frame|right|Examples of non-Kekulé (a) polyenes, (b) quinodimethanes, and (c) polynuclear aromatics
A non-Kekulé molecule is a conjugated hydrocarbon that cannot be assigned a classical Kekulé structure.
Since non-Kekulé molecules have two or more formal charges or
radical centers, their spin-spin interactions can cause electrical conductivity or ferromagnetism (molecule-based magnets), and applications to functional materials are expected. However, as these molecules are quite reactive and most of them are easily decomposed or polymerized at room temperature, strategies for stabilization are needed for their practical use. Synthesis and observation of these reactive molecules are generally accomplished by matrix-isolation methods.
Biradicals
The simplest non-Kekulé molecules are biradicals. A biradical is an even-electron chemical compound with two free radical centres which act independently of each other. They should not be confused with the more general class of diradicals. In 2019, larger homologues of triangulene, consisting of ten ([4]triangulene) and fifteen fused six-membered rings ([5]triangulene) were synthesized in 2019. In 2021, synthesis of the hitherto largest triangulene homologue, consisting of twenty-eight fused six-membered rings ([7]triangulene) was achieved. Scanning tunneling microscopy experiments on triangulene spin chains have revealed the clearest proof yet of the existence of Haldane gap and fractional edge states predicted for spin-1 Heisenberg chain. A related class of biradicals are para-benzynes.
Other studied biradicals are those based on pleiadene,
</references>