Measurement Of Triplet Extinction Coefficients Of Organic Compounds

Abstract

Employing the ultraviolet lines from a krypton ion cw laser as steady-state excitation source, we tested McClure's method for measur-ing the triplet extinction coefficients eT of organic compounds on anthracene and pyrene. Our values for anthracene eT = 82 X 103 e/mole cm at 426 nm and pyrene eT = 37 X 103 e/mole cm at 413 nm agree well with values obtained by other methods. McClure derived his linear relationship from kinetic considerations. One simply measures triplet optical densities (ODT) at a fixed wavelength (e.g., at a triplet absorption maximum) as function(s) of different cw laser excitation intensities (powers) Iex. A plot of 1/ODT against 1/Iex yields a straight line. Extrapolating to the intersection of the ordinate (1/Iex = 0, or Iex = ∞) yields 1/ODT∞. ODT∞ = NseT d. At infinite excitation intensity Iex, the concentration of the triplet state molecules NT is equal to the original concentration Ns of the ground state concentration. d is the thickness of the sample. This allows one to obtain eT. The success of this method requires the production of high concentrations of triplet state molecules, NT, as well as steady-state excitation. cw lasers fulfill all these requirements. We discuss the spectroscopic equipment employed for measuring triplet optical densities in some detail. Methods for reducing heat gradients (noise) in liquid nitrogen and laser excitation spot sizes are reviewed. We varied the cw laser illumination density (laser power/area) by setting the focusing lens at different distances from the sample, and measured 1/ODT as a function of 1/Iex. As long as the size of the excitation area was not below a critical size, all the straight lines obtained at different lens settings converged well into only one value of 1/ODT∞. Measurements were also performed at different concentrations of the solutes.