A method of preparing a compound of formula I is described. In this compound, each of R1-R6 is, independently, in which Y is O, S, NH, or C(R7)═C(R8). Each of R7-R11 is, independently, H, substituted or unsubstituted C1-6 alkyl, substituted or unsubstituted C2-6 alkenyl, substituted or unsubstituted C2-6 alkynyl, substituted or unsubstituted C6-20 aryl, or substituted or unsubstituted C4-20 heteroaryl. Alternatively, each of R7-R11 is OH, C1-6 alkoxy, or N(R12)(R13), in which each of R12 and R13 is, independently, H, substituted or unsubstituted C1-6 alkyl, substituted or unsubstituted C2-6 alkenyl, substituted or unsubstituted C2-6 alkynyl, or substituted or unsubstituted C6-20 aryl. Each of R7-R11 can also be NO2, CN, or CO2R14, in which R14 is H or C1-6 alkyl.