<title>Measurement of triplet optical densities of organic compounds by means of CW laser excitation</title>

Abstract

The application of cw lasers as powerful excitation sources for transient (triplet-) state spectroscopy is demonstrated. By focusing a cw laser beam on a sample with the aid of a lens, high triplet optical densities ODT((lambda) ) can be recorded. Using McClure's method, triplet extinction coefficients (epsilon) T((lambda) ) of many organic compounds are conveniently measured. To illustrate the method, we recorded (epsilon) T values on syn- (dimethoxyphosphinylmethyl,methyl) bimane. Exciting organic compounds with light causes various degrees of photodecomposition. We have developed a method to measure triplet optical densities ODT((lambda) ) in the presence of photodecomposition. The method is demonstrated on Coumarin 120. One records ODT((lambda) ) values as well as the accumulation of absorbing photoproducts as a function of time. Turning off the cw laser excitation, one records the accumulation of photoproducts only. Separating the two processes, one can determine how the triplet optical density declines as a function of time t. The decline curve can be expressed with an equation that is of second order in time. This equation allows one to extrapolate back to t equals 0 and recover ODT when there was no photodecomposition. Because the cw laser light is highly polarized, triplet-triplet (T-T) photoselection spectroscopy can be employed. The degree of polarization P of several syn- binanes was recorded, providing important insight on the polarization of T-T transitions. These data may be used to improve laser action properties of syn-bimanes by substitutions.