Stereochemical effects on monolayer formation of [Ru(dpp)(3)](2+) (dpp=4,7-diphenyl-1,10-phenanthroline) at an air-water interface

Stereochemical effects on monolayer formations have been studied at an air-water interface for the racemic mixture and enantiomer of [Ru(dpp)(3)](ClO4)(2) (dpp = 4.7-diphenyl-1,10-phenanthroline). Stearic acid (SA) was added to reduce the cohesive interaction of the Ru(II) complexes. The ratio of SA to Ru(II) complex, [SA]/[Ru], was varied from 0 to 4. At [SA]/[Ru] = 0 and 1, the ''lift-off'' area per molecule at which the surface pressure increased from zero was smaller for the racemic mixture than for the enantiomer. The results indicated that the racemic mixture of the Ru(II) complex formed a more compact monolayer than the enantiomer. There was no difference observed in the surface pressure-molecular area (pi-A) curves at [SA]/[Ru] larger than 3. The monolayer was transferred onto a hydrophobic glass or silicon wafer substrate as a Langmuir-Blodgett (LB) film of Y-type. At [SA]/[Ru(II)] = 1, the d(001) spacing of the film was determined by X-ray diffraction measurements, leading to d(001) = 50.4 and 37.8 Angstrom for the racemic mixture and the enantiomer, respectively. The IR reflection-absorption spectroscopy showed that the alkyl chains of SA molecules took different orientations for the racemic and enantiomeric films. It is postulated that SA molecules which intervened between the Ru(II) complexes are more perpendicular to the substrate surface due to the closer packing of the racemic Ru(II) complexes than the enantiomers.