Through a combined experimental and theoretical investigation we determine that the fundamental modes of three quinacridones fall in the terahertz spectral range (1-10 THz, ∼30-300 cm-1). In each spectrum the terahertz resonances correspond to wagging, rocking, or twisting of the quinacridone rings, with the most intense absorption being an in-plane rocking vibration of the carbonyl oxygens. In spite of these spectral similarities, we demonstrate that terahertz measurements readily differentiate β-quinacridone, γ-quinacridone, and 2,9-dimethylquinacridone. The spectrum of β-quinacridone has a group of closely spaced modes at ∼4 THz, whereas in contrast the spectrum of γ-quinacridone displays a widely spaced series of modes spread over the range ∼1-5 THz. Both of these have the strongest mode at ∼9 THz, whereas in contrast 2,9-dimethylquinacridone exhibits the strongest mode at ∼7 THz. Because quinacridones are the basis of widely used synthetic pigments of relatively recent origin, our findings offer promising applications in the identification and dating of modern art.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry