Photocontrol on ultrafast excited state transfer processes

Our studies on photo-switching of intramolecular charge transfer in our newly designed anthracene (donor)/pyridinium (acceptor) substituted bistable dithienylethene supermolecule (J. Am. Chem. Soc. 122 (2000) 3037) are extended by applying fs transient-absorption spectroscopy. For the open-ring isomer of the dithienylethene switching unit, the transient-absorption spectra of the supermolecule evolve from the locally-excited to the radical-cation state of the donor anthracene, indicating a two-step photoinduced charge-transfer process with a time-constant of τCT≈1.7 ps. In contrast, after photocyclisation to the closed-ring isomer, no radical-cation signal is detected, while it reappears upon re-opening. Comparison is made with a molecular system without a switching subunit. In a polymer environment the charge transfer is inhibited.