Through-bond excited energy transfer mediated by an amidinium-carboxylate salt bridge in Zn-porphyrin free-base porphyrin dyads

Excited energy-transfer processes were investigated for a supramolecular Zn-porphyrin free-base porphyrin dyad, ZnPA-2·FbPC-2, in which β-octaalkylated meso-diarylporphyrins are connected through an amidinium-carboxylate salt bridge. The rate of energy transfer in the dyad (1.3 × 10⁹ s^-1) is substantially slower than that in the previously reported dyad, ZnPA-1·FbPC-1 (4.0 × 10⁹ s^-1), in which meso-tetraarylporphyrins are connected through the same amidinium-carboxylate salt bridge. The Förster-type mechanism can explain only minor parts of these rates (3.3 × 10⁸ and 5.1 × 10⁸ s^-1, respectively). Thus, Dexter-type through-bond energy transfer may be invoked. Indeed, bridge-mediated electronic processes would be favored in ZnPA-1·FbPC-1 over ZnPA-2·FbPC-2 on the basis of steric and electronic factors. Sterically, the phenyl groups in ZnPA-2 and FbPC-2 are more closely perpendicular to the porphyrin planes than those in ZnPA-1 and FbPC-1. Electronically, the energy and symmetry of the occupied frontier orbitais should favor ZnPA-1·FbPC-1 over ZnPA-2·FbPC-2 in terms of electronic interactions through the bridge. Therefore, the observed trend (ZnPA-1·FbPC-1 > ZnPA-2·FbPC-2), consistent with these considerations, lends further support to the through-bond mechanism. Thus, the amidinium-carboxylate salt bridge is effective in mediating through-bond energy transfer even though the bond is noncovalent.