Using Polarized Spectroscopy to Investigate Order in Thin-Films of Ionic Self-Assembled Materials Based on Azo-Dyes

Three series of ionic self-assembled materials based on anionic azo-dyes and cationic benzalkonium surfactants were synthesized and thin films were prepared by spin-casting. These thin films appear isotropic when investigated with polarized optical microscopy, although they are highly anisotropic. Here, three series of homologous materials were studied to rationalize this observation. Investigating thin films of ordered molecular materials relies to a large extent on advanced experimental methods and large research infrastructure. A statement that in particular is true for thin films with nanoscopic order, where X-ray reflectometry, X-ray and neutron scattering, electron microscopy and atom force microscopy (AFM) has to be used to elucidate film morphology and the underlying molecular structure. Here, the thin films were investigated using AFM, optical microscopy and polarized absorption spectroscopy. It was shown that by using numerical method for treating the polarized absorption spectroscopy data, the molecular structure can be elucidated. Further, it was shown that polarized optical spectroscopy is a general tool that allows determination of the molecular order in thin films. Finally, it was found that full control of thermal history and rigorous control of the ionic self-assembly conditions are required to reproducibly make these materials of high nanoscopic order. Similarly, the conditions for spin-casting are shown to be determining for the overall thin film morphology, while molecular order is maintained.


Data Analysis
Scripts for the programs used for data analysis are supplied as part of the supporting information. Three scripts are supplied.
The data is loaded as x: wavelength in nm; y: absorbance from e.g. CSV files in the first two programs in order to determine and (see main text). The last program plots the orientation triangles used in the main text Figure 4b and   Figure S7. Height distribution taken from a local area in the image given in Figure S6.                               Figure 4 in the main text shows the data and the result from the modelling of the experimental data for Allura Red in stretched polymers. Here, the angle is well defined at 48°, while the angle can vary from ≈30° to ≈60° at the emission maximum, depending on the assumptions made regarding the overall shape of the dye molecule. The assumptions regarding the molecular shape correspond to assuming that the molecules at perfect alignment can be located at a specific point of the orientation triangle. The relation between , the assumed shape of the molecule, and the orientation factor is plotted in Figure 4 in the main text and in Figure S79 below, using equation 5. If the molecule is considered to be rod-like it will follow the top line of the orientation triangle, while a flat-like molecule will be at the bottom edge of the orientation triangle. The numerical analysis allows us to determine the possible values can take as a function of the position in the orientation triangle.

Polarised Spectroscopy Analysis
To analyse the data an assumption of the degree of alignment must be made. With intermediate limiting alignment (region R2) the angle between the transition moment and the stretch direction must be within the range 28°≤ ≤68°. If the limiting alignment with the stretch direction is better (R1) we can predict that the angle must be in the region 43°≤ ≤53, and if the molecule can be perfectly aligned we can determine that will be 48°. These considerations are compiled in Table S1 for all four dyes, while figure S79 shows the orientation triangles and corresponding vales for Allura Red and Trypan Blue. Note that the gray areas in figure S79 are excluded due to the experimentally determined values of kf.
For the purpose of describing the direction of the transition moment in the molecular structure the data is not perfect. For the box-like molecules Allura Red, Bordeaux Red, and Amaranth we can estimate a ≈ 10° range where must lie within when we assume an alignment of 0.7≤kz≤1 (region R1, Figure S79). For the rod-shaped Trypan Blue a higher spread of 33° is found for that range of kz. A much narrower 4 degree range is found if we assume a higher degree of alignment 0.9≤kz≤1 (region R1, Figure S79) than for the other molecules, due to the well-defined shape of the molecule. For the purpose of determining the molecular structure of thin films these results clearly show the orientation of the transition moments (Mf) with respect to the molecular long axis (z). The detailed data is compiled in Table S1.
Trypan Blue provides a clear example. In stretched PVA the rod-like molecule is oriented with z and Z close to the same axis i.e. kz is close to 1. In the thin films, the Z axis is perpendicular to the lamellar, such that Trypan Blue is oriented with the long axis (z) roughly perpendicular to Z. The data shows that ,PVA = 33° while ,thin film = 80° , which assuming ideal alignment ,PVA ≈ 0° ,PVA ≈ 33° corresponds to ,thin film at either ≈53° or ≈113°. From a fundamental standpoint a large different, but as can be seen in Figure 4, the molecular structure in the two materials that would give rise to the different angles is small. As for Trypan Blue ,PVA =33° is the maximum ,PVA angle, when considering ,PVA ≠ 0° we can provide the boundary 53°≤ ,thin ≤113°.  .  Table S1. Summary of the results from the numerical analysis of polarized spectroscopy data obtained on azo-dyes in stretched PVA. The results are presented as a function of the orientation factor (kz) and the assumed shape of the molecular structure. The insert show the orientation of the transition moment (Mf) in the molecular structure of Allura Red, Bordeaux Red, Amaranth, and Trypan Blue in the case of perfect alignment ( = ) represented with black arrows. And in case of intermediate alignment assuming that the molecular shape is rod-like (blue arrows) or flat (red arrows).        Figure S86. (a) Transition dipole angle respect to the uniaxial axes Z against kf; (b) Transition dipole angle respect to the uniaxial axes Z against the dichroic ratio df.