Photophysics of Lanthanide Triple Decker Porphyrin Sandwich Complexes

Abstract

Ground state and time resolved excited state optical absorption spectra and luminescence spectra are reported for La 2 (OEP) 3, Eu 2 (OEP) 3, and Ce 2 (OEP) 3 (OEP = 2,3,7,8,12,13,17,18-octaethylporphyrinate). The porphyrin macrocycles in these molecules are strongly coupled due to their mean separation of ≤3.5 Å. The triple deckers show many optical properties analogous to those found for double decker porphyrin sandwich complexes, including features not exhibited by porphyrin monomers. Following excitation, the lowest 1 (π,π*) state of La 2 (OEP) 3 decays in ∼10 ps predominantly to the phosphorescent 3 (π,π*) state, which lives for ∼10 μs. The lowest 1 (π,π*) state of Eu 2 (OEP) 3 also decays rapidly and with high yield to the lowest 3 (π,π*) state, which has a rather short lifetime of 630 ps. The lack of luminescence from Eu 2 (OEP) 3 is ascribed to deactivation of the 3 (π,π*) state through one or more low-energy ligand-field states of the metal ion. Ce 2 (OEP) 3 is also nonluminescent. Unlike La 2 (OEP) 3 and Eu 2 (OEP) 3, the 1 (π,π*) lifetime of 7 ps for Ce 2 (OEP) 3 largely reflects deactivation within the singlet manifold. The decay of 1 (π,π*) to the ground state likely involves low-lying (f,f) states, although a charge transfer state may play a role. The small (∼20%) population of 3 (π,π*) that forms from 1 (π,π*) also decays rapidly (τ ∼ 300 ps), again via ligand-field states of the metal ions. These combined results further demonstrate the importance of (i) π−π interactions in defining the electronic properties of chromophores brought within van der Waals distance and (ii) metal-centered excited states in determining the relaxation rates and pathways following photoexcitation of tetrapyrrole complexes.