Spectroscopic Evidence for Pt−Pt Interaction in a Langmuir−Blodgett Film of an Amphiphilic Platinum(II) Complex

Abstract

When a chloroform solution of a dicyano(diimine)platinum(II) complex, [Pt(CN)2(5,5‘-dinonyl-2,2‘-bipyridine)], was spread onto pure water, a reversible surface pressure−molecular area curve was obtained. The surface pressure began to increase steeply from 0 at a molecular area of 0.61 nm2 molecule-1. On further compression, the monolayer was stable at 0.2 nm2 molecule-1 and 30 mN m-1. In such a highly compressed state, the complex was thought to orient its planar ligand 2,2‘-bipyridine perpendicular to the water surface. The floating monolayer was vertically deposited onto a hydrophobic quartz substrate as an X-type film. The absorption spectra of the deposited films showed a broad peak at 480 nm, while that band was absent in the solution spectrum. The emission spectra of the films exhibited a broad band around 600 nm. The excitation spectra monitored at 600 nm coincided well with the absorption spectra. The results were interpreted in terms of the formation of the molecular aggregate, in which the platinum complexes were stacked with the overlaying bipyridine moieties. In such an aggregate, the MMLCT state was generated due to the Pt−Pt interaction among the neighboring molecules. On the basis of the model, the absorption band at 480 nm and the emission band at 600 nm were assigned to the 1MMLCT and 3MMLCT transitions from or to the dσ* orbital of Pt(II) to or from the π* orbital of 2,2‘-bipyridine, respectively. The AFM observation showed that the film deposited on a silicon wafer consisted of a grain-shaped aggregate, assisting the intermolecular interactions of composite molecules.