Radiative lifetimes of van der Waals heteroclusters

In this paper we advance and apply a molecular theory of pure radiative lifetimes τr of (aromatic molecule)⋅(rare gas)n heteroclusters. The modification of τr in the heterocluster, relative to the bare molecule value, originates from intermolecular interactions between molecular multicenter transition monopoles, which are described by π electron approximation, and the rare-gas atom transition dipoles, which are specified in terms of the static atomic polarizability α. The calculated changes of τr of 9,10 dichloroanthracene⋅A1 (A=Ar, Kr, Xe) heterodimers are in good agreement with experiment; the theory accounting for the nearly linear dependence of the change of τr on α. Calculations of the radiative lifetimes of 9,10 dichloroanthracene⋅Arn (n=1–34) heteroclusters were performed using structural information from potential optimization for small (n=1–3) heteroclusters, static structural data for small and medium-sized (n=1–8) heteroclusters, and (constant energy) finite-temperature molecular dynamics simulations for medium-sized and large (n=5–34) heteroclusters. The structural sensitivity of τr is manifested by different values of τr for distinct structural isomers, by the nonmonotonous size effects on τr with increasing n, and by pronounced decrease of τr with increasing temperature. For medium-sized clusters (n=5–18), the temperature dependence of τr originates from the enhancement of atomic motion on both sides of the aromatic microsurface and the occupation of the peripheral region, while for large clusters (n≂34) two layer–one layer isomerization processes with increasing temperature result in dramatic changes of τr. Satisfactory overall agreement between theory and experiment for 9,10 dichloroanthracene⋅Arn n=1–34 heteroclusters was accomplished. The dominating 9,10 dichloroanthracene⋅Arn structures for n=1–18, which yield agreement between theory and experimental for τr, correspond to nearly equal distribution of the rare gases on both sides of the aromatic microsurfaces at T≤20 K, while large n=34 two-layered structures in the temperature domain 22 K≤T≤40 K account for the experimental τr result. τr serves as a useful spectroscopic probe for the interrogation of the structure and isomerization dynamics in heteroclusters.