Localization of Excitons by Molecular Layer Formation in a Polymer Film

Abstract

Atactic polystyrene of two different molecular weights (560900 g.mol-1 and 212400 g.mol-1) have been studied as spin-coated films of thickness varying from ~30 nm to ~250 nm, i.e., ~2Rg to ~12Rg where Rg is the unperturbed radius of gyration of polystyrene, using x-ray reflectivity and transmission UV-spectroscopy. Electron density profiles along depth of the films show formation of layers parallel to the substrate surface, when the film thickness is below 4Rg, with layer spacing ~2Rg. The pure electronic singlet 1A1g -> 1E1u indicates exciton interaction throughout film thickness, as evidenced by the lineshape, drop in extinction coefficient and size-dependent blue shift of the corresponding UV peak. Analysis of the total energy of the exciton as a function of film thickness shows that above the thickness of 4Rg the exciton is delocalized over the film thickness through linear, molecular J-aggregates. The separations between these molecules in the two polystyrene samples match with the respective layer spacing obtained from x-ray reflectivity. Transition dipole moments of the molecules in this aggregate are parallel and each dipole is arranged at an angle of ~43 degrees with the axis of the linear chain. Below a film thickness of 4Rg, as the molecular layers are formed, the exciton becomes localized within these layers. This localization is clearly indicated by the lineshape becoming comparable to isolated molecular bands, and by both the extinction coefficient and blue shift reaching maximum values and becoming size independent. Localization of excitons due to layer formation has been explained as caused by decrease of cohesion between layers.