Influence of molecular conformations on the electronic structure of organic charge transfer salts

TL;DR

This study uses ab-initio calculations to explore how molecular conformations in organic charge transfer salts affect their electronic structures, revealing a link between ethylene group orientations and metal-insulator transitions.

Contribution

It demonstrates that molecular conformations critically influence electronic parameters and ground states, providing insights into tuning phase transitions in these materials.

Findings

Conformations predict ground state structures accurately.

Molecular orientations significantly affect electronic parameters.

Configurations correlate with metal-insulator transition status.

Abstract

We report ab-initio calculations for the electronic structure of organic charge transfer salts $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br, $\kappa$-(ET)$_2$Cu[N(CN)$_2$]I, $\kappa^{\prime \prime}$-(ET)$_2$Cu[N(CN)$_2$]Cl and $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$. These materials show an ordering of the relative orientation of terminal ethylene groups in the BEDT-TTF molecules at finite temperature and our calculations correctly predict the experimentally observed ground state molecular conformations (eclipsed or staggered). Further, it was recently demonstrated that the ethylene endgroup relative orientations can be used to reversibly tune $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br through a metal-insulator transition. Using a tight-binding analysis, we show that the molecular conformations of ethylene endgroups are intimately connected to the electronic structure and significantly influence hopping and Hubbard repulsion parameters. Our results place $\kappa$-(ET)$_2$Cu[N(CN)$_2$]Br in eclipsed and staggered configurations on opposite sides of the metal-insulator transition.