Measurement of triplet extinction coefficients of organic compounds by McClure's method

Abstract

We tested M cC lure's method for measuring the triplet extinction coefficients ε T of organic compounds on anthracene and pyrene, using the u.v. lines from a krypton ion cw laser as a steady-state excitation source. Our values for anthracene ε T = 82 × 10 31/mole cm at 426 nm and pyrene ε T = 37 × 10 3l/mole cm at 413 nm agree well with values obtained by other methods. M cC lure derived his linear relationship from kinetic considerations. One simply measures triplet optical densities (OD T ) at a fixed wavelength (e.g. at a triplet absorption maximum) as functions of different cw laser excitation intensities (powers) I ex . A plot of 1/OD T against 1/ I ex yields a straight line. Extrapolating to the intersection of the ordinate (1/ I ex = 0, or I ex = ∞) yields 1/OD ∞ T . Since, at infinite excitation intensity I ex , the concentration of the triplet state molecules N T is equal to the original concentration N S of the ground state concentration, ε T can be easily calculated from ε T = OD ∞ T / N Sd where OD ∞ T is the triplet optical density at infinite excitation intensity, N S is the original concentration of molecules (at ground state), and d is the thickness of the sample. The success of this method requires the production of high concentrations of triplet state molecules N T , 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, laser excitation spot sizes, reduction of spherical aberrations, etc., 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/OD T as a function of 1/ I ex . 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/OD ∞ T . Measurements were also performed at different concentrations of the solutes.