Chasing weakly-bound biological water in aqueous environment near the peptide backbone by ultrafast 2D infrared spectroscopy.

There has been a long-standing debate as to how many hydrogen bonds a peptide backbone amide can form in aqueous solution. Hydrogen-bonding structural dynamics of N-ethylpropionamide (a ß-peptide model) in water was examined using infrared (IR) spectroscopy. Two amide-I sub bands arise mainly from amide C=O group that forms strong H-bonds with solvent water molecules (SHB state), and minorly from that involving one weak H-bond with water (WHB state). This picture is supported by molecular dynamics simulations and ab-initio calculations. Further, thermodynamics and kinetics of the SHB and WHB species were examined mainly by chemical-exchange two-dimensional IR spectroscopy, yielding an activation energy for the SHB-to-WHB exchange of 13.25 ± 0.52 kJ mol‒1, which occurs in half picosecond at room temperature. Our results provided experimental evidence of an unstable water molecule near peptide backbone, allowing us to gain more insights into the dynamics of the protein backbone hydration.

Synthesis of 12-Connected Three-Dimensional Covalent Organic Framework with lnj Topology.

The structural exploration of three-dimensional covalent organic frameworks (3D COFs) is of great significance to the development of COF materials. Different from structurally diverse MOFs, which have a variety of connectivity (3-24), now the valency of 3D COFs is limited to only 4, 6, and 8. Therefore, the exploration of organic building blocks with higher connectivity is a necessary path to broaden the scope of 3D COF structures. Herein, for the first time, we have designed and synthesized a 12-connected triptycene-based precursor (triptycene-12-CHO) with 12 symmetrical distributions of aldehyde groups, which is also the highest valency reported until now. Based on this unique 12-connected structure, we have successfully prepared a novel 3D COF with lnj topology (termed 3D-lnj-COF). The as-synthesized 3D COF exhibits honeycomb main pores and permanent porosity with a Brunauer-Emmett-Teller surface area of 1159.6 m2 g-1. This work not only provides a strategy for synthesizing precursors with a high connectivity but also provides inspiration for enriching the variety of 3D COFs.

Ultralow-background SERS substrates for reliable identification of organic pollutants and degradation intermediates.

Chemical methods for preparing SERS substrates have the advantages of low cost and high productivity, but the strong background signals from the substrate greatly limit their applications in characterization and identification of organic compounds. Herein, we developed a one-step synthesis method to prepare silver nanoparticle substrates with ultralow SERS background using anionic ligands as stabilizing agents and applied the SERS substrate for the reliable and reproducible identification of typical organic pollutants and corresponding degradation intermediates. The synthesis method shows excellent universality to different reducing agents cooperating with different anionic ligands (Cl-, Br-, I-, SCN-). As model applications, the machine learning algorithm can realize the precise prediction of six organophosphorus pesticides and eight sulfonamide antibiotics with 100% accuracy based on SERS training data. More importantly, the ultralow-background SERS substrate enables one to detect and identify the time-dependent degradation intermediates of organophosphorus pesticides by combining them with density functional theory (DFT) calculations. All the results indicate that the ultralow-background SERS substrate will greatly push the development of SERS characterization applications.

Significance of Antisolvents on Solvation Structures Enhancing Interfacial Chemistry in Localized High-Concentration Electrolytes.

Localized high-concentration electrolytes (LHCEs) provide a new way to expand multifunctional electrolytes because of their unique physicochemical properties. LHCEs are generated when high-concentration electrolytes (HCEs) are diluted by antisolvents, while the effect of antisolvents on the lithium-ion solvation structure is negligible. Herein, using one-dimensional infrared spectroscopy and theoretical calculations, we explore the significance of antisolvents in the model electrolyte lithium bis(fluorosulfonyl)imide/dimethyl carbonate (LiFSI/DMC) with hydrofluoroether. We clarify that the role of antisolvent is more than dilution; it is also the formation of a low-dielectric environment and intensification of the inductive effect on the C=O moiety of DMC caused by the antisolvent, which decrease the binding energy of the Li+···solvent and Li+···anion interactions. It also has beneficial effects on interfacial ion desolvation and Li+ transport. Furthermore, antisolvents also favor reducing the lowest unoccupied molecular orbital (LUMO) energy level of the solvated clusters, and FSI- anions show a decreased reduction stability. Consequently, the influence of antisolvents on the interfacial chemical and electrochemical activities of solvation structures cannot be ignored. This finding introduces a new way to improve battery performance.

Ultrafast Structure and Vibrational Dynamics of a Cyano-Containing Non-Fullerene Acceptor for Organic Solar Cells Revealed by Two-Dimensional Infrared Spectroscopy.

Non-fullerene molecules, such as ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene) indanone)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene), are among the most promising non-fullerene acceptors for organic solar cells (OSCs). In this work, using the cyano stretching mode as a vibrational marker, the structural and vibrational energy dynamics of ITIC were examined on an ultrafast time scale with two-dimensional infrared spectroscopy. Two IR-active modes studied here mainly correspond to two anti-symmetric combinations of symmetric and asymmetric stretching vibrations of two C≡N modes originating from two -C(CN)2 chromophores that are located across the ITIC system, which were found to have significantly larger off-diagonal anharmonicity than their corresponding diagonal anharmonicities. This indicates strong anharmonic vibrational coupling between the two modes, which is supported by ab initio anharmonic frequency computations. Transient IR results indicate picosecond intramolecular vibrational energy transfer between the two C≡N modes upon excitation. The structural basis for these vibrational and energetic features is the conjugating molecular frame that is composed of a network of single/double bonds connecting the two -C(CN)2 chromophores and may enable efficient vibration delocalization, in addition to its well-known electron delocalization capability. The importance of the results for the OSC applications is discussed.

Excited-state photophysical processes in a molecular system containing perylene bisimide and zinc porphyrin chromophores.

Multichromophoric systems with efficient photoinduced excited-state processes are important for the conversion of solar energy in artificial photosynthesis. However, a low molecular absorption coefficient of these multichromophoric systems in the near-infrared region limits their power conversion efficiency in organic solar cells. It is critical to design molecules with a broad absorption range in the whole spectral region, to better harvest solar energy, and to reveal their important multiple-step photophysical processes for the design of organic solar cells. Here, we investigate a novel compound having three chromophores, namely two near-by N,N'-bis(1-pentyl)hexyl-3,4,9,10-perylenebiscarboximide (PDI) units linked to a zinc porphyrin core side by side (in the form of PDI-ZnPor-PDI), which absorbs solar energy ranging from the ultraviolet (UV) to near-infrared regions. The photophysical behavior of PDI-ZnPor-PDI in both film and solution forms, has been investigated using steady-state and transient spectroscopy measurements. Charge-transfer species and triplet excited-state species are observed, the excited-state evolutions of which are monitored using molecular vibrations as probes. These observations support the idea that PDI-ZnPor-PDI on photoexcitation generates the radical anion and triplet species of the PDI unit (PDI˙- and 3PDI*). Our results demonstrate the effect of solid film state on the photophysical properties in such multichromophoric system, and are valuable for guiding the design and utilization of novel near-infrared electron donors or acceptors for use in organic solar cells.

Superstretchable Dynamic Polymer Networks.

Superstretchable materials have many applications in advanced technological fields but are difficult to stretch to more than 1000× their original length. A superstretchable dynamic polymer network that can be stretched to 13 000× its original length is designed. It is revealed that superstretchability of the polymer network is derived from the synergistic effect of two different types of dynamic bonds, including a small number of strong dynamic imine bonds to maintain the network integrity during stretching and a large number of weak ionic hydrogen bonds to dissipate energy. This approach provides new insights into the design of superstretchable polymers.

Intensified C≡C Stretching Vibrator and Its Potential Role in Monitoring Ultrafast Energy Transfer in 2D Carbon Material by Nonlinear Vibrational Spectroscopy.

In this work, an intensity-enhanced C≡C stretching infrared (IR) absorption is observed in hexakis[(trimethylsilyl)ethynyl]benzene (HTEB), whose IR transition dipole magnitude becomes comparable to that of a typical C═O stretch, and the enhancement is believed to be due to a joint effect of π-π conjugation and hyperconjugation associated with a terminal trimethylsilyl group. Using dynamical time-dependent two-dimensional infrared (2D IR) spectroscopy, a picosecond intramolecular energy redistribution process is observed between two nondegenerate C≡C stretching modes, whose symmetry breaking is attributed to a noncovalent halogen-bonding interaction between HTEB and solvent CH2Cl2. The rigid structure of HTEB and limited structural dynamics are also inferred from the insignificant initial spectral diffusion value extracted from the 2D IR spectra. This work provides the first nonlinear infrared investigation of the conventionally weak C≡C stretch. The methods outlined are particularly important for detailed understanding of the structure-related processes such as vibrational energy transfer in novel C≡C species containing materials such as graphdiyne.

Solvent-dependent structural dynamics of an azido-platinum complex revealed by linear and nonlinear infrared spectroscopy.

The vibrational and anisotropic relaxation dynamics and structural dynamics of a potential anticancer prodrug, trans,trans,trans-[Pt(N3)2(OH)2(py)2], were investigated using time-resolved infrared pump-probe spectroscopy and ultrafast two-dimensional infrared (2D IR) spectroscopy. Herein, two representative bio-friendly solvents, H2O and DMSO, were used, in which the local structural and dynamical variations were monitored using the antisymmetric linear combination of the two N3 stretching vibrational modes as an infrared probe. It was found that the vibrational relaxation process of the N3 antisymmetric stretching (as) mode in H2O is two to three times faster than that in DMSO. The anisotropic relaxation process of the anticancer prodrug was observed to be hindered in DMSO; this indicated a tighter solvent environment around the sample molecule in this solvent. The vibrational frequency time correlation of the N3 antisymmetric stretching mode in H2O decays with a time constant of 1.94 ps, in agreement with the hydrogen bond formation and breaking times of water. In DMSO, the frequency time correlation of the N3 as mode decays on a much longer time scale; this further indicates its sensitivity to the out-layer DMSO structural dynamics, which are relatively static in the experimental time window.