Structures and Energetics of Neutral and Cationic Pyrene Clusters

Abstract

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.

Threshold collision induced dissociation of pyrene cluster cations

“…All the needed ingredients for the PST calculation (harmonic frequencies and moments of inertia) are taken from DFTB-CI calculations 39 as in our previous work on thermal evaporation 38 . For sake of clarity, note that the theoretical dissociation energies discussed hereafter always correspond to the differences between the energies of global minima of parents and fragments, disregarding dissociation from or toward secondary minima.…”

“…For each cluster size, there was only one free parameter left in the PST calculations, namely the dissociation energy. For n up to 6, all other parameters (harmonic frequencies, moments of inertia) entering the PST calculations were deduced from recent calculations using Density Functional based Tight Binding coupled with a Configuration Interaction scheme (DFTB-CI) and involving global structural search 39 . The DFTB-CI method 40 is designed to circumvent self-interaction and ill-defined electron-delocalization in cationic molecular clusters and correctly describe charge resonance.…”

Density-functional tight-binding: basic concepts and applications to molecules and clusters

“…Note that ground state PES gradients are also available in various extended versions of DFTB such as DFTB3 [55], spin-polarized DFTB [99], CI-DFTB [150] or when LDA+U or pSIC-corrections are included [110].…”

“…When molecular clusters are singly ionized, alternative DFTB-CI schemes (see section 3.3) may be considered to properly describe the charge and excitation resonance over the different units. Its combination with global exploration schemes allowed to identify the most stable structures of cationic pyrene (Py) clusters, showing that the charge is delocalised over a dimer or trimer core [150], and to compute their electronic spectra [149]. This model was further used to interpret various experiments concerned with thermal evaporation of Py þ n clusters [289], photodissociation of Py þ 2 [290], combined photoionisation and dissociation of Py 2 [291] and the determination of Py n ionisation potentials (see Figure 6) [292].…”

“…As for thiolathes, DFTB Figure 6. Left: DFTB most stable structure of the cationic pyrene heptamer with all-atom relaxation [150]. Right: experimental and computed ionization potentials for pyrene clusters.…”