MULTISTEP ELECTRON-TRANSFER BETWEEN PORPHYRIN MODULES ASSEMBLED AROUND A RUTHENIUM CENTER

A new strategy has been devised for the construction of photoactive multicomponent arrays based on metal ion chelation whereby bisporphyrins have been assembled around a central ruthenium(II) bis(terpyridyl) complex. One of the terminal subunits is a gold(III) porphyrin while the second terminus is selected from a gold(III), zinc(II), or free-base porphyrin. Photophysical properties have been measured for each of the tripartite compounds using ultrafast transient absorption and emission spectroscopy. Excitation into the central ruthenium(II) bis(terpyridyl) complex is followed by rapid intramolecular triplet energy transfer to one of the appended porphyrins. Direct excitation into the gold(III) porphyrin subunit generates the corresponding triplet excited state which is unreactive toward energy- or electron-transfer processes. In contrast, excitation into the zinc(II) or free-base porphyrin produces the corresponding excited singlet state which transfers an electron to the adjacent ruthenium(II) bis(terpyridyl) complex. Secondary electron transfer to the appended gold(III) porphyrin competes with reverse electron transfer such that the redox equivalents become separated by about 30 Angstrom. The original ground-state system is restored by relatively slow interporphyrin electron transfer. The energetics for each of these electron-transfer steps have been evaluated from electrochemical measurements and by measuring the rates as a function of temperature and solvent polarity. For the zinc(II) porphyrin-containing triad, electron transfer takes place in the solid state at 77 K. Finally, the performance of the latter triad is compared with that of the bacterial photosynthetic reaction center complex.