"R2 and R7 is an electron-donating group, preferably an alkyl group such as CH3; or all of R2, R3, R4, R5, R7, R8, R9, and R10 are H, and at least one of, preferably both of, R1 and R6 is an electron-donating group, preferably an alkyl group such as CH3." . . . .