Excited-state energy flow in covalently linked multiporphyrin arrays: The essential contribution of energy transfer between nonadjacent chromophores

A series of multiporphyrin arrays has been studied to probe the contribution of energy transfer between second-neighbor ("nonadjacent") porphyrins and to determine the rate of energy transfer between identical porphyrins at adjacent sites. The arrays, organized in linear or branched architectures, contain up to 21 constituents, domains of 2-5 zinc porphyrins, and a single energy trap. The study has involved iterative cycles of molecular design, synthesis, determination of rates via transient absorption spectroscopy, and kinetic analysis. A rate constant of (30 +/- 10 ps)(-1) is deduced for bidirectional energy transfer between adjacent zinc porphyrins joined by a diphenylethyne linker. The value is (50 +/- 10 ps)(-1) when the porphyrin-linker internal rotation is hindered by o,o'-methyl groups on one aryl ring of the linker. Rates of nonadjacent energy transfers are typically only 5-10-fold less than the rates of adjacent transfers. Thus, the nonadjacent pathway has a significant impact on the overall rate of energy flow to the trap, even in architectures as small as triads. These findings provide information that will be essential for the rational design of multichromophore arrays whose function is to transfer excitation energy efficiently over large distances to a trap site.