Why thermal isomerization of the chromic switch spiropyran-merocyanine is enhance in polar protic solvents. A computational study of the reaction mechanism

Spiropyran (SP) is a chromic material that undergoes isomerization to merocyanine (MC) under irradiation. To extend its applicability as a molecular switch, it would be convenient to be able to induce and control its isomerization also in the dark, what seems to be possible in acidic media. We present here a computational study of the mechanism of isomerization of protonated SP. We have found that protonation induces the stabilization of MC, protonated at O, relative to SP, protonated at N. Given the different site of protonation for both species, the ring-opening reaction must take place together with a proton transfer event. We have shown that the barrier of this process decreases if a protic solvent molecule take part in the reaction. In this way, the rate determining step of the process is the isomerization between different rotamers of protonated MC. The detailed knowledge of the reaction mechanism can help in the design of structural or electronic modifications of SP to improve its switchability.