Investigation of the nature of the unpaired electron states in the organic semiconductor N-methyl-N-ethylmorpholinium-tetracyanoquinodimethane

The nature of the unpaired electron states in the dimerized phase of the crystalline organic semiconductor N-methyl-N-ethylmorpholinium-tetracyanoquinodimethane [MEM${\mathrm{}\left(\mathrm{TCNQ}\right)\mathrm{}}_2$] is investigated by the combined means of polarized-optical-reflectance measurements and microscopic theoretical analysis. It is found that each unpaired electron is localized on a dimeric TCNQ unit, and it is demonstrated that the two-site molecular orbital (MO) which accommodates the unpaired electron involves internal molecular distortion of the dimeric unit. Experimental values are deduced for the intradimer $\pi$ MO hopping integral, the TCNQ monomer $a_g$ molecular-vibration frequencies and linear-electron-molecular-vibration coupling constants, and the difference in energy of the slightly nonequivalent TCNQ monomer $\pi$ MO's. The dimer charge oscillation associated with the extremely weak coupling of the unpaired electron to the high-frequency $a_g$ C-H stretch mode of the TCNQ molecule is observed for the first time.