A Minimal Cluster Model of Valence Electrons in Adatom-Assisted Adsorbed Molecules: NCH₃/Cu(110) and OCH₃/Cu(110)

In this study, we found that the local density of states and ionization energy spectrum of the valence electrons of methylnitrene (NCH₃) adsorbed on Cu(110) can be calculated from molecular orbital calculations of a simple artificial isolated NCH₃–Cu₂ molecular cluster in which the two Cu atoms form bonds to the N atom. Such a NCH₃–Cu₂ cluster represents the basic structural unit of a NCH₃ molecule adsorbing on a Cu double-added-row structure. This finite NCH₃–Cu₂ cluster structure is not optimized as a single system but is extracted directly from an optimized surface structure obtained by density functional theory with periodic boundary conditions. With this approach, we obtained excellent agreement between the measured ultraviolet photoemission spectra (UPS) and the theoretical calculation results. To further examine this minimal cluster concept, we analyzed methoxy (OCH₃) adsorption on Cu(110) and found a OCH₃–Cu₃ cluster structure. On the basis of this structure, we calculated UPS and also obtained substantial agreement with the experimental UPS. These results may indicate that, when substrate adatoms bridge the adsorption of a molecule and a surface, a small cluster consisting of the adsorbate and the neighboring bonding substrate adatoms suffices to describe the electronic structure of the valence electrons of the adsorbates

Characteristics of Low-Frequency Molecular Phonon Modes Studied by THz Spectroscopy and Solid-State ab Initio Theory: Polymorphs I and III of Diflunisal

THz absorption spectra of two polymorphs of diflunisal, form I and form III, exhibit distinct features due to the influence of packing conformations on the frequency distributions and IR activities of gamma point phonon modes within the 100 cm^–1 region. In order to understand the origins of these THz modes, we perform a detailed mode analysis. The result shows that although the spectral features are different, these low-frequency phonon modes of the two molecular polymorphs have similar vibrational characteristics in terms of harmonic couplings of intermolecular and intramolecular vibrations

Intramolecular Vibrations in Low-Frequency Normal Modes of Amino Acids: l‑Alanine in the Neat Solid State

This paper presents a theoretical analysis of the low-frequency phonons of l-alanine by using the solid-state density functional theory at the Γ point. We are particularly interested in the intramolecular vibrations accessing low-frequency phonons via harmonic coupling with intermolecular vibrations. A new mode-analysis method is introduced to quantify the vibrational characteristics of such intramolecular vibrations. We find that the torsional motions of COO^– are involved in low-frequency phonons, although COO^– is conventionally assumed to undergo localized torsion. We also find the broad distributions of intramolecular vibrations relevant to important functional groups of amino acids, e.g., the COO^– and NH₃⁺ torsions, in the low-frequency phonons. The latter finding is illustrated by the concept of frequency distribution of vibrations. These findings may lead to immediate implications in other amino acid systems

Deciphering Anomalous Raman Features of Regioregular Poly(3-hexylthiophene) in Ordered Aggregation Form

Poly­(3-hexylthiophene) (P3HT), being a prototypic conjugated polymer, bears a high charge mobility that is sensitive to its packing configuration in the condensed phase. Despite its extensive experimental study with X-ray diffraction, its specified packing structure still remains stymied. This study searched for possible structures of crystalline P3HT and identified the one that holds a simulated Raman spectrum most approximate to the experimental one of ordered P3HT aggregates in the frozen solvent. The spectral correspondence shows that the Raman-active C–C stretch peak exhibits a red shift in frequency, while the CC stretch peak displays a blue shift as the layer planarity of P3HT is relaxed. Moreover, the CC peak splits into two when adjacent thiophene rings in the P3HT chain hold a dihedral angle of 22° with respect to each other. This study demonstrates that Raman spectroscopy plus first-principles simulations can serve as a powerful tool to resolve fine structures of molecular crystals

Scanning Tunneling Microscopy and Density Functional Theory Studies of Adatom-Involved Adsorption of Methylnitrene on Copper(110) Surface

In this study, we used scanning tunneling microscopy (STM) and density functional theory (DFT) to examine the bonding structure of CH₃N adsorbed on the Cu(110) surface. A previous study [<i>Chin. J. Phys.</i> <b>2005</b>, <i>43</i>, 212–218] shows the adsorbed CH₃N aggregate to form a zigzag structure with a <i>p</i>(2 × 3) unit cell, without considering the possibility of adsorbate-induced surface reconstruction. Here, we propose a revised adsorption structure, with the key feature of bonding each CH₃N with two Cu adatoms in a tetrahedral manner. Three structure models (double-row, dimer, and alternative-dimer) are examined by ab initio calculations. We find that the most energetically favorable model is the double-row model with CH₃N bonding alternatingly along either side of double added rows from Cu adatoms

Quantum Switching of π‑Electron Rotations in a Nonplanar Chiral Molecule by Using Linearly Polarized UV Laser Pulses

Nonplanar chiral aromatic molecules are candidates for use as building blocks of multidimensional switching devices because the π electrons can generate ring currents with a variety of directions. We employed (<i>P</i>)-2,2′-biphenol because four patterns of π-electron rotations along the two phenol rings are possible and theoretically determine how quantum switching of the π-electron rotations can be realized. We found that each rotational pattern can be driven by a coherent excitation of two electronic states under two conditions: one is the symmetry of the electronic states and the other is their relative phase. On the basis of the results of quantum dynamics simulations, we propose a quantum control method for sequential switching among the four rotational patterns that can be performed by using ultrashort overlapped pump and dump pulses with properly selected relative phases and photon polarization directions. The results serve as a theoretical basis for the design of confined ultrafast switching of ring currents of nonplanar molecules and further current-induced magnetic fluxes of more sophisticated systems

Elucidation of Chiral Symmetry Breaking in a Racemic Polymer System with Terahertz Vibrational Spectroscopy and Crystal Orbital Density Functional Theory

The conservation of chiral symmetry has been used as a fundamental rule to determine polymer packing conformations in racemic systems. We have illustrated, through the interplay of polarization terahertz (THz) spectroscopy and solid-state density functional theory, that the chiral symmetry is not conserved in a poly­(lactic acid) stereocomplex (scPLA) system. Poly­(l-lactic acid) (PLLA) displays a weaker violation of the 3₁ screw symmetry than poly­(d-lactic acid) (PDLA) and possesses a stronger intramolecular vibrational energy, on average, in the low-frequency gamma-point phonon modes than does PDLA. Polarization THz spectroscopy adds a new dimension to polymer crystallography through which new phenomena are expected to be revealed

Benzoselenadiazole Fluorescent Probes – Near-IR Optical and Ratiometric Fluorescence Sensor for Fluoride Ion

A highly selective and sensitive near-IR optical sensor, benzoselenadiazole based diarylamine (<b>TBS-HN</b>), for fluoride (F^–) has been designed and synthesized. <b>TBS-HN</b> also shows turn-on ratiometric fluorescence signaling in the presence of F^– by inhibiting the excited state intramolecular proton transfer (ESIPT) processes

Quantum Localization of Coherent π‑Electron Angular Momentum in (<i>P</i>)‑2,2′-Biphenol

Controlling π-electrons with delocalized character is one of the fundamental issues in femtosecond and attosecond chemistry. Localization of π-electron rotation by using laser pulses is expected to play an essential role in nanoscience. The π-electron rotation created at a selected aromatic ring of a single molecule induces a local intense electromagnetic field, which is a new type of ultrafast optical control functioning. We propose a quantum localization of coherent π-electron angular momentum in (<i>P</i>)-2,2′-biphenol, which is a simple, covalently linked chiral aromatic ring chain molecule. The localization considered here consists of sequential two steps: the first step is to localize the π-electron angular momentum at a selected ring of the two benzene rings, and the other is to maintain the localization. Optimal control theory was used for obtaining the optimized electric fields of linearly polarized laser pulses to realize the localization. The optimal electric fields and the resultant coherent electronic dynamics are analyzed

Semiconductor Behavior of a Three-Dimensional Strontium-Based Metal–Organic Framework

The self-assembly of a three-dimensional strontium-based metal–organic framework [Sr­(Hbtc)­(H₂O)]_<i>n</i> (<b>1</b>) was achieved through the reaction of Sr­(NO₃)₂ with a 1,2,4-benzenetricarboxylic acid (1,2,4-H₃btc) ligand under hydrothermal conditions. This Sr-based metal–organic framework exhibits remarkable semiconducting behavior, as evidenced by theoretical calculations and experimental measurements. Temperature-dependent DC conductivity, near-room-temperature AC conductivity, diffuse reflection spectra, and photoluminescence spectra provide strong proof that compound <b>1</b> shows a band gap of 2.3 eV, which is comparable to that for other commonly available semiconducting materials (e.g., CdSe, CdTe, ZnTe, GaP, etc.). The optimized molecular structure and electronic properties (density of states and band gap energy) of <b>1</b> were calculated using density functional theory, and the results are consistent with experimental findings. This is the first report on the semiconducting properties of a strontium-based MOF, which will pave the way for further studies in semiconducting MOFs with interesting potential applications in optoelectronic devices