Coherent Dynamics in the Ultrafast Relaxation of red absorbing squaraine Dyes: 2D Electronic Spectroscopic Characterization

In the last decades the energy sources that have been predominantly used, mostly associated with fossil fuels, have had a catastrophic impact on our environment. It is therefore urgent to find new renewable ways to produce energy in a cleaner way for our safety and for the planet. The sun represents one important alternative source, but the challenges in converting the solar energy into electrical energy are hindering the progress of new technologies. Despite that, new implementations in the photovoltaic field are now being implemented. The classical inorganic solid-state junction devices have been replaced by the dye-sensitized solar cells (DSSCs). In this thesis we focused on the optical properties of dye-sensitizers capable of absorbing in the red and near-infrared region of the solar emission, which are spectral regions not efficiently harvested by actual technologies. Among the various classes of organic molecules currently employed in DSSCs as sensitizers in the red and NIR spectral regions, squaraines were selected because they are promising for their intense absorption in red/NIR region and for the feasibility of their synthesis. Since the early 2000’s, several symmetrical and unsymmetrical squaraine dyes have been studied as sensitizers in DSSCs, anyway, despite the large amounts of experimental works and proposed applications, a thorough understanding of their complex photophysics is still limited.This thesis project has the aim of contributing to a better understanding of the static and time-resolved photophysical properties of these molecules and identifying structure-to-properties relationships useful for the design of new molecules with improved and/or controllable performances.In these regards, spectroscopic features related to high frequency nuclear motions or solute-solvent vibrations can be a signature of increasing formation/dissociation of the exciton. It is crucial to follow such charge-recombination and thermal relaxation process and understand their molecular nature, since they are undesired for solar cell efficiency and even more relevant in red adsorbing dyes.In order to characterize the microscopic details regulating these processes so essential for the DSSCs performance we used a combination of linear and non-linear, static and time-resolved optical spectroscopies. In particular, 2D electronic spectroscopy (2DES) that is a four-wave-mixing heterodyne detected spectroscopy with ultrafast temporal resolution. In a 2DES experiment, a sequence of 3 ultrashort laser pulses is used to induce and measure a third-order nonlinear optical response. The detected signal is cast in the form of frequency-frequency evolving along a time axis.The experimental data we obtained using 2DES technique and their comparison with results from Raman spectroscopy, revealed that the vibrational coupling with the environment strongly affects the electronic transitions. We described in detail the third-order signals obtained using Decay associated spectra (DAS), Coherence associated spectra (CAS) and Feynman diagrams .We found out, surprisingly, that several vibrational modes of these molecules involving torsional motions are coupled with each other opening new non-radiative relaxation pathways, possibly involving the crossing of a conical intersection. This might have a potentially crucial effect on the formation/dissociation of the excitons. In summary, in this thesis we applied a combination of several linear, nonlinear, static and time-resolved techniques to characterize the dynamic and photophysical properties of three red-absorbing squaraine dyes. The results we have obtained are important for understanding the photophysical behavior of the three squaraines we investigated. Additionally, these results suggest potential for designing better DSSCs using these molecules in the future.