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

Abstract

The nature of the unpaired electron states in the dimerized phase of the crystalline organic semiconductor N-methyl-N-ethylmorpholinium-tetracyanoquinodimethane [MEM${(\mathrm{TCNQ})}_{2}$] 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 $\ensuremath{\pi}$ 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 $\ensuremath{\pi}$ 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.