Ultrafast Intramolecular Electron Transfer in Peripherally Ruthenated Zinc Tetraarylporphyrins

Abstract

Ultrafast fluorescence quenching has been examined for 5-pyridyl-10,15,20-triphenylporphinatozinc (ZnP), which is coordinatively linked to ruthenium complex (oxo-acetatobridged-triruthenium ions) (Ru), and for its derivatives having cyanopyridyl groups (Ru1) or 4-(N,N-dimethylamino)pyridyl groups (Ru2). The fluorescence for these peripherally elaborated zinc tetraarylporphyrins are > 99% quenched relative to ZnP. Fluorescence decays were measured in different solvents using the technique of femtosecond fluorescence up-conversion for these three compounds: ZnP–Ru⁺, ZnP–Ru⁺–Ru1⁺, and ZnP–Ru⁺–Ru2⁺. The fluorescence lifetime for ZnP^* (τ_f = 1.7—1.8 ns in ZnP monomer) is dramatically decreased by the linkage of ruthenium moieties to the periphery of ZnP; τ_f = 460—530 fs for ZnP^*–Ru⁺, τ_f = 380—400 fs for ZnP–Ru⁺–Ru1⁺, and τ_f = 300—380 fs for ZnP–Ru⁺–Ru2⁺. This fluorescence quenching behavior is in accord with a semiempirical estimation of the free energy for electron transfer from ZnP^* to Ru⁺. Thus the ultrafast fluorescence quenching process in these molecules is attributed to an intramolecular electron transfer governed by the following equations: ZnP^*–Ru⁺ → ZnP⁺–Ru⁰ and ZnP^*–Ru⁺–Ru1⁺ (or Ru2⁺) → ZnP⁺–Ru⁰–Ru1⁺ (or Ru2⁺).