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

Five charge transfer complexes of electron donor N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD or Wurster's blue) with quinoid electron acceptors tetrafluoro- (F4Q), tetrachloro- (Cl4Q), tetrabromoquinone (Br4Q), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) were prepared and studied by X-ray crystallography, impedance spectroscopy, and DFT computations. Mixed donor–acceptor π-stacks were observed in all compounds; in TMPD·F4TCNQ, the moieties formed 2D stacked arrays. All moieties had a partial radical character due to charge transfer. The presence of two-electron multicenter bonding (pancake bonding) was confirmed by quantum chemical computation. In TMPD·F4Q, alternating longer and shorter interplanar distances were consistent with formation of pancake-bonded dimers; in complexes of TMPD with Cl4Q, Br4Q, and TCNQ, the stacks were equidistant implying existence of pancake-bonded polymers.