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

We tested McClure'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 × 10³1/mole cm at 426 nm and pyrene ε_T = 37 × 10³l/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 (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.