A two-step electron-driven proton transfer mechanism for the solution-phase nonradiative decay process of squaraine molecule

The strong solvent polarity dependence observed in the nonradiative decay rates of Bis[4-(dimethylamino)phenyl]squaraine (SQ-DMA) had previously been explained by the twisted intramolecular charge transfer mechanism. However, our recent theoretical study did not support this mechanism. We theoretically study the possibility of electron-driven proton transfer (EDPT) processes between an alcohol solvent molecule and the CO^- groups of SQ-DMA using a complex model. The results actually support the EDPT mechanism that proceeds via two nonadiabatic transitions: the first from the locally excited (π → π∗) state of SQ-DMA to the charge transfer (CT) state, and the second from the CT to S₀ states.