Spin Multiplicity and Solid-State Electrochemical Behavior in Charge-Transfer Co-crystals of DBTTF/F4TCNQ

Federica Solano,Mery Malandrino,Gabriele Kociok-Köhn,Enrico Da Como,Paolo Inaudi,Claudio Fontanesi,Raanan Carmieli,Davide Vanossi,Mario Chiesa,Enrico Salvadori,Agnese Giacomino

doi:10.1021/acs.jpcc.1c00020

Abstract

Charge-transfer crystals exhibit unique electronic and magnetic properties with interesting applications. The charge-transfer single crystal formed by dibenzotetrathiafulvalene (DBTTF) together with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) presents a long-range ordered supramolecular structure of segregated stacks, with a unitary degree of charge transfer. Thus, the crystal structure is composed of dimerized radical molecules with unpaired electrons. The energy levels and the spin degrees of freedom of this material were investigated by solid-state electrochemistry and electron paramagnetic resonance (EPR) spectroscopy. The electrochemical data, supported by density functional theory calculations, show how this organic Mott insulator has an electronic gap in the range of hundreds of meV. EPR experiments show the presence of a ground-state S = 1 triplet spin state along with localized S = 1/2 spins. The calculations also predict a ground-state triplet configuration, with the singlet configuration at 170 meV higher energy. DBTTF/F4TCNQ seems to be a candidate material for organic electronic and spintronic applications.

Highlights

Charge-transfer complexes arise from the combination of two neutral molecules with different electron affinities, which, by mutual polarization, induce the transfer of an electron from a donor (D) to an acceptor (A), commonly referred to as a D−A pair.[1]
Two peculiar features are the segregated stack of donors and acceptors and the dimerization of both DBTTF and F4TCNQ
At 170 K, the interplanar distances between F4TCNQ molecules are 3.223 and 3.416 Å, whereas those between DBTTF molecules are 3.418 and 3.491 Å

Summary

■ INTRODUCTION

Charge-transfer complexes arise from the combination of two neutral molecules with different electron affinities, which, by mutual polarization, induce the transfer of an electron from a donor (D) to an acceptor (A), commonly referred to as a D−A pair.[1]. This is a prerequisite for metallic behavior where a partially filled electronic band contributes to conduction.[6] When complete charge transfer occurs, that is, when both the donor and acceptor exchange a complete unit of charge, an ionic crystal is formed by the cation of the donor and the anion of the acceptor.[7,8] Such systems have not received extensive attention in the literature, mainly because their performances as conductors were observed to be poor.[9] the presence of ionized molecules in the lattice increases the on-site charge repulsion when electrons have to delocalize or move in response to an external field This gives rise to what is commonly known as a Mott insulator or Mott gap behavior in electrical transport.[10] The unpaired electrons, on these molecular ions, within the crystal lattice open up interesting possibilities when looking at the spin degrees of freedom. In the case of the DBTTF/F4TCNQ co-crystal, the data reported in the following are obtained by considering explicitly a two (DBTTF)-by-two (F4TCNQ) cluster

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ REFERENCES