Influence of high pressures and low temperatures on the absorption spectra of α,α-diphenyl-β-picrylhydrazyl

Abstract

The absorption spectrum of α,α-diphenyl-β-picrylhydrazyl (DPPH) has been studied in the 0–40 kbar range at 295°K, 193°K and 77°K. The influence of these environmental perturbations was investigated for DPPH crystals dispersed in a sodium chloride matrix and for DPPH monomolecularly dispersed in cellulose acetate films. Near 30 kbar the broad absorption in the visible spectrum of isolated DPPH, attributed to orbital promotion of the odd electron, becomes broader and is displaced 300–600 cm −1 (in the 295-77°K temperature range) toward longer wavelengths. For the crystalline material a small pressure dependence of the center of gravity of the visible band is observed. At the higher pressures two band peaks, separated by 9̃00 cm −1 emerge from the nearly symmetric band. The second component manifests itself on the short wavelength side of the original, red-shifted peak. The ultraviolet absorption showed a comparable shift for DPPH and a larger shift for the corresponding parent hydrazine in plastics at relatively low pressures. The DPPH system exhibits some unusual environmental responses compared to other π-systems. The crystal spectra suggest phase changes at higher pressures.