Photochemical synthesis and electronic spectra of fulminene ([6]phenacene)

Facile synthesis of fulminene ([6]phenacene) was achieved through the Mallory reaction of 1-(1-naphthyl)-2-(1-phenanthryl)ethene or the 9-fluorenone-sensitized photo-ring-closure of 1-(1-naphthyl)-2-(1-phenanthryl)ethane. The electronic spectral properties of fulminene were investigated for the first time using photoluminescence as well as transient absorption spectroscopy. The spectral features were compared with those of a series of lower phenacene homologs such as phenanthrene ([3]phenacene), chrysene ([4]phenacene), and picene ([5]phenacene). For the [n]phenacene series, both the fluorescence and phosphorescence bands linearly red-shifted with an increase in the number of the benzene rings (n). Trends in the energy levels of the excited singlet (E (S)) and the triplet (E (T)) states were expressed as E (s) = -2.6n + 89.1 (kcal mol(-1)) and E (T) = -1.8n + 66.2 (kcal mol(-1)), respectively. In the case of fulminene, laser flash photolysis displayed a transient spectrum with an absorption maximum (lambda (max) (T-T) ) at 675 nm, which was assigned as the triplet fulminene excited state. The lambda (max) (T-T) values for the [n]phenacene series showed a linear correlation as a function of the ring number n, given by an equation, lambda (max) (T-T) = 60n + 318 (nm).