Luminescent Di‐ and Trinuclear Boron Complexes Based on Aromatic Iminopyrrolyl Spacer Ligands: Synthesis,Characterization, and Application in OLEDs

New bis‐ and tris(iminopyrrole)‐functionalized linear (1,2‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 2 ), 1,3‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 3 ), 1,4‐(HNC₄H₃‐C(H)?N)₂‐C₆H₄ ( 4 ), 4,4′‐(HNC₄H₃‐C(H)?N)₂‐(C₆H₄‐C₆H₄) ( 5 ), 1,5‐(HNC₄H₃C‐(H)?N)₂‐C₁₀H₆ ( 6 ), 2,6‐(HNC₄H₃C‐(H)?N)₂‐C₁₀H₆ ( 7 ), 2,6‐(HNC₄H₃C‐(H)?N)₂‐C₁₄H₈ ( 8 )) and star‐shaped (1,3,5‐(HNC₄H₃‐C(H)?N‐1,4‐C₆H₄)₃‐C₆H₃ ( 9 )) π‐conjugated molecules were synthesized by the condensation reactions of 2‐formylpyrrole ( 1 ) with several aromatic di‐ and triamines. The corresponding linear diboron chelate complexes (Ph₂B[1,3‐bis(iminopyrrolyl)‐phenyl]BPh₂ ( 10 ), Ph₂B[1,4‐bis(iminopyrrolyl)‐phenyl]BPh₂ ( 11 ), Ph₂B[4,4′‐bis(iminopyrrolyl)‐biphenyl]BPh₂ ( 12 ), Ph₂B[1,5‐bis(iminopyrrolyl)‐naphthyl]BPh₂ ( 13 ), Ph₂B[2,6‐bis(iminopyrrolyl)‐naphthyl]BPh₂ ( 14 ), Ph₂B[2,6‐bis(iminopyrrolyl)‐anthracenyl]BPh₂ ( 15 )) and the star‐shaped triboron complex ([4′,4′′,4′′′‐tris(iminopyrrolyl)‐1,3,5‐triphenylbenzene](BPh₂)₃ ( 16 )) were obtained in moderate to good yields, by the treatment of 3 – 9 with B(C₆H₅)₃. The ligand precursors are non‐emissive, whereas most of their boron complexes are highly fluorescent; their emission color depends on the π‐conjugation length. The photophysical properties of the luminescent polyboron compounds were measured, showing good solution fluorescence quantum yields ranging from 0.15 to 0.69. DFT and time‐dependent DFT calculations confirmed that molecules 10 and 16 are blue emitters, because only one of the iminopyrrolyl groups becomes planar in the singlet excited state, whereas the second (and third) keeps the same geometry. Compound 13 , in which planarity is not achieved in any of the groups, is poorly emissive. In the other examples ( 11 , 12 , 14 , and 15 ), the LUMO is stabilized, narrowing the gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO), and the two iminopyrrolyl groups become planar, extending the size of the π‐system, to afford green to yellow emissions. Organic light‐emitting diodes (OLEDs) were fabricated by using the new polyboron complexes and their luminance was found to be in the order of 2400 cd m^?2, for single layer devices, increasing to 4400 cd m^?2 when a hole‐transporting layer is used.     

Tuning the Reactivity of Isocyano Group: Synthesis of Imidazoles and Imidazoliums from Propargylamines and Isonitriles in the Presence of Multiple Catalysts

The reaction of propargyl amines with tert‐butylisonitrile in the presence of a catalytic amount of both Yb(OTf)₃ and AgOTf afforded imidazoles, whereas the same reaction with primary and secondary alkylisonitriles, as well as arylisonitriles, in the presence of three metal salts [Yb(OTf)₃/AgOTf/KOTf] resulted in the 1,3,4,5‐tetrasubstituted imidazoliums in excellent yields. Both chiral amines and chiral isonitriles can be used to provide corresponding chiral heterocycles without racemization. In this multiple catalytic system, Yb(OTf)₃ catalyzed the insertion of isonitriles to the N? H bond of amines, AgOTf catalyzed the 5‐exo‐dig cyclization of the resulting amidine nitrogen to the tethered triple bond, and KOTf promoted the salt metathesis, thus providing at the same time the counterion to the imidazolium. Against common knowledge, the isocyano group acted in these reactions as a polarized triple bond instead of conventional carbene‐like function.     

On the Role of the Electronic Structure of the Heteronuclear Oxide Cluster [Ga2Mg2O5].+ in the Thermal Activation of Methane and Ethane: An Unusual Doping Effect

The reactivity of the heteronuclear oxide cluster [Ga₂Mg₂O₅]^.+, bearing an unpaired electron at a bridging oxygen atom (O_b^.?), towards methane and ethane has been studied using Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS). Hydrogen‐atom transfer (HAT) from both methane and ethane to the cluster ion is identified experimentally. The reaction mechanisms of these reactions are elucidated by state‐of‐the‐art quantum chemical calculations. The roles of spin density and charge distributions in HAT processes, as revealed by theory, not only deepen our mechanistic understanding of C? H bond activation but also provide important guidance for the rational design of catalysts by pointing to the particular role of doping effects.     

Development of Ar‐BINMOL‐Derived Atropisomeric Ligands with Matched Axial and sp3 Central Chirality for Catalytic Asymmetric Transformations

Recently, academic chemists have renewed their interest in the development of 1,1′‐binaphthalene‐2,2′‐diol (BINOL)‐derived chiral ligands. Six years ago, a working hypothesis, that the chirality matching of hybrid chirality on a ligand could probably lead to high levels of stereoselective induction, prompted us to use the axial chirality of BINOL derivatives to generate new stereogenic centers within the same molecule with high stereoselectivity, obtaining as a result sterically favorable ligands for applications in asymmetric catalysis. This Personal Account describes our laboratory's efforts toward the development of a novel class of BINOL‐derived atropisomers bearing both axial and sp³ central chirality, the so‐called Ar‐BINMOLs, for asymmetric synthesis. Furthermore, on the basis of the successful application of Ar‐BINMOLs and their derivatives in asymmetric catalysis, the search for highly efficient and enantioselective processes also compelled us to give special attention to the BINOL‐derived multifunctional ligands with multiple stereogenic centers for use in catalytic asymmetric reactions.

     

Effect of Immunosuppressants Tacrolimus and Mycophenolate Mofetil on the Keratinocyte UVB Response

Nonmelanoma skin cancer, derived from epidermal keratinocytes, is the most common malignancy in organ transplant recipients, causes serious morbidity and mortality, and is strongly associated with solar ultraviolet (UV) exposure. Preventing and treating skin cancer in these individuals has been extraordinarily challenging. Following organ transplantation, the immunosuppressants are used to prevent graft rejection. Until now, immunosuppression has been assumed to be the major factor leading to skin cancer in this setting. However, the mechanism of skin carcinogenesis in organ transplant recipients has not been understood to date; specifically, it remains unknown whether these cancers are immunosuppression‐dependent or ‐independent. In particular, it remains poorly understood what is the mechanistic carcinogenic action of the newer generation of immunosuppressants including tacrolimus and mycophenolate mofetil (MMF). Here, we show that tacrolimus and MMF impairs UVB‐induced DNA damage repair and apoptosis in human epidermal keratinocytes. In addition, tacrolimus inhibits UVB‐induced checkpoint signaling. However, MMF had no effect. Our findings have demonstrated that tacrolimus and MMF compromises proper UVB response in keratinocytes, suggesting an immunosuppression‐independent mechanism in the tumor‐promoting action of these immunosuppressants.     

Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

Blinking of the photoluminescence (PL) emitted from individual conjugated polymer chains is one of the central observations made by single‐molecule spectroscopy (SMS). Important information, for example regarding excitation energy transfer, can be extracted by evaluating dynamic quenching. However, the nature of trap states, which are responsible for PL quenching, often remains obscured. We present a detailed investigation of the photon statistics of single poly(3‐hexylthiophene) (P3HT) chains obtained by SMS. The photon statistics provide a measure of the number and brightness of independently emitting areas on a single chain. These observables can be followed during blinking. A decrease in PL intensity is shown to be correlated with either 1) a decrease in the average brightness of the emitting sites; or 2) a decrease in the number of emitting regions. We attribute these phenomena to the formation of 1) shallow charge traps, which can weakly affect all emitting areas of a single chain at once; and 2) deep traps, which have a strong effect on small regions within the single chains.     

Terpenoids from the Stems of Drypetes congestiflora

Two new eremophilane‐type sesquiterpenoids, 1α‐hydroxyeremophila‐6,9,11‐trien‐8‐one ( 1 ), 4α‐hydroxyeremophila‐1,9‐diene‐3,8‐dione ( 2 ), and a new friedelane‐type triterpenoid, friedelane‐3α,16β‐diol ( 4 ), along with six known terpenoids, 3 and 5 – 9 , have been isolated from the stems of Drypetes congestiflora. Their structures and relative configurations were elucidated on the basis of detailed spectroscopic analyses and by comparison of their NMR data with those reported in the literature. All of the compounds, 1 – 9 , were isolated for the first time from this species. Compound 3 exhibited moderate cytotoxic activities against the A549 and B16F10 cell lines.     

Indachlorins: Nonplanar Indanone‐Annulated Chlorin Analogues with Panchromatic Absorption Spectra between 300 and 900 nm

Indaphyrins, pyrrole‐modified porphyrins containing a cleaved pyrrole β,β′‐bond and two annulated indanone moieties, possess unusually broadened and redshifted UV/Vis spectra because of their π‐expanded chromophores. The parent free base indaphyrin has been crystallographically characterized, highlighting its strongly ruffled conformation incorporating a helimeric twist. It was shown to be susceptible to regiospecific derivatizations at the opposite side of the ring‐cleaved pyrrole (dihydroxylation, followed by functional group transformations of the resulting diol functionality), generating indaphyrin‐based chlorin analogues, indachlorins, that incorporate a dihydroxypyrroline, pyrrolindione, oxazolone, or a morpholine moiety. Structural modifications resulted in further broadening and hyper‐ and bathochromic shifts of the optical spectra, some of which possess a nearly panchromatic absorption between 300 to well above 900 nm. The extents to which these modifications affect their solid‐state conformations were analyzed.     

Engineering Homochiral Metal–Organic Frameworks by Spatially Separating 1D Chiral Metal–Peptide Ladders: Tuning the Pore Size for Enantioselective Adsorption

The reaction of the chiral dipeptide glycyl‐L(S)‐glutamate with Co^II ions produces chiral ladders that can be used as rigid 1D building units. Spatial separation of these building units with linkers of different lengths allows the engineering of homochiral porous MOFs with enhanced pore sizes, pore volumes, and surface areas. This strategy enables the synthesis of a family of isoreticular MOFs, in which the pore size dictates the enantioselective adsorption of chiral molecules (in terms of their size and enantiomeric excess).     

Stochastic/Controlled Symmetry Breaking of the T8‐POSS Cages toward Multifunctional Regioisomeric Nanobuilding Blocks

Precise synthesis of nanobuilding blocks with accurately positioned functional groups presents a daunting challenge. Herein, a practical synthesis and thorough characterization of a series of T₈‐polyhedral oligomeric silsesquioxane (POSS) di‐ and triadducts is reported. Upon addition of triflic acid across the double bonds in octavinylPOSS (V₈T₈) followed by hydrolysis, the cubic symmetry of the T₈‐POSS cage (O_h) is broken into C_2v (ortho‐), C_2v (meta‐), and D_3d (para‐) for diadducts and further to C_s (oom‐), C_s (omp‐), and C_3v (mmm‐) for triadducts in a stochastic fashion. Their structures and regioconfigurations have been unambiguously demonstrated by ¹H, ¹³C, and ²⁹Si NMR spectroscopy, as well as MALDI‐TOF mass spectrometry. The assignment of the diadducts was further corroborated by converting each individual diadduct into triadduct(s), the structure of which is controlled by the symmetry of the precursor. Except for the C_3v triadduct, they can all be prepared in synthetically useful quantities. The presence of two types of highly reactive and mutually orthogonal functional groups facilitates further modification into complex nanostructures and composite materials. These unique regioisomers provide a versatile platform for constructing giant molecules and Janus silsesquioxanes.