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

Abstract

Charge-transfer crystals exhibit unique electronic and magnetic properties with interesting applications. The chargetransfer 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. ■ 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. The charge-transfer interaction is crucial in the solid state and leads to the formation of some typical crystal structures, where the donor and acceptor molecules stack with their π-conjugated orbitals facing each other in an alternating fashion. Another common solid-state arrangement is when the donor and the acceptor form parallel stacks with only a πoverlap between molecules having the same molecular structure, that is, only donors or only acceptors. These are referred to as segregated stack charge-transfer co-crystals. Recently, charge-transfer co-crystals have been used to demonstrate new interesting applications. In the vast literature on these systems, one of the first organic metals, tetrathiafulvalene/2,3,5,6-tetrafluoro-7,7,8,8 tetracyanoquinodimethane (TTF/TCNQ), has emerged as a prototype system and played a central role in the development of the field of organic electronics. In this case, the TTF and TCNQ molecules stack into segregated stacks with a charge transfer that does not correspond to a complete unit of charge. This is a prerequisite for metallic behavior where a partially filled electronic band contributes to conduction. 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. Such systems have not received equally extensive attention in the literature, mainly because their performances as conductors were observed to be poor. In fact, 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. The unpaired electrons, on these molecular ions, within the crystal lattice open up interesting possibilities when looking at the spin degrees of freedom. This aspect has been recently recognized by Wudl and colleagues. Thus, charge-transfer co-crystals combine the high conductivity or semiconductor behavior together with the unpaired or paired (singlet or triplet multiplicity) spins. The charge and Received: January 2, 2021 Revised: April 1, 2021 Published: April 20, 2021 Article pubs.acs.org/JPCC © 2021 The Authors. Published by American Chemical Society 8677 https://doi.org/10.1021/acs.jpcc.1c00020 J. Phys. Chem. C 2021, 125, 8677−8683 D ow nl oa de d vi a U N IV D E G L I ST U D I D I T O R IN O o n M ay 1 2, 2 02 1 at 0 8: 59 :5 8 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s. spin degrees of freedom could be combined toward the design of new organic multiferroic materials. In this paper, a charge-transfer co-crystal with peculiar electronic and magnetic properties based on DBTTF/ F4TCNQ is investigated. A detailed structural characterization by single-crystal X-ray diffraction reveals that the material unit cell is composed of homodimers of DBTTF and F4TCNQ which are arranged in segregated stacks. The spin degrees of freedom were investigated using electron paramagnetic resonance (EPR) spectroscopy, which show the coexistence of spin doublet (S = 1/2) and triplet (S = 1) states at room temperature. The material is an electroactive Mott insulator with well-defined frontier orbitals capable of accepting electrons and holes, as demonstrated by the solid-state electrochemistry data. These data were modeled using a density functional theory (DFT) cluster approach, which captures the EPR observations and is able to describe the novel electrochemistry data. Typically, electrochemical-based cyclic voltammetry (CV) measurements are performed for studies in solution; here, to probe the crystal directly and avoid the dissolution of the solid-state material in solution, a new custom-designed setup was adopted. The approach is based on a solid-state electrochemical configuration fully described in the Supporting Information. The energies of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels and of the related band gap, EBG, were determined from the electrochemical results. EPR experiments reveal a rather complex picture for the electronic structure, where localized doublet radicals on DBTTF coexist along with the triplet species, found at a much lower concentration. Ab initio DFT calculations (CAMB3LYP/cc-pVTZ) were used to rationalize the experimental evidence in terms of a molecular perspective. ■ EXPERIMENTAL AND THEORETICAL METHODS Chemicals. DBTTF (99% purity) was purchased from Sigma-Aldrich, and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (98% purity) was purchased from TCI, UK. The materials were purified using a vacuum sublimation apparatus. Acetonitrile anhydrous, ACN (<99% purity), tetrabutylammonium hexafluorophosphate, TBAPF6 (99% purity), and 5% Nafion 117 solution were obtained from Sigma-Aldrich and were used for electrochemical studies. During all electrochemical analyses, alumina powder was used to renew the electrode surface. All glassware were cleaned by high-purity water (Millipore >20 MΩ cm). Electrochemical Setup. The electrochemical analysis was performed using a PalmSens3 portable potentiostat (PalmSens, Houten, Netherlands). It was interfaced to a tablet or laptop computer, and the software PSTrace 4.6 was used to set the process parameters. A conventional three-electrode cell was used for electrochemical studies of pristine solutions. The working electrode (WE) selected was a glassy carbon electrode (GCE) with a diameter of 3 mm. The counter electrode (CE) was a Pt electrode. The potentials were measured against a Ag/ AgCl/KClsat reference electrode (RE). A peculiar arrangement was adopted in the case of co-crystals to preserve their solid state: a cylindrical Teflon cell featuring a hole (0.8 cm diameter) in the bottom was used in a vertical configuration, where a glassy carbon plate (GCP) (size: 25 mm × 25 mm) was tightened from below; a Teflon ring was used to ensure no solution leakage from the cell. The cell was rested/leaning on a copper plate as the electrical contact. The GCP was used as WE, while a Pt wire and Ag/AgCl/KClsat were used as CE and RE, respectively. CE and RE were placed at the top of the cylinder in contact with the electrolytic solution. The cocrystals were placed over the WE and covered with a Nafion membrane. Then, the membrane was left “to dry” for 5 min, and the electrolytic solution was added. The Nafion membrane prevents direct contact between the solid co-crystal and the electrolytic solution, avoiding the dissolution of the co-crystals but preserving the ion transfer. In fact, if dissolved in solution, the co-crystals would be divided, giving the individual species. Charge-Transfer Co-crystal Preparation and X-ray Structure. DBTTF/F4TCNQ co-crystals were grown using a horizontal quartz tube under an argon stream. This growth method is widely used for organic materials and recently has been employed by our group and others to grow high-quality organic co-crystals. Argon gas was obtained from BOC, UK, with a purity of 6N. The tube was filled with quartz tubes of smaller diameter and 10 cm length, used to harvest the crystals at the end of the growth procedure. The tube, with 150 mg of each material in powder form, was placed in a Carbolite EHC 12/600B three-zone furnace, with the hot zone at 507 K, the center of the furnace at 466 K, and the cold zone at 420 K for the co-crystal DBTTF/F4TCNQ. The tubes were cleaned by ultrasonication in deionized water, 2-propanol, and acetone, sequentially. For structure determination, a single crystal was mounted on a Dual-Thickness MicroMount. Intensity data were collected on a Rigaku Supernova Dual, EosS2 system using monochromated Cu Kα radiation (λ = 1.54184 Å). Unit cell determination, data collection, and data reduction were performed using the CrysAlisPro software. The structure was solved with SHELXT and refined by a full-matrix least-squares procedure based on F2(SHELXL-2018/3). All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed onto calculated positions and refined using a riding model. EPR Spectroscopy. Room-temperature X-band CW EPR spectra were collected with a super-high Q resonator (ER4122 SHQE, operating at ∼9.8 GHz). Low-temperature Q-band CW EPR spectra were collected using the EN 5107D2 Bruker resonator housed in an Oxford CF935 cryostat. Detailed experimen