Segregated Array Tailoring Charge-Transfer Degree of Organic Cocrystal for the Efficient Near-Infrared Emission beyond 760 nm.

Harvesting the narrow bandgap excitons of charge-transfer (CT) complexes for the achievement of near-infrared (NIR) emission has attracted intensive attention for its fundamental importance and practical application. Herein, the triphenylene (TP)-2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4 TCNQ) CT organic complex is designed and fabricated via the supramolecular self-assembly process, which demonstrates the NIR emission with a maximum peak of 770 nm and a photoluminescence quantum yield (PLQY) of 5.4%. The segregated stacking mode of TP-F4 TCNQ CT complex based on the multiple types of intermolecular interaction has a low CT degree of 0.00103 and a small counter pitch angle of 40° between F4 TCNQ and TP molecules, which breaks the forbidden electronic transitions of CT state, resulting in the effective NIR emission. Acting as the promising candidates for the active optical waveguide in the NIR region beyond 760 nm, the self-assembled TP-F4 TCNQ single-crystalline organic microwires display an ultralow optical-loss coefficient of 0.060 dB µm-1 . This work holds considerable insights for the exploration of novel NIR-emissive organic materials via an universal "cocrystal engineering" strategy.

A Rotorlike Supramolecular Assembly, {[K(18-crown-6)]PbI3}∞, with a Reversible Breaking-Symmetry Phase Transition near Room Temperature.

A rotorlike supramolecular crystal, {[K(18-crown-6)]PbI3}∞, is composed of a linear [PbI3]∞ chain acting as a stator and [K(18-crown-6)]+ cations fastened to the [PbI3]∞ chain and K-I bond like rotators and axes, respectively. A reversible breaking-symmetry phase transition occurs at ∼305 K. Variable-temperature 1H NMR spectra and dielectrics were used for the dynamic analysis of [K(18-crown-6)]+ cations in the crystal.

Vapochromic Behavior of a Chair-Shaped Supramolecular Metallacycle with Ultra-Stability.

A new discrete supramolecular metallacycle functionalized with an alkynylplatinum(II) bzimpy moiety was successfully prepared via coordination-driven self-assembly, and it displayed a reversible color change in the solid state between yellow and red, triggered by CH2Cl2 vapor or mechanical grinding. Notably, unlike many known vapochromic systems, the obtained vapochromic metallacycle exhibits ultra-stability, with the red color remaining unchanged in air for several months at room temperature or even under vacuum for >1 week. Further investigation revealed that the chair conformation of the metallacyclic scaffold, which was thought to prevent intermolecular steric repulsion between the alkyl chain and triethylphosphine, favored close molecular stacking through intermolecular Pt···Pt and π-π stacking interactions, thus allowing such vapochromic behavior with ultra-stability.

Rotor-synchronized dipolar-filter sequence at fast MAS in solid-state NMR.

Dipolar filters are of considerable importance for eliminating the (1)H NMR signal of the rigid components of heterogeneous compounds while selecting the signal of their mobile parts. On the basis of such filters, structural and dynamical information of these compounds can often be acquired through further manipulations (e.g. spin diffusion) on the spin systems. To overcome the destructive interferences between the magic angle spinning (MAS) speed and the cycle-time of the widely-used Rotor-Asynchronized Dipolar Filter (RADF) sequence, we introduce a new method called Rotor-Synchronized Dipolar Filter (RSDF). This communication shows that this sequence does not present any interference with the spinning speed and is more compatible than RADF with high MAS frequencies (ν(R)>12 kHz). This new pulse sequence will potentially contribute to future researches on heterogeneous materials, such as multiphase polymer and membrane systems.

Determination of coordination modes and estimation of the 31P-31P distances in heterogeneous catalyst by solid state double quantum filtered 31P NMR spectroscopy.

To overcome the separation difficulty of the palladium-based homogeneous catalyst, the palladium complex can be anchored on various supports such as silica. However, it is difficult to determine the amounts of the two coordination modes of the Pd nucleus, that is, Pd coordinates with one phosphorus atom and Pd coordinates with two phosphorus atoms. Here a (31)P double-quantum filtered (DQ-filtered) method in solid-state NMR is introduced for the palladium-based heterogenous catalyst system. With the DQ-filtered method, we can not only determine the amounts of the two different kinds of palladium coordination modes, we can also estimate the interatomic distance of two (31)P nuclei bonded to a palladium nucleus. With the help of this method, we can quickly estimate interatomic distances in our designed system and accurately re-design the palladium system to accommodate either one (31)P or two (31)P.

An investigation into the effect of potassium ions on the folding of silk fibroin studied by generalized two-dimensional NMR-NMR correlation and Raman spectroscopy.

We used generalized two-dimensional NMR-NMR correlation to examine the effect of potassium ions on the conformation transition in silk fibroin to investigate the possibility that the fairly high K+ ion content found in the distal end of silk-secreting ducts in the silkworms could have a bearing on natural formation of the silk fiber. This has enabled us to propose a detailed mechanism for the transition process. Our evidence indicates that increasing the [K+] from 0 to 3.7 mg.g(-1) in the silk fibroin, as is thought to occur as the silk fibroin moves through the secretory pathway to the spigot, produces a sequence of secondary structural changes: helix and/or random coil-->helix-like-->beta-sheet-like-->beta-sheet. The sequence is the same as that produced in silk fibroin films by decreasing the pH of fibroin from 6.8 to 4.8. In addition, we used Raman spectroscopy to study the effect of K+ ions on the Fermi doublet resonance of the tyrosyl phenolic ring at 850 and 830 cm(-1). The intensity ratio I(850)/I(830) at these wave numbers indicated that the hydrogen bonding formed by the tyrosyl phenolic-OH becomes more stable with an increase in the K+ ion concentration as above. Our investigation on the effect of K+ ions on fibroin may help provide a theoretical basis for understanding the natural silk-spinning process and the conditions required for biomimetic spinning. It may also have relevance to the aggregation of other beta-sheet proteins, including prion proteins, neurofibrillary proteins and amyloid plaques.

Generalized two-dimensional correlation analysis of NMR and Raman spectra for structural evolution characterizations of silk fibroin.

Generalized two-dimensional (2D) correlation spectroscopy was used to characterize the structural evolution of silk fibroin as the pH changed from 6.8 to 4.8, demonstrating that the conformational transitions of silk fibroin are induced step by step as the pH decreases. 2D homo- and hetero-spectral correlation spectroscopy was used to establish the relationship between information extracted from NMR and Raman spectroscopy. This novel method reveals the structural evolution using two probes with different frequency scales (10(5-9) Hz for nuclear spin motion and 10(12-14) Hz for molecular vibration motion), reflecting the different spatial scale sensitivity to the molecular conformational change. The transition order is identified as silk I state (helix dominant) --> silk I intermediate state --> silk II intermediate state --> silk II state (beta-sheet dominant), as the pH decreases. The results may rationalize the silkworm spinning process, which undergoes the conformational transition steadily from the soluble helix state to the insoluble beta-sheet state as the pH decreases from the posterior to anterior glands.

An NMR approach applicable to biomolecular structure characterization.

A generalized 2D correlation NMR (GEN2D-NMR) scheme capable of substantially reducing the experimental time for two-dimensional correlation NMR experiments is described. The experimental time used in GEN2D-NMR is shortened to less than one-tenth of that required in traditional double Fourier transform 2D-NMR (FT2D-NMR) for a 13C-13C spin diffusion experiment for Nephila edulis spider threads. Experimentally, one needs to acquire many fewer points in GEN2D-NMR than that in FT2D-NMR in the evolution time domain. By keeping other acquisition pulse sequence and parameters the same in both methods, the GEN2D-NMR technique can produce a 2D-NMR spectrum using fewer points along the evolution time domain equivalent to that produced by the FT2D-NMR technique using more points. GEN2D-NMR would provide a significant advantage for those molecules that are difficult to highly isotropically label, such as proteins, polypeptides, and polymers, or those which become unstable in a prolonged measurement time.

Effect of pH and copper(II) on the conformation transitions of silk fibroin based on EPR, NMR, and Raman spectroscopy.

Much attention has been paid to the natural mechanism of silkworm spinning due to the impressive mechanical properties of the natural fibers. Our results in the present work show that the fractional changes of the conformational components in regenerated silk fibroin (SF) extracted from Bombyx mori fibers is remarkably pH- and Cu(II)-dependent as demonstrated by Cu(II) EPR, (13)C NMR, and Raman spectroscopy. Cu(II) coordination atoms in SF are changed from four nitrogens to two nitrogens and two oxygens as well as to one nitrogen and three oxygens when the pH is lowered from 8.0 to 4.0. The addition of a given amount of Cu(II) into a SF solution could induce efficiently the SF conformational fractional change from silk I, a soluble helical conformation, to silk II, an insoluble beta-sheet conformation. This behavior is strikingly similar to that seen in prion protein and amyloid beta-peptide. On the basis of the similarity in the relevant sequence in SF to the octapeptide PHGGGWGQ in PrP, we suggest that at basic and neutral pH polypeptide AHGGYSGY in SF may form a 1:1 complex with Cu(II) by coordination of imidazole N(pi) of His together with two deprotonated main-chain nitrogens from two glycine residues and one nitrogen or oxygen from serine. Such a type of coordination may make the interaction between two adjacent beta-form polypeptide chains more difficult, thereby leading to an amorphous structure. Under weakly acidic conditions, however, Cu(II)-amide linkages may be broken and Cu(II) may switch to bind two N(tau) from two histidines in adjacent peptide chains, forming an intermolecular His(N(tau))-Cu(II)-His(N(tau)) bridge. This type of coordination may induce beta-sheet formation and aggregation, leading to a crystalline structure.