Change in reaction mechanism with driving force in photoinduced dissociative electron transfer (PDET) reaction - A subpicosecond transient absorptionstudy

Electron-transfer reactions that are dissociative in nature can proceed via two mechanisms, stepwise and concerted. In this work, the mechanism of pho toinduced dissociative electron transfer between carbon tetrachloride and f our phenothiazine derivatives has been investigated by subpicosecond transi ent absorption study. Clear evidence was obtained for the stepwise mechanis m for the reaction between phenothiazine and carbon tetrachloride, whereas for the other three derivatives concerted reaction is seen to be operative. The mechanism of the dissociative electron-transfer reaction has been corr elated with the driving force of the reactions studied. A crossover of reac tion mechanism from concerted to stepwise was observed with the increase in the reducing power of the phenothiazine donor, i.e., with the increase in the driving force of the reaction. It was also shown that changes in solven t polarity can trigger the crossover of the reaction mechanism. The change in the reaction mechanism with the driving force of the reaction has been e xplained by potential energy surfaces involved in the two different mechani sms.