Stacking of triphenylene: characterization of the potential energy surface

Addressing electronic and dynamical evolution of molecules and molecular clusters: DFTB simulations of energy relaxation in polycyclic aromatic hydrocarbons

We present a review of the capabilities of the density functional based Tight Binding (DFTB) scheme to address the electronic relaxation and dynamical evolution of molecules and molecular clusters following energy deposition via either collision or photoabsorption. The basics and extensions of DFTB for addressing these systems and in particular their electronic states and their dynamical evolution are reviewed. Applications to PAH molecules and clusters, carbonaceous systems of major interest in astrochemical/astrophysical context, are reported. A variety of processes are examined and discussed such as collisional hydrogenation, fast collisional processes and induced electronic and charge dynamics, collision-induced fragmentation, photo-induced fragmentation, relaxation in high electronic states, electronic-to-vibrational energy conversion and statistical versus non-statistical fragmentation. This review illustrates how simulations may help to unravel different relaxation mechanisms depending on various factors such as the system size, specific electronic structure or excitation conditions, in close connection with experiments.

Structures and stabilities of PAH clusters solvated by water aggregates: The case of the pyrene dimer

Although clusters made of polycyclic aromatic hydrocarbon and water monomers are relevant objects in both atmospheric and astrophysical science, little is known about their energetic and structural properties. In this work, we perform global explorations of the potential energy landscapes of neutral clusters made of two pyrene units and one to ten water molecules using a density-functional-based tight-binding (DFTB) potential followed by local optimizations at the density-functional theory level. We discuss the binding energies with respect to various dissociation channels. It shows that cohesion energies of the water clusters interacting with a pyrene dimer are larger than those of the pure water clusters, reaching for the largest clusters an asymptotic limit similar to that of pure water clusters and that, although the hexamer and octamer can be considered magic numbers for isolated water clusters, it is not the case anymore when they are interacting with a pyrene dimer. Ionization potentials are also computed by making use of the configuration interaction extension of DFTB, and we show that in cations, the charge is mostly carried by the pyrene molecules.

Structures and Energetics of Neutral and Cationic Pyrene Clusters

The low energy structures of neutral and cationic pyrene clusters containing up to seven molecules are searched through a global exploration scheme combining parallel tempering Monte Carlo algorithm and local quenches. The potential energies are computed at the density functional based tight binding level for neutrals and configuration interaction density functional based tight binding for cations in order to treat properly the charge resonance. New simplified versions of these schemes are also presented and used during the global exploration. Neutral clusters are shown to be made of compact assemblies of sub-blocs containing up to three units whereas cations present a charged dimer or trimer core surrounded by neutral units. The structural features of the clusters are analyzed and correlated for the cation with the charge distribution. The stability of clusters is also discussed in terms of cohesive and evaporation energies. Adiabatic and vertical ionization potentials are also discussed.

A mixed basis with off-center Gaussian functions for the calculation of the potential energy surfaces for π-stacking interactions: dimers of benzene and planar C₆

A practical mixed basis set was developed to facilitate accurate calculations of potential energy surfaces for π-stacking interactions. Correlation consistent basis sets (cc-PVXZ) were augmented by p-type Gaussian functions placed above and below the planes of C6 moieties. Møller-Plesset (MP2, SCS-MP2) and coupled cluster [CCSD(T)] calculations show that such generated basis sets provide an accurate description of π-stacking systems with favorable computation times compared to the standard augmented basis sets. The addition of these off-center functions eliminates the linear dependence of the augmented basis sets, which is one of the most encountered numerical problems during calculation of the oligomers of polyaromatic hydrocarbons (PAH). In this work, we present a comparative study of the general characteristics of the potential energy surfaces for the parallel stacked and T-shape conformations of benzene and planar C6 clusters, using a combination of cc-PVXZ and our optimized functions. We discuss properties, such as the depth and curvature of the potential functions, short and long distance behavior, and the frictional forces between two model monomers.

Solution-phase dimerization of an oblong shape-persistent macrocycle

A new shape-persistent macrocycle comprising two 2,3-triphenylene moieties bridged by m-phenylene ethynylenes has been synthesized. UV-vis and fluorescence spectroscopies indicate limited interaction between the two triphenylene units. The compound, which has a pronounced oblong shape (the core measures approximately 2.2 × 1.0 nm), aggregates in CDCl3 and toluene-d8 to give stacked dimers, as indicated by the (1)H NMR signals corresponding to protons on or near the core, which shift upfield with increasing concentration. These changes in NMR shielding were modeled using DFT calculations on candidate dimer geometries. The best match to the experimental data was obtained for a dimer consisting of arene-arene stacking by 3.6 Å (on average) with a displacement along the molecular long axis of 3.5-4.0 Å. This displacement is larger than can be explained by the electronic effects of aromatic stacking interactions. Instead, the minimization of steric interactions between the side chains appears to control the dimer geometry, with the alkoxy groups of one molecule sliding into registry with the gaps along the periphery of the other. Such lateral displacement (as opposed to rotation) is a direct consequence of the extended oval shape of the compound.