Ultrafast Energy Transfer from a Carotenoid to a Chlorin in a Simple Artificial Photosynthetic Antenna

A model photosynthetic antenna system consisting of a carotenoid moiety covalently linked to a purpurin has been prepared to study singlet−singlet energy transfer from a carotenoid to a cyclic tetrapyrrole. Ultrafast fluorescence upconversion measurements of the carotenopurpurin dyad and an unlinked reference carotenoid demonstrate that the fluorescent S₂ excited state of the carotenoid model has a lifetime of 150 ± 3 fs, whereas the corresponding excited state of the carotenoid in the carotenopurpurin dyad is quenched to 40 ± 3 fs. This quenching is assigned to energy transfer from the S₂ state to the purpurin with a 73 ± 6% efficiency, which is in accord with the 67 ± 4% quantum yield obtained by steady-state fluorescence excitation measurements. Concomitant with the decay of the carotenoid S₂ excited state, a single-exponential rise of the excited S₁ state of the purpurin moiety was observed at 699 nm with a time constant of 64 fs. However, the decay of the fluorescence anisotropy was faster at this wavelength (40 fs) and isotropic rise times as short as 44 fs were determined at other emission wavelengths. The lifetime of the S₁ state of the carotenoid (7.8 ps) was the same in both the carotenoid model and the dyad. Taken together, these results unequivocally demonstrate that the S₂ state of the carotenoid moiety is the sole donor state in this efficient singlet−singlet energy transfer process. The simple dyad described in this work mimics the ultrafast energy transfer kinetics found in certain naturally occurring pigment protein complexes and is thus able to reproduce the high electronic coupling needed for efficient energy transfer from an extremely short-lived energy donor state.