Transparent and colourless room temperature ionic liquids having high refractive index over 1.60

Transparent and colourless ionic liquids with refractive index over 1.60 were synthesised by combining 1-benzyl-3-methylimidazolium or hexyltriphenylphosphonium cations with suitable anions. There is a positive relation between their refractive index and Kamlet-Taft parameters, especially dipolarity/polarisability, suggested as a potential parameter to design ionic liquids with high refractive index.