Singlet Fission in Molecular Crystals and Covalent Dimers: from Chromophore Design to Nonadiabatic Dynamics Simulation

In this thesis we present an extensive computational investigation on singlet fission, a photophysical process whereby a spin-singlet excited state is converted into two lower energy spin-triplet states. Our goal is twofold. First, we aim to provide a contribution to the fundamental understanding of the singlet fission phenomenon in both molecular crystals and covalently bound dimers. Second, we seek to identify and propose new chromophores and materials which can efficiently undergo singlet fission. After providing a general introduction on the singlet fission process, we present a novel computational procedure devised to construct diabatic electronic states for systems which can be clearly partitioned into subsystems. Such procedure is widely used in the subsequent chapters to study singlet fission in our investigated systems. Next, we investigate how singlet fission can be affected by extending the delocalization of the excited states beyond two chromophores in a crystal. In particular, we present and compare simulations of the excited state dynamics based on quantum mechanics/molecular mechanics (QM/MM) schemes in which either two or three interacting QM molecules of a singlet fission chromophore are embedded in their MM crystal environment. Subsequently, we present several attempts to design not only new potential singlet fission chromophores, but also novel covalent dimers for singlet fission. In the latter case, new covalent dimers are designed by linking the two chromophore units so as to maximize the effective electronic coupling between the initial and final states of singlet fission. Finally, the most promising covalent dimers obtained by applying such strategy are further investigated by performing semiclassical simulations of the excited state dynamics. The simulation results confirm that two of the newly devised dimers undergo singlet fission in the picosecond time scale with high quantum yields