Polarons in light harvesting: Lessons from natural nanoworld

The "learning from nature" - principle continues to produce novel results, both fundamental and applied. Here, a few unresolved problems related to elementary processes in light harvesting materials are addressed, an issue of potential interest for future solar cell technologies.

Nanoscopic aggregates of chromophores play key roles in light harvesting processes of photosynthetic bacteria, algae, and higher plants. The collective electronic excitations commonly known as excitons are generally considered as primary optical excitations in closely spaced molecular aggregates. In nanoscale, however, the physical size and shape become important. Therefore, the exciton models developed on the bases of the bulk solid studies require modification. As an example, the antenna exciton spectra are discreet rather than continuous as in the bulk solids. Also, due to natural variation of the chromophore binding sites, the so-called static disorder, the spectra of small antenna aggregates are always significantly inhomogeneously broadened. The exciton level structure is important, because together with the exciton-phonon coupling (the coupling of excitons with lattice vibrations) it controls the functionally crucial antenna energy transfer pathways and rates.

The ring-shaped antenna aggregates from the purple photosynthetic bacteria of 6-11 nm diameter comprise from 8 to 32 bacteriochlorophyll a molecules. From the structural point of view, the closely spaced chromophores appear as one-dimensional nanocrystals of unique (genetically predetermined) size. The ordered nanostructures are open for straightforward physical modeling, while their quasi-linear spectra allow detailed spectroscopic studies. As such, they constitute great model systems for the photophysical research. Our studies have been concentrated on the very interesting interplay between the exciton interaction, exciton-phonon coupling, and static disorder effects. It was first found that the static disorder results in considerable reduction of the exciton-lattice coupling energy required for initiation of smooth self-trapping transition characteristic for one-dimensional systems. Therefore, excitonic polarons rather than Frenkel excitons are the proper bacterial antenna excitations. Above certain critical exciton-lattice coupling energy these aggregates even support multiple self-trapped excitons with different size, reorganization energy, and spatial location. Such states are viable in regular lattices only in higher dimensions. Self-trapping of the antenna excitons is followed by significant broadening of their fluorescence emission spectra and increase of the emission rate. Both these effects, which promote antenna efficiency, should be considered in realistic description of light harvesting processes. In future, they should also be explored in relation with artificial light harvesting systems. ( Photosynthesis in Silico )