Unusual temperature quenching of bacteriochlorophyll a fluorescence in FMO antenna protein trimers

Strong thermally induced quenching of fluorescence emission intensity of the Fenna–Matthews–Olson trimeric bacteriochlorophyll a – protein antenna complex from the green photosynthetic bacterium Chlorobium tepidum is observed starting already from 2 K, very different from that for other antenna complexes as well as for bacteriochlorophyll a in common solutions where the emission yield decreases only slightly past 100 K. Close spacing of the trimer exciton states within the lowest-energy manifold of absorbing states around 826 nm along with significant increase of the electron–phonon coupling strength toward higher energies is proposed responsible for this effect.