" R1, R2 R3, R4, R5, and R6 each independently represents an alkyl group, which may be substituted, and as the substituent thereof, there are various functional groups, for example, a halogen atom such as chlorine, bromine, etc.; an alkyl group such as methyl, ethyl, t-butyl, etc.; an alkoxy group such as methoxy, ethoxy, etc.; a substituted or unsubstituted aryl group such as phenyl, naphthyl, etc" . . . . .