text: Persistent carbenes tend to exist in the singlet, dimerizing when forced into triplet states.  Nevertheless, Hideo Tomioka and associates used electron delocalization to produce a comparatively stable triplet carbene (bis(9-anthryl)carbene) in 2001.  It has an unusually long half-life of 19 minutes.
  Although the figure below shows the two parts of the molecule in one flat plane, molecular geometry puts the two aromatic parts in orthogonal positions with respect to each other.
  In 2006 a triplet carbene was reported by the same group with a half-life of 40 minutes. This carbene is prepared by a photochemical decomposition of a diazomethane precursor by 300 nm light in benzene with expulsion of nitrogen gas.
  Again the figure below is not an adequate representation of the actual molecular structure: both phenyl rings are positioned orthogonal with respect to each other. The carbene carbon has an sp-hybridisation, the two remaining orthogonal p-orbitals each conjugating with one of the aromatic rings.
  Exposure to oxygen (a triplet diradical) converts this carbene to the corresponding benzophenone. The diphenylmethane compound is formed when it is trapped by cyclohexa-1,4-diene. As with the other carbenes, this species contains large bulky substituents, namely bromine and the trifluoromethyl groups on the phenyl rings, that shield the carbene and prevent or slow down the process of dimerization to a 1,1,2,2-tetra(phenyl)alkene. Based on computer simulations, the distance of the divalent carbon atom to its neighbors is claimed to be 138 picometers with a bond angle of 158.8°. The planes of the phenyl groups are almost at right angles to each other (the dihedral angle being 85.7°).