Energy flow in complex molecular systems: Spectroscopic study by static and time-resolved techniques

This thesis collects the results of a series of experiments aimed at the characterization of energy and charge transfer processes in organic systems, carried out during my PhD course. The complexity of the subject is connected to the identification of the micro-scale phenomena that occur after visible excitation of the systems. The goal is to control these processes in order to maximize the energy migration throughout the systems, aiming to their potential application in OPV devices. Stationary and time-resolved spectroscopic techniques are excellent experimental methods to investigate ET in organic molecular complexes, because the electronic properties are revealed in the absorption and emission spectra. The characterization of the spectral features and their time-evolution, on femtosecond timescale, gives important pieces of information about the nature and dynamics of the excited state, and its interaction with the surrounding. The first experimental work I carried out in the Ultrafast group of Prof. Paolo Foggi at LENS was focused on the spectroscopic investigation of a substituted Zn-Phtalocyanine (ZnPc) dye, that has a potential application as sensitizer in Dye Sensitized Solar Cells (DSSCs). Transient Absorption Spectroscopy (TAS), an ultrafast pump-probe technique with sub-picosecond time resolution, was employed to investigate the excited state dynamics of the dye in ethanol (EtOH) solution and adsorbed on nanocrystalline films of Titania (TiO2) and Zirconia (ZrO2). TAS results allowed us to identify the timescales of the different relaxation processes occurring after visible excitation, and to propose a scheme of the energetic levels involved in the excited state dynamics. Furthermore, we had evidence electron injection in the Titania conduction band, which makes the molecule suitable for DSSCs applications. Many research group are focused toward the realization of artificial light-harvesting antennae for energy capture, based on organic molecules. The optimal electronic properties of such systems are still object of scientific debates, because different ET mechanisms take part in the overall exciton migration, which interplay is not trivial. During a one-year period spent in the M. G. Bawendi group at MIT, I studied the exciton transport properties in self-assembled light-harvesting nanotubes (LHNs), quasi one-dimensional aggregates consisting of ordered amphiphillic cyanine dyes. These systems present interesting excitonic properties, which derive from the unusual electronic coupling between the monomers. Because of the low static disorder, large exciton delocalization and the negligible reorganization energy (low coupling with the environment), LHNs represent an ideal model system to explore the quantum contribution and the influence of energetic disorder on the ET. The experimental investigation of LHNs was carried out by stationary and time resolved spectroscopy, at room temperature and as a function of temperature, from room temperature to 5K: the change in the spectral lineshape was interpreted in terms of homogeneous broadening. Intensity dependent fluorescence measurements gave evidence of exciton-exciton nnihilation (EEA). In order to get insight the effect of energetic disorder, a ballistic model for exciton diffusion was developed: two regimes of exciton migration were identified, depending on the relative value of the 2homogeneous and inhomogeneous timescales. Stationary fluorescence was measured as a function of the visible light irradiation, over several minutes, showing Photobrightening (PB) phenomena followed by Photodarkening (PD) on longer timescales. This results, confirmed also by 2D electronic spectroscopy (2DES), have been interpreted in terms of exciton migration mediated by super-radiance, a collective emission process arising from the coherent exciton delocalization. In order to deeply investigate the view on the molecular aggregate and their influence on exciton dynamics, I performed 2DES measurements on mixed monomer / H-aggregate systems made of ZnPc molecules. When dissolved in particular solvents, such as Cloroform (CHCl 3 ), the ZnPc molecules forms aggregates. Aggregation is one of the main issues that prevents a high efficiency of electron injection in the semiconductor conduction band. In the aggregation process, radiative channels of deactivation from the dye excited state are introduced. 2DES is a powerful multidimensional coherent spectroscopic technique that allows to: investigate the exciton dynamics and the lineshape time-evolution on femtosecond timescale, identifying the homogeneous and inhomogeneous timescales; highlight the coupling between electronic states and the possible coherent contribution. Broadband excitation of the sample allowed to discriminate between the monomer and aggregate kinetic traces. We had evidence of electronic coupling between the aggregate and the isolated molecules in close proximity. Furthermore, electronic coupling mediated by an internal vibrational mode was observed between two non-degenerate Q-states of the monomer.