Ultrafast exciton dynamics after Soret- or Q-band excitation of a directly β, β′-linked bisporphyrin

We report on a comprehensive transient absorption study with beta,beta'-linked bis[tetraphenylporphyrinato-zinc(II)] and its corresponding monomer, covering the ultrafast dynamics from femtoseconds up to several microseconds. By exciting these porphyrins either to their first (S-1) or second (S-2) electronically excited states and by probing the subsequent dynamics, a multitude of reaction pathways have been identified. In the spectral region associated with the ground-state recovery of the bisporphyrin, transient absorption changes occur within the first few picoseconds, which are ascribable to excitonic interaction both in the S-2 (fs time-domain) and in the S-1 (ps time-domain) state. This is substantiated by complementary experiments with the monomeric porphyrin, in which the S-2 state exhibits a longer lifetime. In contrast to the picosecond dynamics the bisporphyrin and the monomer behave similarly on the nanosecond time-scale, that is nearly all excited molecules eventually reach a long-lived triplet excited state.