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Area of Science:

  • Coordination Chemistry
  • Materials Science
  • Chemical Sensing

Background:

  • Spin Crossover (SCO) involves reversible switching between high-spin (HS) and low-spin (LS) states in transition metal complexes, triggered by external stimuli.
  • SCO phenomena can induce observable outputs, such as color changes, offering potential for visual detection of analytes.
  • Vapochromism, a color change in response to volatile analytes, is a promising low-cost sensing technology for environmental and workplace monitoring.

Purpose of the Study:

  • To review iron coordination compounds exhibiting color change coupled with HS ↔ LS spin transition upon exposure to gaseous analytes.
  • To categorize the post-synthetic modification (PSM) mechanisms occurring within these materials upon analyte uptake.
  • To provide a comprehensive compilation of room-temperature SCO systems demonstrating vapochromism linked to spin state transitions.

Main Methods:

  • Literature review and compilation of existing research on iron-based spin crossover coordination compounds.
  • Analysis of reported instances where gas analyte interaction induces a color change linked to a spin transition.
  • Classification of post-synthetic modification (PSM) types: supramolecular contacts, coordinative bonds, and covalent bonds.

Main Results:

  • Identified and categorized iron coordination compounds that display a color change due to SCO triggered by volatile analytes.
  • Detailed the three primary types of post-synthetic modification (PSM) occurring upon analyte adsorption: supramolecular, coordinative, and covalent bonding.
  • Highlighted that covalent bond formation during PSM results in a new compound with altered properties, alongside the spin state switch and color change.

Conclusions:

  • Iron coordination compounds exhibiting vapochromism linked to room-temperature spin crossover offer a direct visual detection method for volatile compounds.
  • Understanding the PSM mechanisms is crucial for designing effective SCO-based sensors.
  • This review consolidates systems where color change is directly associated with a spin transition upon analyte interaction, distinct from previous reviews focusing on thermal SCO or non-spin-transition-related color changes.