Low-frequency spectra of the hexamethylbenzene/tetracyanoethylene electron donor–acceptor complexes in solution studied by terahertz time-domain spectroscopy

Abstract

We have measured the frequency dependent extinction coefficients and refractive indices of electron donor–acceptor (EDA) complexes consisting of hexamethylbenzene (HMB; electron donor) and tetracyanoethylene (TCNE; electron acceptor) in the low-frequency region by terahertz time-domain spectroscopy (THz-TDS). A mixture of the 1∶1 (DA) and 2∶1 (D₂A) EDA complexes exist in carbon tetrachloride solution, and we successfully obtained the spectral components of the 1∶1 and 2∶1 EDA complexes separately by analyzing the concentration dependence of the THz spectra. The 1∶1 and 2∶1 complexes show quite different THz spectra of the extinction coefficient, reflecting unique features of dynamics, fluctuations and intermolecular interactions of these complexes. Polarization-selective THz-TDS on the crystalline DA complex shows two peaks at 53 and 70 cm⁻¹ in the spectral component perpendicular to the crystal axis. On the other hand, the crystalline D₂A complex exhibits peaks at 42 and 50 cm⁻¹ in the perpendicular spectral component. We compare the obtained spectra of the crystalline complex and the results of molecular orbital calculations at the HF/6-31G(d) level of theory to discuss the intermolecular vibrational modes of the complexes.