wherein R1 and R2 can be the same or different; when R1 and R2 are the same, each is a substituted or unsubstituted arylamino, cycloalkylamino, pyridineamino, piperidino, 9-purine-6-amine or thiazoleamino group; when R1 and R2 are different R1=R3???N???R4, wherein each of R3 and R4 are independently the same as or different from each other and are a hydrogen atom, a hydroxyl group, a substituted o