Exploring Coumarin-Based Boron Emissive Complexes as Temperature Thermometers in Polymer-Supported Materials

Three coumarin-based boron complexes (L1, L2 and L3) were designed and successfully incorporated into polymeric matrixes for evaluation as temperature probes. The photophysical properties of the complexes were carried out in different solvents and in the solid state. In solution, compound L1 exhibited the highest fluorescence quantum yield, 33%, with a positive solvatochromism also being observed on the absorption and emission when the polarity of the solvent increased. Additionally in the presence of anions, L1 showed a colour change from yellow to pink, followed by a quenching in the emission intensity, which is due to deprotonation with the formation of a quinone base. Absorption and fluorescence spectra of L1 were calculated at different temperatures by the DFT/B3LYP method. The decrease in fluorescence of compound L1 with an increase in temperature seems to be due to the presence of pronounced torsional vibrations of the donor and acceptor fragments relative to the single bond with C(carbonyl)-C (styrene fragment). L1, L2 and L3, through their incorporation into the polymeric matrixes, became highly emissive by aggregation. These dye@doped polymers were evaluated as temperature sensors, showing an excellent fluorescent response and reversibility after 15 cycles of heating and cooling.

Reversible Vapochromic Luminescence Accompanied by Planar Half-Chair Conformational Change of a Propeller-Shaped Boron β-Diketiminate Complex

Leakage of volatile organic compounds (VOCs) is one of the most severe industrial problems, because it can cause environmental pollution, global warming, fire, and explosion. Hence, the visualization of leakage is an essential technology to detect it at an early stage. Molecular crystals, fluorescence color of which can be changed by the exposure to VOCs could potentially serve as the sensing materials for realizing rapid and facile VOC detection. However, these materials usually require harsh conditions, such as heating or a vacuum, to recover their initial phases for reuse. Therefore, it remains a challenge to obtain completely reversible sensing systems without such energy-consuming recycling processes. Herein, the reversible color change of fluorescence from the crystals of a propeller-shaped boron β-diketiminate complex is reported. The complex was crystallized in distinct crystalline phases having different luminescent colors. Importantly, these phases were interconverted very rapidly (time constant <60 s) and repeatedly upon exposure to the vapors of the appropriate VOCs. The small energy differences between conformers of the complex could lead to this pseudopolymorphic behavior. This finding could be applied for the development of further eco-friendly reversible sensing materials based on four-coordinated boron complexes.

Aggregation-Induced Circularly Polarized Luminescence from Boron Complexes with a Carbazolyl Schiff Base

A variety of carbazolyl-appended Schiff bases were readily synthesized from 1-formylcarbazoles and aniline derivatives. Boron complexation of the resulting ligands allowed for facile preparation of new carbazole-based BODIPY analogues showing solid-state fluorescence. Furthermore, some dyes were converted into chiral compounds through the Et₂ AlCl-mediated incorporation of a binaphthyl unit. The chiral dyes showed aggregation-induced fluorescence and circularly polarized luminescence (CPL) with the Φ_F and g_lum of up to 0.22 and -3.5×10^-3 , respectively, in the solid state. The solid-state fluorescence and CPL were well characterized by the crystal packing analyses and DFT calculations.

Fluorescent Probes from Aromatic Polycyclic Nitrile Oxides: Isoxazoles versus Dihydro-1λ3,3,2λ4-Oxazaborinines

Anthracenenitrile oxide undergoes 1,3‐dipolar cycloaddition reaction with propargyl bromide affording the expected isoxazole as single regioisomer, suitably synthetically elaborated and functionalized with a protected triple bond. The introduction of a bromine atom at the position C10 of the anthracene moiety allows for inserting a variety of aromatic and heterocyclic substituents through Suzuki coupling. A two‐way synthetic route can lead to simple isoxazole derivatives or, after N−O bond reductive cleavage and BF₃ complexation, enamino ketone boron complexes. The photophysical properties of both the substituted isoxazoles and the corresponding boron complexes were investigated to show the potentialities for the employment as fluorescent tags in imaging techniques. The quite good quantum yield values confirm the suitability of these compounds in the cellular environment. Scope and limitations of the methodology are discussed.

A Stable Antiaromatic 5,20-Dibenzoyl [28]Hexaphyrin(1.1.1.1.1.1): Core AuIII Metalation and Subsequent Peripheral BIII Metalation

5,20-Dibenzoyl [28]hexaphyrin(1.1.1.1.1.1) was synthesized as the first hexaphyrin bearing meso-aroyl substituents. The meso-dibenzoyl substituents are hydrogen-bonded with the pyrrolic protons to stabilize an antiaromatic dumbbell conformer. Core metalation of this hexaphyrin with Au^III afforded rectangular and aromatic [26]hexaphyrin bis-Au^III complexes, the major isomer of which was reduced with NaBH₄ to give its antiaromatic 28π bis-Au^III complex. This complex allowed facile peripheral metalation with B^III owing to the peripheral benzoyl substituents.

Synthesis and Characterization of the Hemi-Salen Ligands and Their Triboron Complexes: Spectroscopy and Examination of Anticancer Properties

The synthesis, spectroscopic properties, and in vitro cytotoxicity activity of a series of various salen-based triboron complexes have been designed and prepared from hemi-salen (L₁ H₃  - L₄ H₃ ) ligands and BF₃ ·Et₂ O or BPh₃ under simple reaction conditions. The hemi-salen (L₁ H₃  - L₄ H₃ ) ligands and their BF₂ or BPh₂ chelating triboron complexes were characterized by means of NMR (¹ H, ¹³ C, ¹⁹ F, and ¹¹ B) spectra, FT-IR spectra, UV/VIS spectra, fluorescence spectra, mass spectra, melting point, as well as elemental analysis. The triboron [L_(1 - 4) (BF₂ )₃ ] and [L_(1 - 4) (BPh₂ )₃ ] complexes were investigated for their absorption and emission properties, and these complexes are also good chelates towards boron(III) fragments such as BF₂ or BPh₂ quantum yield in solution reaching up to 38%. The hemi-salen (L₁ H₃  - L₄ H₃ ) ligands and their BF₂ or BPh₂ chelating triboron complexes were tested for the in vitro anticancer activity against various cancer and normal cells (HeLa, DLD-1, ECC-1, PC-3, PNT-1A, and CRL-4010), and it was found that the cell viability of cancer cells was decreased while most of the healthy cells could still be viable. Also, the cytotoxicity studies showed that anticancer activity of hemi-salen (L₁ H₃  - L₄ H₃ ) ligands is higher than that of triboron [L_(1 - 4) (BF₂ )₃ ] and [L_(1 - 4) (BPh₂ )₃ ] complexes. The hemi-salen (L₁ H₃  - L₄ H₃ ) ligands showing the strongest cytotoxic effect in PC-3 cells were found to exhibit anticancer activity with apoptosis by increasing the level of ROS in the PC-3 cells.

Boron Complexes of Glyoxal-Derived Ugi Adducts as a New Class of Aggregation-Induced Emission Photoluminescent Materials

A series of O,O-chelated boron complexes was prepared through a four-component Ugi reaction followed by complexation of the resulting 1,3-dicarbonyl compounds with boron trifluoride diethyl etherate. The optical properties of these novel luminophores were investigated by UV/Vis spectroscopy and spectrofluorometry, revealing pronounced aggregation-induced emission (AIE) features.