The computation of excited state geometries of molecules of the size described in the paper is very problematic, but some insight may be gained for a molecule of moderate symmetry such as 9-methylanthracene (structure 1 in Uchiyama, Prasanna de Silva, and Iwai). Figures 1 and 2 below show the HOMO and LUMO orbitals of 9-methylanthracene resulting from an ab initio 3-21G* calculation. Figures 3 and 4 show the two singly occupied molecular orbitals of the first excited singlet state, from a similar calculation including configuration interaction. The similarity between the orbitals—the HOMO of the ground state with the lowest of the singly occupied orbitals of the excited state, and the LUMO of the ground state with the higher of the singly occupied orbitals of the excited state—is quite apparent. There is minimal excited state distortion, rather, the system is behaving much as a particle in a box and the difference in energy between the two states arises from the redistribution of electron density in a box of essentially the same size.

Fully manipulable (Chime and Jmol) versions of these molecules are available at the JCE Digital Library Web site.