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Atactic polystyrene of two different molecular weights (560900 g.mol-1 and212400 g.mol-1) have been studied as spin-coated films of thickness varyingfrom ~30 nm to ~250 nm, i.e., ~2Rg to ~12Rg where Rg is the unperturbed radiusof gyration of polystyrene, using x-ray reflectivity and transmissionUV-spectroscopy. Electron density profiles along depth of the films showformation of layers parallel to the substrate surface, when the film thicknessis below 4Rg, with layer spacing ~2Rg. The pure electronic singlet 1A1g -> 1E1uindicates exciton interaction throughout film thickness, as evidenced by thelineshape, drop in extinction coefficient and size-dependent blue shift of thecorresponding UV peak. Analysis of the total energy of the exciton as afunction of film thickness shows that above the thickness of 4Rg the exciton isdelocalized over the film thickness through linear, molecular J-aggregates. Theseparations between these molecules in the two polystyrene samples match withthe respective layer spacing obtained from x-ray reflectivity. Transitiondipole moments of the molecules in this aggregate are parallel and each dipoleis arranged at an angle of ~43 degrees with the axis of the linear chain. Belowa film thickness of 4Rg, as the molecular layers are formed, the excitonbecomes localized within these layers. This localization is clearly indicatedby the lineshape becoming comparable to isolated molecular bands, and by boththe extinction coefficient and blue shift reaching maximum values and becomingsize independent. Localization of excitons due to layer formation has beenexplained as caused by decrease of cohesion between layers.Comment: 16 pages, 7 figure

Localization of excitons by molecular layer formation in a polymer film