Stochactic model of charge separation in photo excited molecular triad

Within framework of the stationary stochastic model the dynamics of ultrafast photoinduced electron transfer reactions in the donor–acceptor triad are studied. The molecular triad $DA_1A_2$ contains a primary electron donor $D$ that transfers an electron to consistently to the nearest neighbor acceptor $A_1$ and acceptor $A_2$ in accordance with the scheme $DA_1A_2\stackrel{h\nu}{\longrightarrow}D^*A_1A_2\stackrel{I}{\longrightarrow}D^+A^-_1A_2\stackrel{II}{\longrightarrow}D^+A_1A^-_2$. This separation of charges is of paramount importance for the efficient functioning of the photosynthetic reaction centers containing an ordered array of secondary electron acceptors. The model accounts for the solvent reorganization and decay of intermediate $(D^+A^-_1A_2)$ and final products $(D^+A_1A^-_2)$ of the reaction. An analytical expression for the probability of the charge transfer to secondary acceptor of flowing parallel to the relaxation of polar solvent is received. Quantitative estimation of the influence of the decay reaction products on the charge separation probability is made. It is shown that the decay of the molecular triad final state $(D^+A_1A^-_2)$ increases charge separation probability $W_{CS}$. The most noticeable influence of the product decay time $\tau_{\nu 3}$ on $W_{CS}$ is seen in the Marcus normal region (up $30\%$), and in the Marcus inverted region $W_{CS}$ varies slightly with variation of the product decay time $\tau_{\nu 3}$. The mechanism of the effect is transparent enough. The final reaction state decay constrains reverse flow, which increases the direct charge transfer rate and therefore the probability of charge separation increases. Fast decay of the intermediate $(D^+A^-_1A_2)$ products of reaction blocks up charge transfer to the secondary acceptor.