Energy Level, Crystal Morphology and Fluorescence Emission Tuning in Cocrystals via Molecular-Level Engineering

Organic donor-acceptor complexes as new organic semiconductor class have attracted wide attention, due to their potential applications in functional optoelectronics. Herein, we present two new charge transfer cocrystals of di-cyanodiazafluorene -perylene (DCPE) and di-cyanodiazaflfluorene-pyrene (DCPY) through a rational cocrystal-engineering strategy. Although they are both 1 : 1 mixed stacking cocrystals with similar chemical structures, the DCPE cocrystal possesses a non-centrosymmetric space group and narrower band gap compared to DCPY cocrystal, because of the non-covalent bonding variation. The electrostatic potential accumulated in the lateral facets leads to highly twisted DCPE nanobelts, and the small band gap causes near infrared fluorescence. Meanwhile, the DCPY crystals with centrosymmetric space groups and weaker intermolecular interactions exhibited an untwisted morphology and red emission. This study will be helpful for the design and understanding of functional cocrystal materials that can be used in flexible micro/nano-mechanics, mechanical energy, and optical devices.

Design, Synthesis and Hydrogen Bonding of B3 N6 -[4]Triangulene

Replacement of the allylic C=C-C unit with a N-B-N unit at each of the three zigzag edges of [4]triangulene gives rise to B₃ N₆ -[4]triangulene, which is envisioned to represent a key structural unit of a new hypothetical boron carbon nitride (BC₄ N). A tert-butylated B₃ N₆ -[4]triangulene has been successfully synthesized by three-fold nitrogen-directed borylation, and the X-ray crystallographic analysis indicates that its slightly bent triangular polycyclic framework can be viewed as a 1,3,5-triphenylbenzene connected by three 4π-electron N-B-N units. The HN-B-NH moiety provides a dual hydrogen-bond donor, which forms H-bonds with halide or carboxylate anions in solution, and form DD-AA hydrogen-bond arrays with 2,7-di(tert-butyl)-pyrene-4,5,9,10-tetraone in the co-crystal. Moreover, the blue fluorescence of B₃ N₆ -[4]triangulene in solution is responsive to binding p-nitrobenzoate anion through hydrogen bonds.

Hydrogen-Binding-Initiated Activation of O-H Bonds on a Nitrogen-Doped Surface for the Catalytic Oxidation of Biomass Hydroxyl Compounds

Hydrogen binding of molecules on solid surfaces is an attractive interaction that can be used as the driving force for bond activation, material-directed assembly, protein protection, etc. However, the lack of a quantitative characterization method for hydrogen bonds (HBs) on surfaces seriously limits its application. We measured the standard Gibbs free energy change (ΔG⁰ ) of on-surface HBs using NMR. The HB-accepting ability of the surface was investigated by comparing ΔG⁰ values employing the model biomass platform 5-hydroxymethylfurfural on a series of Co-N-C-n catalysts with adjustable electron-rich nitrogen-doped contents. Decreasing ΔG⁰ improves the HB-accepting ability of the nitrogen-doped surface and promotes the selectively initiated activation of O-H bonds in the oxidation of 5-hydroxymethylfurfural. As a result, the reaction kinetics is accelerated. In addition to the excellent catalytic performance, the turnover frequency (TOF) for this oxidation is much higher than for reported non-noble-metal catalysts.

Color- and Dimension-Tunable Light-Harvesting Organic Charge-Transfer Alloys for Controllable Photon-Transport Photonics

An electron donor/acceptor pair comprising perylene (Pe) and 9,10-dicyanoanthracene (DCA) was specifically designed to construct organic charge-transfer (CT) alloys via weak CT interaction through a solution co-assembly route. By adjusting the molar ratio between Pe and DCA, we achieve color- and dimension-tunable CT alloy assemblies involving one-dimensional (1D) (DCA)_1-x (Pe)_x (0 ≤ x ≤10 %) microribbons and two-dimensional (2D) (Pe)_1-y (DCA)_y (0 ≤ y ≤5 %) nanosheets as a consequence of energy transfer from DCA or α-Pe to Pe-DCA CT complex. Importantly, dimension-related optical waveguiding performances are also revealed: continuously adjustable optical loss in 1D (DCA)_1-x (Pe)_x microribbons and successive conversion from isotropic waveguide to anisotropic waveguide in 2D (Pe)_1-y (DCA)_y nanosheets. The present work provides a desired platform for in-depth investigation of light-harvesting organic CT alloy assemblies, which show promising applications in miniaturized optoelectronic devices.

Dramatic Enhancement of Binding Affinities Between Foldamer-Based Receptors and Anions by Intra-Receptor π-Stacking

As a synthetic model for intra-protein interactions that reinforce binding affinities between proteins and ligands, the energetic interplay of binding and folding was investigated using foldamer-based receptors capable of adopting helical structures. The receptors were designed to have identical hydrogen-bonding sites for anion binding but different aryl appendages that simply provide additional π-stacking within the helical backbones without direct interactions with the bound anions. In particular, the presence of electron-deficient aryl appendages led to dramatic enhancements in the association constant between the receptor and chloride or nitrate ions, by up to three orders of magnitude. Extended stacking within the receptor contributes to the stabilization of the entire folding structure of complexes, thereby enhancing binding affinities.