Friday, october 3 at 2 pm
ISMO Amphitheater

To improve the comprehension of the mechanism of photosynthesis, particularly the efficiency of charge separation within Photosystem II, it is essential to accurately determine the vibronic structure of various chlorophyll pigments. Here we have studied deprotonated anions of these pigments whose spectroscopic properties are very similar to that of their neutral counterparts. To this end, we performed gas-phase photodetachment spectroscopy experiments on cold, deprotonated anions belonging to the chlorophyll family. The properties of these anions, in the absence of perturbations, were characterized by measuring the kinetic energy and/or intensity of the electrons ejected after photodetachment. All experiments were conducted using a dedicated setup combining a nanoelectrospray source, a cryogenic ion trap, and a photoelectron detection system. The studied compounds include pheophytin — a pigment playing a key role in charge transfer during photosynthesis — as well as two simplified derivative models: pheophorbide and methylpheophorbide. In these two molecules, the phytyl chain (C₂₀H₃₉) of pheophytin has been replaced by an H atom or a methyl group, respectively. The first step of this work involved identifying the most stable deprotonation sites of the studied molecules by correlating them with the photodetachment thresholds of their anions. The results indicate that deprotonation occurs at the periphery  of the macrocycle in all molecules, and additionally at the carboxylic group in the case of pheophorbide. In the second step, we successfully recorded action spectra of the allowed electronic states (Qₓ and Qᵧ transitions) of deprotonated pheophorbide and methylpheophorbide. These spectra were measured by monitoring the intensity of photoelectrons resulting from the absorption of two photons as a function of the photodetachment laser wavelength. The resolution of our measurements reveals  a series of  vibrational modes in the Qᵧ band — and, for the first time, in the Qₓ band. Due to the observed similarity in the vibrational spectra of the deprotonated pigments and the neutral ones in matrices, an assignment of these modes can be made comparing with low-temperature matrix fluorescence excitation spectra. It is also supported by previous calculations on the neutral methylpheophorbide molecule performed by A. Freiberg’s group. The hypothesis that the deprotonated anion serves as a valid model for the neutral molecule is fully confirmed.