ABCs of Faraday Rotation in Organic Materials.

Faraday rotation is a magneto-optical effect central to a number of commercial technologies including optical isolation and magneto-optical imaging. Today, the performance needs of these technologies are met by inorganic materials containing paramagnetic heavy elements. However, organic thin films are increasingly being evaluated as replacement materials, promising higher magneto-optical performance and facile fabrication of structures that enable expanded applications. Despite being an object of research for more than 175 years, our understanding of the Faraday effect in solid-state organic materials remains incomplete, hindering our attempts to methodically improve magneto-optical performance. This Perspective aims to place several recent advances in the field of thin-film organic Faraday rotators within the well-established theoretical framework developed by solution-state magnetic circular dichroism spectroscopists: the Faraday A, B, and C terms. Through careful consideration of these quantum mechanical mechanisms in example molecules, an intuitive understanding of the impact of chemical structure in thin-film Faraday rotators can be achieved, including the critical roles of molecular symmetry, rigidity, absorptivity, and magnetism. Future work seeking to maximize the magneto-optical performance of organic thin films may more readily evaluate candidate chromophores based on the Faraday A, B, and C term framework presented herein.