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

Abstract

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 (S₁) or second (S₂) 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 S₂ (fs time-domain) and in the S₁ (ps time-domain) state. This is substantiated by complementary experiments with the monomeric porphyrin, in which the S₂ 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.