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 (λ _max ^T-T) at 675 nm, which was assigned as the triplet fulminene excited state. The λ _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, λ _max ^T-T = 60n + 318 (nm).