Spectral Resemblances of Cata-Condensed Hydrocarbons

The spectra of naphthalene, anthracene, naphthacene, phenanthrene, 1,2-benzanthrene, chrysene, and acenaphthene in n-heptane solution are extended to 1700A. Electronic energy levels of 17 such cata-condensed hydrocarbons are collected and compared. The lowest five or six excited states shift in a regular way with changes of molecular length and shape. Intensities and vibrational structures of corresponding bands are remarkably alike in the different compounds. The total oscillator strength is almost proportional to the number of π-electrons, but the proportionality constant differs from that in polyenes. With new identifications, the positions of energy levels in naphthalene, anthracene, and azulene agree remarkably well with previous LCAO molecular orbital calculations. The lowest singlet state is of one type in benzene, naphthalene, and most non-linear systems; of another type in anthracene and the higher linear polyacenes because of a cross-over. This clears up some controversial questions, such as the relations among the spectra of naphthalene, anthracene, and phenanthrene.