In silico strategy to design an efficient organic photoswitch based on excited-state cation transfer

A new class of photoswitches and the corresponding elementary photoinduced reaction, the so-called Excited-State Cation Transfer (ESCT), are investigated. This reaction relies on an intramolecular photo-release/photo-complexation of cation: after irradiation, the cation is translocated from a complexation site 1 to a site 2 during the excited state lifetime. Our purpose is thus to develop a computational strategy based on Density Functional theory (DFT) and its time-dependent counterpart (TD-DFT) to improve the different properties of the ESCT photoswitches, namely (i) the ground state complexation constant K, (ii) the excited state complexation constant K*, (iii) the photoejection properties and (iv) the population of the triplet states from a singlet state via intersystem crossing to increase the lifetime of the excited state. In this work, we are interested in optimizing the ESCT properties of a betaine pyridinium chromophore substituted by a 15-aza-5-crown, that was previously shown to efficiently photoeject a Ca²⁺ cation from the site 1 but no photo-recapture was observed in the site 2 [Aloïse et al., Phys. Chem. Chem. Phys., 2016, 22, 15384]. To this purpose, we have investigated the impact of the modification of the site 1 on the ESCT properties by introducing different substituents (EDG groups, halogen atoms) at different positions. So far, promising systems have been identified but a simultaneous improvement of all the ESCT photoswitches properties has yet not been achieved.