High Pressure Optical Absorption in Organic Semiconductors (NMe4)2TCNQ3 and Cs2TCNQ3

Abstract

The pressure dependence of the optical absorption spectrum in (NMe 4 ) 2 TCNQ 3 and Cs 2 TCNQ 3 has been measured at room temperature in the spectral range from mid-infrared to visible. At the ambient pressure the optical energy gap is observed at around 0.5 and 0.3 eV in (NMe 4 ) 2 TCNQ 3 and Cs 2 TCNQ 3 , respectively. The energy gap in (NMe 4 ) 2 TCNQ 3 decreases by 0.05 eV with increasing pressure up to 2 GPa, followed by an abrupt decrease towards 0.2 eV between 2 and 3 GPa. At higher pressures up to 5.5 GPa the gap remains almost unchanged.The intra-and inter-molecular optical absorption bands in the near-infrared to visible region are intensified by pressure and broadened out at 2.5 GPa. The absorption spectrum in Cs 2 TCNQ 3 exhibits a similar behavior under pressure. These results suggest that an electronic phase transition is induced by pressure at around 2.5 GPa in both substances. This phase transition resembles the charge-order-disorder (Verwey) transition in Fe 3 O 4 in the electronic mechanism.

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

“…A pressure-induced electronic transition which resembles to the insulator-metal transition has been observed from the electrical resistivity measurement in the present substance [8]. 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].…”

Infrared study on pressure-induced charge delocalization in Cs2TCNQ3

“…The delocalization of the π * -electrons in the high-pressure phase arises from the increasing of the transfer integral t as a result of the decreasing of the intermolecular distances in the TCNQ stack. We have previously shown that at high pressure the bandwidth 4t becomes comparable to the Coulomb on-site energy U [2], which in the framework of the Hubbard model implies that the dimers tend to be unstable.…”

“…Cs 2 TCNQ 3 crystallizes into a columnar structure consisting of a periodic stack of TCNQ 0 /TCNQ − /TCNQ − [1], where TCNQ 0 and TCNQ − are neutral and anion radical molecules, respectively, resulting in a semiconductor with the lowest optical energy gap of 0.3 eV [2]. The radicals TCNQ − are dimerized, so the uppermost valence band is formed by the π * -orbit of spin singlet.…”

“…High-pressure electric resistivity measurement has shown that this material undergoes a phase transition to a metallic state at around 3 GPa [6]. However, the optical energy gap remains unclosed throughout the phase transition indicating that the metallic state in this material should be considered different from those of ordinary metals [2]. The results from a highpressure Raman scattering experiment suggest that the electrons are completely delocalized from TCNQ − radical molecules at high pressure resulting in a homogeneous distribution of charge along the TCNQ molecular stack [7].…”

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

“…It is well known that the size of the gap in TCNQ complexes depends very much on this quantity. This explains why the size of the gap in this substance is significantly smaller than that of Cs 2 TCNQ 3 , which excess electrons are more localized [9].…”