Temperature-Induced Neutral-Ionic Phase Transition in the (EDT-TTF-I₂)₂TCNQF Mixed-Stack Charge-Transfer Salt

Abstract

International audienceWe report the infrared (IR) and Raman study of the 2:1 mixed-stack charge transfer salt (EDT-TTF-I2)2TCNQF, where EDT-TTF = ethylenedithiotetrathiafulvalene and TCNQ = tetracyanoquinodimethane, which undergoes a temperature-induced neutral-ionic phase transition. Polarized infrared (IR) and Raman spectra of single crystals were measured in the 8-293 K temperature range. Temperature variations of vibrational modes that are assigned to both donors and acceptors show that the average degree of charge-transfer is growing continuously from about zero at room-temperature to about 1e at T = 8 K; nevertheless, at about 100 K a regime change is observed. The coexistence of molecular species of different ionicity is detected in the whole temperature range. The IR vibrational features due to electron-molecular vibrational coupling increase their intensities considerably upon cooling giving an evidence of relatively small gradual distortions of crystal structure. Existence of ferroelectric domains is suggested. A scheme of the neutral-ionic phase transition in a 2:1 complex is proposed and discussed. © 2016 American Chemical Society

(Perylene)3-(TCNQF1)2: Yet Another Member in the Series of Perylene–TCNQFx Polymorphic Charge Transfer Crystals

“…These have been already identified for TCNQ, TCNQF 2 and TCNQF 4 [[13] and references therein]. Instead, to the best of our knowledge, a detailed study of TCNQF 1 vibrations is still missing, even though this molecule is increasingly used in the formation of CT crystals [21]. For this reason, the details of the analysis are given here.…”

Pressure-Induced Neutral–Ionic Phase Transition in the Mixed-Stack 2:1 Charge-Transfer Complex (EDT-TTF-I2)2TCNQF

“…Raman data 24 . Evidently, at 100 K and 90 K the crystal undergoes an abrupt but small modification which can be related to a weak change of ionicity and/or deformation of the crystal lattice.…”

“…Recently, a temperature-induced N-I conversion was discovered in the mixed-stack complex (EDT-TTF-I 2 ) 2 TCNQF (where EDT-TTF-I 2 is diiodoethylenedithio-tetrathiafulvalene and TCNQF is monofluoro-tetracyano-p-quinodimethane) 23,24 . To the best of our knowledge, this is the first observation of such effect in a 2:1 complex under ambient pressure.…”

“…To the best of our knowledge, this is the first observation of such effect in a 2:1 complex under ambient pressure. The compound crystallizes in the triclinic system (space group ) with one TCNQF acceptor on inversion 24 . It was suggested that such effect should be common for the 2:1 mixed-stack complexes.…”

Activating both Halogen and Chalcogen Bonding Interactions in Cation Radical Salts of Iodinated Tetrathiafulavalene Derivatives

“…This concept of charge activation upon electron transfer was later on convincingly investigated in solution [5] and illustrated in a series of 2 : 1 charge transfer salts involving EDTÀ TTFÀ I 2 (in Scheme 1) and fluorinated tetracyanoquinodimethanes (TCNQF n , n = 0, 1, 2), [6] where stronger I•••NC XB were found in the charge transfer salt with TCNQF 2 (1 TCNQF2 = À 1) than in the neutral charge-transfer complex with TCNQ (1 TCNQ = 0). The intermediate TCNQF salt exhibits a neutral-ionic transition under temperature [7] and pressure. [8] Most reported examples of these iodinated TTFs involve derivatives bearing either one iodine atom (as in EDTÀ TTFÀ I and EDOÀ TTFÀ I in Scheme 1), or two iodine atoms ortho to each other, on the same dithiole moiety (as in EDOÀ TTFÀ I 2 and EDTÀ TTFÀ I 2 in Scheme 1), together with one single example of salt derived from TTFÀ I 4 .…”