Synthesis and photoisomerization of dithienylethene-bridged diporphyrins

Dithienylethene-bridged diporphyrins 1-6 were prepared as photochemical swi tching molecules. Porphyrin and dithienylethene are directly linked in 1, a nd linked, respectively, through a 1,4-phenylene spacer in 2, through a 4-e thynylphenylene spacer in 3, and through a di-4-phenylethynylene spacer in 4, while meso-ethynylated porphyrin and dithienylethene are directly connec ted in 5 and linked through a 1,4-phenylene spacer in 6. Compounds 1, 2, an d 5 do not undergo any photochemical isomerization, probably due to efficie nt quenching of the excited dithienylethene by the attached porphyrin moiet y via intramolecular energy transfer. Compounds 4 and 6 undergo open-to-clo sed and closed-to-open photoisomerizations in quantum yields of 4.3 x 10(-2 ) and 1.8 x 10(-3), and 2.6 x 10(-3) and 7.5 x 10(-4), respectively, by irr adiation with 313 and 625 nm light, which are considerably smaller than qua ntum yields of 0.52 and 3.8 x 10(-3) for reference dithienylethene molecule 7. The fluorescence of 4 was regulated in a reversible manner by the photo isomerization of the dithienylethene moiety. In addition, the absorption pr operties of the porphyrin in 6 changed in response to the photochromic reac tion of the dithienylethene bridge.