LIGHT QUENCHING OF FLUORESCENCE USING TIME-DELAYED LASER-PULSES AS OBSERVED BY FREQUENCY-DOMAIN FLUOROMETRY

We describe experimental observations of fluorescence quenching by tim e-delayed light pulses whose wavelength overlaps the emission spectrum of 4-(dimethylamino)-4'-cyanostilbene (DCS). The relative cross secti ons for light quenching were proportional to the amplitude of the emis sion spectra at the light quenching wavelength. The frequency-domain i ntensity and anisotropy decay measurements showed oscillations resulti ng from time-delayed light quenching. The amplitude of the oscillation s depends upon the amount of light quenching. The frequency of the osc illations depends upon the delay between the excitation and quenching pulses. To the best of our knowledge, only a stepwise decrease in the intensity or anisotropy could produce such oscillations in the frequen cy-domain data. Light quenching of fluorescence is thus shown to provi de a means to control the number and orientation of the excited fluoro phores. The use of multiple light pulses for excitation and quenching can have far-reaching applications in the use of time-resolved fluores cence in physical chemistry and biophysics.