Synthesis and Excited-State Photodynamics of a Molecular Square Containing Four Mutually Coplanar Porphyrins

To examine the effect of restricted porphyrin-porphyrin rotation on energy transfer in diphenylethyne-linked porphyrin arrays, a square macrocyclic array of four porphyrins has been prepared that locks the porphyrins in a mutually coplanar architecture. The palladium-mediated coupling of Zn(II) 5,10-dimesityl-15,20-bis(4-ethynylphenyl)porphyrin and 5,10-dimesityl-15,20-bis(4-iodophenyl)porphyrin afforded the molecular square (cyclo-Zn₂Fb₂U) with zinc (Zn) and free base (Fb) porphyrins on alternating corners. The yield of cyclo-Zn₂Fb₂U is relatively insensitive to concentration with reactants at 5-0.5 mM but declines significantly at <0.05 mM. Transient absorption data from cyclo-Zn₂Fb₂U in toluene at room temperature indicate that the rate of energy transfer from the photoexcited Zn porphyrin to a neighboring Fb porphyrin is (26 ps)^-1. A ZnFb porphyrin (ZnFbU) with an identical diphenylethyne linker but no constraints on rotation exhibits an identical energy-transfer rate. Resonance Raman spectroscopy shows that the intensities of the νC≡C mode(s) of the cyclo-Zn₂Fb₂U are comparable to those of ZnFbU, indicating that the extent of electronic coupling between the ethyne groups and the π-systems of the porphyrins is comparable in the two types of arrays. Thus, the geometric constraints imposed by the closure to the macrocyclic structure, including the enforced coplanarity of the four porphyrin rings, do not alter the through-bond electronic communication among the porphyrins.