Photostability of pyrromethene 567 laser dye solutions via photoluminescence measurements

The effects of antioxidant addition and concentration on the photostability of methanol solutions of the pyrromethene laser dye 1,3,5,7,8-pentamethyl-2,6diethylpyrromethene-difluoroborate complex (PM-567) were investigated. Specifically, methanol solutions of PM-567 were irradiated with 532 nm Nd:YAG laser pulses (2nd harmonic), and the photoluminescence measured as a function of accumulated laser pulses. The decrease in the photoluminescence-to-initial photoluminescence ratio with accumulated laser pulses was measured as a function of antioxidant addition and dye concentration. The addition of antioxidants, 1,4-diazabicyclo[2.2.2] octane (DABCO) and sodium azide (NaN3), resulted in a dramatic enhancement of photostability, whereas only nominal enhancement was observed upon n-propyl gallate addition. Over the concentration range 6×105 to 5×104 M, dye-photostability was virtually concentration independent, with only a small enhancement at lower concentrations. The normalized photostability, the half-life of accumulated energy per mole of dye, could not explain the concentration dependence of photostability. Reasons why the normalized photostability fails to explain the concentration trends are discussed.