Flash photolysis in the adsorbed state: pyrene on porous glass

The kinetics of the decay of triplet pyrene adsorbed on porous glass have been studied. The samples were photolyzed at ambient temperatures using krypton lamps and a conventional flash photolysis system. A transient absorption, which was formed during the flash, was observed in the wavelength range 400 to 700 nm. This signal decayed according to first-order kinetics and the rate constant was independent of wavelength. The intensity of the absorbance, measured at 520 nm, increased linearly with flash intensity, while the rate of decay remained unchanged. The transient absorption was assigned to that of triplet pyrene formed during the excitation flash. At ambient temperature, the first-order rate constant for triplet decay was 7.95 +- 0.75 s-1, corresponding to an average lifetime of 126 +- ms. Comparable lifetimes observed for pyrene in other systems are: 1.8 ms in n-hexane at 300 K, 333 ms in viscous paraffin at 300 K, 500 to 700 ms in EPA glass at 77 K, and 400 ms in plastic at 300 K. Preliminary studies have shown that the triplet lifetime was affected by temperature as well as by the surface environment, which varied according to the outgassing history of the particular sorbent sample. 2 figures, 1 table