Comparison of Excited-State Energy Transfer in Arrays of Hydroporphyrins (Chlorins, Oxochlorins) versus Porphyrins: Rates, Mechanisms, and Design Criteria

A set of chlorin−chlorin and oxochlorin−oxochlorin dyads has been prepared with components in the same or different metalation states. In each case a 4,4‘-diphenylethyne linker spans the respective 10-position of each macrocycle. The dyads have been studied using static and time-resolved absorption and emission spectroscopy, resonance Raman spectroscopy, and electrochemical techniques. Excited-state energy transfer from a zinc chlorin to a free-base (Fb) chlorin occurs with a rate constant of (110 ps)-1 and an efficiency of 93%; similar values of (140 ps)-1 and 83% are found for the corresponding oxochlorin dyad. Energy transfer in both dyads is slower and less efficient than found previously for the analogous porphyrin dyad, which displays a rate of (24 ps)-1 and a yield of 99%. The slower rates and diminished efficiencies in the ZnFb chlorin and oxochlorin dyads versus the ZnFb porphyrin dyad are attributed to substantially weaker linker-mediated through-bond (TB) electron-exchange coupling (as indicate...