SCIENTIFIC HIGHLIGHTS

Anions are not naïve players in molecular conductors

Anions are usually considered to act simply as electron donors or acceptors in molecular conductors but they play an essential role in directing their structural and electronic properties.

Anions have often been considered to act essentially as a source or a sink of electrons in molecular conductors. However there is now growing evidence that they play an essential role in directing the structural and hence electronic properties of many of these systems. In this review the basic interactions and different ground states occurring in molecular conductors are considered. How anions influence the structure of donor stacks and often guide them toward different types of transitions (charge and anion ordering transitions, charge and/or spin density waves, etc.) is discussed in detail.

The well-known Bechgaard and Fabre salts are used to illustrate how anions play a crucial role in directing these prototype one-dimensional conductors through complex phase diagrams resulting from the competition between different conducting and localized states. For instance, the anion ordering transitions exert a strong control of the low-temperature transport properties (metallic, spin density wave, superconductivity, etc) of the Bechgaard (TMTSF)2X (X: PF6, ClO4, NO3, etc.) salts. The anions also have a crucial role in imposing the nature of the charge localized and charge ordered phases of the Fabre salts, (TMTSF)2X (X: PF6, ClO4, ReO4, etc.).  

Two-dimensional molecular conductors are also subject to the control of anions. The important role played by hydrogen bonding and the conformational flexibility of donors related to BEDT-TTF is illustrated by several examples as for instance: (i)  the concerted action of anion shifts and hydrogen bonding modulation in d-(EDT-TTF-CONMe2)2Br, (ii) the structural and charge ordering instability of suym-(BEDT-TTF)2I3, (iii) the puzzling one-dimensional instability of a-(BEDT-TTF)2KHg(SCN)4, and (iv) the role of the anions in the order-disorder and metal to insulator transitions of q-(BEDT-TTF)2RbM´ (SCN)4 (M´ = Co, Zn) salts.

Authors:  
Jean-Paul Pouget,1 Pere Alemany2 and Enric Canadell3

Affiliations:
1Laboratoire de Physique des Solides, CNRS UMR 8502, Université Paris-Sud, Université Paris Saclay, France
2Departament de Ciència de Materials i Química Física and Institut de Química
Teòrica i Computacional (IQTCUB), Universitat de Barcelona, Spain
3Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Spain

Publication:
Donor–anion interactions in quarter-filled low-dimensional organic conductors
Materials Horizons, 5, 590-640 (2018).
DOI: 10.1039/C8MH00423D

Figure:
Crystal structure of a-(BEDT-TTF)2I3 and diagram showing how the development of a positive s charge in the H atom making a hydrogen bond with the anion and a positive p charge in the central core of BEDT-TTF are correlated through a negative  s charge shift

 

Acknowledgments

Work in Spain was supported by MINECO (Spain) through Grants FIS2015-64886-C5-4-P and CTQ2015-64579-C3-3-P, and Generalitat de Catalunya (2017SGR1506, 2017SGR1289 and XRQTC). E. C. acknowledges support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program under Grant SEV-2015-0496. This review is based on recent collaborative works and pertinent discussions with N. Avarvari, P. Batail, C. Bourbonnais, C. Coulon, M. de Souza, M. Dressel, M. Fourmigué, P. Foury-Leylekian, V. Ilakovac, D. Jérome, M. Lang, E. Molins, Y. Nogami, C. Pasquier, H. Sawa and S. Tomic.

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