Substitution Effect of a Single Nitrogen Atom on π-Electronic Systems of Linear Polycyclic Aromatic Hydrocarbons (PAHs): Theoretically Visualized Coexistence of Mono- and Polycyclic π-Electron Delocalization.

We theoretically investigated the nitrogen substitution effect on the molecular structure and π-electron delocalization in linear nitrogen-substituted polycyclic aromatic hydrocarbons (N-PAHs). Based on the optimized molecular structures and magnetic field-induced parameters of fused bi- and tricyclic linear N-PAHs, we found that the local π-electron delocalization of subcycles (e.g., mono- and bicyclic constituent moieties) in linear N-PAHs is preserved, despite deviation from ideal structures of parent monocycles. The introduction of a fused five-membered ring with a pyrrolic N atom (N-5MR) in linear N-PAHs significantly perturbs the π-electronic condition of the neighboring fused six-membered ring (6MR). Monocyclic pyrrole exhibits substantial bond length alternations, strongly influencing the π-electronic systems of both the fused N-5MR and 6MR in linear N-PAHs, depending on the location of shared covalent bonds. A fused six-membered ring with a graphitic N atom in an indolizine moiety cannot generate monocyclic π-electron delocalization but instead contributes to the formation of polycyclic π-electron delocalization. This is evidenced by bifurcated diatropic ring currents induced by an external magnetic field. In conclusion, the satisfaction of Hückel's 4n + 2 rule for both mono- and polycycles is crucial for understanding the overall π-electron delocalization. It is crucial to consider the unique characteristics of the three types of substituted N atoms and the spatial arrangement of 5MR and 6MR in N-PAHs.

Complex ligand adsorption on 3D atomic surfaces of synthesized nanoparticles investigated by machine-learning accelerated ab initio calculation.

Nanoparticle surfaces are passivated by surface-bound ligands, and their adsorption on synthesized nanoparticles is complicated because of the intricate and low-symmetry surface structures. Thus, it is challenging to precisely investigate ligand adsorption on synthesized nanoparticles. Here, we applied machine-learning-accelerated ab initio calculation to experimentally resolved 3D atomic structures of Pt nanoparticles to analyze the complex adsorption behavior of polyvinylpyrrolidone (PVP) ligands on synthesized nanoparticles. Different angular configurations of large-sized ligands are thoroughly investigated to understand the adsorption behavior on various surface-exposed atoms with intrinsic low-symmetry. It is revealed that the ligand binding energy (Eads) of the large-sized ligand shows a weak positive relationship with the generalized coordination number . This is because the strong positive relationship of short-range direct bonding (Ebind) is attenuated by the negative relationship of long-range van der Waals interaction (EvdW). In addition, it is demonstrated that the PVP ligands prefer to adsorb where the long-range vdW interaction with the surrounding surface structure is maximized. Our results highlight the significant contribution of vdW interactions and the importance of the local geometry of surface atoms to the adsorption behavior of large-sized ligands on synthesized nanoparticle surfaces.

Real-space imaging of nanoparticle transport and interaction dynamics by graphene liquid cell TEM.

Thermal motion of colloidal nanoparticles and their cohesive interactions are of fundamental importance in nanoscience but are difficult to access quantitatively, primarily due to the lack of the appropriate analytical tools to investigate the dynamics of individual particles at nanoscales. Here, we directly monitor the stochastic thermal motion and coalescence dynamics of gold nanoparticles smaller than 5 nm, using graphene liquid cell (GLC) transmission electron microscopy (TEM). We also present a novel model of nanoparticle dynamics, providing a unified, quantitative explanation of our experimental observations. The nanoparticles in a GLC exhibit non-Gaussian, diffusive motion, signifying dynamic fluctuation of the diffusion coefficient due to the dynamically heterogeneous environment surrounding nanoparticles, including organic ligands on the nanoparticle surface. Our study shows that the dynamics of nanoparticle coalescence is controlled by two elementary processes: diffusion-limited encounter complex formation and the subsequent coalescence of the encounter complex through rotational motion, where surface-passivating ligands play a critical role.

Wafer-Scale Production of Transition Metal Dichalcogenides and Alloy Monolayers by Nanocrystal Conversion for Large-Scale Ultrathin Flexible Electronics.

Two-dimensional (2D) transition metal dichalcogenide (TMD) layers are unit-cell thick materials with tunable physical properties according to their size, morphology, and chemical composition. Their transition of lab-scale research to industrial-scale applications requires process development for the wafer-scale growth and scalable device fabrication. Herein, we report on a new type of atmospheric pressure chemical vapor deposition (APCVD) process that utilizes colloidal nanoparticles as process-scalable precursors for the wafer-scale production of TMD monolayers. Facile uniform distribution of nanoparticle precursors on the entire substrate leads to the wafer-scale uniform synthesis of TMD monolayers with the controlled size and morphology. Composition-controlled TMD alloy monolayers with tunable bandgaps can be produced by simply mixing dual nanoparticle precursor solutions in the desired ratio. We also demonstrate the fabrication of ultrathin field-effect transistors and flexible electronics with uniformly controlled performance by using TMD monolayers.

Correlating 3D Surface Atomic Structure and Catalytic Activities of Pt Nanocrystals.

Active sites and catalytic activity of heterogeneous catalysts is determined by their surface atomic structures. However, probing the surface structure at an atomic resolution is difficult, especially for solution ensembles of catalytic nanocrystals, which consist of heterogeneous particles with irregular shapes and surfaces. Here, we constructed 3D maps of the coordination number (CN) and generalized CN (CN_) for individual surface atoms of sub-3 nm Pt nanocrystals. Our results reveal that the synthesized Pt nanocrystals are enclosed by islands of atoms with nonuniform shapes that lead to complex surface structures, including a high ratio of low-coordination surface atoms, reduced domain size of low-index facets, and various types of exposed high-index facets. 3D maps of CN_ are directly correlated to catalytic activities assigned to individual surface atoms with distinct local coordination structures, which explains the origin of high catalytic performance of small Pt nanocrystals in important reactions such as oxygen reduction reactions and CO electro-oxidation.

New Condensation Polymer Precursors Containing Consecutive Silicon Atoms-Decaisopropoxycyclopentasilane and Dodecaethoxyneopentasilane-And Their Sol⁻Gel Polymerization.

The sol-gel polymerization of alkoxysilanes is a convenient and widely used method for the synthesis of silicon polymers and silicon-organic composites. The development of new sol-gel precursors is very important for obtaining new types of sol-gel products. New condensation polymer precursors containing consecutive silicon atoms-decaisopropoxycyclopentasilane (CPS) and dodecaethoxyneopentasilane (NPS)-were synthesized for the preparation of polysilane-polysiloxane material. The CPS and NPS xerogels were prepared by the sol-gel polymerization of CPS and NPS under three reaction conditions (acidic, basic and neutral). The CPS and NPS xerogels were characterized using N2 physisorption measurements (Brunauer-Emmett-Teller; BET and Brunauer-Joyner-Halenda; BJH), solid-state CP/MAS (cross-polarization/magic angle spinning) NMRs (nuclear magnetic resonances), TEM, and SEM. Their porosity and morphology were strongly affected by the structure of the precursors, and partial oxidative cleavage of Si-Si bonds occurred during the sol-gel process. The new condensation polymer precursors are expected to expand the choice of approaches for new polysilane-polysiloxane.

5-Bromo-4',5'-bis(dimethylamino)fluorescein: Synthesis and Photophysical Studies.

In this study, three new fluorescein derivatives-5-bromo-4',5'-dinitrofluorescein (BDNF), 5-bromo-4',5'-diaminofluorescein (BDAF), and 5-bromo-4',5'-bis(dimethylamino)fluorescein (BBDMAF)-were synthesized and their pH-dependent protolytic equilibria were investigated. In particular, BBDMAF exhibited pH-dependent fluorescence, showing strong emission only at pH 3-6. BBDMAF bears a bromine moiety and thus, can be used in various cross-coupling reactions to prepare derivatives and take advantage of its unique emission properties. To confirm this, the Suzuki and Sonogashira reactions of BBDMAF with phenylboronic acid and phenylacetylene, respectively, were performed, and the desired products were successfully obtained.

Self-assembled plasmonic nanoparticles on vertically aligned carbon nanotube electrodes via thermal evaporation.

This study details the development of a large-area, three-dimensional (3D), plasmonic integrated electrode (PIE) system. Vertically aligned multiwalled carbon nanotube (VA-MWNT) electrodes are grown and populated with self-assembling silver nanoparticles via thermal evaporation. Due to the geometric and surface characteristics of VA-MWNTs, evaporated silver atoms form nanoparticles approximately 15-20 nm in diameter. The nanoparticles are well distributed on VA-MWNTs, with a 5-10 nm gap between particles. The size and gap of the self-assembled plasmonic nanoparticles is dependent upon both the length of the MWNT and the thickness of the evaporated silver. The wetting properties of water of the VA-MWNT electrodes change from hydrophilic (∼70°) to hydrophobic (∼120°) as a result of the evaporated silver. This effect is particularly pronounced on the VA-MWNT electrodes with a length of 1 µm, where the contact angle is altered from an initial 8° to 124°. Based on UV-visible spectroscopic analysis, plasmonic resonance of the PIE systems occurs at a wavelength of approximately 400 nm. The optical behavior was found to vary as a function of MWNT length, with the exception of MWNT with a length of 1 µm. Using our PIE systems, we were able to obtain clear surface-enhanced Raman scattering (SERS) spectra with a detection limit of ∼10 nM and an enhancement factor of ∼10(6). This PIE system shows promise for use as a novel electrode system in next-generation optoelectronics such as photovoltaics, light-emitting diodes, and solar water splitting.

Few layer graphene matrix for matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

We present the employment of few layer graphene (FLG) as a matrix for the analysis of low molecular weight polymeric compounds using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The practicality of FLG as a matrix for MALDI experiments is demonstrated by analyzing low molecular weight polymers, polar polyethylene glycol (PEG) of 1000 Da and nonpolar polymethylmethacrylate (PMMA) of 650 Da. The high quality MS spectra without low-mass interference signals without any further sampling procedure were acquired.

SnO2/graphene composites with self-assembled alternating oxide and amine layers for high Li-storage and excellent stability.

An alternating stack (SG/GN) consisting of SnO2-functionalized graphene oxide (SG) and amine-functionalized GO (GN) is prepared in solution. The thermally reduced SG/GN (r(SG/GN)) with decreased micro-mesopores and completely eliminated macropores, results in a high reversible capacity and excellent capacity retention (872 mA h g⁻¹ after 200 cycles at 100 mA g⁻¹) when compared to a composite without GN.

Surface-enhanced Raman scattering of single- and few-layer graphene by the deposition of gold nanoparticles.

Surface-enhanced Raman scattering (SERS) of graphene on a SiO(2)(300 nm)/Si substrate was investigated by depositing Au nanoparticles using thermal evaporation. This provided a maximum enhancement of 120 times for single-layer graphene at 633 nm excitation. SERS spectra and scan images of single-layer and few-layer graphene were acquired. Single-layer graphene provides much larger SERS enhancement compared to few-layer graphene, while in single-layer graphene the enhancement of the G band was larger than that of the 2D band. Furthermore, the D bands were identified in the SERS spectra; these bands were not observed in a normal Raman spectrum without Au deposition. Appearance of the D band is ascribed to the considerable SERS enhancement and not to an Au deposition-induced defect. Lastly, SERS enhancement of graphene on a transparent glass substrate was compared with that on the SiO(2)(300 nm)/Si substrate to exclude enhancement by multiple reflections between the Si substrate and deposited Au nanoparticles. The contribution of multiple reflections to total enhancement on the SiO(2)(300 nm)/Si substrate was 1.6 times out of average SERS enhancement factor, 71 times.

Femtosecond time-resolved stimulated Raman reveals the birth of bacteriorhodopsin's J and K intermediates.

Light-activated proton translocation in halobacteria is driven by photoisomerization of the retinal chromophore within the membrane-bound protein bacteriorhodopsin. The molecular mechanism of this process has been widely debated due to the absence of structural information on the time scale of the reactive dynamics (the initial 0.1-1 ps). Here we use tunable femtosecond stimulated Raman spectroscopy to obtain time-resolved resonance Raman vibrational spectra of bacteriorhodopsin's key J and K photoisomerization intermediates. The appearance of the J state is delayed by approximately 150 fs relative to the zero of time and rises after this dwell with a 450 fs time constant. The J state is characterized by a 16 cm(-1) red-shifted C=C stretch, which blue shifts by 5 cm(-1) coincident with the rise of the K state. The delayed 3 ps rise of the C(15)-H HOOP mode with enhanced intensity in K reveals the appearance of strain near the Schiff's base once the 13-cis configuration is fully formed. The delay in the initial appearance of J is assigned to nuclear dynamics on the excited state that precede the formation of the proper geometry for reactive internal conversion.

Origin of negative and dispersive features in anti-Stokes and resonance femtosecond stimulated Raman spectroscopy.

Femtosecond stimulated Raman spectroscopy is extended to probe ground state anti-Stokes vibrational features. Off resonance, negative anti-Stokes features are seen that are the mirror image of the positive Stokes side spectra. On resonance, the observed dispersive lineshapes are dramatically dependent on the frequencies of the picosecond pump and femtosecond probe pulses used to generate the stimulated Raman spectra. These observations are explained by the contributions of the inverse Raman and hot luminescence four-wave mixing processes discussed by Sun et al. [J. Chem. Phys. 128, 144114 (2008)], which contribute to the overall femtosecond stimulated Raman signal.

Development of a tunable femtosecond stimulated raman apparatus and its application to beta-carotene.

We have developed a tunable femtosecond stimulated Raman spectroscopy (FSRS) apparatus and used it to perform time-resolved resonance Raman experiments with <100 fs temporal and <35 cm(-1) spectral resolution. The key technical change that facilitates this advance is the use of a tunable narrow-bandwidth optical parametric amplifier (NB-OPA) presented recently by Shim et al. (Shim, S.; Mathies, R. A. Appl. Phys. Lett. 2006, 89, 121124). The practicality of tunable FSRS is demonstrated by examining the photophysical dynamics of beta-carotene. Using 560 nm Raman excitation, the resonant S1 state modes are enhanced by a factor of approximately 200 compared with 800 nm FSRS experiments. The improved signal-to-noise ratios facilitate the measurement of definitive time constants for beta-carotene dynamics including the 180 fs appearance of the S1 vibrational features due to direct internal conversion from S2 and their characteristic 9 ps decay to S0. By tuning the FSRS system to 590 nm Raman excitation, we are able to selectively enhance vibrational features of the hot ground state S hot 0 and monitor its approximately 5 ps cooling dynamics. This tunable FSRS system is valuable because it facilitates the direct observation of structural changes of selected resonantly enhanced states and intermediates during photochemical and photobiological reactions.

Ring closure reaction dynamics of diarylethene derivatives in solution.

Photochromic ring closure reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophen-3-yl)hexafluoro cyclopentene and its derivatives in solution has been studied by femtosecond time-resolved fluorescence. Time-resolved spontaneous fluorescence of the open isomer reveals a fast component of around 1 ps and a slow component on the order of 100 ps. Fluorescence time profiles, reaction quantum yields, and relative populations of the parallel (C(s) symmetry) and antiparallel (C(2) symmetry) conformations indicate that both time components are attributable mostly to the C(2) conformer that undergoes the ring closure reaction. The fast component is assigned to the direct ring closure reaction, and the slow component is assigned to the reaction through conformation change. Time constants of the slow component for the derivatives are inversely proportional to the reaction quantum yields, suggesting that the rate of the conformational dynamics is comparable to the rate of other population relaxation processes. The relative amplitude and exact time constant of the fast component depend on the detection wavelength displaying a higher relative amplitude with shorter time constant at longer wavelengths. The results allow us to propose a conformational inhomogeneity model, in which a broad distribution of conformations of the open isomers in the ground state is projected into two minima in the excited electronic potential surface to lead to the slow and the fast reaction pathways.