Symmetry - breaking charge transfer, intersystem crossing and other ultrafast processes in chromophore aggregates

Abstract

This dissertation begins with a theoretical background chapter, where Molecular Orbital theory is discussed, focusing on π-conjugated orbitals. The chapter also addresses the subsequent π-stacking of molecules and the interactions derived from there. Intramolecular Frenkel excitons and intermolecular Charge Transfer excitons are elucidated through the lens of the Exciton Model, along with the effects of their intermixing. Chapter 3 outlines the experimental methods employed, provides theoretical background, and describes the setups. It offers an overview of the fundamentals of linear spectroscopy: absorption and fluorescence. Additionally, the chapter provides an in-depth discussion of time-resolved spectroscopy using pulsed laser systems and the methodology for data collection and analysis. Chapter 4 studies how intermolecular interactions in π-stacked chromophores strongly influence their photophysical properties and function in photonic applications. The photoexcited dynamics of the small molecule semiconductor copper phthalocyanine (CuPc) are characterized in solution and thin film, the latter comprising two different π-stacked architectures, α-CuPc and β-CuPc. In solution, CuPc undergoes ultrafast intersystem crossing (ISC) to the triplet excited state. In the solid state, both α-CuPc and β-CuPc morphologies exhibit a mixing between Frenkel and charge-transfer excitons (Frenkel-CT mixing). This mixing influences the photophysical properties differently based on morphology. In addition to ISC, α-CuPc demonstrates symmetry-breaking charge transfer, which depends on the excitation wavelength. This mechanism is not observed in β-CuPc. These results elucidate how molecular organization mediates the balance of competitive photoexcited decay mechanisms in organic semiconductors. Chapter 5 presents squaraine aggregates slip stacked and in amorphous form. The effect of aggregation of n-alkyl SQ can be seen when, upon photoexcitation, SQ relax through a complex pathway that involves variable charge transfer character that decreases for longer alkyl chain length. The possibility of excitation and decay to triplet states is also explored because, due to the weak SOC in SQ, the triplets are not expected to be from ISC but eventually from the spin-allowed Singlet Fission process; it is usually CT mediated.