high sensitivity, resolution, and pronounced signal detection depth, respectively. For improved contrast, both techniques require imaging agents possessing high absorption in the red-NIR range. Herein, we took advantage of a ternary complex formed with the anionic ytterbium(III) tetrakis(2-thenoyltrifluoroacetonate) ([Yb(tta)(4)](-)) and the cationic NIR-absorbing chromophore, 1,1 '-diethyl-2,2 '-dicarbocyanine (Cy+), to evaluate its potential to act as a dual-mode NIR luminescence and PA imaging agent. We demonstrated that, upon excitation with red-NIR light, Cy[Yb(tta)(4)] encapsulated into polystyrene nanoparticles is able to generate both NIR Yb3+ emission and a PA signal in an imaging experiment performed in a tissue-mimicking phantom.

The interest in Cr(III) complexes has been renewed over the past decades for building practical guidelines in the design of efficient earth-abundant phosphorescent near-infrared emitters. In that context, we report the first family of homoleptic tri(didentate) Cr(III) complexes [CrL3](3+) based on polyaromatic ligands inducing 6-membered chelate rings, namely, the bis(1-methylimidazol-2-yl)ketone (L = bik), bis(1-methylimidazol-2-yl)methane (L = bim), and bis(1-methylimidazol-2-yl)ethane (L = bie) ligands. The programmed close-to-perfect octahedral microsymmetry of {(CrN6)-N-III} chromophores found in [Cr(bik)(3)](OTf)(3) (1), [Cr(bim)(3)](OTf)(3) (2), and [Cr(bie)(3)](BF4)(3) (3) ensures a ligand-field strength large enough to induce intense and long-lived Cr-based phosphorescence. Impressive excited-state lifetimes (5.0-8.2 ms) were obtained at low temperatures for the [Cr(L)(3)](3+) series. Additionally, the photoluminescent quantum yield climbs to 0.8% for compound 1 in deaerated solutions. Moreover, the photophysical features of the three homoleptic complexes are barely influenced by the presence of dioxygen presumably because of the poor overlap between the Cr-based phosphorescence spectra (ca. 14100 cm(-1)) and the (1)Sigma(+)(g) <- (3)Sigma(-)(g) transition in the absorption spectrum of dioxygen (13100 cm(-1)). The multiredox electrochemical pattern of 1 is evidenced by cyclic voltammetry as well as its strong photooxidant behavior. The pH sensitivity of 2 and 3 luminescence is discussed, along with the reactivity of their beta-diketiminate derivatives.

The structurally related compounds NiI_2 and CoI_2 are multiferroic van der Waals materials, in which helimagnetic orders exist simultaneously with electric polarization. Here, we report on the evolution of the crystal structure and of the magnetic properties across the solid solution Co_1-xNi_xI_2. We have successfully grown crystals of the whole range of the solid solution, i.e. x = 0-1, by employing the self-selecting vapor growth (SSVG) technique and by carefully tuning the synthesis conditions according to the chemical composition. Our structural investigations show that the crystal symmetry changes from P3̅m1 to R3̅m when Ni substitutes for Co beyond x = 0.2. Both the lattice parameters and magnetic properties evolve continuously and smoothly from one end member to the other, showing that they can be finely tuned by the chemical composition. We also observe that the Ni substitution degree in the solid solution affects the metamagnetic transition typical for CoI_2 at high magnetic fields. In particular, we find the existence of the metamagnetic transition similar to that for CoI_2 in the NiI_2 structure. Based on magnetic measurements we construct the phase diagram of the Co_1-xNi_xI_2 system. Controlling the magnetic properties by the chemical composition may open new pathways for the fabrication of electronic devices made of two-dimensional (2D) flakes of multiferroic van der Waals materials.