Computational Investigation of the Photoisomerization of Novel N-Alkylated Indanylidene Pyrroline Biomimetic Switches

The biological function of Rhodopsin (Rh), the G-protein-coupled photoreceptor responsible for twilight vision in vertebrates, is based on a very efficient process of photo-isomerization in the 11-cis retinal protonated Schiff base (PSB11) that is covalently linked to an apoprotein: opsin. The role of the opsin in the efficiency of the process is dramatical and PSB11 that is isolated from the protein environment (i.e., in methanol solution) exhibits different photo-chemical properties. When considering possible applications, the photo-isomerization is the process that can be used to change the properties of the system by means of an external light stimulus (photo-switches) or to exploit the light energy into unidirectional motion at the molecular level (molecular motors). N-alkylated indanylidene pyrroline (NAIP) switches are compounds that are designed to mimic, in solution, several aspects of the photochemistry of Rhodopsin. In this work, both the photoisomerization process of NAIP-switches, and the photochemistry of PSB11 in the opsin environment are investigated. The ab initio multi-configurational QM/MM approach (CASPT2//CASSCF/AMBER) is supported by time-resolved spectroscopy studies. Results show that NAIP switches exhibit several properties similar to that of Rhodopsin, such as stereoselectivity of the photo-isomerization , unidirectional motion, a sub-picosecond life time, and a barierless Minimum Energy Path leading from FC point to Conical Intersection. These properties make these compounds promising photo-switches or molecular motors. However, in spite of these remarkable similarities, the quantum yield of the photoisomerization of NAIPs is 2 to 3 times lower then that of Rh. These facts suggest that NAIPs not only provide a route to new materials but that they also constitute attractive systems for the investigation of fundamental problems such as the relationship between excited-state evolution and quantum yields. The invesigation of the dynamics of photo-isomerization of Rh and bathoRh provided in the second part of this thesis can be considered to be the first step in understanding the factors that are responsible for the quantum yield of the process