Hydrogen-bonded maleate counter anions in tetrathiafulvalene crystals

Intramolecular and/or intermolecular hydrogen-bonded maleate (HMA(-)) monoanions were introduced into cation radical salts of tetrathiafulvalene (TTF), forming three kinds of crystals: (TTF+)(H2MA)(HMA(-)) (1), (TTF+)(3)(HMA(-))(3)(CH3OH)(H2O)(0.5) (2), and (TTF+)(3)(TTF0)(5)(HMA(-))(3) (3). In crystals 1 and 2, the monovalent TTF+ cation formed a strong pi-dimer of (TTF+)(2) and a pi-trimer of (TTF+)(3), respectively. In crystal 3, a non-uniform pi-stack of TTF0 and TTF+ was formed, which was further surrounded by neutral TTF through S center dot center dot center dot S and S center dot center dot center dot pi interactions, forming a cylindrical TTF0 array around the pi-stack. A charge-separated electronic state of (TTF+)(3)(TTF0)(5)(HMA(-))(3) in crystal 3 was observed in the Raman spectra. Crystal 3 showed a semiconducting temperature dependent behavior with a room temperature conductivity of 3 x 10(-3) S cm(-1), which was six orders of magnitude higher than those of crystals 1 and 2. The protonated state of the (H2MA)(HMA(-)) anion in crystal 1 was representative of an (HMA(-)center dot center dot center dot H+center dot center dot center dot HMA(-)) dimer, where the bridging proton between the two HMA(-) anions was located at the midpoint between the two oxygen sites. A very strong intramolecular symmetrical O-H center dot center dot center dot O- hydrogen-bonding interaction was observed in crystals 1-3 according to potential energy calculations of proton coordination in the O-H center dot center dot center dot O- hydrogen bonding.