Effects of Residual Water on Proton Transfer-Switching Molecular Device

The excited state proton transfer (ESPT) reaction plays a crucial role in DNA defense and ON-OFF proton-switching molecular devices. o-Hydroxybenzaldehyde (OHBA) is the simplest model-molecule for the ESPT reactions where a proton is transferred from OH to C=O carbonyl groups by photo-excitation. In the present study, the reaction mechanism of ESPT in OHBA was investigated by means of the direct ab initio molecular dynamics (AIMD) method. The triplet (T1) state of OHBA, OHBA(T1), was considered as the excited state of OHBA. The dynamic calculations showed that fast PT occurred from OH to C=O carbonyl groups at the T1 state. The time of PT was calculated to be 34–57 fs in OHBA(T1). The spin density was mainly distributed on the benzene ring (Bz) at time zero. The density was gradually transferred from Bz to C=O as a function of time on the T1 surface. When the spin density on C=O was larger than that on Bz (at time = 35–43 fs), the proton of OH was rapidly transferred to C=O. The localization of spin density on C=O dominated strongly the PT rate. Next, the effects of residual water (H2O) on the PT rate were investigated using OHBA-H2O 1:1-complexes to elucidate the effects of H2O on the PT rate in the ON-OFF proton-switching molecular devices. The PT rates were strongly dependent on the position of H2O around OHBA. The reaction mechanism is discussed based on theoretical results.