ELECTRONIC-STRUCTURE OF THE NAPHTHALENE RADICAL-CATION AND SOME SIMPLE ALKYLATED DERIVATIVES

The excited states of the radical cations of naphthalene (N), dihydroa cenaphthylene (A) and pyracene (P) are probed experimentally by photoe lectron (PE) and by electronic absorption (EA) spectroscopy. Their ele ctronic structure is discussed in some detail on the basis of ab initi o CASSCF/CASPT2 calculations which yield a description in good accord with experiment, both with regard to band positions and intensities. F or example, they help to explain why a prominent band in the EA spectr um of N.+ is absent in the spectra of A(.+) and P.+. This is not due t o a spectral shift induced by the alkyl bridges, but rather to a cance llation of transition dipole moments that is only partial in N.+ but m ore complete in the two derivatives. It is found that-in spite of the relatively small relaxation energies of the vertically formed radical cations-the accompanying geometry changes on ionization may lead to qu ite substantial shift ih some of the excited state energies, notably t hat bf the first (unobserved) one. Therefore, the common assumption th at the good correlation between the PE spectra of polycyclic aromatic hydrocarbons and the EA spectra of their radical cations is due to the rigidity of these compounds is not well founded.