The Journal of Physical Chemistry Letters最新文献

It is a common belief that the negative heat capacity change (ΔC_p) associated with protein folding, which is a manifestation of the hydrophobic effect, results from a decrease in the solvent accessible hydrophobic surface area. Herein, we investigate the conformational energy landscape and dynamics of a tetrapeptide composed of two glycine and two 4-cyanotryptophan residues using time-resolved fluorescence spectroscopy, molecular dynamics simulations, and density functional theory calculations and find that, contrary to this expectation, the hydrophobic association of two 4-cyanotryptophan side chains leads to a positive ΔC_p (approximately 543 J K^-1 mol^-1). Furthermore, we find that promoting one of the 4-cyanotryptophans to its excited electronic state strengthens this self-association. Taken together, our results provide not only insight into how modification of an aromatic amino acid can affect its hydrophobicity but also a potential strategy for designing protein sequences that can fold (unfold) at high (low) temperatures.

By hydrogenating carbon dioxide to value-added products such as methanol, heterogeneous catalysts can lower greenhouse gas emissions and generate alternative liquid fuels. The most common commercial catalyst for the reduction of CO₂ to methanol is Cu/ZnO/Al₂O₃, where ZnO improves conversion and selectivity toward methanol. The structure of this catalyst is thought to be Zn oxy(hydroxyl) overlayers on the nanometer scale on Cu. In the presence of CO₂ and H₂ under reaction conditions, the Cu substrate itself can be restructured and/or partially oxidized at its interface with ZnO, or the Zn might be reduced, possibly completely to a CuZn alloy, making the exact structure and stoichiometry of the active site a topic of active debate. In this study, we examine Zn₃ clusters on Cu(100) and Cu(111), as a subnano model of the catalyst. We use a grand canonical genetic algorithm to sample the system structure and stoichiometry under catalytic conditions: T of 550 K, initial partial pressures of H₂ of 4.5 atm and CO₂ of 0.5 atm, and 1% conversion. We uncover a strong dependence of the catalyst stoichiometry on the surface coverage. At the optimal 0.2 ML surface coverage, chains of Zn(OH) form on both Cu surfaces. On Cu(100), the catalyst has many thermally accessible metastable minima, whereas on Cu(111), it does not. No oxidation or reconstruction of the Cu is found. However, at a lower coverage of Zn, Zn₃ clusters take on a metallic form on Cu(100), and slightly oxidized Zn₃O on Cu(111), while the surface uptakes H to form a variety of low hydrides of Cu. We thus hypothesize that the 0.2 ML Zn coverage is optimal, as found experimentally, because of the stronger yet incomplete oxidation afforded by Zn at this coverage.

This study investigates lead-free organic-inorganic hybrids (C₃NH₈)₂[BiCl₅] (ABC) and (C₃NH₈)₂[BiBr₅] (ABB), focusing on their structural, dielectric, ferroelectric, and optical properties. Both compounds exhibit paraelectric (I) to ferroelectric (II) phase transitions (PTs) at 230/233 K and 228/229 K, respectively, transitioning from orthorhombic (Pnma) to monoclinic (P2₁) phases, with distorted [BiX₆]^3- octahedra forming 1D chains. Quasielastic neutron scattering and solid-state ¹H NMR studies reveal the localized motion of azetidinium cations. Dielectric measurements of ABC and ABB show step-like permittivity changes by 11-12 units post-transition I → II, demonstrating reversible switching behavior. Absorption studies reveal band gaps of 3.24 eV for ABC and 2.76 eV for ABB, classifying them as insulators. Luminescence spectra at 77K display 578 nm (ABC) and 708 nm (ABB) emissions, attributed to ³P_1,0 → ¹S₀ transitions. Both compounds demonstrate stable second harmonic generation (SHG) switching of the on-off type. The switching performance is evaluated over multiple thermal cycles using the t_req metric, which decreases with increasing temperature change rate and indicates that ABB's SHG switching is approximately 30% faster than that of ABC.

Since its appearance in [Cheng, X.; Zhang, Y.; Jónsson,E.; Jónsson, H.; Weber, P. M. Nat. Commun. 2016, 7, 11013] and recent re-investigation in [Gałyńska,M.; Ásgeirsson, V.; Jónsson, H.; Bjornsson, R. J. Phys. Chem. Lett., 2021, 12, 1250−1255], the dimethylpiperazine cation (DMP⁺) has generated considerable discussion and controversy in the scientific literature over the existence of stable, local energy minima in this molecular system. Specifically, prior assumptions that the Rydberg state and radical cation of DMP are similar have led to significant confusion and debate regarding the accuracy of various quantum chemistry methods and the existence of stable configurations of DMP⁺ itself. The purpose of this Viewpoint is to highlight recent studies that call into question the main findings in the previously mentioned works as well as present new CCSDT (Coupled-Cluster with Single, Double, and Triple excitations) calculations to finally bring closure to this controversy.

Stable triarylmethyl radicals are the most common carbon radical building blocks and have recently attracted much attention for their luminescent properties. However, magnetic circular dichroism (MCD) discovered by Michael Faraday and magnetic circularly polarized luminescence (MCPL) have not been observed for simple triarylmethyl radicals, probably due to their photodegradability. Here we report the first observation of MCD and MCPL of triarylmethyl radicals in solution using racemic mixtures of (3,5-dichloro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (PyBTM) and (3,5-difluoro-4-pyridyl)bis(2,4,6-trichlorophenyl)methyl radical (F₂PyBTM), which are much more photostable than simple triphenylmethyl radical derivatives. Faraday B terms, which are at the origin of magnetic dichroism in nondegenerate systems, were calculated using TD-DFT, and the line shape of MCD spectra was well reproduced. This study provides new circular dichroism properties for luminescent triarylmethyl radicals in solution without separating enantiomers and also clarifies the origin of magnetic circular dichroism properties of stable organic radicals for the first time.

Enhancers regulate gene expression by forming contacts with distant promoters. Phase-separated condensates or clusters formed by transcription factors (TFs) and cofactors are thought to facilitate these enhancer–promoter (E–P) interactions. Using polymer physics, we developed distinct coarse-grained chromatin models that produce similar ensemble-averaged Hi-C maps but with “stable” and “dynamic” characteristics. Our findings, consistent with recent experiments, reveal a multistep E–P communication process. The dynamic model facilitates E–P proximity by enhancing TF clustering and subsequently promotes direct E–P interactions by destabilizing the TF clusters through chain flexibility. Our study promotes physical understanding of the molecular mechanisms governing E–P communication in transcriptional regulation.

The Bi₂Se₃ family can exhibit many intriguing topological insulator properties, including a narrow bandgap and strong surface states, which show excellent nonlinear optical properties. Thinning bulk Bi₂Se₃ family materials to create a low-cost photoelectric modulation device and explaining the mechanisms of nonlinear optical differences in different types of materials remain challenges. Based on liquid-phase exfoliation technology and tapered fiber, this work prepared optoelectronic modulation devices for various samples within the Bi₂Se₃ family, quantitatively compared their nonlinear optical properties, and analyzed the sources of differentiation using the occupied and unoccupied multiband structure theory. The results correspond well to the phenomena observed in the ultrafast laser, which will provide strong support for the design of higher performance photoelectric devices based on topological insulators.

In this study, we investigate the atomic layer deposition (ALD) process on all-inorganic CsPbBr₃ perovskite nanocrystals (PNCs) to introduce an inorganic electron transport layer (ETL) in light-emitting diode (LED) devices. Two types of CsPbBr₃ PNCs were synthesized with oleate (OA) and oleylammonium (OLA) ligands on the surface. We found that CsPbBr₃ PNCs with Cs oleate surfaces experienced severe photoluminescence (PL) quenching after the ALD process, while those with oleylammonium bromide surfaces did not show any significant PL drop. Transmission electron microscopy and X-ray photoelectron spectroscopy revealed that significant Pb metal formation and Ruddlesden–Popper planar faults, linked to uncoordinated Pb²⁺ ion defects, were generated in CsPbBr₃ PNCs terminated with Cs oleate after ALD ZnO. Finally, we fabricated LEDs using PNCs with an ALD ZnO process to introduce inorganic ZnMgO nanoparticles as the ETL. The devices processed with ALD exhibited superior luminance and external quantum efficiency compared to those without the ALD process. This research provides crucial insights into the surface-dependent chemistry of PNCs and the surface-dependent performance of perovskite-based optoelectronic devices.

Metal-oxide interfaces play a critical role in catalytic processes, such as methanol adsorption and decomposition reactions. In this work, we investigated methanol reactions on the inverse model CeO₂/Ag(111) catalyst surfaces, i.e., submonolayer CeO₂ films on Ag(111), under ultrahigh vacuum (UHV) conditions to specially address the role of CeO₂–Ag interface in the catalytic methanol decomposition reactions. Using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and synchrotron radiation photoemission spectroscopy (SRPES), we found that, at the submonolayer ceria coverages, the CeO₂ nanoislands exhibit a hexagonal CeO₂(111) lattice with fully oxidized Ce⁴⁺ on Ag(111). At higher ceria coverages, multilayer ceria nanoislands form on the Ag(111) surface instead of a well-ordered film. A combination of temperature-programmed desorption (TPD) and SRPES reveals that methanol adsorbs dissociatively on the CeO₂/Ag(111) surfaces at 110 K, resulting in the formation of methoxy groups. These methoxy groups subsequently decompose via two pathways: (i) interaction with lattice oxygen to produce formate species at 230 K, which then decompose to CO, and (ii) direct dehydrogenation of methoxy to formaldehyde. Notably, the surface with submonolayer CeO₂ film on Ag(111) demonstrates low-temperature reactivity (440 K) for methoxy dehydrogenation to formaldehyde, which occurs at a much lower temperature, compared to the surface of multilayer CeO₂ on Ag(111) surface (530 K). This finding emphasizes that the CeO₂–Ag(111) interfaces provide unique active sites for methoxy dehydrogenation reactions.