Electron transfer in porphyrin−quinone cyclophanes studied on the pico− and femto−second time scale

Electron transfer in porphyrin−quinone cyclophanes is investigated by fluorescence and absorption spectroscopy with pico− and femto−second pulses. In nonpolar solvents, the S1 state of the porphyrin shows a lifetime from 300 ps up to several nanoseconds, depending upon the number of quinones and upon their electron affinity. Comparative measurements in polar solvents demonstrate very fast electron transfer on a time scale between 1 and 10 ps. The results are analyzed with the aid of quantum−chemical calculations which give the energy of the charge transfer states and the relevant coupling strengths. For nonpolar solvents, theory suggests fluctuation−induced charge separation and/or direct radiationless internal conversion from the porphyrin S1 to the ground state. In polar solution, the molecules exist in a tilted configuration with strong electronic coupling and charge transfer states well below the S1 level, resulting in fast electron transfer and subsequent charge recombination within 10−40 p