referably aryl groups containing 6 to 14 carbon atoms, such as a phenyl group, a tolyl group and a naphthyl group. [0251] Examples of halogen atoms represented by R8 to R12 include a fluorine atom, a chlorine atom, a bromine atom and an iodine atom. [0252] The alkyl groups represented by Y3 to Y6 are preferably alkyl groups containing 1 to 30 carbon atoms.