Infrared study on pressure-induced charge delocalization in Cs₂TCNQ₃

Abstract

Pressure dependence of several molecular vibration modes in Cs 2 TCNQ 3 has been observed through infrared absorption measurements. The behaviour of the C-CN and C-H stretching modes suggests that a phase transition takes place at around 3.6 GPa. The π * -electrons, which are localized on TCNQ − molecules in the low-pressure phase, are delocalized significantly in the high-pressure phase. However, the radical-like and neutral-like molecules still coexist in the highpressure phase indicating that the electrons are not completely delocalized. The electron-molecular-vibration coupled mode disappears in the high-pressure phase in coincidence with the disappearance of the inter-radical charge-transfer band S 1 , proving the strong coupling between them.

High-pressure infrared absorption in Cs2TCNQ3 crystals grown under magnetic field

“…The situation of the dimer-monomer transition in the spin-Pierls system K-TCNQ and Rb-TCNQ [16], as well as the slipped-eclipsed dimer transition in (NMe 4 ) 2 TCNQ 3 [17], for example, can be revealed very well by infrared spectroscopy. Our previous work on the insulator to metal transition in the present substance explained above has also demonstrated that both information on the electronic distribution and crystal structure can be obtained at the same time by this method [12]. Furthermore, as mentioned previously, the effect of magnetic field cannot be detected by X-ray diffraction, but very well observed by infrared spectroscopy [1].…”

“…This metal-like state, however, is considered different from that of ordinary metals, since no Drude tail is observed in the high-pressure phase [9]. High-pressure Raman scattering as well as infrared absorption studies have shown that π* electrons of the TCNQ − are significantly delocalized in the high-pressure phase [10,11]. The delocalization, however, is not complete so that the above-mentioned localization character is maintained, and therefore the substance maintains its semiconducting character.…”

“…In addition, the infrared absorption data have also suggested that this phase transition is accompanied by a symmetry change in the relative position of the TCNQ − radical molecules. In the low-pressure phase they are displaced against each other along the short molecular axis forming slipped dimers, and in the high-pressure phase those dimers turn into eclipsed ones [12].…”

Pressure Dependence of Electron Distribution along the TCNQ Columns in (TEDA)2 TCNQ3

“…Let the actual valence of A and B molecules be -ρ A and -ρ B , respectively. Then, since the frequency difference ∆ν of a given infrared mode between the radical-like and neutral-like molecules is known to vary nearly linearly to ρ A − ρ B [5], the following relationship is expected to hold to a good approximation at a pressure P:…”

“…However, to the best of authors' knowledge, none of these complexes has been reported to exhibit such transition. In Cs 2 TCNQ 3 for example, although excess electrons are significantly delocalized at around 3.6 GPa [3], which corresponds to significantly high electrical conductivity compared to its value it ambient pressure [4], the semiconducting character remains up to 8 GPa [5]. Nevertheless, the change in the distribution of electrons along the TCNQ column that is induced by pressure is interesting.…”