Electronic structure of the naphthalene radical cation and some simple alkylated derivatives

The excited states of the radical cations of naphthalene (N), dihydroacenaphthylene (A) and pyracene (P) are probed experimentally by photoelectron (PE) and by electronic absorption (EA) spectroscopy. Their electronic structure is discussed in some detail on the basis of ab initio CASSCF/CASPT2 calculations which yield a description in good accord with experiment, both with regard to band positions and intensities. For example, they help to explain why a prominent band in the EA spectrum of N.+ is absent in the spectra of A(.+) and P.+. This is not due to a spectral shift induced by the alkyl bridges, but rather to a cancellation of transition dipole moments that is only partial in N.+ but more 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 quite substantial shift ih some of the excited state energies, notably that bf the first (unobserved) one. Therefore, the common assumption that 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.