Fluoran

Fluoran refers to a class of synthetic, colorless organic compounds that become intensely colored upon contact with acidic media or under pressure. They are commonly used as leuco dyes in carbonless copy paper, thermal paper, and other applications where a color change is desired in response to a specific trigger.

Fluorans are typically derivatives of xanthene, a heterocyclic organic compound. The core structure is modified with various substituents to fine-tune the compound's properties, such as its sensitivity to acid or pressure, its color when activated, and its stability.

The mechanism behind the color change involves the opening of a lactone ring within the fluoran molecule. This ring-opening is triggered by protonation in acidic conditions or by mechanical stress. The resulting change in the molecule's structure alters its electronic properties, causing it to absorb light in the visible spectrum and thus appear colored. The intensity of the color is directly proportional to the amount of fluoran converted to its colored form.

Commonly used fluorans include 2-amino-6-diethylamino-3-methylfluoran (ODB-2), 2'-anilino-6'-(diethylamino)-3'-methylfluoran (ODB), and 3-(N-ethyl-N-isopentylamino)-6-methyl-7-anilinofluoran. These compounds exhibit a variety of colors upon activation, ranging from blue and green to red and black, depending on their specific structure and substituents.

The use of fluorans is subject to regulatory scrutiny due to concerns regarding their potential toxicity and environmental impact. Some fluorans have been shown to exhibit endocrine-disrupting properties, leading to restrictions on their use in certain applications, particularly those involving direct contact with skin or food. Research is ongoing to develop safer and more environmentally friendly alternatives to fluoran-based leuco dyes.