Ultrafast Photoinitiated Long-Range Electron Transfer in Cyclophane-Bridged Zincporphyrin−Quinone Complexes via Conical Intersections

Upon photoexcitation of the spectroscopically known Q states of cyclophane-bridged zincporphyrin−quinone complexes, an electron is transferred from the porphyrin to the quinone ring on a subpicosecond time scale. The details of the molecular mechanism of this ultrafast process are investigated with modern quantum chemical methods. Two excited-state crossings between the initially excited Q states and an appropriate zincporphyrin-to-quinone charge-transfer state could be identified, which are located in the vicinity of the ground-state equilibrium geometry. One state crossing occurs along a vibrational mode corresponding to a twisting of the cyclophane bridges, the so-called twist mode, while a second crossing appears along a so-called swinging-bridge mode, in which the quinone ring swings versus the porphyrin ring. Arguments are given that these state crossings correspond to conical intersections, which present here a highly efficient pathway for long-range electron transfer. General aspects of the presented mechanism and its relevance for biological electron-transfer processes are discussed.