Excited Electronic Levels in Conjugated Molecules: I. Long Wavelength Ultra-Violet Absorption of Naphthalene, Anthracene and Homologs

The method of molecular orbitals has been applied to study the long wave ultra-violet absorption of the polyacene series, naphthalene, anthracene,..... The longest wavelength in an allowed excitation probably arises from a transition between mobile electron levels of symmetry A1g → B2u, and polarized across the width of the molecule. In the first four or five members of the series, the second wavelength is due to a transition A1g → B1u, polarized along the length of the molecule. Discrepancies between this theory and both the valence-bond resonance treatment and the Lewis-Calvin theory of colour are elucidated. This assignment of transitions is in agreement with (1) calculation of absolute energies as a function of chain length, (2) calculation of relative energies of the B2u and B1u transitions, (3) limiting condition for a large number of rings, (4) calculations of intensities, and (5) some experimental work on fluorescence.