Electric field effect on charge and lattice dynamics in TTF-QCl4 and TTF-QBrCl3

Electroreflectance (ER) spectra are measured on the mixed-stack charge transfer (CT) complexes, tetrathiafulvalane (TTF)-2-bromo-3,5,6-trichloro-p-benzoquinone (QBrCl3) and TTF-p-chloranil (QCl4), showing the neutral–ionic (N–I) phase transition. In the N phase of TTF-QBrCl3, we observe a large ER signal, which originates from the electric field-induced I domains in the N stacks. In contrast, no ER signal is obtained in the N phase of TTF-QCl4. The difference of the ER signal intensity between the two compounds is discussed by taking account of the difference in the degree of the valence instability.