Why is azulene blue and anthracene white? a simple mo picture

In spite of their almost identical ionization potentials (IP's) and electron affinities (EA's), azulene and anthracene differ considrably in their lowest singlet-singlet excitation energies. This is hard to understand in the simple Hückel picture in which excitation energies are expressed as orbital energy differences, if orbital energies are taken from IP's and EA's. The difficulty is removed when electron repulsion is introduced explicitly. This is shown using simple inuitive concepts, which also account for the magnitude of singlet-triplet splitting and single out the structural features responsible for the difference between azulene and anthracene, so that generalizations are possible