Spectral characterization of the main pigments in the plant photosynthetic apparatus by theory and experiment

Comprehensive experimental spectroscopic and TD-CAM-B3LYP/6-31G*, DFT/MRCI libraries of the most rele-vant plant light harnessing pigments are presented. Included are beta-carotene, zeaxanthin, violaxanthin, lutein, neoxanthin, chlorophyll a and chlorophyll b. We employ vibrational broadening with various approximations to enable direct comparison of computational to experimental data. Spectroscopic shifts based on continuum sol-vents and protein matrices were computed as well, employing CPCM or (for chlorophylls) QM/MM models. We also provide insight into the transient behavior after photoexcitation through optimization of the corresponding excited states. It is found, that the investigated carotenoids only differ in their excitation energies but display analogous subsequent photophysics. Vibrational broadening of the carotenoid spectra compares well to experimental data regardless of the actual approach chosen. This appears to be a result of a strong gradient at the Franck-Condon point. Thus, it is not trivial to assess which computational method is actually suited for these purposes. Further, Q and Soret band reorganization energies are very similar in both, chlorophylls a and b. Both, TD-DFT, as well as DFT/MRCI predict the existence of an intramolecular charge transfer state in the Soret region of chlorophylls. The existence/absence of this state has previously not been experimentally corroborated. However, measure-ments of chlorophyll a absorption spectra in different solvents support this assumption. The results suggest, that the relaxation of the chlorophyll Soret bands might be affected by the presence of such an intramolecular charge transfer state.

Automated summary

This study presents comprehensive experimental spectroscopic and computational data for relevant plant light harnessing pigments. The investigated pigments show similar photophysics despite differences in excitation energies. The vibrational broadening of carotenoid spectra matches well with experimental data, while Q and Soret band reorganization energies are similar in chlorophylls a and b. Furthermore, this study suggests the existence of an intramolecular charge transfer state in the Soret region of chlorophylls, which has not been experimentally confirmed before.