Calicene or triapentafulvalene is a hydrocarbon of the fulvalene class with chemical formula C8H6, composed of a cyclopentadiene ring and a cyclopropene ring linked by a double bond. Its name is derived from the Latin calix meaning "goblet", from its shape.
Very high resonance energy is predicted by the Hückel method,[1] however its resonance energy is not high.The central double bond is polarized with a partial positive charge on the carbon atom of triangular ring and a partial negative charge on the carbon atom of pentagonal ring, in keeping with added Hückel's rule stability of rings containing 2 π electrons and 6 π electrons respectively. Calicene's dipole moment has been computed to be 4.66 D.[2] Several compounds that contains two or more calicene subunits are aromatic, such as trans-bicalicene (ring compound) or poly-2,7-[N]calicenes (chain compound)[3]
Despite several attempts to prepare it, the parent calicene has so far defied attempts at synthesis.[4] However, 1,2,3,4,5,6-hexaphenylcalicene has been prepared and an experimental dipole moment of 6.3 D was measured.[5]