The [Au25(SR)18]− and [Au13(dppe)5Cl2]3+ [dppe = 1,2-bis(diphenylphosphino)ethane] nanoclusters both possess a 13-atom icosahedral core with 8 delocalized superatomic electrons (8e), but their emission properties and time-resolved electron dynamics differ significantly. In this work, experimental photoluminescence and photoluminescence decay measurements are combined with time-dependent density functional theory calculations of radiative and nonradiative decay properties and lifetimes to elucidate the similarities and differences in the emission of these two nanoclusters with similar cores. In this work, the photodynamic properties of [Au13(dppe)5Cl2]3+ are elucidated theoretically for the first time. [Au13(dppe)5Cl2]3+ exhibits a single strong emission peak compared to the weaker bimodal luminescence of [Au25(SR)18]− (modeled here as [Au25(SH)18]−). The strongly emissive state is found to arise from deexcitation out of the S1 state, similar to what is seen for [Au25(SH)18]−. Both theory and experiment exhibit microsecond lifetimes for this state. Transient absorption measurements and theoretical calculations demonstrate that the excited-state lifetimes for higher excited states are typically less than 1 ps. The decay times for the higher excited states of [Au13(dppe)5Cl2]3+ and its model compound [Au13(pe)5Cl2]3+ [pe = 1,2-bis(phosphino)ethane] are observed to be shorter than the lifetimes of the corresponding states of [Au25(SR)18]−; this occurs because the energy gap separating degenerate sets of unoccupied orbitals is only ∼0.2 eV in [Au13(dppe)5Cl2]3+ compared to a ∼0.6 eV energy gap in [Au25(SH)18]−.
We investigate the doping process theoretically for singly doped MAu24, MAg24, and MAu37 (M = Ni, Pd, Pt, Cu, Ag/Au, Zn, Cd, Hg, Ga, In, and Tl) clusters using density functional theory (DFT). For all clusters, the group X dopants (Ni, Pd, and Pt) prefer the central location due to the relative stability of d electrons in the dopant. For dopants in groups XI–XIII, doping on the surface of the core and the ligand shell in MAu24 becomes thermodynamically more preferable as a result of symmetry-dictated coupling between dopant atomic orbitals and superatomic levels as well as because of relativistic contraction of s and p orbitals. The same mechanisms are also found to be responsible for the relative isomer energies in MAu37 clusters. For these clusters, DFT calculations predict that it is unlikely for the dopant atom to occupy the central location. We found similar trends for different dopants across the periodic table in relative isomer energies of MAu24 and MAg24; however, center-doped clusters are somewhat more stable in the case of MAg24 due to the smaller relativistic stabilization of s and p levels in Ag compared to Au. We also found that the metallic radii of the dopant can affect the geometries and relative stabilities of the isomers for the doped clusters significantly.
Nitrogen bond dissociation is one of the important steps in the Haber–Bosch process, where N2 is catalytically converted to NH3; however, the dissociation of the nitrogen triple bond is difficult to achieve. In this study, we investigate the possibility of nitrogen activation using plasmonic excitation of an icosahedral aluminum nanocluster. Real-time time-dependent density functional theory is employed to study the electron dynamics of the Al13 –1 and [Al13N2]−1 systems. Step and trapezoidal electric fields with field strengths of 0.001 and 0.01 au and different polarization directions are applied to the systems, and the electron dynamics are analyzed. Because the occupation of nitrogen antibonding orbitals could potentially activate the N–N bond, we investigated the single-particle electronic transitions corresponding to an excitation from an occupied (O) to virtual (V) molecular orbitals (P OV) of [Al13N2]−1. We found that N2 antibonding orbitals are more likely to become populated with stronger fields and also by using off-resonance fields.
Multireference calculations can provide accurate information of systems with strong correlation, which have increasing importance in the development of new molecules and materials. However, selecting a suitable active space for multireference calculations is nontrivial, and the selection of an unsuitable active space can sometimes lead to results that are not physically meaningful. Active space selection often requires significant human input, and the selection that leads to reasonable results often goes beyond chemical intuition. In this work, we have developed and evaluated two protocols for automated selection of the active space for multireference calculations based on a simple physical observable, the dipole moment, for molecules with nonzero ground-state dipole moments. One protocol is based on the ground-state dipole moment, and the other is based on the excited-state dipole moments. To evaluate the protocols, we constructed a dataset of 1275 active spaces from 25 molecules, each with 51 active space sizes considered, and have mapped out the relationship between the active space, dipole moments, and vertical excitation energies. We have demonstrated that, within this dataset, our protocols allow one to choose among a number of accessible active spaces one that is likely to give reasonable vertical excitation energies, especially for the first three excitations, with no parameters manually decided by the user. We show that, with large active spaces removed from consideration, the accuracy is similar and the time-to-solution can be reduced by more than 10 fold. We also show that the protocols can be applied to potential energy surface scans and determining the spin states of transition metal oxides.
Plasmonic nanoparticles are well-known for their properties of electromagnetic field enhancement and surface spectroscopy enhancement. We used the plasmon hybridization method and group theory to study parallel dimers and dolmen trimers of Ag n (n = 4, 6, and 10) nanoparticles. Interactions between the plasmon modes were studied with decreasing interparticle separation distances. Time-dependent density functional calculations are performed on the structures using the BP86/DZ level of theory. In dimers, the decrease of the interparticle separation blue-shifts the longitudinal peak, but the transverse peak position is not affected significantly. In trimers, a new peak is also observed as a shoulder of the longitudinal peak. When the interparticle separation reduces to 0.6 nm in dimers and trimers, a new peak emerges between the longitudinal and transverse peaks. This new peak red-shifts and increases in intensity upon further decreasing the interparticle separation. Analysis of the transition densities and symmetries for the respective peaks shows that the new peak arises from a charge transfer excitation.
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