Effect of Donor Density on FRET efficiency in Artificial Light-Harvesting System

초록

Artificial light-harvesting systems have been synthesized and studied to understand the energy transfer phenomena occurring in natural light-harvesting systems. However, even with relatively simple artificial light harvesting systems, they are inherently composed of a large number of chromophores, making it difficult to fully understand the energy transfer phenomena within them. Here, we studied the Forster resonance energy transfer (FRET) characteristics of the multi-porphyrin dendrimers mimicking LH2, a typical light harvesting system, while maintaining the distance between the donor and the acceptor, giving systematic differences only in the donor density and number. The multi-porphyrin dendrimers are composed of zinc porphyrins (energy donors) in the periphery and free base porphyrin (energy acceptor) in the core. In these systems, energy migration between zinc porphyrins and energy transfer from zinc porphyrin to free-base porphyrin can occur. These energy transfers were confirmed by steady state emission, transient absorption and, two-dimensional electronic spectroscopy. We show that the more the donor, the higher the efficiency of FRET when the energy migration between donors is faster than the energy transfer from donor to acceptor. On the contrary, in the absence of energy migration between zinc porphyrins, the efficiency of FRET depends solely on the distance between zinc porphyrin and free-base porphyrin. These results suggest that energy migration between the zinc porphyrins provides an additional pathway for energy transfer to the acceptor, and this additional path compensates for the low efficiency energy transfer that occurs in donors with orientation or distance that is relatively unfavorable to FRET.