Circular Dichroism and Isotropy – Polarity Reversal of Ellipticity in Molecular Films of 1, 1'‐Bi‐2‐Naphtol. Issue 1 (4th December 2018)

• Record Type: Journal Article
• Authors: von Weber, Alexander; Hooper, David C.; Jakob, Matthias; Valev, Ventsislav K.; Kartouzian, Aras; Heiz, Ueli
• Is Part Of: Chemphyschem. Volume 20:Issue 1(2019)
• Page Start: 62
• Page End: 69
• Publication Date: 2018-12-04
• Subjects: circular dichroism -- ellipticity -- films -- isotropy -- second-harmonic generation; Chemistry, Physical and theoretical -- Periodicals; 541.05
• DOI: 10.1002/cphc.201800950
• Languages: English
• ISSNs: 1439-4235

Abstract

Abstract: We have studied the circular dichroism (CD), in the ultraviolet and visible regions, of the transparent, chiral molecule 1, 1'‐Bi‐2‐naphtol (BINOL) in 1.5 μm thick films. The initial transparent film shows an additional negative cotton effect in the CD compared to solution. With time under room temperature the film undergoes a structural phase transition. This goes hand in hand with a cotton effect at the low energy absorption band which inverts with opposite propagation direction of light through the film which is revealed as a polarity reversal of ellipticity (PRE). After completion of the phase transition the film exhibits circular differential scattering throughout the visible range which also shows PRE. The structure change was studied with Raman, microscopy under cross polarization conditions and nonlinear second‐harmonic generation circular dichroism (SHG‐CD). The superposition of the optical activity of individual molecules and isotropy effects makes an interpretation challenging. Yet overcoming this challenge by finding a suitable model structural information can be derived from CD measurements. Abstract : Isotropy vs. CD : Circular dichroism (CD) and second‐harmonic generation CD are powerful tools to study the chirality at interfaces and surfaces, for example, the chiroptical response of asymmetric heterogeneous catalysts. However in the solid state, additional effects related to isotropy and scattering could affect the observed spectra. The authors connect such effects with crystallization and domain formation in a molecular film and observe a polarity reversal of ellipticity.