Picosecond kinetics of chlorophyll and chlorophyll/quinone solutions in ethanol

The mechanism of quenching by quinones of the lowest excited singlet state of chlorophyll has been investigated using picosecond laser spectroscopy. With chlorophyll alone, laser excitation resulted in immediate (< 10 ps) bleaching of the 665 nm band and production of new absorption bands in the regions 460-550 and 800-830 nm. The lifetimes of these changes were greater than 500 ps. Addition of 2,6-dimethyl-benzoquinone caused quenching of these absorbance changes. No indication of chlorophyll cation radical formation was obtained. Thus, the interaction between quinone and the chlorophyll excited singlet state results in energy dissipation without measurable formation of radical species having lifetimes longer than 10 ps. This is in marked contrast to the quenching of the chlorophyll lowest triplet state by quinones, during which easily detectable stable radical formation has been observed.