Photoinduced electron transfer in multiporphyrinic interlocked structures: the effect of copper(I) coordination in the central site.

Abstract

Photoinduced processes have been determined in a [2]catenane containing a zinc(II) porphyrin, a gold(III) porphyrin, and two free phenanthroline binding sites, Zn-Au(+), and in the corresponding copper(I) phenanthroline complex, Zn-Cu(+)-Au(+). In acetonitrile solution Zn-Au(+) is present in two different conformations: an extended one, L, which accounts for 40 % of the total, and a compact one, S. In the L conformation, the electron transfer from the excited state of the Zn porphyrin to the gold-porphyrin unit (k = 1.3x10(9) s(-1)) is followed by a slow recombination (k = 8.3x10(7) s(-1)) to the ground state. The processes in the S conformation cannot be clearly resolved but a charge-separated (CS) state is rapidly formed and decays with a lifetime on the order of fifty picoseconds. In the catenate Zn-Cu(+)-Au(+), the zinc-porphyrin excited state initially transfers energy to the Cu(I)-phenantholine unit, producing a metal-to-ligand charge-transfer (MLCT) excited state localized on the copper complex with a rate k = 1.4x10(9) s(-1). From this excited state the transfer of an electron to the gold-porphyrin unit takes place, producing the CS state Zn-Cu(2+)-Au(.), which decays with a lifetime of 10 ns. The results are discussed in comparison with the closely related [2]rotaxane, in which a further charge shift from the copper center to the zinc-porphyrin unit leads to the fully CS state. Even in the absence of such full charge separation, it is shown that the lifetimes of the CS states are increased by a factor of about 2-2.5 over those of the corresponding rotaxanes.