Polycyclic Pyrazolidines by Tandem Diazomalonate Dipolar Cycloadditions and CpRu-Catalyzed Carbene Additions.

Thanks to the ability of diazo derivatives to react either as 1,3-dipoles and as carbenes after dinitrogen extrusion, combinations of oxa or aza benzonorbornadienes and diazomalonates afford polycyclic pyrazolidines via a three-step sequence of (i) a highly diastereoselective [3+2]-cycloaddition, (ii) a CpRu-catalyzed carbene addition, and (iii) a second dipolar cycloaddition. Of importance, step (II) represents a unique access to novel bench-stable N,N-cyclic azomethine imines, which behave as effective 1,3-dipoles in combination with electron-poor dipolarophiles. Each step proceeds efficiently and the 3-step process can be performed in one-pot to yield a polycyclic pyrazolidine in excellent overall yield (90 %).

Axially-chiral boramidine for detailed (chir)optical studies.

The inclusion of boron atoms into chiral π-conjugated systems is an effective strategy to unlock unique chiroptical properties. Herein, the preparation and characterization of a configurationally stable axially-chiral boramidine are reported, showcasing absorption in the UV domain, deep-blue fluorescence (Φ up to 94%), and ca. |10-3| gabs and glum values. Detailed photophysical studies and quantum-chemical calculations clearly elucidate the deactivation pathways of the emissive state to triplet excited states, involving increased spin-orbit coupling between the lowest singlet excited state and an upper triplet state.

Controlling Symmetry-Breaking Charge Separation in Pyrene Bichromophores.

So far, symmetry-breaking charge separation (SB-CS) has been observed with a limited number of chromophores and is usually inhibited by the formation of an excimer. We show here that thanks to fine-tuning of the interchromophore coupling via structural control, SB-CS can be operative with pyrene, despite its high propensity to form an excimer. This is realized with a bichromophoric system consisting of two pyrenes attached to a crown ether macrocycle, which can bind cations of different sizes. By combining stationary and time-resolved spectroscopy together with molecular dynamics simulations, we demonstrate that the excited-state dynamics can be totally changed depending on the binding cation. Whereas strong coupling leads to rapid excimer formation, too weak coupling results in noninteracting chromophores. However, intermediate coupling, achieved upon binding of Mg2+, allows for SB-CS to be operative.

Triple para-Functionalized Cations and Neutral Radicals of Enantiopure Diaza[4]helicenes.

Modulation of absorbance and emission is key for the design of chiral chromophores. Accessing a series of compounds absorbing and emitting (circularly polarized) light over a wide spectral window and often toward near-infrared is of practical value in (chir)optical applications. Herein, by late-stage functionalization on derivatives bridging triaryl methyl and helicene domains, we have achieved the regioselective triple introduction of para electron-donating or electron-withdrawing substituents. Extended tuning of electronic (e.g., E1/2red -1.50 V → -0.68 V) and optical (e.g., emission covering from 550 to 850 nm) properties is achieved for the cations and neutral radicals; the latter compounds being easily prepared by mono electron reductions under electrochemical or chemical conditions. While luminescence quantum yields can be increased up to 70% in the cationic series, strong Cotton effects are obtained for certain radicals at low energies (λabs ∼ 700-900 nm) with gabs values above 10-3. The open-shell electronic nature of the radicals was further characterized by electron paramagnetic resonance revealing an important spin density delocalization that contributes to their persistence.

Concave P-Stereogenic Phosphorodiamidite Ligands for Enantioselective Rh(I) Catalysis.

The development of a new class of concave P-stereogenic phosphorodiamidite ligands, derived from Tröger's bases (TBs), is explored. These ligands, characterized by their remarkable stability and unique structural geometry around the P atom, are used in asymmetric Rh(I)-catalyzed additions of boronic acids to NH-isatins resulting in excellent reactivities and reasonable enantioselectivity (er up to 92:8).

Modulation of circularly polarized luminescence by swelling of microgels functionalized with enantiopure [Ru(bpy)3]2+ luminophores.

Chemoresponsive microgels functionalized with enantiomeric Δ- or Λ-[Ru(bpy)3]2+ showed tunable chiroptical properties upon swelling and shrinking. The tuning is triggered by a modulation of the local mobility of [Ru(bpy)3]2+ upon addition of fructose, controlling interactions and distances between [Ru(bpy)3]2+ and phenylboronic acid.

General Ir-Catalyzed N-H Insertions of Diazomalonates into Aliphatic and Aromatic Amines.

A general N-H insertion reactivity of acceptor-acceptor diazo malonate reagents is reported using [Ir(cod)Cl]2 as catalyst. A large range of amines, primary and secondary, aliphatic and aromatic, is possible. Mild temperatures, perfect substrate/reactant stoichiometry, and good functional group compatibility render the process particularly attractive for the (late-stage) functionalization of amines.

Pfeiffer effect on configurationally labile dyes within ternary complexes with metal ions and enantiopure macrocycles.

A configurationally-labile helical dye, 2,4,5,7-tetranitrofluorenone oximate, is used to probe complexes made of enantiopure macrocycles and mono/divalent metal ions. Induced electronic circular dichroism (ECD) and 1H NMR responses are amplified at room temperature only in the presence of K+ and Na+ ions despite larger binding efficiency with alkaline earth metal ions.

2,4,5,7-Tetranitrofluorenone Oximate for the Naked-Eye Detection of H-Bond Donors and the Chiroptical Sensing of Enantiopure Reagents.

Hydrogen bonding greatly influences rates and equilibrium positions of chemical reactions, conformations, and sometimes even stereochemistry. This study reports on tetranitrofluorenone oximate, a novel dye capable of naked-eye detection of hydrogen-bond donating species (HBDs) and of rapid determination of H-bond donation strength by hypsochromic shift monitoring. In addition, the molecule possesses atropisomeric conformations, of M and P configuration, as evidenced in solid and solution state studies by X-ray diffraction and electronic circular dichroism (ECD), respectively. In the latter case, enantiopure bis-thioureas were the most effective HBDs to promote a chiral induction (diastereoselective recognition, Pfeiffer effect); the ECD results being rationalized by time-dependent density functional theory (TDDFT) calculations. Based on these experiments, bis-thioureas were used as chiral reagents in asymmetric 1,3-dipolar cycloadditions of structurally-related nitrones; the ECD sensing of the stereoinduction between bis-thioureas and the oximate serving as an indirect method of selection of the most effective HBD for asymmetric synthesis.

Multistate Aggregation-Induced Chiroptical Properties of Enantiopure Disulfide-Mediated Bispyrene Macrocycles.

A chiral bispyrene macrocycle designed for exclusive intermolecular excimer fluorescence upon aggregation was synthesized by a double hydrothiolation of a bis-enol ether macrocycle followed by intramolecular oxidation of free thiols. Unusually high stereoselectivity was achieved for the thiol-ene additions under templated conditions and Et3 B/O2 radical initiation. After enantiomer separation (chiral stationary phase HPLC), aqueous conditions provoked aggregation. Detailed structural evolution was afforded by ECD/CPL monitoring. Three regimes can be observed and characterized by strong modifications in chiroptical patterns under, at, or above a 70 % H2 O : THF threshold. In luminescence, high glum dissymmetry factors values were obtained, up to 0.022, as well as a double sign inversion of CPL signals during the aggregation, a behavior rationalized by time-dependent density functional theory (TDDFT) calculations. Langmuir layers of enantiopure disulfide macrocycles were formed at the air-water interface and transferred onto solid substrates to afford Langmuir-Blodgett films, which were then studied by AFM and UV/ECD/fluorescence/CPL.

Looking for chiral recognition in photoinduced bimolecular electron transfer using ultrafast spectroscopy.

Occurrence of chiral recognition in bimolecular photoinduced electron transfer (ET) is difficult to identify because of the predominant role of diffusion. To circumvent this problem, we apply a combination of ultrafast time-resolved fluorescence and transient electronic absorption to look for stereoselectivity in the initial, static stage of ET quenching, where diffusion is not relevant. The fluorophore and electron acceptor is a cationic hexahelicene, whereas the quencher has either stereocentered (tryptophan) or axial (binaphthol) chirality. We found that, in all cases, the quenching dynamics are the same, within the limit of error, for different diastereomeric pairs in polar and medium-polar solvents. The same absence of chiral effect is observed for the recombination of the radical pair, which results from the quenching. Molecular dynamics simulations suggest that the distribution of inter-reactant distance is independent of the chirality of the acceptor and the donor. Close contact resulting in large electronic coupling is predicted to be possible with all diastereomeric pairs. In this case, ET is an adiabatic process, whose dynamics do no longer depend on the coupling, but are rather controlled by high-frequency intramolecular modes.

Circularly polarized luminescence from Tb(III) interacting with chiral polyether macrocycles.

A straightforward two-step synthesis protocol affords a series of chiral amide-based bis-pyridine substituted polyether macrocycles. One ligand is particularly able to complex terbium(III) ions spontaneously. Upon complexation, interesting chiroptical properties are observed both in absorbance (ECD) and in fluorescence (CPL). In ligand-centered electronic circular dichroism, a sign inversion coupled with a signal enhancement is measured; while an easily detectable metal-centered circularly polarized luminescence with a glum of 0.05 is obtained for the main 5D4 → 7F5 terbium transition. The coordination mode and structure of the complex was studied using different analysis methods (NMR analysis, spectrophotometric titration and solid-state elucidation).

Chemo- and Regioselective Multiple C(sp2 )-H Insertions of Malonate Metal Carbenes for Late-Stage Functionalizations of Azahelicenes.

Chiral quinacridines react up to four times, step-by-step, with α-diazomalonates under RuII and RhII catalysis. By selecting the catalyst, [CpRu(CH3 CN)3 ][PF6 ] (Cp=cyclopentadienyl) or Rh2 (oct)4 , chemo and regioselective insertions of derived metal carbenes are achieved in favor of mono- or bis-functionalized malonate derivatives, respectively, (r.r.>49 : 1, up to 77 % yield, 12 examples). This multi-introduction of malonate groups is particularly useful to tune optical and chemical properties such as absorption, emission or Brønsted acidity but also cellular bioimaging. Density-functional theory further elucidates the origin of the carbene insertion selectivity and also showcases the importance of conformations in the optical response.

Triple Regioselective Functionalization of Cationic [4]Helicenes via Iridium-Catalyzed Borylation and Suzuki Cross-Coupling Reactivity.

In essentially one-pot, using Ir- and Pd-catalysis, tris(arene)-functionalized cationic [4]helicenes are synthesized with full regioselectivity and enantiospecificity starting from a trivial precursor (17 examples). This poly-addition of aryl groups improves key optical properties, that is, fluorescence quantum yields and lifetimes. Electronic circular dichroism and circularly polarized luminescence signatures are observed up to the far-red domain, in particular with additional arenes prone to aggregation.

Chiral control of spin-crossover dynamics in Fe(II) complexes.

Iron-based spin-crossover complexes hold tremendous promise as multifunctional switches in molecular devices. However, real-world technological applications require the excited high-spin state to be kinetically stable-a feature that has been achieved only at cryogenic temperatures. Here we demonstrate high-spin-state trapping by controlling the chiral configuration of the prototypical iron(II)tris(4,4'-dimethyl-2,2'-bipyridine) in solution, associated for stereocontrol with the enantiopure Δ- or Λ-enantiomer of tris(3,4,5,6-tetrachlorobenzene-1,2-diolato-κ2O1,O2)phosphorus(V) (P(O2C6Cl4)3- or TRISPHAT) anions. We characterize the high-spin-state relaxation using broadband ultrafast circular dichroism spectroscopy in the deep ultraviolet in combination with transient absorption and anisotropy measurements. We find that the high-spin-state decay is accompanied by ultrafast changes of its optical activity, reflecting the coupling to a symmetry-breaking torsional twisting mode, contrary to the commonly assumed picture. The diastereoselective ion pairing suppresses the vibrational population of the identified reaction coordinate, thereby achieving a fourfold increase of the high-spin-state lifetime. More generally, our results motivate the synthetic control of the torsional modes of iron(II) complexes as a complementary route to manipulate their spin-crossover dynamics.

Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes - Access to Red Emitters and Planar Chiral Stereochemical Traits.

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late-stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron-donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near-infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG-functionalized derivatives, C≡C- functionalized products remain broadly fluorescent, with red-shifted absorptions (Δλabs up to 25 nm) and emissions (Δλem up to 73 nm, ΦPL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in 1 H NMR spectroscopy (218 K), N-CH2 protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol-1 . This phenomenon is due to planar chiral conformations (Sp and Rp configurations), induced by the geometry of the alkyl (n-propyl) side-chains next to the acetylenic substituents. Ion pairing studies with Δ-TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.

Straightforward Synthesis of Indenes by Gold-Catalyzed Intramolecular Hydroalkylation of Ynamides.

An original and straightforward entry to polysubstituted indenes from readily available ynamides is reported. Upon reaction with a N-heterocyclic carbene-gold complex under mild conditions, activated keteniminium ions are generated whose unique electrophilicity triggers a [1,5]-hydride shift and a subsequent cyclization. The presence of an endocyclic enamide in the densely functionalized resulting indenes was shown to be especially useful and versatile, offering a range of opportunities for their further postfunctionalization.

Combined spectroscopic studies on post-functionalized Au25 cluster as an ATR-FTIR sensor for cations.

Recently, significant research activity has been devoted to thiolate-protected gold clusters due to their attractive optical and electronic properties. These properties as well as solubility and stability can be controlled by post-synthetic modification strategies. Herein, the ligand exchange reaction between Au25(2-PET)18 cluster (where 2-PET is 2-phenylethanethiol) and di-thiolated crown ether (t-CE) ligands bearing two chromophores was studied. The post-functionalization aimed to endow the cluster with ion binding properties. The exchange reaction was followed in situ by UV-vis, 1H NMR and HPLC. MALDI mass analysis revealed the incorporation of up to 5 t-CE ligands into the ligand shell. Once functionalized MALDI furthermore showed complexation of sodium ions to the cluster. ATR-FTIR spectroscopic studies using aqueous solutions of K+, Ba2+, Gd3+ and Eu3+ showed noticeable spectral shifts of the C-O stretching band around 1100 cm-1 upon complexation. Further spectral changes point towards a conformational change of the two chromophores that are attached to the crown ether. Density functional theory calculations indicate that the di-thiol ligand bridges two staple units on the cluster. The calculations furthermore reproduce the spectral shift of the C-O stretching vibrations upon complex formation and reveal a conformational change that involves the two chromophores attached to the crown ether. The functionalized clusters have therefore attractive ion sensing properties due to the combination of binding properties, mainly due to the crown ether, and the possibility for signal transduction via an induced conformational change involving chromophore units.

Hybrids of cationic [4]helicene and N-heterocyclic carbene as ligands for complexes exhibiting (chir)optical properties in the far red spectral window.

Synthesis, electronic and structural properties of a chiral NHC bearing a N-bonded cationic [4]helicene moiety are reported. This ligand is used to construct AuI, AuIII and RhI complexes exhibiting far-red (chir)optical properties regardless of the metal.