Low-temperature frequency domain study of excitation energy transfer in ethynyl-linked chlorophyll trefoils and aggregates

Using hole-burning spectroscopy, we show that excitation energy transfer (EET) time in ethynyl-linked chlorophyll trefoil (ChlT1) monomer is very fast (∼2.5 ps) at liquid helium temperature. This is consistent with data obtained by femtosecond transient spectroscopy experiments performed at room temperature, in which an EET time of 1.8 ps was observed (Kelley, R. F. et al. Angew. Chem. Int. Ed. 2006, 45, 7979). This finding further supports the importance of through-bond electronic coupling at low temperature. In addition, we show that ChlT1 (even at very low concentrations) in methyl tetrahydrofuran-ethanol glass (1:200 v/v; T ∼ 5 K) forms different types of aggregates. It is demonstrated that the relative distribution of various types of aggregates (whose possible structures are briefly discussed) depends on the cooling rate and matrix composition. For example, the EET time in two types of ChlT1-based aggregates is slower by a factor of ∼5-7 with respect to that observed for ChlT1 monomer. This indicates that ChlT1 aggregates can retain ultrafast energy transfer properties similar to those observed in natural photosynthetic antennas. It is anticipated that such building blocks could be utilized in future photovoltaic devices.