Modelling Spectral Tunability and Photoisomerization Mechanisms in Natural and Artificial Retinal Systems

The photoinduced ultrafast isomerizations of the retinal chromophore (RPSB) in visual rhodopsins are characterized by high speed and efficiency due to conical intersection between the excited and ground electronic states that drives its radiationless decay via an efficient internal conversion. The retinal chromophore displays also a cationic structure and the charge transfer character of its lowest excited bright state makes it sensitive to the environment, namely to the presence of an external electric field. This is in turn able to tune its absorption energy in order to cover the whole visible range, hus enabling color vision. All these properties call for the RPSB as an ideal candidate for design fast, efficient and spectrally tunable molecular machines that work via reversible and controlled photo−induced reactions. The aim of this thesis is to investigate the mechanisms driving these reactions in different natural and artificial environments using high-level ab initio calculations and hybrid QM/MM methods. Eventually, the aim is to understand how these systems work and provide guidelines for design artificial photoactive retinal inspired systems that may mimic Nature. Firstly, we investigate the effect of an homogeneous electric field on the spectral tunability, photoisomerization efficiency and selectivity of the native RPSB. One of many intersting outcomes, is the possibility to selectively switch on/off the isomerization by adjusting the electric field to a specific critical value via judicious substitutions/functionalizations. Secondly, the modeling of the isomerization mechanism of three different pigments were investigated to predict the rate and efficency of the reaction. Our results show that the Landau−Zener rule does not apply for visual pigments. Since the dynamics are essentially multi−dimensional. lastly, we shed some light on the photoisomerization mechanism of the Siberian hamster ultraviolet visual pigment, a so far poorly investigated. The results suggest the photoinduced proton transfer as possible efficient photoisomerization mechanism.