The thermochromic behavior of aromatic amine-SO₂ charge transfer complexes

Area of Science:

• Physical Chemistry
• Spectroscopy
• Chemical Physics

Background:

• Thermochromism is a color change in response to temperature variations.
• Charge transfer (CT) complexes are known to form between electron-donating amines and electron-accepting molecules like sulfur dioxide (SO2).

Purpose of the Study:

• To investigate the molecular mechanisms behind the distinct thermochromism in N,N-dimethylaniline (DMA) and N,N-diethylaniline (DEA) solutions with SO2.
• To identify the specific species responsible for the color change and their temperature-dependent behavior.

Main Methods:

• Resonance Raman spectroscopy was employed across a wide temperature range.
• Quantum chemical calculations using time-dependent density functional theory (TD-DFT) were performed to support experimental findings.

Main Results:

• The study identified both 1:1 charge transfer (CT) complexes (DMA-SO2, DEA-SO2) and collision complexes.
• Collision complexes, formed by CT complexes and excess amine molecules, were found to be the primary chromophores responsible for the observed color.
• A distinct Raman band at 1140 cm⁻¹ (νs(SO2)) was attributed to the collision complex, differing from the 1:1 complex band at 1115 cm⁻¹.
• The intensity of the collision complex band is favored at higher temperatures and influenced by amine steric hindrance and SO2 concentration.

Conclusions:

• The thermochromism in DMA/DEA-SO2 systems originates from collision complexes, not solely from 1:1 CT complexes.
• These collision complexes act as the key chromophores, with their formation and spectral properties being temperature-dependent and influenced by molecular structure and composition.