ABCs of Faraday Rotation in Organic Materials

Abstract

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.

A Long-Lived Water-Soluble Phenazine Radical Cation

“…As UV–vis absorption reflects the electric dipole moment in the molecule, magnetic circular dichroism (MCD) measurement was used to investigate the magnetic dipole moment of the unpaired electron in PSPR . It showed obvious MCD response in the area corresponding to the absorption bands when a magnetic field of +1.0 or −1.0 T was applied, which is a sign of a Faraday effect that reveals degeneracy information of its electronic states, indicating the potential of PSPR to be used as a magneto-optical (MO) material (Figure f). − Measured by UV–vis spectroscopy, PSPR exhibited a high solubility of 1.4 M in water owing to the existence of two sulfonate groups (Figures S8 and S9), enabling it to be further characterized and studied in aqueous media. The stability of PSPR under aqueous conditions was monitored by UV–vis spectroscopy and time-dependent EPR.…”

C-Term Faraday Rotation in Low Symmetry tert-Butyl Substituted Polyferroceniums

“…The Faraday rotation, named after its famous discoverer, is caused by magnetic field-induced circular birefringence (MCD) and is a ubiquitous property of matter. Figure depicts the effect as it would appear in an experiment . Faraday or magneto-optical (MO) rotation θ is proportional to the Verdet constant V and thickness d of the medium and the magnetic field along the direction of light propagation B z .…”

“…Figure 1 depicts the effect as it would appear in an experiment. 1 Faraday or magneto-optical (MO) rotation θ is proportional to the Verdet constant V and thickness d of the medium and the magnetic field along the direction of light propagation B z . The Faraday effect is currently used in commercial technologies, mainly in optical isolation and modulation, 2−5 and MO imaging, 6−12 but new devices enabled by next generation materials will present new opportunities.…”

“…15 Molecules and polymers have been shown to exhibit Verdet constants that are at least 1 order of magnitude larger than inorganic crystals, with the highest values being close in energy to electronic transitions. 1 An intuitive understanding of the Faraday rotation in organic molecules can be gained from the modern interpretation of Serber's theory of magnetic circular birefringence. 16 This model describes three quantum mechanical mechanisms which could result in large Faraday rotation: the A-, B-, and C-terms.…”

“…16 This model describes three quantum mechanical mechanisms which could result in large Faraday rotation: the A-, B-, and C-terms. For further theoretical details, we would refer the readers to the Perspective by Nelson et al 1 All three terms operate through resonance-enhanced mechanisms that unfortunately often result in large optical absorption where Faraday rotation is at its maximum. This can be detrimental for many applications where optical output is paramount, such as optical isolation.…”