Excitation Energy Transfer Pathways in Asymmetric Covalent Chlorophyll a Tetramers

Abstract

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 Chlx-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 Qy 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 depend...