Simple molecular model for crystalline tetrathiofulvalene-tetracyanoquinodimethane (TTF-TCNQ)

The energy-level structure of solid tetrathiofulvalene-tetracyanoquinodimethane(TTF-TCNQ) is approximated by superimposing the energy levels of the radical ions TT${\mathrm{F}}^{+}$ and TCN${\mathrm{Q}}^{-}$. The energy levels for these ions were obtained by carrying out self-consistent statistical-exchange multiple-scattering calculations using overlapping-atomic-sphere molecular models. It is possible to account for many of the principal features of the experimental photoemission spectrum of solid TTF-TCNQ by shifting the free cation TT${\mathrm{F}}^{+}$ and the free anion TCN${\mathrm{Q}}^{-}$ energy levels upwards and downwards by 3.8 eV, respectively. These features can also be accounted for by assuming less than unity charge transfer and shifting the constituent-ion energy levels by proportionally smaller amounts. The next to the highest experimental photoemission peak cannot be reproduced by energy level models of the type just described. We propose that this peak is due to electronic transitions from neutral TCNQ or neutral TTF molecules lying on the surface immediately above TT${\mathrm{F}}^{+}$ or TCN${\mathrm{Q}}^{-}$ ions, respectively.