We report the infrared and optical reflectance spectrum of semiconducting N-dimethyl thiomorpholinium (tetracyanoquinodimethane${)}_{2}$ at temperatures above and below the structural phase transition. Polarized measurements were made on several faces of large single crystal samples. For polarization close to the tetracyanoquinodimethane (TCNQ) stacking axis, the spectrum resembles other many weakly conducting TCNQ salts, with strong vibrational features and two charge-transfer bands. The infrared data in this polarization were analyzed within the framework of both an isolated dimer model and a twofold-commensurate charge-density-wave model. We have obtained experimental values of the unperturbed phonon frequencies and linear electron-molecular-vibration coupling constants, both of which are nearly independent of temperature. The dimer model fails to reproduce the phonon intensities and line shapes and underestimates the coupling constants, whereas the charge-density-wave model produces better results in both cases. We have also investigated whether the structural phase transition leads to a change of electronic structure (because of crystal-field distortion) that can account for the observed dc conductivity. For light polarized close to the b crystallographic axis, the spectral data are, overall, not in good agreement with the proposed band structure. In the low-temperature phase, several expected low-energy excitations are absent, most notably the low-energy interband expected near 430 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$. In the absence of spectral evidence for this low-energy transition, the activation energy for interchain transport is \ensuremath{\approxeq}0.25 eV both above and below ${\mathit{T}}_{\mathit{c}}$. Consequently, our data do not support the idea that the unusual transport properties of semiconducting DMTM(TCNQ${)}_{2}$ are caused by low-energy interchain charge transfer.