Introducing heterocyclic moieties in photoswitchable compounds provides an intrinsic benefit in terms of applications. Some heterocycles are naturally occurring and biologically compatible, allowing for diverse functionality and varied electronic effects that can be advantageous for tailoring the photoswitch’s behaviour. The Crespi group is interested in developing novel compounds based on heterocyclic scaffolds and understanding the fundamental isomerization properties of model structures. To obtain these goals, the group employs computational chemistry as a key tool to understand and predict the excited and ground state properties of photochemical switches, such as relative stabilities of the various isomeric forms, interactions with the environment, light absorption properties, simulation of quantum yields, and excited state lifetimes. Our objectives are: 1) To understand the excited and ground state movement of novel photoswitchable compounds based on heterocycles, focusing in particular on the family of hemipiperazines, and ethylidenethiophenones. Both structures are key components of photochemical switches with potential applications in biology and medicine, thanks to their ability to control biological functions with high spatio-temporal precision. Moreover, they represent minimal, synthetically relevant chromophores that can be used as a blueprint to rationalize the excited and ground state mechanisms of more complex families of switches. We will study the motion with high-level multireference theoretical simulations (e.g. CASPT2 or MCPDFT), nonadiabatic molecular dynamics, QM/MM and semiempirical methods (e.g. OM2-MRCI). The first goal will initially focus on the reported compounds based on C=C isomerizing bonds. 2) We will explore computationally the isomerization pathways of alternative chromophores based on C=N and C=P bonds containing hemipiperazines and ethylidenethiophenones. 3) To introduce a computational workflow based on semiempirical methods (GFN0-xTB ) to be interfaced with software packages that allow nonadiabatic molecular dynamics simulations based on Tully’s surface hopping (e.g. Newton-X ). This method will allow to speed-up the exploration of the excited state potential energy surface while keeping a high level of precision in the results obtained. 4) To benchmark and extend the previous workflow to larger structures (e.g. HTIs) and more complex ensembles, e.g. explicit solvent cavities and protein pockets. Expected Results: 1) Elucidation of the essential excited and ground state properties of the hemipiperazine and ethylidenthiophenone molecular switches. These results will lead to the possibility to rationally design new switches based on these same scaffolds, in terms of photophysical and photochemical properties. These results will be shared with the groups studying the compounds experimentally, driving the synthesis while helping to understand the results obtained experimentally. 2) Obtain a computational workflow for semiempirical methods that can be used to simulate the pathways of isomerization of large (e.g. comprising multiple aromatic rings) molecular photoswitches in “realistic” explicit environment (in cavities and explicit solvents). 3) Establishing and benchmarking a nonadiabatic molecular dynamics computational protocol for photoswitches based on tight-binding semiempirical approaches. This result will be a key step for fast and reliable predictions of the excited state properties of molecular switches.