Orientational changes of supported chiral 2,2′-dihydroxy-1,1′binaphthyl molecules

Well defined thin molecular films of 2,2′-dihydroxy-1,1′binaphthyl (binol) molecules at coverages between 5 × 1015 molecules per cm2 and 1017 molecules per cm2 on thin glass (BK7) substrates were investigated under ultra-high-vacuum (UHV) conditions. Second-Harmonic-Generation Optical-Rotatory-Dispersion measurements (SHG-ORD) were performed using a dedicated spectroscopic setup which allows for the determination of the rotation angle of the SH-signal of two enantiomers. Rotation angles of up to 38 degrees were measured. The chirality of the two enantiomers has been studied at 674 nm (337 nm resonance wavelength) in the transmission mode. Coverage dependent orientation evolution of binol molecular films was revealed by precise monitoring of the surface coverage while performing SHG-ORD experiments. We show that the molecules reach their final orientation at a surface coverage of 5 × 1016 molecules per cm2. From the obtained experimental data the ratio of chiral and achiral susceptibility components could be calculated and was observed to change with coverage.