on the top. The changes in intensity and shifts in position of the bands are clearly visible.[]{data-label="20100.fig"}](gesamt20_100.eps){width="8.8cm"}

Energetics
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Table \[energies\] summarizes the ground state energies and the energy difference between (5,7)-member ring defected and ground states. Adiabatic ionization energies and electron affinities as obtained at the B3LYP/4-31G level of theory for pyrene and coronene are shown in table \[ioni\]. Literature values for the ionization energies and electron affinities of pyrene and coronene are 7.7, 7.21, 0.59 and 0.47 eV, respectively (Kazakov et al. [@kazakov]; Schroeder et al. [@schroeder]; Chen & Cooks [@chen]; Duncan et al. [@duncan]). By comparison it turns out that both are underestimated by 0.5 - 1 eV for both molecules. This is not surprising since the 4-31G basis set is far from being complete. For the energy difference between ground and defected states we note, however, that the estimates yielded by the B3LYP/4-31G model chemistry are close to the ones resulting from a more accurate treatment using G4(MP2) extrapolations in order to investigate the minimum energy reaction paths on the potential energy surfaces (PES) for the neutral species. The use of these accurate calculations results in energy differences of 2.23 eV in case of pyrene and 