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) and the triplet ( E) states were expressed as E = −2.6 n + 89.1 (kcal mol) and E = −1.8 n + 66.2 (kcal mol), respectively. In the case of fulminene, laser flash photolysis displayed a transient spectrum with an absorption maximum ( λ) at 675 nm, which was assigned as the triplet fulminene excited state. The λ values for the [ n]phenacene series showed a linear correlation as a function of the ring number n, given by an equation, λ = 60 n + 318 (nm). [ABSTRACT FROM AUTHOR]