An unusual energy transfer process from free-base porphyrin guests to a zinc porphyrin host in self-assembled systems

The “V” shaped bis-porphyrinic host ZnH can complex the dipyridyl free-base porphyrin guests FBP and FBE with an association constant of 10⁸ M⁻¹. Complexation by ZnH of a reference photo-inactive guest, MN, occurs with a similar association constant and induces a perturbation in the luminescence properties of the host i.e. lower energy and decreased intensity and lifetime of the excited state localized on ZnH. A comparison of the effect of MN complexation by ZnH with those occurring in the complexes of FBP and FBE with ZnH, allowed us to establish the occurrence of an efficient energy transfer from each guest to the host ZnH. The rate constants are 2.5 × 10¹⁰ s⁻¹ (ΔG⁰=−0.2 eV) and 6.5 × 10⁹ s⁻¹ (ΔG⁰=−0.13 eV) for FBE and FBP respectively. The results are consistent with a Förster type process with a very low orientation factor (κ²), even if a Dexter type mechanism with a weak intercomponent electronic interaction term (H), cannot be excluded.