Study of photo-induced electron transfer in pyrene-(CH2)n-N,N′-dimethylaniline system by molecular dynamic simulation

The dynamic behavior of pyrene-(CH2)n-N,N′-dimethylaniline (PnD; n = 1-3) in acetonitrile and cyclohexane was investigated with molecular dynamic simulation (MD). Then, changes in distance (R) between pyrene and N,N-dimethylaniline moieties with time were obtained at time intervals of 1 ps for P3D, 0.1 ps for P2D and 0.02 ps for P1D. Photo-induced electron transfer (ET) rates were evaluated with Kakitani, Yoshimori and Mataga theory (KYM theory) with R at each time. The observed fluorescence intensities with lifetimes, 11 ps for P3D, 6.1 ps for P2D and 1.7 ps for P1D were obtained from observed rise times of transient absorbance of pyrene anion radicals in PnD in acetonitrile. Parameters contained in KYM theory, frequency factor (ν0), coefficient for criterion of adiabatic and non-adiabatic processes (β), critical distance between adiabatic and non-adiabatic processes (R0) and standard free energy change (ΔG0) were determined by a non-linear least squares method, so as to fit the calculated fluorescence intensities obtained with the observed fluorescence intensities. The calculated intensities were in good agreement with the observed ones. The values of best-fit parameters, ν0, β, R0, and ΔG0 were 82.04 ps−1, 5.56 nm−1, 0.568 nm and −2.12 eV, respectively, in P3D, 82.05 ps−1, 5.56 nm−1, 0.569 nm and −2.12 eV, respectively, for P2D, and 93.19 ps−1, 5.58 nm−1, 0.698 nm and −2.08 eV, respectively, for P1D. In all systems, the ET processes were non-adiabatic. The dependence of ET rate on R were calculated with these parameters, which suggests that ET rate also exhibits a “bell shape”, such as the one obtained in the variation of ΔG0. Therefore, ET rate may decrease with R less than 0.55 nm for all PnD (n = 1-3) systems.