Energy transfer in supramolecular artificial antennae units of synthetic zinc chlorins and co-aggregated energy traps.: A time-resolved fluorescence study

Using time-resolved fluorescence. we explored the energy transfer process(es) in supramolecular zinc chlorin aggregates co-aggregated with various kinds of energy traps. The energy transfer times from the antenna aggregate to the trap are in the picosecond time range (7-9 ps) under reducing conditions (addition of dithionite to avoid oxidative quenching) and were resolved in all cases. Under nonreducing conditions substantial fluorescence quenching occurred in the antenna aggregates. We tentatively suggest that a small amount of chlorin cations acts as a quencher. We find that in the aggregates the excitation is delocalized over at least 10-15 pigments, on the basis of the corresponding strong increase of the pure radiative rate vs a monomeric chlorin. In bacteriochlorophyll-based aggregates the transfer to the energy trap is biexponential (27 and 91 ps), which is reminiscent of native isolated chlorosomes from green sulfur bacteria. For the zinc chlorin-based light-harvesting units the overall efficiency of the energy collection reaches up to 70%. We conclude that these units have suitable proper-ties as artificial antenna systems for solar energy utilization.