Electronic spectral shifts of aromatic molecule–rare-gas heteroclusters

Abstract

In this paper, a semiempirical theory for the spectral shifts of the electronic origin of the S0→S1 transition of (aromatic molecule)⋅(rare-gas)n heteroclusters is advanced and applied. Neglecting the modifications of intermolecular overlap and exchange interactions upon electronic excitation, the dispersive contributions to the spectral shift are evaluated to second order, accounting for finite-size structural features of the large molecule by the utilization of the multicenter monopole representation of the intermolecular interactions. The spectral shifts for nonpolar aromatic hydrocarbons in or on rare-gas heteroclusters are represented in terms of differences between electrostatic interactions involving an electrostatic field (due to the molecular transition monopoles charge distribution) and an induced dipole (originating from the rare-gas polarizability). The transition monopoles incorporated all the one- and two-electron ππ* excitations of the aromatic molecule, which were represented by Hückel or self-consistent molecular orbitals (MO). The dispersive spectral shifts were semiempirically scaled to correct the systematic overestimate of the transition monopoles within these simple MO schemes. The red spectral shift was recast in terms of a sum of two-atom (carbon atom–rare-gas) and three-atom (carbon atom–rare-gas–carbon atom) contributions, which are subsequently summed over the contributions of all the rare-gas atoms, with each term involving products of an electronic factor and a geometric factor. The electronic factors exhibit a linear dependence of the spectral shift on the rare-gas polarizability, while the geometric factors incorporate the structural effects of the contributions of the individual rare-gas atoms to the spectral shift, predicting the nonadditivity of the spectral shift per added rare-gas atom and the exhibition of distinct spectral shifts for different structural isomers. The semiempirical theory in conjunction with structural information emerging from (pairwise) potential optimization for small and medium-sized (n=1–8) heteroclusters and from molecular dynamics simulation for medium-sized and large heteroclusters (n=5–34) was applied. The theory accounts for the spectral shifts of small and medium-sized rare-gas heteroclusters of pentacene and tetracene as well as for small, medium-sized, and large heteroclusters of 9,10 dichloroanthracene, but not for the general pattern of the spectral shifts for anthracene⋅An (A=Ar, Kr; n=1–8) and for perylene⋅Arn (n=2). The confrontation between theory and experiment for the spectral shifts of pentacene⋅Arn (n=1–8), tetracene⋅Arn (n=1–8), tetracene⋅Krn (n=1–8), 9,10 dichloroanthracene⋅Arn (n=1–34) and 9,10 dichloroanthracene⋅Krn (n=1–20), allows for the elucidation of the size dependence of spectral shifts, for the identification of structural isomers in small and medium-sized heteroclusters, and for the exploration of spectral inhomogeneous broadening in large heteroclusters.