Infrared optical spectra have been investigated for a charge-transfer complex of tetrathiafulvalene and 2-bromo-3,5,6-trichloro-p-benzoquinone, which undergoes a neutral-ionic (N-I) transition at T c 68 K. With the decrease of temperature toward T c , the far-infrared reflectivity spectrum is gradually transformed into a Drude-like high reflectance band edging around 100 cm 21 , and the optical conductivity shows an anomalous peak around 10 cm 21 . The far-infrared anomaly is related to the gigantic dielectric response just above T c , which are both discussed in terms of dynamics of N-I domain walls with fractional charge.Charge-transfer (CT) complexes composed of alternative stacks of donor ͑D͒ and acceptor ͑A͒ molecules have drawn considerable attention since the discovery of the neutral-ionic (N-I) transition [1]. The N-I transition is characterized by an abrupt change of degree of CT (r in . . . D 1r A 2r D 1r A 2r D 1r A 2r . . .), as well as dimerization of the DA stack. Among a number of the CT complexes, tetrathiafulvalene (TTF)-p-chloranil (QCl 4 ) is the prototypical system that shows the N-I transition from the N phase ͑r N ഠ 0.3͒ to the I phase ͑r I ഠ 0.6͒ with lowering temperature (T c ഠ 81 K) [2,3]. Such a temperature-induced N-I transition is realized due to the subtle balance in free energy between the two phases. Under these circumstances, the N-I transition can be caused by other external stimuli than the decrease of temperature in the TTF-QCl 4 crystal, e.g., application of external pressure [4-6], current injection [7], and photoirradiation [8]. All the unconventional phase controls are considered as relevant to the unique electronic structure of TTF-QCl 4 .Another important aspect in the N-I transition is ferroelectricity driven by the dimerization of the DA stack [9]. In TTF-QCl 4 , the dimerization gives birth to an electric dipole moment along the stack axis, so that dielectric constant shows a huge increase toward T c [10]. Besides, the dimerized DA stack chain possesses two degenerate configurations with the opposite electric dipole moment [11] and hence bears unconventional excited states such as charge and spin solitons [12]. For the DA stack with nearly degenerate N and I states, Nagaosa et al. proposed that the lowest-lying excitation is the domain wall with fractional charge as the boundary between the N and I domain (N-I domain wall, NIDW).One of the isostructural series of TTF-QCl 4 , a Brsubstituted analog, QBrCl 3 [2-bromo-3,5,6-trichloro-pbenzoquinone; for its molecular structure, see the inset to Fig. 1(a)] provides a 1:1 TTF complex, which displays the significant N-I valence fluctuation and the gigantic dielectric response near the lowered N-I transition temperature[13], as shown below. We show in Figs. 1(a) and 1(b) the temperature ͑T͒ dependence of the dielectric constant (´, measured at 30 KHz) along the a axis (stack axis) and molecular ionicity ͑r͒ in TTF-QBrCl 3 , respectively. The´-T curve shows a sharp peak and the r-T curve an abrupt jump around 68 K ͑ T c ͒, indic...