Diboron and Triboron Compounds Based on Linear and Star-Shaped Conjugated Ligands with 8-Hydroxyquinolate Functionality: Impact of Intermolecular Interaction and Boron Coordination on Luminescence

Recent Advances in the Synthesis of Borinic Acid Derivatives

Borinic acids [R2B(OH)] and their chelate derivatives are a subclass of organoborane compounds used in cross-coupling reactions, catalysis, medicinal chemistry, polymer or optoelectronics materials. In this paper, we review the recent advances in the synthesis of diarylborinic acids and their four-coordinated analogs. The main strategies to build up borinic acids rely either on the addition of organometallic reagents to boranes (B(OR)3, BX3, aminoborane, arylboronic esters) or the reaction of triarylboranes with a ligand (diol, amino alcohol, etc.). After general practical considerations of borinic acids, an overview of the main synthetic methods, their scope and limitations is provided. We also discuss some mechanistic aspects.

Four-Coordinate Monoboron Complexes with 8-Hydroxyquinolin-5-Sulfonate: Synthesis, Crystal Structures, Theoretical Studies, and Luminescence Properties

8-Hydroxyquinolin-5-sulfonic acid (8HQSA) was combined with 3-pyridineboronic acid (3PBA) or 4-pyridineboronic acid (4PBA) to give two zwitterionic monoboron complexes in crystalline form. The compounds were characterized by elemental analysis, single-crystal X-ray diffraction studies, and IR, 1H NMR, UV-Visible, and luminescence spectroscopy. The analyses revealed compounds with boron atoms adopting tetrahedral geometry. In the solid state, the molecular components are linked by charge-assisted (B)(O-H···−O(S) and N+-H···O(S) hydrogen bonds aside from C-H···O contacts and π···π interactions, as shown by Hirshfeld surface analyses and 2D fingerprint plots. The luminescence properties were characterized in terms of the emission behavior in solution and the solid state, showing emission in the bluish-green region in solution and large positive solvatofluorochromism, caused by intramolecular charge transfer. According to TD-DFT calculations at the M06-2X/6-31G(d) level of theory simulating an ethanol solvent environment, the emission properties are originated from π-π * and n-π * HOMO-LUMO transitions.

Benzimidazole-Based N,O Boron Complexes as Deep Blue Solid-State Fluorophores

Benzimidazole-based boranils were designed and synthesized in order to assess the influence of halogen substituents on their optoelectronic properties. All compounds are photoluminescent in solution and solid state. Compared to the free ligands, the new boranils emit at a lower wavelength, by elimination of the excited-state intramolecular proton transfer observed with the ligands. In the solid state, some of the boranils exhibit a deep blue emission, presenting Commission Internationale de l’Éclairage (CIE) coordinates with an x-component of less than 0.16 and a y-component smaller than 0.04, highly desired values for the development of blue emitting materials.

An overview on synthetic strategies for the construction of star-shaped molecules

Strategies for the synthesis of star-shaped molecules have been in high demand in the last decades due to the importance of those compounds in various fields. The distinctly different properties of these compounds compared to their linear analogues make them versatile building blocks for the formation of mesophases of interesting mesomorphic and photophysical properties. Moreover, the applications of star-shaped molecules as building units for dendrimers as well as in supramolecular host-guest chemistry have also been recently studied. The star-shaped molecules mentioned in this review are classified according to the central core as well as the type of side arms. The properties and applications of these compounds are described in the appropriate contexts. This report summarizes the recent advances in this area.

Terthiophene Functionalized Conjugated Triarm Polymers Containing Poly(fluorene-2,7-vinylene) Arms Having Different Cores-Synthesis and Their Unique Optical Properties

Optical properties of three types of terthiophene (3T) functionalized conjugated triarm (star-shaped) polymers consisting of poly(9,9-di-n-octyl-fluorene-2,7-vinylene) (PFV) arms and different [2,4,6-tri(biphenyl)benzene (TBP), 1,3,5-tri(benzyl)benzene (TBB), and triphenylamine (TPA)] cores, prepared by combined olefin metathesis with Wittig coupling, have been studied. Relative intensities [increases in the higher vibronic bands, (0, 1) fluorescence (FL)] of the fully conjugated TPA-core polymers, TPA(PFV-3T)3, in the fluorescence (FL) spectra in tetrahydrofuran (toluene) solution were higher than those in the other triarm polymers, TBP(PFV-3T)3, TBB(PFV-3T)3, whereas no significant differences were observed in their UV-vis spectra; notable temperature dependences were not observed in the UV-vis and FL spectra (at -5, 25, and 55 °C). Remarkable differences were not observed in the spectra in these polymer thin films, whereas λmax values red-shifted due to the formation of J-type aggregates. The observation for the time-resolved study well corresponds to results for the steady-state fluorescence measurements. The observed unique emission by the star-shaped (triarm) polymer containing the TPA core would be assumed to be due to a difference in nature of the core (higher coplanarity) compared to that of the others.

Ground-State Charge-Density Distribution in a Crystal of the Luminescent ortho-Phenylenediboronic Acid Complex with 8-Hydroxyquinoline

This contribution is devoted to the first electron density studies of a luminescent oxyquinolinato boron complex in the solid state. ortho-Phenylenediboronic acid mixed with 8-hydroxyquinoline in dioxane forms high-quality single crystals via slow solvent evaporation, which allows successful high resolution data collection (sin θ/λ = 1.2 Å^-1) and charge density distribution modeling. Particular attention has been paid to the boron-oxygen fragment connecting the two parts of the complex, and to the solvent species exhibiting anharmonic thermal motion. The experiment and theory compared rather well in terms of atomic charges and volumes, except for the boron centers. Boron atoms, as expected, constitute the most electron-deficient species in the complex molecule, whereas the neighboring oxygen and carbon atoms are the most significantly negatively charged ones. This part of the molecule appears to be very much involved in the charge transfer occurring between the acid fragment and oxyquinoline moiety leading to the observed fluorescence, as supported by the time-dependent density functional theory (TDDFT) results and the generated transition density maps. TDDFT calculations indicated that p-type atomic orbitals contributing to the HOMO-1, HOMO, and LUMO play the major role in the lowest energy transitions, and enabled further comparison with the charge density features, which is discussed in details. Furthermore, the results confirmed the known fact the Q ligand character is most important for the spectroscopic properties of this class of complexes.

2-(2-Hydroxyphenyl)imidazole-based four-coordinate organoboron compounds with efficient deep blue photoluminescence and electroluminescence

Four-coordinate organoboron compounds that can realize efficient electroluminescence with a CIE coordinate close to that of standard blue light (0.14, 0.08) have been developed.

Reunderstanding the Fluorescent Behavior of Four-Coordinate Monoboron Complexes Containing Monoanionic Bidentate Ligands

We demonstrated for the first time that, at temperatures below the melting point of a given polar solvent, the emission of some four-coordinate monoboron complexes containing monoanionic bidentate (NO) ligands shifted to lower wavelengths, but no such shift was observed for studies conducted in nonpolar solvents. This means that the emission from a polar solvent appears at shorter wavelengths if compared with that from a nonpolar solvent when the measurement was performed at low temperatures, a phenomenon totally different from that observed for conventional solvatochromic fluorophores. The finding was rationalized by considering the temperature-dependent conformational relaxation of the tetrahedron monoboron complexes from their local excited (LE) state to their relaxed excited (RE) state. Further studies revealed that variating the structure of the chelating ligands could result in remarkable changes in the fluorescent colors of the monoboron complexes. However, changing the structure of other two monodentate ligands showed little effect upon the fluorescence property of the compounds. Therefore, it is anticipated that the monoboron complexes may be taken as a platform to construct a variety of functional molecular systems via alternating the structure of the chelating ligand and that of the monodentate ligand. As an example, naphthalene was introduced as a monodentate ligand, and independent emissions from naphthalene unit and the other part of the monoboron complex as well as intramolecular energy transfer between them were observed. It is believed that the present work provides a new insight into the monoboron complexes, laying the foundation for them to be explored for developing novel molecular systems.

Boron complexes of aromatic ring fused iminopyrrolyl ligands: synthesis, structure, and luminescence properties

New fluorescent aromatic ring-fused 2-iminopyrrolyl diphenyl boron complexes are emitters in the range blue to orange.

Asymmetric multibranched conjugated molecules: synthesis, structure and photophysical properties

The symmetric multibranched π-conjugated compounds with C3 or C6 configuration have been intensively studied. The reports on asymmetric multibranched compounds are very limited. In this work, we designed and synthesized two asymmetric multibranched π-conjugated molecules using truxene as the central core, diphenylamino and thiophenyl (or thiophenylethynyl) groups as the different branches respectively: 2,7-di(N,N-diphenylamino)-12-(2-thiophenyl)-5,5',10,10',15,15'-hexaethyltruxene and 2,7-di(N,N-diphenylamino)-12-(2-thiophenylethynyl)-5,5',10,10',15,15'-hexaethyltruxene. Their photophysical properties have been explored combining with their theoretical calculation and X-ray single-crystal structure of a key intermediate. Though their different π-conjugation length of branches, the two title compounds exhibit almost same absorption maxima. However, their emission peaks behave a gradual red-shift with the increase of the conjugation length. The theoretical calculation results indicate that the two asymmetric compounds behave a main transition from the HOMO-1 to the LUMO or from the HOMO to the LUMO+1 upon excited.

Diboron complexes with bis-spiro structures as high-performance blue emitters for OLEDs

By using blue-emissive diboron complexes with bis-spiro structures as emitting layers, the most efficient blue OLEDs based on four-coordinate organoboron compounds have been achieved.

Precise one-pot synthesis of fully conjugated end-functionalized star polymers containing poly(fluorene-2,7-vinylene) (PFV) arms

A facile one-pot synthesis of end-functionalized star polymers consisting of PFVs has been achieved by olefin metathesis and Wittig-type coupling.

Tuning of the colour and chemical stability of model boranils: a strong effect of structural modifications

An improved approach to luminescent diphenylborinic complexes with functionalized salicydeneaniline ligands was developed. A strong effect of structural modifications on their stability and optical properties was established.

Facile synthesis of highly fluorescent BF2 complexes bearing isoindolin-1-one ligand

A new class of isoindolin-1-one based BF₂ complexes containing pyridine or benzothiazole groups has been prepared from a “one-pot” reaction.

Syntheses and photophysical properties of BF2 complexes of curcumin analogues

Highly photostable π-extended curcumin-BF₂ complexes with strong absorption and fluorescence ranging from 400 to 800 nm were reported.

Structural modification strategies for the rational design of red/NIR region BODIPYs

The structure–property relationships of red/NIR region BODIPY dyes is analyzed, so that trends in their photophysical properties can be readily compared.

Copper(I) coordination polymers of 2,2'-dipyridylamine derivatives: syntheses, structures, and luminescence

Reactions of CuX (X = Br(-), I(-) or CN(-)) with various types of 2,2'-dipyridylamine (dpa) derivatives have been performed via a hydrothermal-solvothermal method and the products have been structurally characterized by X-ray crystallography. Four ligands with different coordination motifs were employed in the reactions, including angular N,N,N',N'-tetra(2-pyridyl)-2,6-pyridinediamine (tppda); linear N,N,N',N'-tetra(2-pyridyl)-1,4-phenylenediamine (tppa) and N,N,N',N'-tetra(2-pyridyl)biphenyl-4,4'-diamine (tpbpa); and star-shaped tris-[4-(2,2'-dipyridylamino)-phenyl]amine (tdpa), which yielded eight copper(I) complexes exhibiting different stoichiometries of Cu-dpa and variable coordination modes of dpa. The compound [Cu(2)(tppda)(μ-I)(2)](n) (1) forms a one dimensional (1D) coordination polymer exclusively through double μ(2)-I bridges, which arranges to two dimensional (2D) metal-organic frameworks (MOFs) via the face-to-face π···π stacking interactions from pyridyl rings. The compound [Cu(6)(tppa)(μ(3)-Br)(6)](n) (2) forms a 2D network linked through multiple μ(3)-Br bridges. The compound [Cu(2)(tppa)(μ-CN)(2)](n) (3) is also a 2D MOF containing 1D (CuCN)(n) chains. The compounds [Cu(tpbpa)Br](n) (4) and [Cu(4)(tpbpa)(2)(μ-I)(4)](n) (5) display two different 1D assemblies: a zig-zag chain for 4 and a linear structure for 5. The compound [Cu(4)(tpbpa)(μ-CN)(4)](n) (6) shows a pseudo-4,8(2) topological net, while the compound [Cu(8)(tpbpa)(μ-CN)(8)](n)·2nH(2)O (7) exhibits a three-dimensional (3D) framework containing a ···PM··· double helical structure, although both of them contain (CuCN)(n) chains. The compound [Cu(2)(tdpa)(μ-I)(2)](n) (8) is a zig-zag chain based on the star-shaped molecule tpda, in which one of three dpa-arms is free of coordination to metal ions. All complexes exhibit luminescence in the solid state.

Four-coordinate organoboron compounds with a π-conjugated chelate ligand for optoelectronic applications

Four-coordinate organoboron compounds with a π-conjugated chelate backbone have emerged recently as highly attractive materials for a number of applications including use as emitters and electron-transport materials for organic light-emitting diodes (OLEDs) or organic field transistors, photoresponsive materials, and sensory and imaging materials. Many applications of this class of boron compounds stem from the electronic properties of the π-conjugated chelate backbone. Charge-transfer transitions from an aromatic substituent attached to the boron center of the π-conjugated chelate backbone and steric congestion have also been found to play important roles in the luminescent and photochromic properties of the four-coordinate boron compounds. This article provides an update-to-date account on the application aspects of this important class of compounds in materials science with the emphasis on OLED applications and photochromic switching.

Donor-acceptor assisted alkyne hydration: a luminescent boron-stabilized enol

An alkyne molecule 2-(2'-BMes(2)-phenylethynyl)pyridine (1) that contains a Lewis acidic BMes(2) group and a Lewis basic pyridyl group has been found to undergo facile hydration under ambient conditions in the presence of CuI, forming a highly emissive and stable enol ester (2). The lack of any hydration reactivity by a control compound 2-(2'-BMes(2)-phenylethynyl)benzene (3) under the same conditions supports that the cooperativity between the Lewis donor and acceptor groups plays a key role in the formation of 2.

Star-Shaped Conjugated Systems

The present review deals with the preparation and the properties of star-shaped conjugated compounds. Three, four or six conjugated arms are attached to cross-conjugated cores, which consist of single atoms (B, C⁺, N), benzene or azine rings or polycyclic ring systems, as for example triphenylene or tristriazolotriazine. Many of these shape-persistent [n]star compounds tend to π-stacking and self-organization, and exhibit interesting properties in materials science: Linear and non-linear optics, electrical conductivity, electroluminescence, formation of liquid crystalline phases, etc.