Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)₂ClO₄

Abstract

We report a detailed structural and spectroscopic study of the one-dimensional 2:1 cation radical salt (tTTF)2ClO4, where tTTF = trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T = 137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers. The reorganization of tTTF stacks is accompanied with an orderdisorder transition in anion sublattice. Polarized infrared and Raman spectra of (tTTF)2ClO4 are measured in the broad frequency range as a function of the temperature (10-293 K). The structural and vibrational features are investigated to elucidate the origin of the semiconductorsemiconductor phase transition. We discuss the electron-intramolecular vibration coupling effects in the vibrational spectra of (tTTF)2ClO4 and identify signatures of high-and lowtemperature states of charge localization in the tetramerized system. Both the C=C and C-S stretching modes of tTTF give evidence of strong charge distribution fluctuations in conducting stacks for T >137 K, which are responsible for appearance of molecules with charge +1e , and charge localization in tTTF tetramers for T <137 K. Uniqueness of the salt (tTTF)2ClO4 in comparison with other tetramerized one-dimensional systems is discussed.

High pressure behaviour of the organic semiconductor salt (TTF-BTD)2I3

“…Thereby, it has emerged that the vast architectural flexibility of molecules possessing a highly delocalized p-electron system, and the enormous diversity of their crystal structures, allow tuning the electrical conductivity of these materials from semiconducting to metallic or even to a superconducting regime. [1][2][3][4][5][6][7][8][9] Very early on in the development of molecular organic conductors, tetrathiafulvalene (TTF) assumed a key role as an electron donor (D) building block in crystal engineering, as illustrated by the chargetransfer salt TTF-TCNQ (TCNQ = tetracyano-p-quinodimethane) which forms a highly conducting complex, 10 or by the conductive salt a-(BEDT-TTF) 2 I 3 (BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene). 11 Generally, (BEDT-TTF) 2 I 3 is known to have the most diverse phases of crystal structures among BEDT-TTF salts, whereby the crystals comprise alternating cationic (BEDT-TTF) + and anionic I 3 À layers.…”