Faraday rotation in conjugated organic molecules

Faraday rotation, which is the rotation of the plane of polarization of light under the influence of a longitudinal magnetic field, was discovered almost two centuries ago. Nowadays, mostly inorganic paramagnetic materials are used in applications based on Faraday rotation. However, organic diamagnetic materials provide many advantages over their inorganic counterparts, such as a smaller temperature dependency of their Verdet constant, magnetic saturation at higher magnetic fields and more flexibility in synthesis and processing. In this dissertation, we report and study several organic diamagnetic materials with exceptionally high Faraday rotation responses. Furthermore, we have explored several parameters that determine the magnitude of Faraday rotation in these materials.

We started our search for efficient Faraday materials by studying a series of conjugated, rod–like molecules in the crystalline and liquid state. We found extremely high Faraday responses but it was not entirely clear where the large magneto-optic response of these molecules originates from. We hypothesized that the supramolecular ordering of the molecules in their crystalline state has a big influence on the magneto-optic response. This was corroborated by the fact that in the liquid state we did not observe large Faraday rotation. To what extent the molecular structure influences the Faraday response is still an open question. We learned that conjugation is crucial to obtain a high response, but other factors like symmetry and the role of substituents are very difficult to study since they also influence the supramolecular stacking in the solid phase.

Since our first results indicated that supramolecular organization might be a key factor, we focused our efforts on liquid crystals. These are materials that provide us with two or more organized phases (the liquid crystalline phase(s) and the crystalline state) that can be studied and could give us more information about the influence of the supramolecular organisation on the observed Faraday rotation. Indeed, we found that in discotic liquid crystals, the method of aligning the molecules in their liquid crystalline state has a dramatic influence on their magneto-optic properties. Upon cooling from the isotropic (liquid) state to the liquid crystalline state, the Faraday response in the liquid crystalline state can change by orders of magnitude by straightforwardly optimizing the cooling rate. Furthermore, the Faraday response in the crystalline state was also highly dependent on the cooling rate. Since cooling rate is known to have an influence on the alignment of the material, both in the liquid crystalline and crystalline state, we concluded that supramolecular organization is indeed a key factor.

We also studied a material that was liquid crystalline at room temperature. The material is in a discotic columnar phase and as a consequence it is highly anisotropic in its liquid crystalline phase. We found that the Faraday response is also highly anisotropic. Large Faraday rotation was only observed when the magnetic field was perpendicular to the axis of the liquid crystalline columns and the electric field of the light had a component along the column axis. Because the electric field of the light drives the electrons in the structure and determines the optical response, our hypothesis is that we need long range electron movement through the columns to obtain a high Faraday response. Again, this confirms that a good alignment of the molecules in the material is needed to obtain a high Faraday response.

In summary, we were able to identify a number of organic, diamagnetic materials with very high Verdet constants. Our main conclusion is that supramolecular organization is the most important factor known to date that determines the magnitude of the Faraday response and that by optimizing this organization we can design materials with extremely high magneto-optic responses. Especially discotic liquid crystals promise to be an important class for future research on Faraday rotation.