Circularly Polarized Luminescence of Boronic Acid-Derived Salicylidenehydrazone Complexes Containing Chiral Boron as Stereogenic Unit

Tetra-coordinated organoboron complexes with triaminoguanidine-salicylidene based ligands: aggregation induced enhanced emission and mechanoresponsive features

Organoboron complexes have garnered significant attention due to their remarkable optical properties and diverse applications. However, synthesizing stable fused five-, six- and seven-membered organoboron complexes possess significant challenges. In this study, we successfully developed novel mono-nuclear (6-8 & 10) and di-nuclear (9) organoboron complexes supported by triaminoguanidine-salicylidene based C3-symmetric Schiff base ligands via one-step condensation reaction with excess phenylboronic acid. Single-crystal X-ray diffraction analysis revealed that in the mononuclear complexes (6-8 & 10), boron atoms adopt tetrahedral geometry with fused five-membered N-B-N and six-membered O-B-N chelate ring whereas in the dinuclear complex (9), two boron atoms exist in distinct coordination environment, forming four fused boron-incorporated rings, including six-membered N-B-N, six-membered O-B-N, seven-membered N-B-O and five-membered N-B-N chelate rings. Our findings provide a unique family of mononuclear organoboron complexes and dinuclear organoboron complex with ESIPT unit. Aggregation induced enhanced emission features of the compounds were established in THF-water mixture and supported by DLS and SEM analyses. Interestingly, compound 6, 9 and 10 shows interesting mechanoresponsive features upon grinding.

Catalytic asymmetric C-N cross-coupling towards boron-stereogenic 3-amino-BODIPYs

3-Amino boron dipyrromethenes (BODIPYs) are a versatile class of fluorophores widely utilized in live cell imaging, photodynamic therapy, and fluorescent materials science. Despite the growing demand for optically active BODIPYs, the synthesis of chiral 3-amino-BODIPYs, particularly the catalytic asymmetric version, remains a challenge. Herein, we report the synthesis of boron-stereogenic 3-amino-BODIPYs via a palladium-catalyzed desymmetric C-N cross-coupling of prochiral 3,5-dihalogen-BODIPYs. This approach features a broad substrate scope, excellent functional group tolerance, high efficiency, and remarkable enantioselectivities, under mild reaction conditions. Further stereospecific formation of chiral 3,5-diamino-BODIPYs, along with an investigation into the photophysical properties of the resulting optical BODIPYs are also explored. This asymmetric protocol not only enriches the chemical space of chiroptical BODIPY dyes but also contributes to the realm of chiral boron chemistry.

Multicomponent synthesis of stereogenic-at-boron fluorophores (BOSPYR) from boronic acids, salicylaldehydes, and 2-formylpyrrole hydrazones

This work describes one-step syntheses of various stereogenic-at-boron fluorochromes (BOSPYR) via multicomponent reactions involving readily accessible boronic acids, salicylaldehydes, and 2-formylpyrrole hydrazones. The dyes absorb and emit in the visible region of the electromagnetic radiation, and are characterized by large Stokes shifts (2850-4930 cm-1) with weak fluorescence emissions (Φfl: 1.5-9.1%). Notably, the dimmed fluorescence of BOSPYRs recovers upon transition to viscous media (21-fold for 1a). The representative compound 1a exhibits clear Cotton effects with dissymmetry factors of ca. |gabs| ∼ 1.9 × 10-3 in the visible region, indicating efficient asymmetry induction to the chromophore. The X-ray molecular structure of 1a shows that the chromophore deviates from planarity by 17.2°, which may contribute significantly to the inherent chirality of the fluorophore. A computational examination of excited states by time-dependent density functional theory (TD-DFT) identifies the emission mechanism as arising from a locally-excited (LE) state.

Boron enabled bioconjugation chemistries

Novel bioconjugation reactions have been heavily pursued for the past two decades. A myriad of conjugation reactions have been developed for labeling molecules of interest in their native context as well as for constructing multifunctional molecular entities or stimuli-responsive materials. A growing cluster of bioconjugation reactions were realized by tapping into the unique properties of boron. As a rare element in human biology, boronic acids and esters exhibit remarkable biocompatibility. A number of organoboron reagents have been evaluated for bioconjugation, targeting the reactivity of either native biomolecules or those incorporating bioorthogonal functional groups. Owing to the dynamic nature of B-O and B-N bond formation, a significant portion of the boron-enabled bioconjugations exhibit rapid reversibility and accordingly have found applications in the development of reversible covalent inhibitors. On the other hand, stable bioconjugations have been developed that display fast kinetics and significantly expand the repertoire of bioorthogonal chemistry. This contribution presents a summary and comparative analysis of the recently developed boron-mediated bioconjugations. Importantly, this article seeks to provide an in-depth discussion of the thermodynamic and kinetic profiles of these boron-enabled bioconjugations, which reveals structure-reactivity relationships and provides guidelines for bioapplications.

Modular enantioselective assembly of multi-substituted boron-stereogenic BODIPYs

Boron dipyrromethenes (BODIPYs) are some of the most popular and indispensable tetracoordinate boron compounds and have found widespread applications owing to their excellent spectroscopic and photophysical properties. BODIPYs possessing boron-stereogenic centres are scarce, and strategies for the synthesis of enantioenriched boron-stereogenic BODIPYs with structural diversity remain underdeveloped. In theory, the BODIPY core skeleton has several sites that could be decorated with different substituents. However, due to the lack of general and efficient asymmetric synthetic methods, this potential diversity of chiral BODIPYs has not been exploited. Here we demonstrate a modular enantioselective assembly of multi-substituted boron-stereogenic BODIPYs in high efficiency with excellent enantioselectivities. Key to the success is the Pd-catalysed desymmetric Suzuki cross-coupling, enabling the precise discrimination of the two α C-Cl bonds of the designed prochiral BODIPY scaffold, giving access to a wide range of highly functionalized boron-stereogenic BODIPYs. Derivatizations, photophysical properties and applications in chiral recognition of the obtained optical BODIPYs are further explored.

Luminescent solvent-free liquids based on Schiff-base boron difluoride complexes with polyethylene glycol chains

A series of Schiff-base boron difluoride complexes with polyethylene glycol chains were synthesized and their photophysical properties were examined. These complexes maintained the solvent-free liquid state even at room temperature and their glass transition temperatures (Tg) were determined to be around -40 °C. The complexes showed blue to yellow luminescence under UV irradiation in the solvent-free liquid state with good emission quantum yields (Φ) of up to 0.26. The luminescence colour could also be tuned by dissolving organic dyes in the blue luminescent liquid sample. Density functional theory (DFT) and time-dependent (TD) DFT calculations were performed to further understand the photophysical properties.

Construction of Ge-Stereogenic Center by Desymmetric Carbene Insertion of Dihydrogermanes

We developed a method for the enantioselective synthesis of germanium-stereogenic compounds by the desymmetric carbene insertion of dihydrogermanes. A chiral rhodium phosphate catalyst decomposes diaryldiazo-methanes to generate rhodium carbenes that insert enantioselectively into one of the two Ge-H bonds of dihydrogermanes to form germanium-stereogenic compounds under mild reaction conditions. By this method, a variety of chiral germanes with germanium-stereogenic centers were synthesized in high yields and excellent enantioselectivities. Kinetic studies of the reaction showed that the diazo decomposition process was the rate-determining step. The remaining Ge-H bond of the chiral germane products provides a possibility for preparing chiral tetra-substituted germanium-stereogenic compounds.

BASHY Dyes Are Highly Efficient Lipid Droplet-Targeting Photosensitizers that Induce Ferroptosis through Lipid Peroxidation

Ferroptosis is an iron-dependent lipid-peroxidation-driven mechanism of cell death and a promising therapeutic target to eradicate cancer cells. In this study, we discovered that boronic acid-derived salicylidenehydrazone (BASHY) dyes are highly efficient singlet-oxygen photosensitizers (PSs; ΦΔ up to 0.8) that induce ferroptosis triggered by photodynamic therapy. The best-performing BASHY dye displayed a high phototoxicity against the human glioblastoma multiform U87 cell line, with an IC50 value in the low nanomolar range (4.40 nM) and a remarkable phototoxicity index (PI > 22,700). Importantly, BASHY dyes were shown to accumulate in lipid droplets (LDs) and this intracellular partition was found to be essential for the enhanced phototoxicity and the induction of ferroptosis through lipid peroxidation. The safety and phototoxicity of this platform were validated using an in vivo zebrafish model (Danio rerio).

Multi-colour circularly polarized luminescence properties of chiral Schiff-base boron difluoride complexes

A series of chiral Schiff-base boron difluoride complexes was synthesized and their photophysical properties were examined. These complexes showed multi-colour (blue, yellow and red) photoluminescence in solution and in the solid state with good emission quantum yield (Φ) depending on the π-systems of the ligands. The chiral complexes exhibited circularly polarized luminescence (CPL) with an absolute luminescence dissymmetry factor (g_lum) of up to the 1.3 × 10^-3 in solution and 1.9 × 10^-2 in the drop-cast film state. Density functional theory (DFT) and time-dependent (TD) DFT calculations were conducted to further understand the photophysical properties.

Construction of boron-stereogenic compounds via enantioselective Cu-catalyzed desymmetric B-H bond insertion reaction

Compared with the well-developed carbon-stereogenic chemistry, the construction of boron-stereogenic compounds remains undeveloped and challenging. Herein, the previously elusive catalytic enantioselective construction of boron-stereogenic compounds has been achieved through enantioselective desymmetric B-H bond insertion reaction. The B-H bond insertion reaction of 2-arylpyridine-boranes with versatile diazo compounds under chiral copper catalyst can afford boron-stereogenic compounds with good to excellent enantioselectivity. Moreover, the synthetic utility of this reaction is demonstrated by the scalability and downstream transformations. DFT calculations provide insights into the reaction mechanism and the origin of stereoselectivity.

Synthesis and stereochemistry of chiral aza-boraspirobifluorenes with tetrahedral boron-stereogenic centers

We have synthesized chiral aza-boraspirobifluorenes and evaluated their structural and photophysical properties.

A Boron Dipyrromethene Chiral-at-Boron and Carbon with a Bent Geometry: Synthesis, Resolution and Chiroptical Properties

We report a boron dipyrromethene chiral-at-boron and carbon (B*C*-BODIPY) accessible through a two-pot, one-step synthesis—an interrupted Knoevenagel condensation. The ECD spectra of chiral HPLC resolved enantiomers show clear Cotton effects...

The BASHY Platform Enables the Assembly of a Fluorescent Bortezomib-GV1001 Conjugate

In this study, we show that fluorescent boronic-acid derived salicylidenehydrazone complexes (BASHY) can function as fluorescent linkers for bioconjugates that were used to monitor the delivery of the proteasome inhibitor bortezomib (Btz) to HT-29 cancer cells. BASHY complexes were structurally optimized to improve the stability of the complex in buffered conditions (ammonium acetate, pH 7 up to t _1/2 = 40 h), photophysically characterized regarding their fluorescence properties and used in confocal microscopy colocalization studies that revealed their intracellular sequestration by lipid droplets. The accumulation in these hydrophobic organelles limited the hydrolysis of the complex and consequently the drug release, a problem that was circumvented by the conjugation of the BASHY-Btz complex with a cell-penetrating peptide GV1001-C. The conjugate exhibited an improved cytoplasmic availability as confirmed by confocal fluorescence microscopy studies and an improved potency against HT-29 cancer cells (IC₅₀ = 100 nM) as compared to the nontargeted complex (IC₅₀ = 450 nM).

Catalytic Enantioselective Construction of Chiroptical Boron-Stereogenic Compounds

The construction of main group heteroatom-stereogenic compounds is of great importance due to their intriguing chemical, physical, biological, and stereoelectronic properties. Despite that organoboron compounds are widely used in organic chemistry, the creation of a tetrahedral boron-stereogenic center in one enantiomeric form remains highly challenging. Given the labile nature of ligands attached to the tetracoordinate boron atom, only a handful of enantioenriched boron-stereogenic compounds have been reported via resolution or a chiral substrate-induced diastereoselective approach. To date catalytic asymmetric synthesis of boron-stereogenic compounds has remained unknown. Here, we demonstrate the first catalytic enantioselective construction of boron-stereogenic compounds via an asymmetric copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. This enantioselective CuAAC reaction not only gives access to a wide range of novel highly functionalized boron-stereogenic heterocycles in high yields with good to excellent enantioselectivities but also produces optically active terminal alkyne and triazole moieties with various potential application prospects. Further transformation of the chiral tetracoordinate boron compounds delivers several complex heterocyclic entities bearing boron-stereogenic centers without the loss of enantiopurity. Moreover, the X-ray structure, the barrier to racemization, and the HOMO/LUMO gap of selected tetracoordinate boron compounds are investigated. Notably, these novel N,N π-conjugated boron-stereogenic compounds exhibit bright fluorescence. The optical properties, including circular dichroism, quantum yield, and circular polarized luminescence spectroscopies, are examined. These features expand the chemical space of the chiroptical boron-based dye platform, which could have great potential applications in chiral optoelectronic materials.

From fluorogens to fluorophores by elucidation and suppression of ultrafast excited state processes of a Schiff base

Strategies have been explored for developing strongly fluorescent species out of a weakly fluorescent Schiff base, 2-(((pyridin-2-ylmethyl)imino)methyl)phenol (salampy). The locally excited enolic state of salampy undergoes an intramolecular proton transfer with a time constant of ca. 200 fs. The emissive cis keto state thus formed decays completely within 50 ps. Its fast decay and miniscule fluorescence quantum yield are attributed to efficient non-radiative channels associated with conformational relaxation. The anionic form, salampy^-, has a significantly longer fluorescence lifetime of 800 ps. Its emissive state evolves in tens of picoseconds, from the locally excited state, by solvent and conformational relaxation. Both the neutral and anionic forms have a fluorescence lifetime of about 6 ns at 77 K, a temperature at which all activated nonradiative channels are blocked. This lifetime is similar to that obtained at room temperature, upon rigidification of the anion by complexation with Zn²⁺. Two such complexes have been studied. The first is binuclear, with acetate bridge between the two Zn²⁺ ions. The second, with ClO₄^- as the counterion, is mononuclear with two salampy ligands ligating the metal ion. Unlike a previous report on a different Schiff base, in which the ligands are π-stacked in its dimeric Zn²⁺ complex, no additional nonradiative deactivation pathway opens up in the Zn complexes of salampy, which are devoid of such stacking. The complex of salampy with Al³⁺ has an even longer fluorescence lifetime of 9 ns, indicating a greater degree of rigidification and consequent suppression of nonradiative processes.

Circularly Polarized Fluorescent Helicene-Boranils: Synthesis, Photophysical and Chiroptical Properties

Mono- and di-boranil-substituted helicenes were prepared by BF₂ -borylation of the corresponding anils, readily synthesized by condensation of 2-amino- and 2,15-diamino-helicenes with 4-(diethylamino)salicylaldehyde. After enantiomeric resolution using HPLC, their chiroptical properties including circularly polarized fluorescence in solution and in PMMA films were investigated and rationalized with the help of NMR, X-ray and quantum-chemical calculations.

Two-photon excitable boron complex based on tridentate imidazo[1,5-a]pyridine ligand for heavy-atom-free mitochondria-targeted photodynamic therapy

We have synthesized a cyan fluorescent boron complex based on a tridentate imidazo[1,5-a]pyridine ligand. The boron complex was found to have potential applications as not only a chiroptical material but also a heavy-atom-free mitochondria-targeted photosensitizer for cancer treatment.

We have synthesized a cyan fluorescent boron complex based on a tridentate imidazo[1,5-a]pyridine ligand.

Multicomponent Synthesis of Luminescent Iminoboronates

A family of iminoboronates was prepared through a one-pot multicomponent reaction, starting from boronic acid, anthranilic acid, and different salicylaldehydes. Their synthesis was straightforward and the complexes were obtained in good to excellent yields. Their photophysical properties were assessed in a diluted solution, and the complexes proved to be faintly luminescent. These chelates demonstrated remarkable Aggregation-Induced Emission Enhancement, which was rationalized using crystal structures.

Facile strategy for obtaining luminescent polymorphs based on the chirality of a boron-fused azomethine complex

A chloro-substituted boron-fused azomethine complex (BAmCl) having a stereogenic boron center was synthesized for obtaining a luminescent chiral crystal. We succeeded in isolating the (R)- and (S)-enantiomers of BAmCl and preparing the homochiral polymorphic crystal, while we obtained the racemic crystal with rac-BAmCl. Single crystal X-ray diffraction analyses suggest that a variety of intermolecular interaction patterns and intrinsic flexibility of the molecular framework should play a significant role in stabilizing the homochiral crystal. We found the difference in molecular arrangements between the racemic and the homochiral crystals, and we observed distinctly different emission colors. In particular, we observed heat-initiated homogeneous racemization without the need for a solvent or catalyst in the molten state of the homochiral crystal (R)-BAmCl. Our results mean that chiral resolution of a flexible fused-skeleton having a stereogenic boron center can be a platform for creating luminescent polymorphic materials.

Amplifying the excited state chirality through self-assembly and subsequent enhancement via plasmonic silver nanowires

The development of circularly polarized luminescent materials with a large luminescence dissymmetry factor (g_lum) is continuing to be a big challenge. Here, we present a general approach for amplifying circular polarization of circularly polarized luminescence (CPL) through intergrating molecular self-assembly and surface plasmon resonance (SPR). Molecular self-assembly could amplify the CPL performance. Subsequently, the composites built of nanoassemblies and achiral silver nanowires (AgNWs) show intense CPL activity with an amplified g_lum value. By applying an external magnetic field, the CPL activity of the nanoassemblies/AgNWs composites has been significantly enhanced, confirming a plasmon-enhanced circular polarization. Our design strategy based on SPR-enhanced circular polarization of the chiral emissive systems suggests that combining plasmonic nanomaterials with chiral organic materials could aid in the development of novel CPL active nanomaterials.

π-Extended Four-Coordinate Organoboron N,C-Chelates as Two-Photon Absorbing Chromophores

Four-coordinate N,C-chelate organoboron dyes with alkynyl spacers were synthesized by Heck alkynylation. These dyes are π-extended analogues of the recently reported class of four-coordinate borylated arylisoquinolines (BAI). Depending on the electron-donor substitution, they feature an intramolecular charge-transfer (ICT) character in the excited state. This translates into pronounced apparent Stokes shifts (up to 8500 cm^-1) and a solvatofluorochromic behavior. In general, the observed emission quantum yields are high in nonpolar media (Φ_F ca. 0.5-0.6). For the dye with the most pronounced ICT rather high emission quantum yields (Φ_F ca. 0.4) are observed for emissions with maxima longer than 600 nm in solvents of moderate polarity. The π-extended dyes show interesting two-photon absorption (TPA) properties, maintaining high cross sections (up to 60 GM) in the near-infrared wavelength window (>900 nm). One of the dyes was designed as dimeric chromophore, integrating the acceptor-π-acceptor (A-π-A) format. This alternative design showed no ICT behavior but led to the observation of high two-photon-absorption (TPA) cross sections (ca. 220 GM at 700 nm). All investigated dyes show pronounced photostability, providing added value to this structural and photofunctional extension of the BAI dye platform.

Boronic acids as building blocks for the construction of therapeutically useful bioconjugates

This review summarizes boronic acid's contribution to the development of bioconjugates with a particular focus on the molecular mechanisms underlying its role in the construction and function of the bioconjugate, namely as a bioconjugation warhead, as a payload and as part of a bioconjugate linker.