Photoregulated Transmembrane Charge Separation by Linked Spiropyran-Anthraquinone Molecules

Amide-linked spiropyran−anthraquinone (SP−AQ) conjugates were shown to mediate ZnTPPS4--photosensitized transmembrane reduction of occluded Co(bpy)33+ within unilamellar phosphatidylcholine vesicles by external EDTA. Overall quantum yields for these reactions were dependent upon the isomeric state of the dye; specifically, 30−35% photoconversion of the closed-ring spiropyran (SP) moiety to the open-ring merocyanine (MC) form caused the quantum yield to decrease by 6-fold in the simple conjugate and 3-fold for an analogue containing a lipophilic 4-dodecylphenoxy substituent on the anthraquinone moiety. Transient spectroscopic and fluorescence quenching measurements revealed that two factors contributed to these photoisomerization-induced changes in quantum yields:  increased efficiencies of fluorescence quenching of 1ZnTPPS4- by the merocyanine group and lowered transmembrane diffusion rates of the merocyanine-containing redox carriers. Transient spectrophotometry also revealed the sequential formation and...