When each of R1 to R7 is an organic group, such an organic group is preferably a hydrocarbon group such as an alkyl group, an alkenyl group, a cycloalkyl group or an aryl group, a halogenated hydrocarbon group such as a chloroalkyl group or a polyfluoroalkyl group, a (halogenated)hydrocarbon group having a hydroxyl group, an epoxy group, an amino group, a mercapto group, a carboxy group or other f