and mixtures thereof, wherein R1 to R19 are independently selected from the group comprised of a hydrogen atom, an alkyl such as having from 1 to about 20 carbon atoms, a cyclic alkyl such as having from 4 to about 20 carbon atoms, an alkoxyl group such as having from 1 to about 20 carbon atoms, and halogen, and subscripts a to p each independently represents an integer of 1 or 2.