Development of Multiscale Models for the Static and Dynamic Description of Photoinduced Processes in Embedded Systems

The detailed knowledge of the processes following photoexcitation is extremely important, but the understanding of the underlying mechanisms, as well as the proper description of the corresponding non-adiabatic dynamics are still real challenges for theoretical chemistry. This is true especially for biologically relevant systems, where not only the chromophores, but also the complex environment need to be included in the simulation in order to obtain a correct picture. The environment, in fact, can play an important role in the mechanisms of the photoinduced processes by tuning both the electronic and structural properties of the chromophore(s). In this thesis, we present novel computational multiscale models, based on a hybrid quantum mechanical/molecular mechanical (QM/MM) description of the system able to study such processes. First, the theoretical formulation and implementation of excited states gradients of a polarizable QM/MM method using an induced dipole scheme are introduced within the framework of time-dependent density functional theory. Secondly, the development and the implementation of a novel exciton approach are presented and discussed, where the environment is included via an electrostatic or mechanical embedding scheme. Consistent hybrid QM/MM energies, gradients and non-adiabatic couplings were developed, allowing ab initio non-adiabatic dynamics simulations in multichromophoric systems, using the surface hopping approach. The methods are finally applied to the study of (i) the effects of a DNA pocket on the excitation process and the corresponding excited state properties of an organic dye, and (ii) the excitation energy transfer process in an orthogonal molecular dyad.