Ultrafast singlet-singlet energy transfer in self-assembled via metal-ligand axial coordination of free-base porphyrin-zinc phthalocyanine and free-base porphyrin-zinc naphthalocyanine dyads.

Singlet-singlet energy transfer in self-assembled via axial coordination of imidazole-appended (at different positions of one of the meso-phenyl entities) free-base tetraphenylporphyrin, H(2)PIm, to either zinc phthalocyanine, ZnPc, or zinc naphthalocyanine, ZnNc, dyads is investigated in noncoordinating solvents, o-dichlorobenzene and toluene, using both steady-state and time-resolved transient absorption techniques. The newly formed supramolecular dyads were fully characterized by spectroscopic, computational, and electrochemical methods. The binding constants measured from optical absorption spectral data were found to be in the range of 10(4)-10(5) M(-1) for the 1:1 dyads, suggesting fairly stable complex formation. Electrochemical and computational studies suggested that photoinduced electron transfer is a thermodynamically unfavorable process when free-base porphyrin is excited in these dyads. Selective excitation of the donor free-base porphyrin entity was possible in both types of dyads formed by either of the ZnPc or ZnNc energy acceptors. Efficient singlet-singlet energy transfer was observed in these dyads, and the position of imidazole linkage on the free-base porphyrin entity, although flexible, seems to have some control over the overall efficiency of excited energy transfer process. Kinetics of energy transfer was monitored by performing transient absorption measurements using both up-conversion and pump-probe techniques. Such studies revealed ultrafast singlet-singlet energy transfer in the studied dyads with time constants on the order of 2-25 ps depending upon the type of the dyad.