Spectroscopy and Chirality in Supramolecular Aggregates: A Joint Experimental and Theoretical Study

This thesis presents an extensive theoretical and experimental study on the optical properties of supramolecular aggregates, with a particular focus on the modelling of chiroptical properties in complex systems. The first part of the thesis describe the interactions responsible for supramolecular aggregation, how supramolecular aggregation affects the optical properties of the system, and how the environment can be used to induce or control supramolecular aggregation. Aggregates are prepared either by exploiting hydrophobic effect (chapter 2), the formation of hydrogen bonds (chapter 3), the covalent linking of monomers (chapter 4) or the high local concentration of monomers inside organic nanoparticles (chapter 5). In each chapter, the optical properties of aggregates are rationalized by theoretical models which help to explain the experimental data. Chapter 2 focuses on the different screening regimes of ground state and excited state interactions in a polar solvent, devising an ESM model able to discriminate the contributions and reproduce both linear and non linear spectra of aggregates. Chapter 3 concerns a naphtyridine dye which is in monomeric form in polar solvent DMSO and forms dimers associated by an array of hydrogen bonds in chloroform. The ground state properties of both monomer and dimers are elucidated by MD and DFT calculations. The effects of the different functionalization of monomers on fluorescence properties are explained by TD-DFT calculations. Last, the optical properties of dimers are well understood within the theory of exciton coupling. Chapters 4 and 5 exploit the stimuli-responsive fluorescence of intramolecular J-aggregates and excimers to use them as sensors of environmental polarity and temperature. The second part of the thesis is devoted to the study of chirality, both at the molecular and supramolecular levels. Special attention is devoted at modelling the chiroptical properties, in particular CD and CPL, of chiral assemblies. In chapter 7 the intrinsic chirality of a series of π-extended helicenes, and their properties of CPL emitters, are addressed. TD-DFT calculations allow to attribute the absolute configuration to each of the enenatiomers, previously separated by chiral chromatography. Chapters 8 and 9 derive the chiroptical properties of chiral supramolecular aggregates within the framework of quantum mechanical exciton model. In particular, chapter 8 explores the effect of chiral coupling between electric transition dipole moments using two different approaches, which converge to the same results, while chapter 9 introduces the role of magnetic transition dipole moments and intrinsic chirality in determining the overall CD of an assembly. These concepts are applied to a real case study in chapter 10, as the peculiar CD and CPL features of hierarchical aggregates of cyanines are rationalized with the exciton model framework. The introduction of intrinsic chirality in the model allows to replicate the peculiar monosignate CD signal of the cyanine tubes, while the chiral coupling between electric dipole moments of the tubes well reproduces the bisignate CD and CPL of the bundles.