A dye appears colorless because its chromophore was reduced; the reversible form is called which term?

• A. Leuco compound
• B. Azo Dye
• C. Chromophore
• D. Fluorophore

Answer: (A)

The concept here is that dyes can exist in two redox forms: the colored chromophore form and a colorless reduced form. When the chromophore is reduced, its conjugation is disrupted, so the molecule no longer absorbs visible light and appears colorless. This colorless, reduced form is called the leuco compound. It is reversible because oxidation can restore the chromophore and bring back the color. The other terms don’t describe this specific reversible colorless form: an azo dye is a class of dyes containing an azo group; a chromophore is the color-bearing part of a dye; a fluorophore is the part that emits fluorescence.