ULTRAFAST DYNAMICS OF HIGHLY CONJUGATED PORPHYRIN ARRAYS

The photophysical properties of a series of ethynyl-bridged (porphinat o)zinc(II) oligomers have been investigated over the femtosecond and p icosecond time scales using ultrafast magic angle and polarized pump-p robe spectroscopy. Bis[(2,2',-5,10, 15,20-tetraphenylporphinato)zinc(I I)]ethyne, {[(5,5',-10,20-diphenylporphinato)zinc(II)]ethyne, and inc( II)]ethynyl}[10,20-diphenylporphinato]zinc(TI) exhibit rapid (less tha n or equal to 50 fs) formation of the emitting state following Soret p hotoexcitation, nearly an order of magnitude faster than the similar t o 1 ps S-2 --> S-1 internal conversion observed for a monomeric (porph inato)zinc(II) complex that bears an ethyne moiety fused directly to i ts macrocycle carbon framework rimethylsilylethynyl-10,20-diphenylporp hinato)zinc (II)]. The femtosecond and picosecond dynamics are strongl y influenced by the porphyrin-to-porphyrin linkage topology: bis[(2,2' ,-5,10,15,20-tetraphenylporphin zinc(II)]ethyne, in which a beta-to-be ta ethyne bridge links the two (porphinato)zinc(II) moieties, maintain s 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-bridg ed linkage topology between (porphinato)zinc(II) units exhibit propert ies 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 system s suggest some degree of conformational heterogeneity for the ground s tate structures in solution, which differ with respect to the degree o f conjugation of porphyrin rings; these conformers interconvert in the S-1 excited state on a similar to 30 ps time scale to produce a confo rmationally uniform, coplanar emitting state.