Triplet Excited State Energies and Phosphorescence Spectra of (Bacterio)Chlorophylls

Abstract

(Bacterio)­Chlorophyll ((B)­Chl) molecules play a major role in photosynthetic light-harvesting proteins, and the knowledge of their triplet state energies is essential to understand the mechanisms of photodamage and photoprotection, as the triplet excitation energy of (B)­Chl molecules can readily generate highly reactive singlet oxygen. The triplet state energies of 10 natural chlorophyll (Chl <i>a</i>, <i>b</i>, <i>c</i>₂, <i>d</i>) and bacteriochlorophyll (BChl <i>a</i>, <i>b</i>, <i>c</i>, <i>d</i>, <i>e</i>, <i>g</i>) molecules and one bacteriopheophytin (BPheo <i>g</i>) have been directly determined via their phosphorescence spectra. Phosphorescence of four molecules (Chl <i>c</i>₂, BChl <i>e</i> and <i>g</i>, BPheo <i>g</i>) was characterized for the first time. Additionally, the relative phosphorescence to fluorescence quantum yield for each molecule was determined. The measurements were performed at 77K using solvents providing a six-coordinate environment of the Mg²⁺ ion, which allows direct comparison of these (B)­Chls. Density functional calculations of the triplet state energies show good correlation with the experimentally determined energies. The correlation determined computationally was used to predict the triplet energies of three additional (B)­Chl molecules: Chl <i>c</i>₁, Chl <i>f</i>, and BChl <i>f</i>