Comparison of pulsed field gradient NMR and holographic relaxation spectroscopy in the study of diffusion of photochromic molecules

Abstract

Pulsed field gradient NMR (PFG-NMR) and holographic relaxation spectroscopy (HRS) have been compared in a study of tracer diffusion rates for the photochromic molecules azobenzene and aminoazobenzene in acetone and 1,4-dioxane. These techniques are applicable to exactly the same range of diffusion rates but can provide complementary information. The cis and trans forms of azobenzene can be distinguished in the NMR experiment, and the tracer diffusion coefficient for the cis form is found to be 5% lower than for the trans form. In the HRS experiment a single diffusion coefficient is determined from the rate of decay of the laser-induced concentration pattern of cis azobenzene. Results obtained by the two methods agree, and the magnitudes of the errors are similar. In general HRS is a much more sensitive method, but only NMR permits the diffusing species to be identified.