Multistep photoinduced electron transfer in self-organised nanoscale porphyrin triads

Abstract

Well-deÿned structurally organised porphyrin triads of a controlled geometry and nanoscale size have been formed in liquid solutions using the combination of a covalent approach and non-covalent self-assembly. The triads contain zinc-octaethylporphyrin chemical dimer, (ZnOEP)2Ph, with covalently linked electron acceptors (p-benzoquinone, Q or pyromellitimide, Pim), and additional dipyridyl-substituted tetrapyrrole extra-ligands. Steady-state, picosecond uorescence ( t 1=2 ≈ 75 ps) and femtosecond pump-probe ( 1=2 ≈ 280 fs) data show that non-radiative deactivation of the dimer S1-states ( S ¡ 1 ps) is due to both the S-S energy transfer (ZnOEP)2Ph→extra-ligand and the sequential photoinduced electron transfer (ZnOEP)2Ph→Q (or Pim) at rDA = 10:8 A. The additional decay shortening of the extra-ligand S1-states by 3-6 times (toluene, 293 K) is attributed to the increased "superexchange" mediated long distant (rDA ≈ 18-21 A) one-step electron transfer extra-ligand→Q (or Pim). ?