Semiconductive 2D arrays of pancake-bonded oligomers of partially charged TCNQ radicals

Abstract

Multicentre two-electron (mc/2e or `pancake bonding') bonding between 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anions was studied on its 14 novel salts with planar organic cations. The formal charges of the TCNQδ− moieties are −1/2 and −2/3, and they form mc/2e bonded dimers, trimers and tetramers which are further stacked into extended arrays. Multicentre bonding within these oligomers is characterized by short interplanar separations of 2.9–3.2 Å; distances between the oligomers are larger, typically >3.3 Å. The stacks are laterally connected by C—H...N hydrogen bonding, forming 2D arrays. The nature of mc/2e bonding is characterized by structural, magnetic and electrical data. The compounds are found to be semiconductors, and high conductivity [10−2 (Ω cm)−1] correlates with short interplanar distances between pancake-bonded oligomers.

Two Electron Multicenter Bonding (Pancake Bonding) between Electron Donors and Acceptors in Charge-Transfer Compounds ofN,N,N′,N′-Tetramethyl-p-phenylenediamine and (Semi)quinoid Electron Acceptors

“…Two types of crystal packing are observed: TMPD·F 4 Q , TMPD·Cl 4 Q , TMPD·Br 4 Q , and TMPD·TCNQ form 1D stacks with alternating donor and acceptor molecules, similar to those observed in other charge-transfer systems, such as TTF·Cl 4 Q. − They involve parallel stacks, which are held together by weak hydrogen bonding (Figure S9). In TMPD·F 4 TCNQ , the donor and acceptor molecules form 2D arrays, similar to those observed in salts of a TCNQ radical anion …”

“…The 2D stacked array of TMPD·F 4 TCNQ involves two symmetry-independent contacts (Table ), one between a TMPD and a F 4 TCNQ and the other between two F 4 TCNQ moieties. Therefore, it is logical to expect that they would form a 2D array of pancake bonding, similar to those found in some salts of the TCNQ radical anion . Molecular orbitals computed for dimers representing both contacts (Figure a) show that significant orbital overlap necessary for formation of pancake bonding is present only in the second one, the parallel slipped F 4 TCNQ···F 4 TCNQ (δ = 0°).…”

“…All structures involve stacking interactions between TMPD and the quinoid acceptor, and charge transfer is apparent due to black color of the crystals. However, the degree of charge transfer is difficult to estimate from molecular geometries. ,, Therefore, molecular charges were assessed using DFT computations (see section Analysis of Charge Transfer and Multicenter Bonding below).…”

“…This indicates that mutual orientation of molecules, as defined by the parameter δ (angle between molecular axes), plays an important role in pancake bonding. Our previous studies of semiquinones , and TCNQ radicals revealed that stacking and pancake bonding occurs only if molecular axes are parallel (i.e., if δ is 0° or 180°). So far, we encountered no stacks of semiquinones with perpendicular molecular axes.…”

“…Another well-studied system is a complex with 7,7,8,8-tetracyanoquinodimethane (TMPD·TCNQ); there is also an analogue with 2:1 stoichiometry . The degree of charge transfer estimated from the molecular geometry is 0.5; , however, this type of estimate is known to be unreliable. , According to vibrational spectroscopy, ρ is 90%, and a phase transformation involving “dimerization” of the stacks takes place between 180 and 220 K . However, the crystal structure of the putative low-temperature phase has not been determined.…”