common thermochromic systems studied are leuco dyes and pigments, liquid crystalline polymers, and transition metal inorganic complexes. [2,15,[16][17][18][19][20][21][22][23] However, finished thermochromic materials commonly consist of or are incorporated into media that are or contain toxic, hazardous, or nondegradable components. [8,15,24] This limits their usage in applications that can pose the risk of cytotoxicity to the human body, such as food packaging or body temperature sensing devices for infants. Further, such hazardous and nondegradable materials exhibit high levels of persistence in soil as well as water systems and can pollute and disrupt these critical ecological systems. [24][25][26] "Green" thermochromic materials is a new and emerging field of research. Breakthroughs in this field will seek to replace toxic or inorganic materials which dominate the current thermochromic literature and applications with newly developed greener ones. Being a field in its infancy, there has yet to be a body of work that accumulates the current state of research in this field. This brief review seeks to explore and identify the constituent requirements for green materials as well as equip researchers to design thermochromic systems utilizing green chemistry. The history of thermochromism is reviewed in this paper. The mechanisms of thermochromism in common solids, liquids, and mesophases are considered as well as how the mechanistic pathway translates into their current applications. A general distinction between reversible and irreversible thermochromism is also discussed. Recent advances in the emerging field of green thermochromic materials are discussed critically and future applications mentioned.

Initially, vanadium dioxide seems to be an ideal first-order phase transition case study due to its deceptively simple structure and composition, but upon closer inspection there are nuances to the driving mechanism of the metal-insulator transition (MIT) that are still unexplained. In this study, a local structure analysis across a bulk powder tungsten-substitution series is utilized to tease out the nuances of this first-order phase transition. A comparison of the average structure to the local structure using synchrotron x-ray diffraction and total scattering pair-distribution function methods, respectively, is discussed as well as comparison to bright field transmission electron microscopy imaging through a similar temperature-series as the local structure characterization. Extended x-ray absorption fine structure fitting of thin film data across the substitution-series is also presented and compared to bulk. Machine learning technique, non-negative matrix factorization, is applied to analyze the total scattering data. The bulk MIT is probed through magnetic susceptibility as well as differential scanning calorimetry. The findings indicate the local transition temperature ($$T_c$$ T c ) is less than the average $$T_c$$ T c supporting the Peierls-Mott MIT mechanism, and demonstrate that in bulk powder and thin-films, increasing tungsten-substitution instigates local V-oxidation through the phase pathway VO$$_2\, \rightarrow$$ 2 → V$$_6$$ 6 O$$_{13} \, \rightarrow$$ 13 → V$$_2$$ 2 O$$_5$$ 5 .