Monolayer formation and aggregation of nickel(II) complexes coordinated with salen substituted by non-linear alkyl side chains

In order to investigate the effect of α-methylene groups of alkyl-substituted [Ni(salen)] on the aggregating nature, the monolayers of nickel(II) complexes coordinated with N,N′-disalicylideneethylenediamine (salen) substituted by propyl, 1-methyl-propyl and 1,1-dimethylpropyl side chains have been prepared at a water–air interface using the Langmuir–Blodgett (LB) method. The π-A isotherm measurement revealed the formation of two different types of monolayers, condensed phase and expanded phase. When the hydrogen atoms of α-methylene groups were totally substituted by the methyl groups, the transition pressure of the expanded phase to the condensed phase decreased considerably from 5.4 to 1.0 mN m−1, and the critical area of the expanded phase increased by ca. 20%. This observation suggests the presence of CH-π interaction between the α-methylene group and the π-conjugated system in the monolayer state as has been found in the crystalline state.