  A new approach is introduced for calculating the spectral shifts of the most
bathochromic pi -> pi* transition of an aromatic chromophore in apolar
environments. As an example, perylene in solid and liquid n-alkane matrices was
chosen, and all shifts are calculated relative to one well-defined
solid-inclusion system. It is shown that a simple two-level treatment of the
solute using Huckel theory yields spectral shifts in excellent agreement with
experimental results for the most prominent inclusion sites of perylene in
solid n-alkane surroundings and for the dilute solutions in liquid n-alkanes.
The idea is general enough to be applied to any aromatic chromophore in a
nonpolar solvent matrix. In contrast to earlier treatments, this approach is
based on geometry-dependent polarizabilities, employs a r^-4 dependence for the
dispersion energy, is conceptually simple and computationally efficient.
Different simple models based on our general approach to compute the UV
spectral shifts due to solvation indicate that the dispersive part of the
van-der-Waals energy, which stabilizes the LUMO of perylene more than the HOMO,
falls off with a distance dependence of r^-4 in a range up to ~1 nm and not as
r^-6, as has been assumed for a long time. This finding corresponds to the
interpretation of temporary dipoles as being equivalent to weak permanent
dipoles with fluctuating orientation.
