Watching the Interplay between Photoinduced Ultrafast Charge Dynamics and Nuclear Vibrations.

Here is presented the ultrafast hole-electron dynamics of photoinduced metal to ligand charge-transfer (MLCT) states in a Ru(II) complex, [Ru(dcbpy)2(NCS)2]4- (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine), a photoactive molecule employed in dye sensitized solar cells. Via cutting-edge computational techniques, a tailored computational protocol is here presented and developed to provide a detailed analysis of the electronic manifold coupled with nuclear vibrations to better understand the nonradiative pathways and the resulting overall dye performances in light-harvesting processes (electron injection). Thus, the effects of different vibrational modes were investigated on both the electronic levels and charge transfer dynamics through a theoretical-computational approach. First, the linear response time-dependent density functional (LR-TDDFT) formalism was employed to characterize excitation energies and spacing among electronic levels (the electronic layouts). Then, to understand the ultrafast (femtosecond) charge dynamics on the molecular scale, we relied on the nonperturbative mean-field quantum electronic dynamics via real-time (RT-) TDDFT. Three vibrational modes were selected, representative for collective nuclear movements that can have a significant influence on the electronic structure: two involving NCS- ligands and one involving dcbpy ligands. As main results, we observed that such MLCT states, under vibrational distortions, are strongly affected and a faster interligand electron transfer mechanism is observed along with an increasing MLCT character of the adiabatic electronic states approaching closer in energy due to the vibrations. Such findings can help both in providing a molecular picture of multidimensional vibro-electronic spectroscopic techniques, used to characterize ultrafast coherent and noncoherent dynamics of complex systems, and to improve dye performances with particular attention to the study of energy or charge transport processes and vibronic couplings.

Influence of halogen bonding on gold(i)-ligand bond components and DFT characterization of a gold-iodine halogen bond.

A gold(i) complex bearing nitrogen acyclic carbene (NAC) and selenourea (SeU) has been used to verify whether the second-sphere SeI halogen bond (XB) is able to modify the Dewar-Chatt-Duncanson components of the Au-C and Au-Se bonds. The chosen system was found to be thermically unstable but it allowed an in-depth theoretical study by means of Energy Decomposition Analysis, Natural Bond Orbital and Natural Orbitals for Chemical Valence methods, coupled with Charge Displacement analysis. Indeed, in the presence of iodoperfluoroalkanes as XB donors, iodine interacts with the lone pair of the coordinated selenium, enhancing the Au ← C σ donation and depressing the Au → C π back-donation, as demonstrated also by the increase of the rotational barrier of the C-N bond of the NAC (see G. Ciancaleoni and others, Chem. - Eur. J., 2015, 21, 2467). On the other hand, in the presence of N-iodosuccinimide (NIS), the gold directly establishes a XB with the iodine by using its d lone pairs. This AuI XB is favored by the low steric hindrance of the ligands coordinated to the gold and the presence of the amino protons of SeU, which establish additional hydrogen bonds with the NIS. Also in this case, the effect is to increase the σ acidity and decrease the π basicity of the metal.