Photochromic Control of Photoinduced Electron Transfer. Molecular Double-Throw Switch

A molecular double-throw switch that employs a photochromic moiety to direct photoinduced electron transfer from an excited state donor down either of two pathways has been prepared. The molecular triad consists of a free base porphyrin (P) linked to both a C(60) electron acceptor and a dihydroindolizine (DHI) photochrome. Excitation of the porphyrin moiety of DHI-P-C(60) results in photoinduced electron transfer with a time constant of 2.3 ns to give the DHI-P(*)(+)-C(60)(*)(-) charge-separated state with a quantum yield of 82%. UV (366 nm) light photoisomerizes the DHI moiety to the betaine (BT) form, which has a higher reduction potential than DHI. Excitation of the porphyrin of BT-P-C(60) is followed by photoinduced electron transfer with a time constant of 56 ps to produce BT(*)(-)-P(*)(+)-C(60) in 99% yield. Isomerization of BT-P-C(60) back to DHI-P-C(60) may be achieved with visible light, or thermally. Thus, photoinduced charge separation originating from the porphyrin is reversibly directed down either of two different pathways by photoisomerization of the dihydroindolizine. The switch may be cycled many times.