Identifying (BN)2-pyrene as a new class of singlet fission chromophores: significance of azaborine substitution

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)2-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiment indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)2-pyrene suggests an in situ chromophore generation in photovoltaic devices. Two more (BN)2-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry

Functionalization of N2 via Formal 1,3-Haloboration of a Tungsten(0) σ-Dinitrogen Complex

Boron tribromide and aryldihaloboranes were found to undergo 1,3-haloboration across one W−N≡N moiety of a group 6 end-on dinitrogen complex (i.e. trans-[W(N2)2(dppe)2]). The N-borylated products consist of a reduced diazenido unit sandwiched between a WII center and a trivalent boron substituent (W−N=N−BXAr), and have all been fully characterized by NMR and IR spectroscopy, elemental analysis, and single-crystal X-ray diffraction. Both the terminal N atom and boron center in the W−N=N−BXAr unit can be further derivatized using electrophiles and nucleophiles/Lewis bases, respectively. This mild reduction and functionalization of a weakly activated N2 ligand with boron halides is unprecedented, and hints at the possibility of generating value-added nitrogen compounds directly from molecular dinitrogen

B–B Cleavage and Ring-Expansion of a 1,4,2,3-Diazadiborinine with N-Heterocyclic Carbenes

A 1,4,2,3‐diazadiborinine derivative was found to form Lewis adducts with strong two‐electron donors such as N‐heterocyclic and cyclic (alkyl)(amino)carbenes. Depending on the donor, some of these Lewis pairs are thermally unstable, converting to sole B,N‐embedded products upon gentle heating. The products of these reactions, which have been fully characterized by NMR spectroscopy, elemental analysis, and single‐crystal X‐ray diffraction, were identified as B,N‐heterocycles with fused 1,5,2,4‐diazadiborepine and 1,4,2‐diazaborinine rings. Computational modelling of the reaction mechanism provides insight into the formation of these unique structures, suggesting that a series of B−H, C−N, and B−B bond activation steps are responsible for these “intercalation” reactions between the 1,4,2,3‐diazadiborinine and NHCs

Benzothiaoline Three-Coordinated Organoboron Compounds with a BN Bond: Dual Emission and Temperature-Dependent Excimer Fluorescence

A series of 2,2-disubstituted benzothiazoline-BMes₂ (Mes = mesityl) compounds containing a BN bond have been prepared and fully characterized. Their photophysical properties were investigated by UV–vis and fluorescence spectroscopy, which revealed the presence of solvent- and concentration-dependent dual emission. On the basis of the spectroscopic data, the dual emission was assigned to monomer and excimer fluorescence of the molecule, respectively. Experimental and TD-DFT computational data indicated that the purple-blue monomer emission of these compounds is mainly from an intramolecular charge transfer (CT) transition between the benzo-sulfur moiety and the boron center. The yellow-green excimer emission is attributed to intermolecular interactions involving the benzo-sulfur unit. Furthermore, the excimer emission maxima of all compounds were found to be sensitive to temperature, shifting to lower energy with decreasing temperature, which illustrates the potential for this class of compounds to be used as luminescent thermometers

Tuning the Colors of the Dark Isomers of Photochromic Boron Compounds with Fluoride Ions: Four-State Color Switching

Combining a three-coordinated boron (BMes₂) moiety with a four-coordinated photochromic organoboron unit leads to a series of new diboron compounds that undergo four-state reversible color switching in response to stimuli of light, heat, and fluoride ions. Thus, these hybrid diboron systems allow both convenient color tuning/switching of such photochromic systems, as well as visual fluoride sensing by color or fluorescent emission color change

Triplet Energy and π‑Conjugation Effects on Photoisomerization of Chiral N,C-Chelate Organoborons with PAH Substituents

Chiral, PAH substituted N,C-chelate boron compounds are systematically investigated to establish the effect of triplet energy and substitution position on their photoreactivity. They all undergo regioselective photoisomerization, forming new dark isomers with quantum efficiencies reflecting these various factors. New PAH fused 4b<i>H</i>-azaborepins are obtained via thermal isomerization of the dark isomers. These results further implicate a photoactive triplet state in the photoisomerization process and its utility in achieving rare PAH-fused azaborepin-like heterocycles

Spiro-BODIPYs with a Diaryl Chelate: Impact on Aggregation and Luminescence

Spiro-BODIPYs with a diaryl chelate unit have been found to form J-aggregates in methanol–water solvent mixture and brightly emissive in the solid state. The diaryl chelate unit has a significant impact on J-aggregates and fluorescence of BODIPYs. Crystal structural analysis reveals that the spiro-structures facilitate J-stacking in the solid state

Spiro-BODIPYs with a Diaryl Chelate: Impact on Aggregation and Luminescence

Spiro-BODIPYs with a diaryl chelate unit have been found to form J-aggregates in methanol–water solvent mixture and brightly emissive in the solid state. The diaryl chelate unit has a significant impact on J-aggregates and fluorescence of BODIPYs. Crystal structural analysis reveals that the spiro-structures facilitate J-stacking in the solid state

Binding Modes and Reactivity of Pyrido[2,1‑<i>a</i>]isoindole as a Neutral Carbon Donor with Main-Group and Transition-Metal Elements

Various binding modes of pyrido­[2,1-<i>a</i>]­isoindole with main-group and transition-metal elements have been established. The carbon atom at position 6 of pyrido­[2,1-<i>a</i>]­isoindole is highly nucleophilic, forming a σ complex with Pt­(II) ion. The benzene ring of pyrido­[2,1-<i>a</i>]­isoindole forms an η⁶-π complex with Cr(0). The reaction of pyrido­[2,1-<i>a</i>]­isoindole with PPh₂Cl in the presence of Proton Sponge leads to a PPh₂-functionalized product, which can further react with a BH₃ molecule, forming a P→B adduct. Pyrido­[2,1-<i>a</i>]­isoindole was also found to undergo a dehydrogenative C–C coupling reaction in the presence of Cu­(I) ions, forming a dimer. These interesting reactivities and binding modes demonstrate the rich chemistry of pyrido­[2,1-<i>a</i>]­isoindole, as well as its potential application in main-group and transition-metal chemistry

Identifying (BN)₂‑pyrenes as a New Class of Singlet Fission Chromophores: Significance of Azaborine Substitution

Singlet fission converts one photoexcited singlet state to two triplet excited states and raises photoelectric conversion efficiency in photovoltaic devices. However, only a handful of chromophores have been known to undergo this process, which greatly limits the application of singlet fission in photovoltaics. We hereby identify a recently synthesized diazadiborine-pyrene ((BN)₂-pyrene) as a singlet fission chromophore. Theoretical calculations indicate that it satisfies the thermodynamics criteria for singlet fission. More importantly, the calculations provide a physical chemistry insight into how the BN substitution makes this happen. Both calculation and transient absorption spectroscopy experiments indicate that the chromophore has a better absorption than pentacene. The convenient synthesis pathway of the (BN)₂-pyrene suggests an <i>in situ</i> chromophore generation in photovoltaic devices. Two more (BN)₂-pyrene isomers are proposed as singlet fission chromophores. This study sets a step forward in the cross-link of singlet fission and azaborine chemistry