Unusual stoichiometry, band structure and band filling in conducting enantiopure radical cation salts of TM-BEDT-TTF showing helical packing of the donors

Electrocrystallization of tetramethyl-bis(ethylenedithio)-tetrathiafulvalene (TM-BEDT-TTF) (1) as pure (S,S,S,S) and (R,R,R,R) enantiomers in the presence of (n-Bu4N)2(Mo6O19) and chloroform or bromoform afforded a series of four isostructural enantiopure radical cation salts [(S/R)-1]9(Mo6O19)5·(CHX3)2 (X = Cl, Br) crystallizing in the trigonal non-centrosymmetric space group R32. In the formula unit there are six donors of type A and three donors of type B showing, respectively, (ax, ax, eq, eq) and all-ax conformations (ax = axial, eq = equatorial) of the methyl substituents. The donors form a hexagonal network in the ab plane with a helical twist between them leading to lateral orbital overlap interactions. Electrocrystallization of the racemic donor provided the compound [(rac)-1]2(Mo6O19) which crystallized in the monoclinic system P21/n. Single crystal resistivity measurements show semiconducting behaviour of the enantiopure materials with a relatively high room temperature conductivity of 0.8–1.2 S cm−1, but rather insensitive to applied pressures of up to 2.3 GPa. Analysis of the electronic structure of the conducting solids through extended Hückel tight-binding band structure calculations indicates a Mott insulator behaviour explaining the semiconducting character and suggests that these compounds are valuable candidates for Dirac cone materials. Further insight into the conducting properties is provided by preliminary field effect transistor measurements.This work was supported in France by the National Agency for Research (ANR, Project 15-CE29-0006-01 ChiraMolCo), the CNRS and the University of Angers. Work at Bellaterra (Spain) was supported by the Spanish MICIU through Grant PGC2018-096955-B-C44 and the Severo Ochoa FUNFUTURE (CEX2019-000917-S) Excellence Centre distinction as well as by Generalitat de Catalunya (2017SGR1506). This work was also partially supported in Chiba, Japan by MEXT = JSPJ KAKENHI under Grants No. 16H06346 and 20K03870. Work in Okazaki was supported by Grant-in-Aid for Scientific Research (A) (Grant Number 19H00891) and Grant-in-Aid for Challenging Research (Exploratory) (Grant Number 20K20903) from JSPS as well as PRESTO (Grant Number JPMJPR20L9) from JST.Peer reviewe

Double Heterohelicenes Composed of Benzo[b]- and Dibenzo[b,i]phenoxazine: A Comprehensive Comparison of Their Electronic and Chiroptical Properties

Heterohelicenes are potential materials in molecular electronics and optics because of their inherent chirality and various electronic properties originating from the introduced heteroatoms. In this work, we comprehensively investigated two kinds of double NO-hetero[5]helicenes composed of 12H-benzo[b]phenoxazine (BPO) and 13H-dibenzo[b,i]phenoxazine (DBPO). These helicenes exhibit good electron donor property reflecting the electron-rich character of their monomers and were demonstrated to work as p-type semiconductors. The enantiomers of these helicenes show the largest class of dissymmetry factors for circularly polarized luminescence (CPL) (|gCPL| > 10−2) among the previously reported helicenes. Interestingly, the signs of CPL are opposite between BPO- and DBPO-double helicenes of the same helicity. The origin of the large gCPL values and the inversion of the CPL signs was addressed by analysis of the transition electronic dipole moments (TEDM) and transition magnetic dipole moments (TMDM) based on the TD-DFT calculations