Emitting excitonic polaron states in core LH1 and peripheral LH2 bacterial light-harvesting complexes

Hole-burned absorption and line-narrowed fluorescence spectra along with nanosecond fluorescence decay kinetics have been studied at 5 K in core LH1 and peripheral LH2 antenna complexes isolated from the photosynthetic purple bacterium Rhodobacter sphaeroides. A dual nature for the respective emission bands has been confirmed in both complexes and has been assigned to the nearly free excitons weakly coupled to lattice vibrations and to the strongly coupled self-trapped excitons. The apparent phonon structure of quasi-free excitons has been analyzed resulting in a total Huang-Rhys factor, a characteristic of the electron-phonon coupling strength, equal to S = 0.85 +/- 0.10 in LH1 and to S = 1.05 +/- 0.10 in LH2. An estimate for self-trapped excitons is a few times larger. Excitonic polarons are thus proper excitations in LH1 and LH2 complexes, as the electron-phonon coupling cannot be ignored.