2,2-Dimethyl-2H-dibenzo[cd,k]fluoranthene, the First Kekulé Hydrocarbon with a Triplet Ground State

Experimental consideration of covalent‐bonding interactions in stacks of singlet biradicals having Kekulé structures

“…Their weak covalent-bonding interaction leads to a high reactivity and an unusual electronic structure, which have fascinated chemists with their use as synthetic precursor, in theoretical treatment, in spectroscopic characterization, and in material application. [1][2][3][4][5][6][7][8][9][10][11][12][13] Singlet biradicals can be categorized into conjugated and nonconjugated systems, and the former is further divided into Kekulé and non-Kekulé systems. Unlike nonconjugated and non-Kekulé biradicals, it is difficult to intuitively recognize the openshell character of singlet biradicals having Kekulé forms because no unpaired electron appears in their chemical formula.…”

Bis-periazulene: a simple Kekul� biradical with a triplet ground state

“…Houk 1 The cation corresponding to replacement of the 4-CH by a 4-NH + has been synthesized [4] (r 1.383 A Ê ) by relatively long bonds (r 1.45± 1.47 A Ê ). Houk 1 The cation corresponding to replacement of the 4-CH by a 4-NH + has been synthesized [4] (r 1.383 A Ê ) by relatively long bonds (r 1.45± 1.47 A Ê ).…”

“…Antiaromatic Kekule hydrocarbons, such as cyclobutadiene, or large extended conjugated systems such as polyacenes, have low-lying triplet states, but until recently, there were no examples of a Kekule hydrocarbon with a triplet ground state [1]. Such molecules normally have singlet ground states and are relatively unreactive.…”

“…Surprisingly, the structure with the aromatic 14p-electron periphery and an ethylene center, 1a, is expected on this basis to be less stable than the diradical structure, 1b. 1 Why is this potentially aromatic compound, a biradical, too reactive to be isolated, and most probably a ground-state triplet hydrocarbon? 1 Why is this potentially aromatic compound, a biradical, too reactive to be isolated, and most probably a ground-state triplet hydrocarbon?…”

Diradical character of some fluoranthenes

“…1). 3,25 The compound was generated photochemically and examined spectroscopically in a cryogenic matrix and in solution using nanosecond laser flash photolysis. On the basis of spectroscopic measurements, trapping experiments and DFT calculations, the singlet-triplet gap of 6 was estimated to be 3.3-5.4 kJ/mol in favor of the triplet.…”