Optical properties and photonic applications of molecular spin-crossover materials

We review the current knowledge on the optical properties of molecular spin-crossover (SCO) materials, which exhibit reversible switching between low-spin and high-spin electronic configurations, as well as their use for practical applications in the fields of optics and photonics. We focus on the physical origins and magnitude of the complex refractive index change (both real and imaginary parts) in SCO materials, including a brief excursion to less-studied lower frequency ranges (THz–Hz). Until now, the optical property changes have primarily been used as a means of detection to characterize the SCO phenomenon itself (particularly at the nanometric scale) or to develop optical sensors for monitoring variations in an external stimulus. However, more recently, optical signal processing and modulation have also been explored using SCO nanomaterials as active elements in tunable photonic devices. Herein, we review the current state of the art of such functional devices, showing that molecular SCO compounds are emerging as a promising class of phase-change materials with high potential for specific active photonic applications, particularly in the visible spectral range. Key results on light-induced spin transitions and the photoswitching dynamics of SCO materials are also briefly reviewed in the context of photonic devices. Finally, the results of recent studies reporting strong light-matter coupling phenomena between SCO molecules and confined electromagnetic fields in resonant optical cavities are also discussed, with, in sight, the fascinating perspectives of manipulating the molecular properties using light.