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 β,β'-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.