. . . . "ing or aromatic hetero ring which may have a substituent, each of R1, R2, R3, R4 and R5 is a hydrogen atom, or an alkyl, aralkyl, aromatic hydrocarbon or heterocyclic group which may have a substituent, each of R6 and R7 is an alkyl, aralkyl, aryl, aromatic hydrocarbon or heterocyclic group which may have a substituent, l is an integer of 0 or 1, m is an integer of 0, 1 or 2, and n is an integer o" . . .