Temperature dependence of electronic optical absorption in (NMe4++ · TCNQ−·)2TCNQ

The temperature dependence of the linearly polarized optical absorption spectrum has been measured in single crystals of (NMe4+·TCNQ−·)2TCNQ, where NMe4 and TCNQ are tetramethylammonium and tetracyanoqui nodi methane, respectively, in the temperature range between −100 and +150 °C. In addition to intense TCNQ−· dimer bands, strongly temperature-dependent satellite bands appear at around 1.3 eV and 1.9 eV. The singular behavior of the intensity and peak position of the 1.3 eV band shows that a second order phase transition takes place at 126 °C. The origin of this band is discussed in relation to the mechanism of the phase transition.