Nonlinear Optical and Structural Properties of Langmuir-Blodgett Films of Thiohelicenebisquinones

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• 作者：Mikael Siltanen ; Elina Vuorimaa ; Helge Lemmetyinen ; Petri Ihalainen ; Jouko Peltonen ; Martti Kauranen
• 刊名：Journal of Physical Chemistry B
• 出版年：2008
• 出版时间：February 21, 2008
• 年：2008
• 卷：112
• 期：7
• 页码：1940 - 1945
• 全文大小：236K
• 年卷期：v.112,no.7(February 21, 2008)
• ISSN：1520-5207

文摘

We provide a detailed investigation of the second-order nonlinear optical and structural properties of Langmuir-Blodgett (LB) films of nonracemic thiohelicenebisquinone (THBQ). We prepare both X- and Y-type films ofdifferent thicknesses and characterize them using optical second-harmonic generation and atomic-forcemicroscopy (AFM). We find that the overall nonlinear properties of the samples are essentially independentof the film thickness and the deposition type and arise from susceptibility tensor components associated withchirality. Both X- and Y-type films can be described by D₂ symmetry, which is a higher symmetry than thepreviously assumed C₂ of LB films of THBQ and a similar helicenebisquinone (HBQ). However, the twotypes of films are shown to differ significantly with respect to the orientation of the in-plane axis. For Y type,the axis follows the direction of vertical sample deposition, but for X type, the direction of the axis variesrandomly and significantly between different samples. The Y-type samples are therefore more ordered thanthe X-type samples. This was confirmed by AFM measurements in which the Y type exhibits uniform orderinginto columnar structures. Similar structures in X type, on the other hand, are shorter and more randomlyoriented, like those earlier observed for racemic samples of HBQ [Verbiest, T., et al. Science 1998, 282,913]. The common nonlinear properties and different high-level ordering observed here for two differenttypes of nonracemic samples reinforces that the nonlinearity of THBQ (and probably HBQ, as well) originatesfrom the low-level columnar aggregation of the molecules with the higher-level structures playing a lesserrole. In addition, within the columns, the molecules likely assume fairly random azimuthal orientations sothat the columns themselves exhibit approximate D symmetry.