Laser-induced fluorescence quenching of chlorophyll

He-Ne laser-induced fluorescence quenching of chlorophyll extracted from spinach leaves was studied. Chlorophyll solutions in mixed acetone/ethylene glycol solvents, containing differing amounts of p-benzoquinone added as a fluorescence quencher were irradiated with the laser. The fluorescence radiation was passed through a monochromator set at a suitable wavelength, detected with a photomultiplier, and the voltage was measured by a chart recorder. The decrease in fluorescence intensity with increasing amount of quencher is believed to be due to collisional energy-transfer from the chlorophyll molecule to the quencher molecule. The increase in fluorescence intensity with increasing solvent viscosity was explained as being due to the reduction in the number of collisions between solute molecules. The values for the rate constants were found to be 48(18) x 109, 20(4) x 109, 5.8(1) x 109 and 1.9(4) x 109 M-1s-1, respectively, in acetone, 80/20, 50/50 and 20/80 (v/v) mixtures of acetone/ethylene glycol. The radius ratio of chlorophyll to p-benzoquinone was found to be 8.9(8).