Excitation Energy Transfer Pathways in Asymmetric Covalent Chlorophyll a Tetramers

We report the observation of multiple excitation energy transfer (EET) pathways in a covalently linked, chlorophyll (Chl) tetramer having three different fixed Chl−Chl distances. The tetramer was synthesized by covalently attaching 20-(4-ethynylphenyl)Chl to the 1,3,6,8-positions of pyrene to give Chl4-py. Reference Chl oligomers were prepared by attaching 20-(4-ethynylphenyl)Chl to the 1-position of pyrene (Chl-py), the 1,8 and 3,6 positions of pyrene (para-Chl2-py and ortho-Chl2-py, respectively), and the 1,3,5-positions of benzene (meta-Chl3). The Chl x -py derivatives were studied using femtosecond transient absorption (TA) and transient absorption anisotropy (TAA) spectroscopy, and compared with data obtained earlier on meta-Chl3. Using femtosecond TA, the decay of 1* Chl4-py was monitored after photoexcitation of its Q y band with a 655 nm, 130 fs laser pulse. A triexponential decay of 1* Chl4-py (τ = 7 ps, 152 ps, and 4.2 ns) was observed, with the two shorter time constants being laser intensity dependent. This dependency is indicative of singlet−singlet annihilation and is attributed to EET via two pathways, those involving the two closest Chl’s (ortho/meta), which are kinetically indistinguishable, and the two most distant (para) Chl’s. Further confirmation of multiple EET pathways was made by femtosecond TAA measurements. Förster energy transfer lifetimes were calculated for the three possible energy transfer pathways (ortho, 20 ps; meta, 34 ps; and para, 150 ps) and compared to the experimental results. Our results indicate that EET between nonadjacent chromophores having high oscillator strengths, such as the para Chl’s in Chl4-py, is significant, and the additional utilization of the minor EET contributing pathways may provide greater avenues for designing efficient light harvesting in future artificial photosynthetic systems.