Triplet-Triplet Excitation Transfer in Palladium Porphyrin Fullerene and Platinum Porphyrin -Fullerene Dyads

Covalently linked donor-acceptor dyads involving palladium(II) and platinum(II) porphyrins as triplet sensitizers and fullerene as an acceptor have been newly synthesized. These dyads were characterized by optical absorbance, emission, and electrochemical methods. In contrast to the earlier reported zinc(II) porphyrin and free-base porphyrin-based dyads of similar structures, photoinduced electron transfer from the short-lived singlet and long-lived triplet excited metalloporphyrin to the fullerene was not observed, although these processes are energetically possible according to the energy level diagrams. That is, diagnostic transient bands corresponding to MP center dot+ [M = Pd(II) or Pt(II)] in the 600-650-nm range and C-60(center dot-) in the 1000-nm range were absent in the femtosecond and nanosecond transient absorption spectra. Interestingly, excited energy transfer from the triplet excited metalloporphyrin to the fullerene was witnessed in both palladium and platinum porphyrin-derived dyads by nanosecond transient absorption studies. Three solvents with different polarities were employed to visualize the medium effects. The determined rate of energy transfer, k(EnT), was found to be higher for the PdP-based dyad than the PtP-based dyad in a given solvent and that the rates were higher for polar solvents than for nonpolar solvent. The present investigation demonstrates how the heavy-metal ion in the porphyrin cavity modulates photoinduced processes and the solvent-dependent kinetics of these events.