The thermochromic behavior of aromatic amine-SO₂ charge transfer complexes

The distinct thermochromism observed in solutions containing N,N-dimethylaniline (DMA) and N,N-diethylaniline (DEA) and SO₂ was investigated by resonance Raman spectroscopy in a wide range of temperatures. The results indicate in addition to the charge transfer (CT) complexes DMA-SO₂ and DEA-SO₂, the presence of collision complexes involving the CT complexes and excess DMA and DEA molecules. The latter in fact is the chromophore responsible for the long wavelength absorption originating the color. The Raman signature of the collision complex was attributed to the distinct enhancement of a band at 1140cm^-1 assigned to ν_s(SO₂), in contrast to the same mode in the 1:1 complex at 1115cm^-1. The intensity of such band, assigned to the collision complex is favored at high temperatures and depends on the steric hindrance associated to amines, as well as the SO₂ molar fraction. Quantum chemical calculations based on time-dependent density functional theory (TDDFT) support the proposed interpretation.