Shaping molecular excited-state properties by means of localized surface plasmon resonances

Controlling light-matter interactions at the nanoscale holds promise for successfully dealing with ever-increasing worldwide issues like energy consumption and shortage. In this regard, metallic nanostructures featuring light-induced localized surface plasmon resonances proved to be an efficacious way of manipulating light at the nanoscale, thus paving the way for controlling the energy flow at molecular scale. Indeed, in recent years many works have illustrated the possibility of modifying molecular properties, e.g. molecular photoluminescence, Raman scattering, energy transfer and so forth, by cleverly harnessing plasmonic effects of metallic nanostructures. Nowadays, these findings have even led to state-of-the-art experimental techniques where single-molecule imaging with sub molecular resolution is possible by using visible light, thus incredibly going beyond light diffraction limit. These complex phenomena are often affected by different system features and span various length and time scales, which makes the rationalization of experimental results an arduous task. In this respect, theory can be instrumental not only to unravel processes which are typically hidden behind experimental observables, but also to investigate new effects that could be later experimentally probed. This thesis aims at shining light on the complex and rich physico-chemical properties arising from coupling molecules with plasmonic nanostructures by combining ab initio molecular modelling with a classical or quantum description of arbitrarily shaped metallic nanostructures, the latter described as homogeneous polarizable objects. Novel methods development and applications to systems of much scientific interest are shown, ranging from plasmon-enhanced single-molecule photoluminescence, plasmon-mediated chirality to collective plasmon-molecules strong-coupling. In many of those cases, a direct comparison between theoretical simulations and state of the art experimental evidence reveals that nanostructures features, such as metal geometrical details and plasmon dynamics drastically impact on the resulting molecular properties, therefore constituting a possible control knob to further manipulate energy at the nanoscale.