C-Term Faraday Rotation in Metallocene Containing Thin Films

Abstract

The Faraday effect is a magneto-optical (MO) phenomenon that causes the plane of linearly polarized light to rotate when passing through a medium subjected to a parallel magnetic field. Informed by the established quantum mechanical model developed by Buckingham and Stephens, we sought to identify molecules that would exhibit large MO responses. Magnetic circular dichroism studies of ferrocenium in the 1970s revealed its potential as an MO material; however, it has not been evaluated in the context of Faraday rotation and thin-film optical applications. Herein, we report near-infrared (NIR) Faraday rotation in thin films of decamethylferrocenium/poly­(methyl methacrylate) composites with maximum Verdet constants of −3.45 × 104 deg T–1 m–1 at 810 nm (absorbance = 0.09) and −1.44 × 104 deg T–1 m–1 at 870 nm (absorbance = 0.01). These polymer–metallocene thin films deliver larger Verdet constants than commercially used NIR inorganic Faraday rotators and are facile and inexpensive to produce. The temperature dependence and distinct lineshape of the MO responses observed in decamethylferrocenium radical cations, decamethylmanganocene, and chromocene are in accordance with the quantum mechanical model. The observation of a strong C-term Faraday rotation in solid-state organometallic materials provides the groundwork for the development of high-performance metallocene-based Faraday rotators.

ABCs of Faraday Rotation in Organic Materials

“…(b) Faraday rotation (top) and absorbance spectrum (bottom). Adapted with permission from ref . Copyright 2021 American Chemical Society.…”

“…Examples of the Faraday C Term. Significant Faraday rotation has been attributed to the Faraday C term in the solid state for two classes of materials: decamethylferrocenium cation 35 and terbium(III) complexes, 41,42 both in matrices of poly(methyl methacrylate) (PMMA). The Faraday C term in both complexes is a result of orbitally degenerate electronic ground states originating from incompletely filled degenerate HOMOs.…”

“…Unlike other proposed mechanisms of large Faraday rotation, the Faraday A, B, and C terms are well described theoretically and experimentally in small molecules, having been originally developed in the 1930s. 45 In this Perspective, we suggest and discuss general molecular design principles for maximizing Faraday rotation in the subset of organic thin films for which A term −33 (800) poly(arylene ethynylene) 25 π-conjugation 25.0 (633) C 3 -symmetric tolane 26 liquid crystallinity 25 (520) porphyrin 1 A term −24 (530) tolane 27,28 liquid crystallinity −15 (500) triphenylene 29 liquid crystallinity 15 (440) Fe 3 O 4 nanoparticle/PMMA 30 magnetism 11 (980) poly(3-hexyl thiophene) 31,32 π-conjugation 8.8 (532) chiral polythiophene 33 π-conjugation/chirality 7.7 (532) FePt nanoparticle/polymer 34 magnetism −6 (845) ferrocenium/PMMA 35 C term −3.5 (810) poly(fluorene-alt-thiophene) 36 π-conjugation/magnetism −2.…”

High Verdet Constant Materials for Magneto-Optical Faraday Rotation: A Review

“…Although these reports demonstrated the ability to prepare high Verdet constant materials using conjugated polymers, one drawback of this approach was the multistep synthetic methods required for these materials which complicates production of these materials on multigram quantities. One alternative approach to alleviate this issue was also developed by the Swager group to achieve high Verdet constant materials ( V ∼ 3.45 × 10 4 °/T·m at 532 nm) by blending paramagnetic decamethyferrocenium derivatives with optically transparent polymers, in this case, poly­(methyl methacrylate) . Although the physics of these organic MO materials are still being explored, it is clear that further development of conjugated polymers is a promising approach toward high Verdet constants (10 4 –10 5 °/T·m; Figure e).…”

“…One alternative approach to alleviate this issue was also developed by the Swager group to achieve high Verdet constant materials (V ∼ 3.45 × 10 4 °/T•m at 532 nm) by blending paramagnetic decamethyferrocenium derivatives with optically transparent polymers, in this case, poly(methyl methacrylate). 131 Although the physics of these organic MO materials are still being explored, it is clear that further development of conjugated polymers is a promising approach toward high Verdet constants (10 4 −10 5 °/T•m; Figure 5e). One potential drawback with these materials is that their high Verdet constants are often reported at wavelengths that are on-resonance with strong absorption bands, limiting their transmission.…”