ENERGY-TRANSFER MODELING FOR THE RATIONAL DESIGN OF MULTIPORPHYRIN LIGHT-HARVESTING ARRAYS

Excited-state energy migration among a collection of pigments forms th e basis for natural light-harvesting processes and synthetic molecular photonic devices. The rational design of efficient energy-transfer de vices requires the ability to analyze the expected performance charact eristics of target molecular architectures comprised of various pigmen ts. Toward that goal, we present a general tool for modeling the kinet ics of energy migration in weakly coupled multipigment arrays. A matri x-formulated eigenvalue/eigenvector approach has been implemented, usi ng empirical data from a small set of prototypical molecules, to predi ct the quantum efficiency (QE) of energy migration in a variety of arr ays as a function of rate, competitive processes, and architecture. Tr ends in the results point to useful design strategies including the fo llowing: (1) The QE for energy transfer to a terminal acceptor upon ra ndom excitation within a linear array of isoenergetic pigments decreas es rapidly as the length of the array is increased. (2) Increasing the rate of transfer and/or the lifetime of the competitive deactivation processes significantly improves QE. (3) Qualitatively similar results are obtained in simulations of linear molecular photonic wires in whi ch excitation and trapping occur at opposite ends of the array. (4) Br anched and cyclic array architectures exhibit higher QEs than linear a rchitectures with equal numbers of pigments. (5) Dramatic improvements in QE are achieved when energy transfer is directed by a progressive downward cascade in excited-state energy. (6) The most effective light -harvesting architectures are those where isolated pools of donors eac h have independent paths directly to the terminal acceptor. Collective ly, these results provide valuable insight into the types of molecular designs that are expected to exhibit high efficiency in overall energ y transfer.