Self-Assembled Light-Harvesting System from Chromophores in Lipid Vesicles

Lipid vesicles are used as the organizational structure of self-assembled light-harvesting systems. Following analysis of 17 chromophores, six were selected for inclusion in vesicle-based antennas. The complementary absorption features of the chromophores span the near-ultraviolet, visible, and near-infrared region. Although the overall concentration of the pigments is low (similar to 1 mu M for quantitative spectroscopic studies) in a cuvette, the lipid-vesicle system affords high concentration (>= 10 mM) in the bilayer for efficient energy flow from donor to acceptor. Energy transfer was characterized in 13 representative binary mixtures using static techniques (fluorescence excitation versus absorptance spectra, quenching of donor fluorescence, modeling emission spectra of a mixture versus components) and time-resolved spectroscopy (fluorescence, ultrafast absorption). Binary donor acceptor systems that employ a boron-dipyrrin donor (S-0 <-> S-1 absorption/emission in the blue-green) and a chlorin or bacteriochlorin acceptor (S-0 <-> S-1, absorption/emission in the red or near-infrared) have an average excitation-energy-transfer efficiency (Phi(EET)) of similar to 50%. Binary systems with a chlorin donor and a chlorin or bacteriochlorin acceptor have Phi(EET) similar to 85%. The differences in Phi(EET) generally track the donor-fluorescence/acceptor-absorption spectral overlap within a dipole dipole coupling (Forster) mechanism. Substantial deviation from single-exponential decay of the excited donor (due to the dispersion of donor acceptor distances) is expected and observed. The time profiles and resulting Phi(EET) are modeled on the basis of (Forster) energy transfer between chromophores relatively densely packed in a two-dimensional compartment. Initial studies of two ternary and one quaternary combination of chromophores show the enhanced spectral coverage and energy-transfer efficacy expected on the basis of the binary systems. Collectively, this approach may provide one of the simplest designs for self-assembled light-harvesting systems that afford broad solar collection and efficient energy transfer.