2020 Vol. 33, No. 5

2020, 33(5): i-iv.
2020, 33(5): v-vi.
Si(111) electrode has been widely used in electrochemical and photoelectrochemical studies. The potential dependent measurements of the second harmonic generation (SHG) were performed to study Si(111) electrode interface. At different azimuthal angles of the Si(111) and under different polarization combinations, the curve of the intensity of SHG with extern potential has a different form of line or parabola. Quantitative analysis showed that these differences in the potential-dependence can be explained by the isotropic and anisotropic contribution of the Si(111) electrode. The change in the isotropic and anisotropic contribution of the Si(111) electrode may be attributed to the increase in the doping concentration of Si(111) electrodes.
Two thin-film 2D organic-inorganic hybrid perovskites, i.e., 2-phenylethylammonium lead iodide (PEPI) and 4-phenyl-1-butylammonium lead iodide (PBPI) were synthesized and investigated by steady-state absorption, temperature-dependent photoluminescence, and temperature-dependent ultrafast transient absorption spectroscopy. PBPI has a longer organic chain (via introducing extra ethyl groups) than PEPI, thus its inorganic skeleton can be distorted, bringing on structural disorder. The comparative analyses of spectral profiles and temporal dynamics revealed that the greater structural disorder in PBPI results in more defect states serving as trap states to promote exciton dynamics. In addition, the fine-structuring of excitonic resonances was unveiled by temperature-dependent ultrafast spectroscopy, suggesting its correlation with inorganic skeleton rather than organic chain. Moreover, the photoexcited coherent phonons were observed in both PEPI and PBPI, pointing to a subtle impact of structural disorder on the low-frequency Raman-active vibrations of inorganic skeleton. This work provides valuable insights into the optical properties, excitonic behaviors and dynamics, as well as coherent phonon effects in 2D hybrid perovskites.
Highly luminescent bulk two-dimensional covalent organic frameworks (COFs) attract much attention recently. Origin of their luminescence and their large Stokes shift is an open question. After first-principles calculations on two kinds of COFs using the GW method and Bethe-Salpeter equation, we find that monolayer COF has a direct band gap, while bulk COF is an indirect band-gap material. The calculated optical gap and optical absorption spectrum for the direct excitons of bulk COF agree with the experiment. However, the calculated energy of the indirect exciton, in which the photoelectron and the hole locate at the conduction band minimum and the valence band maximum of bulk COF respectively, is too low compared to the fluorescence spectrum in experiment. This may exclude the possible assistance of phonons in the luminescence of bulk COF. Luminescence of bulk COF might result from exciton recombination at the defects sites. The indirect band-gap character of bulk COF originates from its AA-stacked conformation. If the conformation is changed to the AB-stacked one, the band gap of COF becomes direct which may enhance the luminescence.
The simple homodinuclear M$ - $M single bonds for group II and XII elements are difficult to obtain as a result of the fulfilled s$ ^2 $ electronic configurations, consequently, a dicationic prototype is often utilized to design the M$ ^+ $$ - $M$ ^+ $ single bond. Existing studies generally use sterically bulky organic ligands L$ ^- $ to synthesize the compounds in the L$ ^- $$ - $M$ ^+ $$ - $M$ ^+ $$ - $L$ ^- $ manner. However, here we report the design of Mg$ - $Mg and Zn$ - $Zn single bonds in two ligandless clusters, Mg$ _2 $B$ _7 $$ ^- $ and Zn$ _2 $B$ _7 $$ ^- $, using density functional theory methods. The global minima of both of the clusters are in the form of M$ _2 $$ ^{2+} $(B$ _7 $$ ^{3-} $), where the M$ - $M single bonds are positioned above a quasi-planar hexagonal B$ _7 $ moiety. Chemical bonding analyses further confirm the existence of Mg$ - $Mg and Zn$ - $Zn single bonds in these clusters, which are driven by the unusually stable B$ _7 $$ ^{3-} $ moiety that is both $ \sigma $ and $ \pi $ aromatic. Vertical detachment energies of Mg$ _2 $B$ _7 $$ ^- $ and Zn$ _2 $B$ _7 $$ ^- $ are calculated to be 2.79 eV and 2.94 eV, respectively, for the future comparisons with experimental data.
We report a study on photo-ionization of benzene and aniline with incidental subsequent dissociation by the customized reflection time-of-flight mass spectrometer utilizing a deep ultraviolet 177.3 nm laser. Highly efficient ionization of benzene is observed with a weak C$ _4 $H$ _3 $$ ^+ $ fragment formed by undergoing disproportional C$ - $C bond dissociation. In comparison, a major C$ _5 $H$ _6 $$ ^{+\cdot} $ fragment and a minor C$ _6 $H$ _6 $$ ^{+\cdot} $ radical are produced in the ionization of aniline pertaining to the removal of CNH$ ^\cdot $ and NH$ ^\cdot $ radicals, respectively. First-principles calculation is employed to reveal the photo-dissociation pathways of these two molecules having a structural difference of just an amino group. It is demonstrated that hydrogen atom transfer plays an important role in the cleavage of C$ - $C or C$ - $N bonds in benzene and aniline ions. This study is helpful to understand the underlying mechanisms of chemical bond fracture of benzene ring and related aromatic molecules.
The infrared multiphoton dissociation (IRMPD) spectrum of the protonated heterodimer of ProPheH$ ^+ $, in the range of 2700-3700 cm$ ^{-1} $, has been obtained with a Fourier-transform ion cyclotron mass spectrometer combined with an IR OPO laser. The experimental spectrum shows one peak at 3565 cm$ ^{-1} $ corresponding to the free carboxyl O-H stretching vibration, and two broad peaks centered at 2935 and 3195 cm$ ^{-1} $. Theoretical calculations were performed on the level of M062X/6-311++G(d, p). Results show that the most stable isomer is characterized by a charge-solvated structure in which the proton is bound to the unit of proline. Its predicted spectrum is in good agreement with the experimental one, although the coexistence of salt-bridged structures cannot be entirely excluded.
Methyl vinyl ketone oxide, an unsaturated four-carbon Criegee intermediate produced from the ozonolysis of isoprene has been recognized to play a key role in determining the tropospheric OH concentration. It exists in four configurations ($ anti $-$ anti $, $ anti $-$ syn $, $ syn $-$ anti $, and $ syn $-$ syn $) due to two different substituents of saturated methyl and unsaturated vinyl groups. In this study, we have carried out the electronic structure calculation at the multi-configurational CASSCF and multi-state MS-CASPT2 levels, as well as the trajectory surface-hopping nonadiabatic dynamics simulation at the CASSCF level to reveal the different fates of $ syn $/$ anti $ configurations in photochemical process. Our results show that the dominant channel for the S$ _1 $-state decay is a ring closure, isomerization to dioxirane, during which, the $ syn $(C$ - $O) configuration with an intramolecular hydrogen bond shows slower nonadiabatic photoisomerization. More importantly, it has been found for the first time in photochemistry of Criegee intermediate that the cooperation of two heavy groups (methyl and vinyl) leads to an evident pyramidalization of C3 atom in methyl-vinyl Criegee intermediate, which then results in two structurally-independent minimal-energy crossing points (CIs) towards the $ syn $(C$ - $O) and $ anti $(C$ - $O) sides, respectively. The preference of surface hopping for a certain CI is responsible for the different dynamics of each configuration.
Inspired by the branching corrected surface hopping (BCSH) method [J. Xu and L. Wang, J. Chem. Phys. 150 , 164101 (2019)], we present two new decoherence time formulas for trajectory surface hopping. Both the proposed linear and exponential formulas characterize the decoherence time as functions of the energy difference between adiabatic states and correctly capture the decoherence effect due to wave packet reflection as predicted by BCSH. The relevant parameters are trained in a series of 200 diverse models with different initial nuclear momenta, and the exact quantum solutions are utilized as references. As demonstrated in the three standard Tully models, the two new approaches exhibit significantly higher reliability than the widely used counterpart algorithm while holding the appealing efficiency, thus promising for nonadiabatic dynamics simulations of general systems.
Formaldehyde and hydrogen peroxide are two important realistic molecules in atmospheric chemistry. We implement path integral Liouville dynamics (PILD) to calculate the dipole-derivative autocorrelation function for obtaining the infrared spectrum. In comparison to exact vibrational frequencies, PILD faithfully captures most nuclear quantum effects in vibrational dynamics as temperature changes and as the isotopic substitution occurs.
Two non-ionic hydro-fluorocarbon hybrid surfactants with and without hydroxyl groups were synthesized and compared. They exhibited good thermal stability and superior surface activity. It was observed that the hydroxyl group had a profound effect on modifying the surface tension of their solutions and the morphology of the formed micelles. This effect may be attributed to the rearranging of the alkane group from above the air/aqueous surface to below it and the disrupting of the interfacial water structure induced by the hydroxyl groups. This work provides a strategy to weaken the immiscibility between hydrocarbon and fluorocarbon chains by modifying their orientational structure at the interface, thus it is helpful for the design of surfactants with varied interfacial properties.
A fundamental study on C-C coupling, that is the crucial step in the Fischer-Tropsch synthesis (FTS) process to obtain multi-carbon products, is of great importance to tailor catalysts and then guide a more promising pathway. It has been demonstrated that the coupling of CO with the metal carbide can represent the early stage in the FTS process, while the related mechanism is elusive. Herein, the reactions of the CuC$ _3 $H$ ^- $ and CuC$ _3 $$ ^- $ cluster anions with CO have been studied by using mass spectrometry and theoretical calculations. The experimental results showed that the coupling of CO with the C$ _3 $H$ ^- $ moiety of CuC$ _3 $H$ ^- $ can generate the exclusive ion product COC$ _3 $H$ ^- $. The reactivity and selectivity of this reaction of CuC$ _3 $H$ ^- $ with CO are greatly higher than that of the reaction of CuC$ _3 $$ ^- $ with CO, and this H-assisted C-C coupling process was rationalized by theoretical calculations.
Photo-induced proton coupled electron transfer (PCET) is essential in the biological, photosynthesis, catalysis and solar energy conversion processes. Recently, $ p $-nitrophenylphenol (HO-Bp-NO2) has been used as a model compound to study the photo-induced PCET mechanism by using ultrafast spectroscopy. In transient absorption spectra both singlet and triplet states were observed to exhibit PCET behavior upon laser excitation of HO-Bp-NO2. When we focused on the PCET in the triplet state, a new sharp band attracted us. This band was recorded upon excitation of HO-Bp-NO2 in aprotic polar solvents, and has not been observed for $ p $-nitrobiphenyl which is without hydroxyl substitution. In order to find out what the new band represents, acidic solutions were used as an additional proton donor considering the acidity of HO-Bp-NO2. With the help of results in strong ($ \sim $10$ ^{-1} $ mol/L) and weak ($ \sim $10$ ^{-4} $ mol/L) acidic solutions, the new band is identified as open shell singlet O-Bp-NO2H, which is generated through protonation of nitro O in $ ^3 $HO-Bp-NO2 followed by deprotonation of hydroxyl. Kinetics analysis indicates that the formation of radical $ \cdot $O-Bp-NO2 competes with O-Bp-NO2H in the way of concerted electron-proton transfer and/or proton followed electron transfers and is responsible for the low yield of O-Bp-NO2H. The results in the present work will make it clear how the $ ^3 $HO-Bp-NO2 deactivates in aprotic polar solvents and provide a solid benchmark for the deeply studying the PCET mechanism in triplets of analogous aromatic nitro compounds.
Recent experiments report the rotation of FA (FA = HC[NH2]2$ ^+ $) cations significantly influence the excited-state lifetime of FAPbI3. However, the underlying mechanism remains unclear. Using ab initio nonadiabatic (NA) molecular dynamics combined with time-domain density functional simulations, we have demonstrated that reorientation of partial FA cations significantly inhibits nonradiative electron-hole recombination with respect to the pristine FAPbI3 due to the decreased NA coupling by localizing electron and hole in different positions and the suppressed atomic motions. Slow nuclear motions simultaneously increase the decoherence time, which is overcome by the reduced NA coupling, extending electron-hole recombination time scales to several nanoseconds and being about 3.9 times longer than that in pristine FAPbI3, which occurs within sub-nanosecond and agrees with experiment. Our study established the mechanism for the experimentally reported prolonged excited-state lifetime, providing a rational strategy for design of high performance of perovskite solar cells and optoelectronic devices.
The photodissociation dynamics of AlO at 193 nm is studied using time-sliced ion velocity mapping. Two dissociation channels are found through the speed and angular distributions of aluminum ions: one is one-photon dissociation of the neutral AlO to generate Al($ ^2 $P$ _ \rm{u} $)+O($ ^3 $P$ _ \rm{g} $), and the other is two-photon ionization and then dissociation of AlO$ ^+ $ to generate Al$ ^+ $($ ^1 $S$ _ \rm{g} $)+O($ ^3 $P$ _ \rm{g} $). Each dissociation channel includes the contribution of AlO in the vibrational states $ v $ = 0-2. The anisotropy parameter of the neutral dissociation channel is more dependent on the vibration state of AlO than the ion dissociation channel.