Ultrafast Dynamics of Highly Conjugated Porphyrin Arrays

The photophysical properties of a series of ethynyl-bridged (porphinato) zinc (II) oligomers have been investigated over the femtosecond and picosecond time scales using ultrafast magic angle and polarized pump−probe spectroscopy. Bis (2, 2‘, -5, 10, 15, 20-tetraphenylporphinato) zinc (II) ethyne, bis (5, 5‘, -10, 20-diphenylporphinato) zinc (II) ethyne, and 5, 15-bis (5‘, -10‘, 20‘-diphenylporphinato) zinc (II) ethynyl10, 20-diphenylporphinatozinc (II) exhibit rapid (≤150 fs) formation of the emitting state following Soret photoexcitation, nearly an order of magnitude faster than the ∼1 ps S2 → S1 internal conversion observed for a monomeric (porphinato) zinc (II) complex that bears an ethyne moiety fused directly to its macrocycle carbon framework (5-trimethylsilylethynyl-10, 20-diphenylporphinato) zinc (II). The femtosecond and picosecond dynamics are strongly influenced by the porphyrin-to-porphyrin linkage topology: bis (2, 2‘, -5, 10, 15, 20-tetraphenylporphinato) zinc (II) ethyne, in which a β-to-β ethyne bridge links the two (porphinato) zinc (II) moieties, maintains the x−y degeneracy of the emitting state, the dynamics of which are consistent with energy equilibration within a weakly coupled porphyrin pair. In contrast, systems which feature a meso-to-meso ethynyl-bridged linkage topology between (porphinato) zinc (II) units exhibit properties consistent with significant inter-ring conjugation and loss of x−y degeneracy for the (π, π*) excited states. Collectively, the steady-state and time-resolved results for these meso-to-meso bridged systems suggest some degree of conformational heterogeneity for the ground state structures in solution, which differ with respect to the degree of conjugation of porphyrin rings; these conformers interconvert in the S1 excited state on a ∼30 ps time scale to produce a conformationally uniform, coplanar emitting state.