Preparation of gel-supported plexcitonic nanohybrids and their optical characterization by 2D electronic spectroscopy

Plasmonic nanostructures possess the intriguing ability to confine and manipulate light at the nanoscale, thanks to their ability to support localized surface plasmons, i.e., collective oscillations of free electrons within the metallic nanoparticles. When an excitonic molecular medium is in close proximity to the plasmonic nanoparticle, a strong interaction might place between the two moieties, leading to the formation of new hybrid states known as "plexcitons". Plexcitons inherit features of both plasmons and excitons, resulting in unique photophysical properties that differ from those of the uncoupled components. This class of light-matter coupled nanosystems is currently gaining great interest due to the possibility of controlling and manipulating light-matter interactions at the nanoscale. By finely controlling the parameters of the plasmonic nanostructures (such as their shape or size), as well as the properties of the excitonic system, it is possible to tailor the optical and dynamic properties of plexcitons. This offers new avenues in the field of integrated optics, optoelectronics, imaging, and sensing. Most plexcitonic materials presented in the literature are built using plasmonic arrays. Recently, also colloidal nanoparticles have been reported as suitable plasmonic moieties. With respect to other families of plexcitonic materials, colloidal plexcitonic materials are highly promising because they are cheap and easy to prepare through wet chemistry methodologies. Yet, the exploration of their photophysical, dynamic, and quantum mechanical properties is still in its infancy. Furthermore, colloidal nanohybrids often suffer from stability issues due to the well-known aggregation phenomena that cause easy precipitation. In this Thesis, inspired by prior literature of our group, we designed, prepared, and characterized gel-supported colloidal nanohybrids built starting from colloidal gold nanoparticles (NPs) and tetra sulphonate phenyl porphyrin (TPPS) dyes. By manipulating the relative ratio between NPs and dyes, it is possible to induce variations in their mutual supramolecular interactions and selectively activate or deactivate two different sets of plexcitonic resonances, which arise from the coupling between different states of the porphyrin dye and gold nanospheres. To avoid precipitation issues and increase the sample stability, the functionalized NPs have been transferred from water suspensions to suitably selected gel matrixes, able to preserve the nanohybrids' properties and prevent NPs’ aggregation and precipitation. The samples in the gel matrixes were revealed to be stable enough to be studied by linear and nonlinear optical techniques. In particular, the ultrafast relaxation dynamics of the nanohybrids were investigated by 2D electronic spectroscopy (2DES), which allows us to characterize the ultrafast coherent and incoherent dynamics of the plexcitonic states with an unprecedented level of detail. The results obtained by applying 2DES to plexciton samples have been compared with reference studies of the uncoupled species (i.e., NPs and the porphyrin dye in the monomeric and aggregated form) conducted under the same experimental conditions. The aim was to uncover the changes in the photophysical and dynamical properties that occur as a result of the coupling between the plasmonic and molecular components. The results obtained revealed the presence of interesting relaxation mechanisms, interpreted in the light of recently proposed theoretical models. Overall, the results of this Thesis represent a big step forward toward a more mature comprehension of the still underexplored yet exciting dynamic of colloidal plexcitonic nanohybrids.