ROLE OF COHERENT PHENOMENA IN ULTRAFAST BIOLOGICAL PROCESSES

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An important question is whether the coherence introduced by optical excitation in the initial step of a light driven reaction is maintained during the primary process or not Do the dynamics of the initially formed wavepacket on the reactive surface involve coherent motion? The observations of coherent wavepacket propagation provides an additional probe of the nature of the potential energy surface upon which the primary dynamical processes are occurring. 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. They 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 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: if a beta-to-beta ethyne bridge links the two (porphinato)zinc(II) moieties, this 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 whi ch 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 S1 excited state on a similar to 30 ps time scale to produce a conformationally uniform, coplanar emitting state.