Insights on the Molecular Orientation of Oligo(p-phenylene vinylene) Derivatives with Alkyl Chains in Langmuir Films

One of the oligo(p-phenylene vinylene) derivatives having alkyl chains, OPV-1, is known to form characteristic rod-shaped nanoaggregates on the water surface, and thin films with aligned nanoaggregates can be obtained by the Langmuir-Blodgett (LB) technique. Although the molecular aggregation and orientation of the "conjugated rings part" in the LB films have already been investigated, analysis of the detailed molecular structure involving the "alkyl chains" is left behind. In the present study, to reveal the role of the alkyl chains in the molecular aggregation in Langmuir films prior to the LB transfer, infrared (IR) external reflection spectroscopic measurements on the water surface are carried out. It has been revealed that not only the conjugated rings of OPV-1 but also the alkyl chains with an ordered conformation are lying on the water surface. The characteristic orientation is found to be kept on a solid substrate even after the LB transfer by IR p-polarized multiple-angle incidence resolution spectrometry (pMAIRS). In addition, pMAIRS was first employed for analyses of the biaxial orientation in the LB films, and the molecular orientations for the conjugated rings and the alkyl chains were quantitatively evaluated from both in-plane and out-of plane perspectives.