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)]ethynyl}[10,20-diphenylporphinato]zinc(TI) exhibit rapid (less than or equal to 50 fs) formation of the emitting state following Soret photoexcitation, nearly an order of magnitude faster than the similar to 1 ps S-2 --> S-1 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-tetraphenylporphin zinc(II)]ethyne, in which a beta-to-beta 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 (pi, pi*) 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 S-1 excited state on a similar to 30 ps time scale to produce a conformationally uniform, coplanar emitting state.