For color developing dyes, various types of dye such as fluoran, phthalide, lactam, triphenylmethane, spiropyran and other types of leuco dyes can be used, but usable dyes are not limited to these only. Besides, these color developing dyes can be mixed in different combinations.
30.3 g of bis(p-hydroxyphenyl)sulfone, 18.5 g of isopropylbromide, and 22.2 g of potassium carbonate as acid-binding agent were added to 200 ml of dimethylformamide, then stirred for 4 hours at about 120° C. and dimethylformamide was distilled off under reduced pressure. When this was accomplished, chloroform was added to the residue to extract crude 4-hydroxy-4'-isopropoxydiphenylsulfone. Then this crude compound was refined by using a silica gel column chromatography and 15.7 g of 4-hydroxy-4'-isopropoxydiphenylsulfone was obtained in white microfine crystals with a melting point of 129.0°~131.0° C.
