Photodetachment Thresholds of Deprotonated Chlorophyll Pigments and Structural Characterization of Their Deprotomers

Chlorophyll pigments and derivatives were studied under isolated conditions and at low temperatures in their anionic deprotonated form. Neutral and deprotonated chlorophyll pigments are isoelectronic, which likens their electronic properties, as is confirmed by electronic structure calculations. The study of these anionic pigments is directed toward understanding the electronic structure of their neutral counterparts involved in photosynthetic processes. In this goal, we measured the photodetachment thresholds that characterize the deprotonated anions unequivocally. Four systems have been studied: pheophytin a, its simpler ester equivalents methyl pheophorbide and metalated zinc methyl pheophorbide, and pheophorbide. By a combination of direct laser one photon detachment measurements and quantum chemical calculations, we could characterize the localization of the charge on these deprotonated anions. Two major sites have been found, one on the carboxylate group of pheophorbide and the other on the carbon atom in the α position of the peripheral methyl ester of the chlorophyll cycle. The experimental electron binding energies to these sites ∼2.7 eV and ∼3.1 eV agree with quantum chemistry calculations and correspond to macrocycle and carboxylate attachment, respectively. In addition, the HOMOs are similar for neutral and both anionic deprotonated pigments, allowing for the comparison of the spectroscopic properties of both species and using anionic deprotonated pigments as models for their neutral counterparts.