Molecular solids with cooperative electronic properties based purely on pi electrons from carbon atoms offer a fertile ground in the search for exotic states of matter, including unconventional superconductivity and quantum magnetism. The field was ignited by reports of hightemperature superconductivity in materials obtained by the reaction of alkali metals with polyaromatic hydrocarbons, such as phenanthrene and picene, but the composition and structure of any compound in this family remained unknown. In this work we reported on the binary caesium salts of phenanthrene, Cs(C14H10) and Cs2(C14H10), and show that they are multiorbital strongly correlated Mott insulators. Whereas Cs2(C14H10) is diamagnetic because of orbital polarization, Cs(C14H10) is a Heisenberg antiferromagnet with a gapped spin-liquid state that emerges from the coupled highly frustrated ?-chain magnetic topology of the alternating-exchange spiral tubes of S=1 (C14H10)•- radical anions. The absence of long-range magnetic order down to 1.8 K (T/J ? 0.02; J is the dominant exchange constant) renders the compound an excellent candidate for a spin-1 quantumspin liquid (QSL) that arises purely from carbon pi electrons.
The reduction of polycyclic aromatic hydrocarbons with alkali metals results in solids having intriguing magnetic properties the understanding of which has been hitherto severely hampered by the lack of single-phase samples. Here, we report on the successful reduction of triphenylene with stoichiometric amount of potassium in 1,2-dimethoxyethane (DME) solution. Comprehensive diffraction measurements of the obtained K2(C18H12)2(DME) solid demonstrate the importance of cation-? interactions as responsible for the characteristic stacking of the triphenylide molecular ions. Electron paramagnetic resonance and magnetization measurements reveal K2(C18H12)2(DME) is a Mott insulator with strikingly strong nearest neighbor antiferromagnetic interactions between S = 1/2 spins of (C18H12)•- radical anions. Low dimensionality hinders long-range magnetic ordering and establishes a spin state that resembles gapped quantum spin liquid state.