Lifetime of the triplet exciplexes of porphyrins with electron acceptors in toluene solution
The quenching of porphyrin triplets by electron acceptors occurs via the formation of triplet exciplexes. The majority of the long-lived triplet exciplexes (τ≈ 10–3 s) are formed by quenchers having Ered1/2≈–0.5 V. On passing to either weaker or stronger electron acceptors, the lifetime of the triplet exciplexes decreases. The number of quenchers which are capable of forming long-lived triplex exciplexes depends on the nature of the porphyrin. The magnesium monopyridinate-meso-tetraphenylporphyrin complex (the most electron-donating among those studied) forms the greatest number of long-lived exciplexes. Triplet exciplex formation is accompanied by a quencher-induded deactivation of the triplets. Both processes involve some common steps. The reaction of an excited molecule with the acceptor produces an ion-radical pair as the initial product. The recombination of the latter gives triplet exciplexes and molecules in the ground state. The triplet exciplexes can be deactivated through intersystem crossing or via the formation of an ion-radical pair followed by recombination. The mechanisms for these processes are discussed.