A dual-mode photoswitching mechanism and charge transfer on chiroptical systems — theoretical study

The synthesis of dual-mode optical molecular switching systems based on chiral helical-shaped alkenes in which chirality can be reversely modulated by light has been recently reported. The chiroptical molecular system is based on donor–acceptor substituted inherently asymmetric thioxanthene derivatives. The switching process can be regulated by reversible protonation in the dimethylamino group. Due to this remarkable properties these systems are very promising building blocks for optical memory devices as well as good model system for theoretical analysis. In this work, we report a theoretical study on the geometric and spectroscopic properties of these systems using the well-known PM3 and Zerner’s intermediate neglect of differential overlap–spectroscopic-configuration interaction (ZINDO/S-CI) methods. Although our calculations are for isolated molecules in vacuum the simulated absorption spectra are in very good agreement with the available experimental data (calculated absorption peak of 5.5 eV in comparison with observed 5.64 eV). Our results show that there are two stable conformers very close in energy for each possible molecular helicity and with a barrier for rotation about C1C2 axis of 40 kcal mol−1. Under protonation, these barriers increase and might explain why the protonation leads to the blocking of the switching process.