In the formula, R1 and R2 may be the same as or different from each other and are a hydroxyl group, an alkyl group, an alkoxy group, --NR1 R2 group (R1 and R2 are an alkyl group or an aryl group), --NR1 COR2 (R1 and R2 are an alkyl group or an aryl group), --COR1 group (R1 is an alkyl group or an aryl group), --OCOR1 group (R1 is an alkyl group or an aryl group), an aryl group, --ONR1 R2 group (R1