Acetylene Derivatives of Cationic Diazaoxatriangulenes and Diaza [4]Helicenes - Access to Red Emitters and Planar Chiral Stereochemical Traits

Cationic triangulenes, and related helicenes, constitute a rich class of dyes and fluorophores, usually absorbing and emitting light at low energy, in the orange to red domains. Recently, to broaden the scope of applications, regioselective late-stage functionalizations on these core moieties have been developed. For instance, with the introduction of electron-donating groups (EDGs), important bathochromic shifts are observed pushing absorptions towards or in the near-infrared (NIR) spectral domain while emissive properties disappear essentially completely. Herein, to upset this drawback, acetylene derivatives of cationic diazaoxa triangulenes (DAOTA) and [4]helicenes are prepared (16 examples). Contrary to other EDG-functionalized derivatives, C≡C− functionalized products remain broadly fluorescent, with red-shifted absorptions (Δλ_abs up to 25 nm) and emissions (Δλ_em up to 73 nm, Φ_PL up to 51 %). Quite interestingly, a general dynamic stereoisomerism phenomenon is evidenced for the compounds derived from achiral DAOTA cores. At low temperature in ¹H NMR spectroscopy (218 K), N−CH₂ protons become diastereotopic with chemical shifts differences (Δδ) as high as +1.64 ppm. The signal coalescence occurs around 273 K with a barrier of ∼12 kcal mol⁻¹. This phenomenon is due to planar chiral conformations (S_p and R_p configurations), induced by the geometry of the alkyl (n-propyl) side-chains next to the acetylenic substituents. Ion pairing studies with Δ-TRISPHAT anion not only confirm the occurrence of the chiral conformations but evidence a moderate but definite asymmetric induction from the chiral anion onto the cations. Finally, DFT calculations offer a valuable insight on the geometries, the corresponding stereodynamics and also on the very large difference in NMR for some of the diastereotopic protons.     

Learning Visible Thermal Person Re-Identification via Spatial Dependence and Dual-Constraint Loss

Visible thermal person re-identification (VT Re-ID) is the task of matching pedestrian images collected by thermal and visible light cameras. The two main challenges presented by VT Re-ID are the intra-class variation between pedestrian images and the cross-modality difference between visible and thermal images. Existing works have principally focused on local representation through cross-modality feature distribution, but ignore the internal connection of the local features of pedestrian body parts. Therefore, this paper proposes a dual-path attention network model to establish the spatial dependency relationship between the local features of the pedestrian feature map and to effectively enhance the feature extraction. Meanwhile, we propose cross-modality dual-constraint loss, which adds the center and boundary constraints for each class distribution in the embedding space to promote compactness within the class and enhance the separability between classes. Our experimental results show that our proposed approach has advantages over the state-of-the-art methods on the two public datasets SYSU-MM01 and RegDB. The result for the SYSU-MM01 is Rank-1/mAP 57.74%/54.35%, and the result for the RegDB is Rank-1/mAP 76.07%/69.43%.     

Synthesis and characterisation of medium-sized ring systems by oxidative cleavage. Part 2: Insights from the study of ring expanded analogues

Variable temperature NMR analysis and computational methods have been used to develop a detailed understanding of the ¹H NMR spectra of a family of medium-sized ring containing compounds. The family consists of analogues containing 10-, 11- and 12-membered rings and in all cases the NMR spectra at room temperature showed a series of diastereotopic methylene signals despite the lack of a stereogenic centre in these systems. On repeating the NMR analysis at higher temperatures, all the signals coalesced for the 12-membered ring system consistent with full interconversion of ring conformers. This was not observed in the analogous 10- and 11-membered ring systems with the interchange of conformers remaining slow on the NMR timescale. However, 1D gs-NOESY/EXSY NMR experiments showed that in the smaller ring systems interconversion of diastereotopic protons did occur. Computational studies suggest that the dynamic process observed by NMR for the 10- and 11-membered rings systems is different from that observed in the 12-membered ring containing compound.     

The chemical reactivity of a known anti-psoriasis drug. Part 1: Further insights into the products resulting from oxidative cleavage

The oxidative cleavage of the known anti-psoriasis drug 1 to give 2 has been reported previously. Due to the importance of accessing medium-sized ring containing systems via oxidative cleavage, this reaction has been revisited revealing additional information about the structure of 2. Alternative reaction products were identified when the reaction was carried out in the presence of water. The conversion of 1 to 2 has also been carried out using ruthenium tetroxide. A detailed variable temperature NMR and computational study of the restricted rotation of the N-aryl ring in 2 is presented.     

Synthesis and VT-NMR studies of cationic hydride platinum clusters

The synthesis of three trinuclear platinum hydrides [Pt₃(L-L)₃(H)₃]⁺ (L-L = 1,2-bis(diphenylphosphino)ethane, dppe, l; 1-diphenylphosphino-2-diphenylarsinoethane, dppae. 2; 1,2-bis(diphenylarsino)ethane, dpae, 3) is reported. The complexes were characterized by IR, FAB-MS, and NMR (¹H,³¹P and¹⁹⁵Pt) spectroscopic techniques. The fast exchange of the hydride ligands, observed at ambient temperature, is frozen out at low temperature. The low-temperature¹H and³¹P NMR spectra are consistent with an open array of Pt atoms in the clusters, in keeping with a 16-electron configuration on each platinum atom. Two of the hydride ligands are terminally bonded to two metal centers, whereas the third one is µ₃-coordinated, interacting more tightly with the unique platinum atom.     

Studies on supramolecular gel formation using DOSY NMR

Herein, we present the results obtained from our studies on supramolecular self‐assembly and molecular mobility of low‐molecular‐weight gelators (LMWGs) in organic solvents using pulsed field gradient (PFG) diffusion ordered spectroscopy (DOSY) NMR. A series of concentration‐dependent DOSY NMR experiments were performed on selected LMWGs to determine the critical gelation concentration (CGC) as well as to understand the behaviour of the gelator molecules in the gel state. In addition, variable‐temperature DOSY NMR experiments were performed to determine the gel‐to‐sol transition. The PFG NMR experiments performed as a function of gradient strength were further analyzed using monoexponential DOSY processing, and the results were compared with the automated Bayesian DOSY transformation to obtain 2D plots. Our results provide useful information on the stepwise self‐assembly of small molecules leading to gelation. We believe that the results obtained from these experiments are applicable in determining the CGC and gel melting temperatures of supramolecular gels. Copyright © 2015 John Wiley & Sons, Ltd.     

Anti to syn isomerization of oxacalix[4]arene bearing two methyl groups at the intra-annular distal positions

The thermal isomerization of anti to syn stereoisomers of oxacalix[4]arene bearing two methyl groups at the intra-annular distal positions was investigated by temperature-dependent NMR spectroscopy. The conversion followed a first-order kinetics, and very slowly proceeded at 473 K in nitrobenzene-d₅ with a half-life of 7.2 h. The free energy of activation (ΔG^≠ 139 kJ mol⁻¹) is much higher than those for the ring inversion of related calix[4]arene derivatives.     

Novel pseudorotaxanes based on fluorine-containing cyclophanes and phenyl ether derivatives

Self-assembly of π-electron-deficient cyclophanes, in particular π-electron-deficient fluorine-containing bipyridine cyclophanes, is still the important branch of supramolecular chemistry today. Two kinds of novel pseudorotaxanes were constructed by the self-assembly of fluorine-containing cyclophanes with phenyl ether derivatives. Interactions between two fluorine-containing cyclophanes 2 and 3 with phenyl ether BHEEB (1,4-Bis[2-(2-hydroxyethoxy)ethoxy]benzene) 1 were well investigated by variable-temperature nuclear magnetic resonance (VT NMR), ¹H NMR titrations, solid-state findings (single-crystal X-ray diffraction) and a theoretical study. The introduction of the fluorine atom to the host cyclobis (paraquat-p-phenylene) (CBPQT) decreased π–π stacking interactions between CBPQT and BHEEB. The average binding energies of 5FCBPQT–BHEEB 1·3 were lower than the average binding energies of 1FCBPQT–BHEEB 1·2 since 5FCBPQT 3 was introduced having more electron-withdrawing fluorine atoms to CBPQT which is not good for the π–π stacking interaction.     

奥拉西坦的核磁共振波谱特征与构象研究

奥拉西坦是一种改善脑功能药物. 该文研究了该药的核磁共振波谱特征和构象. 吡咯环的各项异性效应使侧链亚甲基的质子的磁不等价. 由于吡咯环内酰胺键的部分双键性质,观测到了远程耦合现象. 信封式构象为吡咯环的优势构象. 同时研究了环外侧链上的酰胺键的受阻旋转.     

A Masked Phosphinidene Trapped in a Fluxional NCN Pincer

The trapping of a phosphinidene (R‐P) in an NCN pincer is presented. Stabilized phosphinidene 1 was characterized by ³¹P{¹H}, ¹H, and ¹³C{¹H} NMR spectroscopy, exhibiting an averaged C_2v symmetry in solution between ?60 and 60 °C. In the solid state, the phosphinidene is coordinated by one adjacent N atom featuring a formal P?N bond (1.757(2) Å) to give a five‐membered ring with some aromatic character, confirmed by DFT calculations (B3LYP‐D3/6‐311G**++) to be the ground‐state structure. Equilibration of the two N ligands occurs rapidly in solution via a “bell‐clapper”‐type process through an associative symmetric transition state calculated to lie 4.0 kcal mol^?1 above the ground state.