Spectroscopy of deprotonated anions of cold chlorophyll pigments, an approach to their photosynthetic properties

Abstract: We introduce here a new approach to the study of isolated chlorophyll pigments with the electronic spectroscopy of their deprotonated anions in a cold environment at 9K. These anions are isoelectronic to the corresponding neutral molecules, therefore their electronic properties are comparable. This comparison is pursued through this article. The electronic spectrum of the Q bands of deprotonated pheophorbide and methyl pheophorbide are recorded by resonant two photon electron detachment with vibrational resolution and show well-resolved bands extending through the visible spectrum in a very similar fashion to their neutral counterparts in solution. The results are compared with data of neutral pheophytin obtained by fluorescence line narrowing spectroscopy at 4K and resonance Raman spectroscopy and found very similar. The theoretical analysis is achieved by density functional (ground state DFT WB97XD, hybrid exchange-correlation functionals and 6-311++G(d,p) basis sets) and time dependent density functional (TDDFT) calculations. The comparison of the active orbitals in the Q transitions shows a great similarity between neutral and anionic forms together with some orbital and related level inversions. The vibronic spectra are simulated by Franck-Condon calculations to assign the dominant transitions of the electronic spectrum. Globally, this is a new perspective in the electronic spectroscopy of chlorophyll pigments that has been opened.