Modeling charge and energy transfer in organic molecular materials

The understanding of nanoscale physics, chemistry and biology still poses unanswered questions such as how the optical and electrical properties of materials evolve from those of individual molecules, and organic semiconductors fall in this class of materials. The main processes occurring in such systems are both charge and energy transfer, responsible for the practical operation of electronic devices. Therefore, an understanding at a fundamental level of the electronic properties of the involved molecules can help the optimization of each process, for a better global performance of the material. My three years PhD activity was developed along two major lines of research: charge and energy transport, both based on the computational investigation of intramolecular properties and intermolecular interactions. Strictly related to energy transport are the optical properties of condensed phase materials and how they evolve from those of isolated molecular components. The charge transport properties were investigated for several organic molecular crystals showing semiconducting behavior, whose experimental crystal structure and charge mobilities are available. As the same interactions that drive the transport of charge play also a role in determining the optical properties and the energy transport in molecular aggregates, in my research activity I investigated such processes as well. In this regard, I took into account a dimer of perylene-bisimide, with the aim of elucidating the role of charge transfer states and their effect on optoelectronic properties. Additionally, to assess the propagation of excited states in a molecular material a kinetic constant is required, similarly to charge transport, but the expression in this case includes the overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor. To this end I also developed a code devoted to the simulation of linear absorption and emission spectra of an isolated molecule, starting from computed quantum mechanical properties.