Photoinduced energy and electron transfers in the porphyrin triad (zinc octaethylporphyrin-4,4′ bipyridinium-tetraphenylporphyrin) 2+, 2C1O4 −)

Abstract

Intramolecular electron and energy transfers were investigated in a series of porphyrin molecules linked to a bipyridinium cation, by steady state absorption and emission spectroscopies, time resolved fluorescence and voltammetry. A strong quenching of the fluorescence intensity related to an important intramolecular electron transfer was observed when bipyridinium moiety V was attached either to the zinc (A) or to the free base porphyrin (B) in dyads AV and BV. This intramolecular electron transfer operates from the photoexcited singlet state of the zine porphyrin (1A) or of the free-base porphyrin (1B) to the bipyridinium mono or dication moiety resulting in charge separation and formation of biradical state A· −V· orB · +V·. The electron transfer rate constants of this process in AV + and BV were 3.8 × 10 9 s −1 and 7.7 × 10 9 s −1, respectively. In the case of heteroporphyrin triad AV 2+B where metallo and free-base porphyrins were attached to the bipyridinium spacer, both energy transfer and electron transfer processes were observed. The quantum yield of the singlet-singlet energy transfer which explains most of the quenching was 0.77 and the corresponding rate constant was 4.9 × 10 9 s −1. In addition, both first excited states 1 AV 2+ B and AV 2+ 1B decayed via photoindueed electron transfers to produce radical ion pairs A · +V · +B and AV · +B · +. An energy levels diagram of the various S 1 states of triad AVB is proposed and the efficiency of the various transfer processes also discussed.