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SummaryIn order to understand the diversity in electronic transitionenergy of the chlorophyll molecules in biological lightharvesting complexes, we studied model systems of fourporphyrin derivatives, porphin, chlorin, Mg-chlorin, and Chla interacting with an external positive point charge locatedabove the tetrapyrollic ring plane. We calculated molecularorbitals and electronic transition energies to S1 and S2 statesfor the four molecules interacting with the point charge byusing TD-DFT method. The transition energies calculatedby the same computational method in the absence of thecharge are in reasonably good correlation to experimentaldata and then we examined the quantum chemical charac-teristics of electronic transitions of the porphyrin derivativesinfluenced by the external point charge. The transition energies of non-metallic derivatives arefound to be generally blue-shifted by the interaction with anexternal positive charge due to stabilization of occupiedmolecular orbitals. In metal-containing derivatives, Mg-chlorin and Chl a, the atomic orbital of the central Mg atomhas been found to play an important role in electronictransition in the presence of an external charge. Due to thestrong interaction of the atomic orbital with the charge, themolecular orbital in mixing with the atomic orbital is muchstabilized by the presence of nearby positive charges andthen the transition energy to S1 state can be red-shifted orblue-shifted from that of isolated molecules depending onthe detailed location of the external charge. Furthermore, afew new transition bands have been shown to emerge as aresult of the interaction with the charge. This may lead to abroadening mechanism of absorption line shape of chloro-phyll molecules in biological environments where variousdistributions of external charges are often found. Acknowledgments. This work was supported by a researchgrant from Chungbuk National University in 2011.References

Computational Study on the Dependence of Electronic Transition Energies of Porphin, Chlorin, Mg-Chlorin and Chlorophyll a on an External Charge