Vibronic coupling explains the ultrafast carotenoid-to-bacteriochlorophyll energy transfer rate

Jürgen Hauera, Václav Perlíkb, Joachim Seibtb, Tomáš Mancalb, František Šandab, Richard J. Cogdellc
aPhotonics Institute, TU Wien, Gusshausstrasse 27, 1040 Vienna, Austria; bFaculty of Mathematics and Physics, Charles University in Prague, Ke Karlovu 5, 121 16 Praha 2, Czech Republic; cInstitute of Molecular Cell and System Biology, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow Biomedical Research Centre, 120 University Place, Glasgow G12 8TA, Scotland

After photo-excitation of a carotenoid (green in Fig. 1a) in light harvesting complex 2 (LH2), population is transferred to bacteriochlolorphylls (red, BChls). This initial transfer step is happening on the truly ultrafast timescale of 40 to 160 fs, depending on the organism in question.[1] Such rapid transfer is a challenge for Förster theory, tending to overestimate the electronic coupling constant J.[2] We show that such high values of J cannot be reconciled with the shape of electronic 2D spectra or even linear absorption spectra of the investigated light harvesting complex, namely LH2 of Marichromatium purpuratum. We propose a vibronic coupling model, in which the fast carotenoid vibrational modes couple strongly to the optical transitions and are part of the system’s Hamiltonian. Fig. 1(b) shows the retrieved transfer times as a function of J and energetic detuning between donor and acceptor Δε.

We further validate the proposed vibronic coupling mechanism by testing it on an artificial carotenoid-purpurin dyad, where donor (carotenoid) and acceptor (a purpurin made to mimic the spectral properties of BChl) are covalently linked. The key ingredient to the mechanism, which dramatically speeds up energy transfer, is the deposition of excess excitation energy into the ground state vibrations of the carotenoid (donor) molecule.[3]

References

[1] R. E. Blankenship, Molecular Mechanisms of Photosynthesis (Wiley-Blackwell, 2014).
[2] B. P. Krueger et al., JPCB 102, 5378 (1998).
[3] V. Perlik et al., JCP 142, 212434 (2015).