Physical Chemistry Chemical Physics最新文献

We use time-dependent density functional theory and many-body perturbation theory to compare and contrast the electronic and optical properties of hydrogen capped carbon, silicon and germanium group IV nanoparticles. Hydrogen-capped silicon nanoparticles are model systems for quantum confinement and hence it is an interesting question if carbon and germanium nanoparticles similarly behave as archetypes of quantum confinement. We find that for several properties, such as the trends in the fundamental and optical gaps with particle size, all group IV systems behave similarly. However, there are also other properties, such as the variation of the electron affinity with particle size and the electronic character of the low-energy excitons and frontier quasiparticle states, for which the carbon particles behave fundamentally differently from their silicon and germanium counterparts and deviate from the idealised cartoon picture of quantum confinement. We speculate that the fact that hydrogen is more electropositive than carbon, and more electronegative than silicon and germanium, combined with the large gap of the carbon nanoparticles, pushes the unoccupied C-H σ-bonds into the gap, which then pins the lowest unoccupied quasiparticle state and excited electron component of the excitons on the surface.

Phthalocyanine (Pc)-based molecular thin films have emerged in recent years as a promising class of organic semiconductor materials for optoelectronic applications owing to their long exciton coherence length and fast exciton diffusion. However, the dependence of their exciton properties on the dimensionality and thermodynamic conditions, presence of metal ions, and effects of chemical modifications to the PC systems is not yet fully understood. As a first step towards a more comprehensive theoretical understanding of the excitonic properties of Pc thin films, we model their low-temperature exciton absorption spectra by employing the Frenkel Hamiltonian. The latter is derived from quantum-chemical estimates of site energies and exciton-exciton couplings. The predicted exciton absorption spectra of octabutoxy phthalocyanine (H₂OBPc) is found to be strongly dependent on the dimensionality of the model as well as the distance cutoff for including the monomer-monomer exciton coupling. We also caution that the widely used dipole-dipole approximation could substantially overestimate the excitonic coupling between different monomers compared to a more accurate evaluation using the respective transition densities.

CO/NaCl(100) at monolayer coverage is a weakly bound adsorbate system for which unusual phenomena after infrared laser excitation of adsorbate vibrations have been observed. Among them are very long vibrational lifetimes in the CO stretch mode (∼4 ms), efficient vibrational energy pooling, and orientational isomerization from a "C-bound" to an "O-bound" configuration. Here we use non-equilibrium ab initio molecular dynamics (neq-AIMD), coupled with time correlation function (TCF) techniques, to study vibrational relaxation dynamics and related processes after selectively exciting specific normal modes (CO internal stretch, frustrated rotation) across submonolayer and monolayer coverages of this system. We are mostly concerned with picosecond dynamics, and in one case we use recently proposed machine-learned, AIMD-based potentials for CO/NaCl, to explore significantly longer timescales (∼ns). To monitor the dynamics, we present atom- and mode-resolved transient vibrational spectra, kinetic energies and qualitative time-dependent local mode couplings. Time-dependent vibrational sum frequency (VSF) spectra are reported for a flipping CO in the CO/NaCl(100) monolayer, following frustrated bending mode excitation. Important highlights of this work are as follows: (i) localization of vibrational energy in the CO stretch modes on ps timescales and signatures of vibrational energy pooling on ns timescales; (ii) sub- to few-picosecond lifetimes of frustrated bending modes of both "C-bound" and "O-bound" CO adsorbates resulting from two dissipative channels, namely, the surface phonons and neighbouring adsorbates causing successive flipping, desorption and diffusion events; and (iii) identification of vibrational non-adiabatic channels in metastable "O-bound" geometries following CO stretch excitation.

The central channel of the nuclear pore complex (NPC) exhibits polydispersity in the FG-nucleoporin (FG-Nup) sequence length, with longer FG-Nups on the periphery and shorter FG-Nups in the interior of the pore. A minimal, coarse-grained model and Langevin dynamics simulations were used to investigate the functional role of FG-Nup polydispersity in NPC transport. The NPC was modelled as a cylindrical pore lined with a random copolymer brush composed of hydrophobic and hydrophilic segments, mimicking FG-Nups. Two model NPCs were considered to simulate the translocation of a karyopherin-bound spherical cargo (12 nm spherical tracers): a homogeneous NPC (h-NPC) with uniform FG-Nup lengths and an inhomogeneous NPC (ih-NPC) featuring shorter FG-Nups in the middle and longer FG-Nups at the periphery. The ih-NPC demonstrated enhanced selectivity and significantly higher passage probabilities for karyopherin-bound tracers, with an increase of up to 90% compared to the h-NPC. Analysis of binding contacts between tracers and FG-Nup hydrophobic segments revealed that tracer translocation was facilitated by a handover process between successive FG-Nups along the NPC length. This enhanced selectivity of the ih-NPC was attributed to an increase in binding contacts of the tracer with the shorter FG-Nups in its middle region. These findings provide a biophysical basis for the evolutionary significance of FG-Nup polydispersity in selective NPC transport.