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. One of the first biradicals was synthesized by Wilhelm Schlenk in 1915 following the same methodology as Moses Gomberg's triphenylmethyl radical. The so-called Schlenk-Brauns hydrocarbons are: Eugene Müller, with the aid of a Gouy balance, established for the first time that these compounds are paramagnetic with a triplet ground state. Another classic biradical was synthesised by Aleksei Chichibabin in 1907. Other classical examples are the biradicals described by Yang in 1960 and by Coppinger in 1962. Trimethylenemethane A well studied biradical is trimethylenemethane (TMM), . In 1966 Paul Dowd determined with electron spin resonance that this compound also has a triplet state. In a crystalline host the 6 hydrogen atoms in TMM are identical. Quinodimethanes and PAHs Other examples of non-Kekulé molecules are the biradicaloid quinodimethanes, that have a six-membered ring with methylene substituents. Non-Kekulé polynuclear aromatic hydrocarbons are composed of several fused six-membered rings. The simplest member of this class is triangulene. After unsuccessful attempts by Erich Clar in 1953, trioxytriangulene was synthesized by Richard J. Bushby in 1995, and kinetically stabilized triangulene by Kazuhiro Nakasuji in 2001. However, in 2017 a project led by David Fox and Anish Mistry from the University of Warwick in collaboration with IBM synthesized and imaged triangulene. 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