Optoelectronic Properties of Carbon-Bound Boron Difluoride Hydrazone Dimers

The creation of dimeric boron difluoride complexes of chelating N-donor ligands is a proven strategy for the enhancement of the optoelectronic properties of fluorescent dyes. We report dimers based on the boron difluoride hydrazone (BODIHY) framework, which offer unique and sometimes unexpected substituent-dependent absorption, emission, and electrochemical properties. BODIHY dimers have low-energy absorption bands (λ_max=421 to 479 nm, ϵ=17 200 to 39 900 m ⁻¹ cm⁻¹) that are red-shifted relative to monomeric analogues. THF solutions of these dimers exhibit aggregation-induced emission upon addition of water, with emission enhancement factors ranging from 5 to 18. Thin films of BODIHY dimers are weakly emissive as a result of the inner-filter effect, attributed to intermolecular π-type interactions. BODIHY dimers are redox-active and display two one-electron oxidation and two one-electron reduction waves that strongly depend on the N-aryl substituents. These properties are rationalized using density-functional theory calculations and X-ray crystallography experiments.     

Aluminum(I)/Boron(III) Redox Reactions

Reactions between B^III species and the novel nucleophilic cyclopentadienyl‐stabilized Al^I reagent ( 1 ) result in a diversity of complexes bearing different Al/B oxidation states and coordination geometries. With the triarylborane B(C₆F₅)₃, a simple Al^I→B^III adduct is formed. In contrast, a bulky aryldihaloborane undergoes oxidative addition with the formation of a covalent bora‐alane species. With an N‐heterocyclic carbene‐stabilized amino(bromo)borenium ion, a redox reaction was observed, where the product is a borylene‐alane B^I→Al^III complex. Additionally, reaction of 1 with BI₃ results in complete scrambling of all of the Al/B‐bound substituents, and the formation of a cyclopentadienylboron(I)→AlI₃ complex. These latter reactions are the first examples of the reduction of a boron(III) compound to a borylene by a p‐block reagent, and illustrate how subtle changes in the nature of the borane can result in highly divergent reaction outcomes.     

Thiazole Boron Difluoride Dyes with Large Stokes Shift,Solid State Emission and Room-Temperature Phosphorescence

The small Stokes shift and weak emission in the solid state are two main shortcomings associated with the boron-dipyrromethene (BODIPY) family of dyes. This study presents the design, synthesis and luminescent properties of boron difluoro complexes of 2-aryl-5-alkylamino-4-alkylaminocarbonylthiazoles. These dyes display Stokes shifts (Δλ, 77–101 nm) with quantum yields (ϕ_FL) up to 64.9 and 34.7 % in toluene solution and in solid state, respectively. Some of these compounds exhibit dual fluorescence and room-temperature phosphorescence (RTP) emission properties with modulable phosphorescence quantum yields (ϕ_PL) and lifetime (τ_p up to 251 μs). The presence of intramolecular H-bonds and negligible π-π stacking revealed by X-ray crystal structure might account for the observed large Stokes shift and significant solid-state emission of these fluorophores, while the enhanced spin-orbit coupling (SOC) of iodine and the self-assembly driven by halogen bonding, π-π and C−H^…π interactions could be responsible for the observed RTP of iodine containing phosphors.     

Hexaarylbutadiene: A Versatile Scaffold with Tunable Redox Properties towards Organic Near‐Infrared Electrochromic Material

When the 1,1,4,4‐tetraanilinobutadiene skeleton is attached with two halogenated aryl units at the 2,3‐positions, they undergo facile two‐electron oxidation to give stable dicationic dyes which exhibit a near‐infrared (NIR) absorption whereas the neutral dienes show only pale color. Therefore, a distinct electrochromic response with an absorption change in the NIR region is achieved, which is attracting considerable recent attention from the viewpoint of bioimaging. Herein, we demonstrate that the redox potentials of the 1,1,4,4‐tetraanilinobutadiene can be precisely controlled by the donating properties of the amino group on the aniline unit as well as the number of halogen atoms on the aryl units at 2,3‐positions on the butadiene. In contrast, the NIR absorption bands mainly depend on the number of halogen atoms irrespective to the donating properties of aniline unit. Thus, the hexaarylbutadiene skeleton is proven to be a versatile scaffold to develop less‐explored organic NIR electrochromic materials, whose redox and spectroscopic properties can be finely tuned by modifying/attaching the proper substituents.     

新型香豆素硼氟荧光染料的合成及其光学性能

以邻氨基苯甲酸甲酯和4-二乙胺基水杨醛为原料,合成了一个新的香豆素喹啉衍生物3-{2-［8-(1H-苯并咪唑-2-基)喹啉-2-基］乙烯基}-7-二乙胺基香豆素(QMC),再与BF₃·Et₂O配位合成了硼氟配合物(BQMC),其结构经¹H NMR和MS(ESI)表征。并对BQMC的光学性能进行了研究。结果表明：BQMC的最大吸收波长在二氧六环中为490 nm,在DMSO中为532 nm; BQMC的最大发射波长在正己烷中为618 nm,在DMSO中为679 nm,与配体QMC相比,最大吸收波长红移了近50 nm,最大发射波长红移了近100 nm, BQMC的Stokes位移值从115 nm增至183 nm。在固态下,BQMC在750~825 nm之间有较宽的荧光发射峰,具有较强荧光。     

Switching of Redox Properties Triggered by a Thermal Equilibrium between Closed‐Shell Folded and Open‐Shell Twisted Species

Thermally switchable redox properties have been reported to be due to a change in the spin state of newly designed overcrowded ethylenes, which can adopt closed‐shell folded and open‐shell twisted forms. In this study, tetrathienylanthraquinodimethane derivatives were designed to be in thermal equilibrium between a more stable folded form and less stable but more donating twisted diradical in solution, so that the oxidation potential can be controlled by heating/cooling. This is the first example of a switching of redox properties based on a thermally equilibrated twisted diradical, which can be more readily oxidized to the twisted dication.     

Preparation and Redox Properties of N,N,N‐1,3,5‐Trialkylated Flavin Derivatives and Their Activity as Redox Catalysts

Eight different flavin derivatives have been synthesized and the electronic effects of substituents in various positions on the flavin redox chemistry were investigated. The redox potentials of the flavins, determined by cyclic voltammetry, correlated with their efficiency as catalysts in the H₂O₂ oxidation of methyl p‐tolyl sulfide. Introduction of electron‐withdrawing groups increased the stability of the reduced catalyst precursor.     

Redox‐Dependent Transformation of a Hydrazinobuckybowl between Curved and Planar Geometries

A red‐fluorescent heterobuckybowl with an embedded hydrazine structure was synthesized from a cyclobiphenothiazine derivative via a strained cyclobicarbazole. The hydrazinobuckybowl was found to possess bowl and twist structures in the neutral state, a shallow bowl structure in the monocation state, and a planar structure in the dication state by means of X‐ray crystallographic analysis, DFT calculations, and a comparison of experimental and calculated ¹³C NMR chemical shifts. The hydrazinobuckybowl is the first buckybowl that changes its geometry between curved bowl/twist structures and a planar structure depending on the oxidation state. The drastic geometrical change was possible as a result of the presence of two heteroatoms in the bowl skeleton and the multiple reversible redox reactions of the compound. Owing to the two kinds of bowl and twist conformations, the bowl‐inversion dynamics of the hydrazinobuckybowl were found to follow a triple‐well potential model.     

A Comprehensive Study on Redox Behavior and Halogen Exchange in Telluropyran Derivatives

Incorporation of tellurium into polycyclic compounds may endow them with unique chemical and optoelectronic properties which are not observed in their lighter chalcogen analogues. Herein, a telluropyran-containing polycyclic compound ( T1 ) synthesized through a ring-expansion reaction from the corresponding tellurophene analogue can be reversibly oxidized into halogen adducts T1•X₂ (X=Cl, Br, I) with the formation of two Te−X bonds. Their chemical structures have been verified by two-dimensional ¹H-¹H correlation spectroscopy and single crystal X-ray diffraction analysis. The halogen oxidations of T1 and the reverse thermal eliminations as well as the halogen exchange in halogen adducts T1•X₂ have been systematically investigated and compared by UV-vis absorption titration, electrochemical measurements, thermogravimetric analysis, and density functional calculations (DFT). The oxidation of Te(II) in T1 to Te(IV) in T1•X₂ results in the switch from aromaticity to nonaromaticity for the six-membered telluropyran ring, as revealed by nucleus-independent chemical shift calculations. It is also found that the halides in the halogen adducts can be exchanged by lighter ones, but not vice versa. The stabilities of the oxidized products are in the order of T1•Cl₂ > T1•Br₂ > T1•I₂ , which are consistent with the calculated rate constants and energy barriers of the elimination reactions.     

Why Boron Nitride is such a Selective Catalyst for the Oxidative Dehydrogenation of Propane

Boron‐containing materials, and in particular boron nitride, have recently been identified as highly selective catalysts for the oxidative dehydrogenation of alkanes such as propane. To date, no mechanism exists that can explain both the unprecedented selectivity, the observed surface oxyfunctionalization, and the peculiar kinetic features of this reaction. We combine catalytic activity measurements with quantum chemical calculations to put forward a bold new hypothesis. We argue that the remarkable product distribution can be rationalized by a combination of surface‐mediated formation of radicals over metastable sites, and their sequential propagation in the gas phase. Based on known radical propagation steps, we quantitatively describe the oxygen pressure‐dependent relative formation of the main product propylene and by‐product ethylene. Free radical intermediates most likely differentiate this catalytic system from less selective vanadium‐based catalysts.     

Boron(II) Cations: Interplay between Lewis‐Pair‐Acceptor and Electron‐Donor Properties

Boron(III) cations are widely used as highly Lewis acidic reagents in synthetic chemistry. In contrast, boron(II) cations are extremely rare and their chemistry almost completely unknown. They are both Lewis acids and electron donors, properties that are commonly associated with catalytically active late‐transition‐metal complexes. This double reactivity pattern ensures a rich and diverse chemistry. Herein we report the facile synthesis of several new boron(II) cations starting with a special diborane with two easily exchangeable triflate substituents. By increasing the π‐acceptor character of the neutral σ‐donor reaction partners, first reactions were developed in which the combined Lewis acidity and electron‐donor properties of boron(II) cations are applied for the reduction of organic molecules. The results of our study pave the way for applications of these unusual compounds in synthetic chemistry.     

Reductive Insertion of Elemental Chalcogens into Boron–Boron Multiple Bonds

The syntheses of sulfur‐ and selenium‐bridged cyclic compounds containing boron stabilized by N‐heterocyclic carbenes (NHCs) have been achieved by the reductive insertion of elemental chalcogens into boron–boron multiple bonds. The three pairs of bonding electrons between the boron atoms in the triply bonded diboryne enabled six‐electron reduction reactions, resulting in the formation of [2.2.1]‐bicyclic systems wherein bridgehead boron atoms are spanned by three chalcogen bridges. A similar reaction using a diborene (boron–boron double bond) resulted in the reductive transfer of both pairs of bonding electrons to three sulfur atoms, yielding a NHC‐stabilized trisulfidodiborolane. The demonstration of these six‐ and four‐electron reductions lends support to the presence of three and two pairs of bonding electrons between the boron atoms of the diboryne and diborene, respectively, a fact that may be useful in future discussions on bond order.     

Synthesis and Redox and Photophysical Properties of Benzodifuran–Spiropyran Ensembles

Two benzodifuran (BDF)‐coupled spiropyran (SP) systems and their BDF reference compounds were obtained in good yields through Huisgen–Meldal–Sharpless “click” chemistry and then subjected to investigation of their electrochemical and photophysical properties. In both SP and merocyanine (MC) forms of the coupled molecules, the BDF‐based emission is quenched to around 1 % of the quantum yield of emission from the BDF reference compounds. Based on electrochemical data, this quenching is attributed to oxidative electron‐transfer quenching. Irradiation at 366 nm results in ring opening to the MC forms of the BDF‐coupled SP compounds and the SP reference compound with a quantum efficiency of about 50 %. The rate constants for the thermal ring closing are approximately 3.4×10^?3 s^?1. However, in the photostationary states the MC fractions of the coupled molecules are substantially lower than that of the reference SP compound, attributed to the observed acceleration of the ring‐closing reaction upon irradiation. As irradiation at 366 nm invariably also excites higher‐energy transitions of the BDF units in the coupled compounds, the ring‐opening reaction is accelerated relative to the SP reference, which results in lower MC fractions in the photostationary state. Reversible photochromism of these BDF‐coupled SP compounds renders them promising in the field of molecular switches.     

Syntheses and Properties of (Nitronyl nitroxide)-substituted Tri-phenylamine ortho-Bridged by Two Oxygen and Sulfur Atoms

A nitronyl nitroxide unit ( NN ) was linked with a triphenylamine-based condensed polycyclic skeleton DOTT to form a radical substituted donor NN - DOTT . X-ray crystal structure analysis demonstrated a flat bowl shape of the DOTT unit. EPR spectra showed the localization of electron spin on the NN unit. Chemical oxidation of the DOTT unit produced radical-substituted radical cation salts NN - DOTT ⁺ ⋅ SbF₆⁻ and NN - DOTT ⁺ ⋅ FeBr₄⁻ that are stable under ambient conditions. The magnetic behavior of NN - DOTT ⁺ ⋅ SbF₆⁻ is characterized by the strong intramolecular ferromagnetic interaction between NN and DOTT ⁺. The X-ray structural analysis of NN - DOTT ⁺ ⋅ FeBr₄⁻ shows planar structure of DOTT and 1D mixed-stack column of NN-DOTT ⁺ and FeBr₄⁻. Magnetic measurements established that NN - DOTT ⁺ ⋅ FeBr₄⁻ undergoes magnetic phase transition into a weak ferromagnet at 7 K.     

A Crystalline Diazadiborinine Radical Cation and Its Boron‐Centered Radical Reactivity

One‐electron oxidation of 1,4,2,5‐diazadiborinine 1 has been studied. While the reaction of 1 a bearing phenyl groups on the B atoms with AgAl{OC(CF₃)₃}₄ afforded a complex mixture, the same oxidation reaction with 1 b featuring bulky mesityl substituents on the B atoms rendered the corresponding cation radical 2 b as an isolable species. X‐ray diffraction analysis, EPR spectroscopy, and DFT calculations of 2 b revealed the delocalization of the unpaired electron over the entire π‐system of 2 b , as well as a large spin density (0.76 in total) on the two equivalent boron atoms. The chemical trapping reaction of 2 b with p‐benzoquinone and triphenyltin hydride afforded the dicationic species 3 containing two newly formed B?O bonds and the monocationic product 2b‐H containing a B?H bond, respectively, thus confirming the boron‐centered radical reactivity of 2 b .     

Theoretical Insight into the Origin of Large Stokes Shift and Photophysical Properties of Anilido‐Pyridine Boron Difluoride Dyes

The geometric and electronic structures and photophysical properties of anilido‐pyridine boron difluoride dyes 1 – 4 , a series of scarce 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) derivatives with large Stokes shift, are investigated by employing density functional theory (DFT) and time‐dependent DFT (TD‐DFT) calculations to shed light on the origin of their large Stokes shifts. To this end, a suitable functional is first determined based on functional tests and a recently proposed index—the charge‐transfer distance. It is found that PBE0 provides satisfactory overall results. An in‐depth insight into Huang–Rhys (HR) factors, Wiberg bond indices, and transition density matrices is provided to scrutinize the geometric distortions and the character of excited states pertaining to absorption and emission. The results show that the pronounced geometric distortion due to the rotation of unlocked phenyl groups and intramolecular charge transfer are responsible for the large Stokes shift of 1 and 2 , while 3 shows a relatively blue‐shifted emission wavelength due to its mild geometric distortion upon photoemission, although it has a comparable energy gap to 1 . Finally, compound 4 , which is designed to realize the rare red emission in BODIPY derivatives, shows desirable and expected properties, such as high Stokes shift (4847 cm^?1), red emission at 660 nm, and reasonable fluorescence efficiency. These properties give it great potential as an ideal emitter in organic light‐emitting diodes. The theoretical results could complement and assist in the development of BODIPY‐based dyes with both large Stokes shift and high quantum efficiency.