Chromotropism of imidazole derivatives. Part 1. 4,5-Bis-(4-methoxyphenyl)-2-(4-nitrophenyl)imidazolium acetate dihydrate

Abstract

4,5-Bis-(4-methoxyphenyl)-2-(4-nitrophenyl)imidazolium acetate dihydrate (1) in the crystalline state exhibits a sensitive colour change from orange-yellow to red on treatment by various methods (drying, trituration, or heating). The colour change was reversible below 70 °C but above 70 °C the change was irreversible and the colour deepened. Compound (1) was deprotonated with the loss of one mole of water of crystallization under treatment and can revert to the original by absorption of one mole of water. On heating to 100 °C, (1) lost all solvent of crystallization to give a deep red material (2). (2) Was considered to be 4,5-bis-(4-methoxyphenyl)-2-(4-nitrophenyl)imidazole which was linked by hydrogen bond from the imino group in one molecule to the nitro oxygen in another. Compound (2) reverted to (1) in saturated AcOH–H₂O vapour.