Synthesis of novel tert-butyl substituted fluorans and an investigation of their thermochromic behavior

• Six novel fluorans (FL1-FL6) were synthesized to analyze the substituent effect.
• Fluorans were used as thermal indicator in solid state.
• Thermochromic tape has been used in real situation and repeatedly used.

In the present study, six tert-butyl substituted fluorans have been synthesized, and their reversible thermochromic properties have been investigated. This work is the first direct demonstration of the role of the substituent on the lactone concerning the thermochromic reversibility of the fluoran. The electron donating substituents induced greater chromic effects than the electron withdrawing substituents. Furthermore, the three-component mixtures that contained fluorans, bisphenol-A, and methyl stearate or 1-dodecanol were used to examine the thermochromic behavior of the fluorans as bulk samples with steady heating and cooling process. The thermochromic properties of the fluorans were evaluated using solid-state UV–Vis, reflectance and FT-IR spectroscopic techniques. DFT calculations were conducted to rationalize the optical behavior of the fluoran in spirolactam form and cationic form. Finally, one of the synthesized fluoran has been successfully converted into the form of a test paper similar to pH paper and acrylic fiber tape for use as thermal indicators.

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