Characterization of the angular orientation distribution of discotic molecules in thin-film assemblies: Combinations of polarized transmission and reflection - absorption infrared spectroscopies

We present a protocol to determine the angular orientation of surface-confined discotic molecules in ordered thin-film assemblies through a combination of polarized reflection absorption infrared spectroscopy (RAIRS) and polarized transmission infrared (IR) spectroscopy. We focus here on the determination of the orientation of side-chain-modified disk-like phthalocyanine molecules that self-organize to form parallel columnar aggregates in Langmuir - Blodgett (LB) films. This work complements and follows on work published previously (Langmuir 2005,21,360-368) where we used the visible absorbance dichroism in similar assemblies to determine the tilt angles of discotic molecules. The approach described here is applicable to all disk-like molecules that possess distinct in-plane or out-of-plane IR transitions. Molecular orientation is determined by comparison of absorbance intensities that are measured using (i) a RAIRS spectrum of the thin-film material on a gold surface, (ii) transmission spectra taken at two different polarizations of the thin-film material on an IR-transparent silicon substrate, and (iii) a transmission IR spectrum of an isotropic sample. We chose two distinct IR transitions of this molecule (a C-O-C "in-plane" stretch, υ_C-O-C, and a C-H out-of-plane bend, δ_C-H), which we initially assume are orthogonal to each other. The υ_C-O-C transition is modeled as an in-plane circular dipole, and the δ_C-H transition is modeled as a linear out-of-plane dipole. Formalisms are described that allow the determination of two independent values for order parameters within the thin film using the dichroism in these transitions in all three IR spectra, assuming that the microstructure of these thin films is the same on both the gold and silicon substrates. The spectra from the δ_C-H transition appear to best describe the orientation of these molecules within the molecular assembly, these IR results agreeing best with earlier visible absorbance and X-ray diffraction studies. The preferential alignment of the δ_C-H transition is parallel to the substrate, arising from Pc molecules that are nearly upright, edge-on to the substrate plane. The approach described here provides a means of complete description of the tilt angles of the monomer building blocks in any discotic assembly in thin-film formats, where distinct in-plane and out-of-plane optical transitions are available.