Photophysics of a cerium(IV)-porphyrin sandwich complex: Picosecond deactivation via neutral exciton or charge-transfer excited states

We have investigated, using time-resolved and steady-state optical techniques, some of the photophysical properties of a lanthanide-porphyrin dimer. The complex contains a Ce(IV) ion sandwiched between two octaethylporphyrin macrocycles. The dimer is nonfluorescent (φ_f 2 × 10^-5). The absorption changes produced by pulse excitation decay largely (>90%) by return of the molecules to the ground state in <10 ps. There is evidence for low energy quenching states in the ground state absorption spectrum. In order to explain these observations, we have considered the importance of neutral (π,π*) exciton states of the two rings, ring-to-ring charge-transfer states and ring-to-metal charge-transfer states. We believe that excited states having substantial charge-transfer character may provide the most effective routes for rapid radiationless decay.