provided that in the formula (a1), each of X1 and X2 which are independent of each other, is a hydrogen atom, a chlorine atom or a fluorine atom, and X3 is a chlorine atom, a fluorine atom or ???CY1Y2Y3, wherein each of Y1, Y2 and Y3 which are independent of one another, is a hydrogen atom, a chlorine atom or a fluorine atom;