Important role of the position of a functional group in isomers for photophysical and antibacterial properties: a case study with naphthalenemaleonitrile positional isomers

Abstract

The naphthalenemaleonitrile positional isomers, (1N: 2-amino-3-(((E)-naphthalene-1-ylmethylene)amino)maleonitrile) and (2N: 2-amino-3-(((E)-naphthalene-2-ylmethylene)amino)maleonitrile), were synthesized and then studied comparatively. Their molecular configurations exhibit an extraordinary ability to affect photophysical properties such as aggregation induced emission (AIE) and aggregation caused quenching (ACQ) properties and an unexpected structural dependence of antibacterial activity in biological cells. The 2N (naphthalene and aminomaleonitrile moieties are planar to each other) shows interesting AIE photophysical behaviours, whereas 1N (the naphthalene and aminomaleonitrile moieties are twisted around each other) exhibits the usual ACQ. The answer to the apparent contradiction of the general conception that planar molecules show ACQ in aggregates is hidden in the packing of molecules and the interactions present therein. Furthermore, 1N shows antimicrobial activity in a biological cell, whereas 2N does not. Although, the presence of a particular functional group such as imine is reported to be a determinant factor for a molecule to show antimicrobial activity, as far as is known, functional group position-dependent biological activities in aminomaleonitrile group containing molecules are not yet reported. Therefore, this report demonstrates that just the presence is not enough; the position of the functional group, which affects the structure of the molecule, is equally important.