Localization of excitons by molecular layer formation in a polymer film

Spin coated films of atactic polystyrene of two different molecular weights have been studied with uv spectroscopy and x-ray reflectivity, the film thickness (d) varying from ∼2Rg to ∼12Rg where Rg is the unperturbed radius of gyration of the polymer. uv extinction due to the pure electronic singlet 1A1g→1E1u is seen to increase with d-1 for 4Rg≤d≤12Rg (region 1). This suggests excitonic interaction along d. The variation of total exciton energy (E) of the A1g→E1u singlet with d in region 1 can be well explained by formation of linear J-aggregates of polystyrene molecules, in a lattice with spacing 'a' (in A) Rg0 at d≅4Rg, where the exciton is still delocalized. Layering reduces the Hamaker constant (AH), deciding the cohesive force, between the layers and this reduction, ΔAH, is found to be less than Ae at d≥4Rg, where iAe/(ℎ/2π) is the amplitude for exciton transfer between neighboring molecules in the excitonic lattice of region 1. On the other hand, ΔAH in region 2 starts from a value larger than Ae. This indicates that ΔAH acts as a barrier between the layer, which localizes the exciton within the layers