First-Principles Crystal Structure Prediction of Cu(I)-TCNQ Polymorphs Riccarda Caputo, † Samet Demir, † and Adem Tekin* ,†,‡ † Informatics Institute, Istanbul Technical University, Maslak, 34469 Istanbul, Turkey ‡ Research Institute for Fundamental Sciences (TU ̈ BI ̇ TAK-TBAE), 41470 Gebze, Kocaeli, Turkey * S Supporting Information ABSTRACT: There is a lack of consensus on the crystal structure and polymorphism of Cu(I)-TCNQ, even though an enormous work and great achievements have been reported for decades. This motivated us to perform a crystal structure prediction study [by using the Fast and Flexible CrystAl Structure Predictor (FFCASP)] combined with total energy calculations at the dispersion-corrected density functional theory (DFT-D) level. The ensemble of the optimized structures falls in two distinct regions based on the energy-density phase space. Our predictions located the thermodynamic phase (as a global minimum) and a conformationally similar local minimum structure to the experimentally proposed phase I, in the low- and high-density regions, respectively. The Rietveld refinement of the P2 1 2 1 2 local minimum structure to the experimental X-ray diffraction (XRD) pattern resulted in a structure with a density between the two regions. This polymorph, the kinetic product, becomes a high-energy local minimum structure after the full geometry optimization, being 35.03 kJ/mol above the global minimum. In addition, we found that the transition from the high- to the low-density region occurs via square planar coordination of copper atoms. 1. INTRODUCTION Substituted quinodimethanes have been studied since 1960s 1,2 when they were accurately synthesized and their chemical properties investigated in detail. In particular, the 7,7,8,8- tetracyanoquinodimethane (TCNQ) was reported to exhibit remarkable properties, one of which is being an electron acceptor to form stable anion-radical derivatives. The solid- state properties of the derivatives, particularly the exceptional low resistivity, have boosted a prolific research activity aiming at tuning the resistance-switching properties for memory storage material applications. Being TCNQ a strong π-acid, it can be easily reduced when treated with metal iodides or certain metals. 2 The facile synthesis of simple salts of TCNQ, in particular, Cu(I)-TCNQ, gained enormous interest in the late 1990s and in the beginning of 2000s for the possible applications as semiconducting solids in electronic and optical switching devices. Starting with the work by Heintz et al., 3 who correlated the two distinct forms (I, II) with the electrical conductivity, further investigations 4-6 do not clarify unambiguously the polymorphs of Cu(I)-TCNQ. The description of the two phases, in terms of space group representation and structure determination from the powder X-ray diffraction (XRD) patterns reported by Heintz et al., 3 does not identify clearly the crystal structures, and the subsequent works 4-6 do not distinguish clearly the different phases. Cu(I)-TCNQ is reported 3 to crystallize in two polymorphic forms, identified as the kinetic and the thermodynamic product, both represented 3 in the monoclinic space groups, though the exact symmetry representation is not unambiguously deter- mined in the experimental works to date. 3-6 The poly- morphism of Cu(I)-TCNQ is governed by the peculiar properties of TCNQ ligands combined with the chemistry of the coordinating metal center Cu(I). As an unsaturated polynitrile, TCNQ ligands can adopt various oxidation states in their complexes with metals. In Cu(I)-TCNQ, the reduction Received: September 23, 2019 Revised: December 3, 2019 Published: December 4, 2019 Article pubs.acs.org/JPCC Cite This: J. Phys. Chem. C XXXX, XXX, XXX-XXX © XXXX American Chemical Society A DOI: 10.1021/acs.jpcc.9b09007 J. Phys. Chem. C XXXX, XXX, XXX-XXX Downloaded via CHALMERS UNIV OF TECHNOLOGY on December 22, 2019 at 04:10:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.