A compound having the formula: wherein X is S, SO or SO2; one of R1, R2, and R3 is O and the others of R1, R2 and R3 are independently, the same or different, CH2, or CR13 wherein, R13 is an alkyl group, an alkenyl group, an alkynyl group, a trialkylsilyl group, or —(CH2)mOR15, wherein R15 is an alkyl group or an aryl group and m is an integer in the range of 1 to 10, and one of R5, R6, and R7 is O and the others of R5, R6 and R7 are independently, the same or different, CH2, or CR14 wherein, R14 is an alkyl group, an alkenyl group, an alkynyl group, a trialkylsilyl group, or —(CH2)nOR16, wherein R16 is an alkyl group or an aryl group and n is an integer in the range of 1 to 10; R4 and R8 are independently, the same or different, H, an alkyl group, an alkenyl group, an alkynyl group, an aryl group, a heteroaryl group, a C1-C3 alkoxy group, an aryloxy group, or —(CH2)qOR17, wherein R17 is an alkyl group or an aryl group and q is an integer in the range of 1 to 10, provided that R4 is not a C1-C3 alkoxy group or an aryloxy group when R1 or R3 is O and R8 is not a C1-C3 alkoxy group or an aryloxy group when R5 or R7 is O; R9, R10, R11 and R12 are independently, the same or different, H, an alkyl group, an alkenyl group, an alkynyl group, or an aryl group.