Important role of the position of a functional group in isomers for Photophysical and Antibacterial Properties: A case study with Naphthalenemaleonitrile Positional Isomers

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 studied comparatively here. Their molecular configurations exhibit an extraordinary ability to affect photophysical properties like Aggregation Induced Emission (AIE) and Aggregation Caused Quenching (ACQ) properties and 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, while 1N ((naphthalene and aminomaleonitrile moieties are twisted to each other) exhibit usual ACQ. This answer for apparent contradiction of general conception that planar molecules shows ACQ in aggregates is hidden in the packing of molecules and interaction present therein. Furthermore, fascinatingly 1N shows antimicrobial activity in a biological cell, while 2N does not. Though, the presence of particular functional group such as imine is reported to determinant factor for a molecule to show antimicrobial activity, to the best of our knowledge, 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 condition; the position of the functional group, which affects the structure of the molecule is equally important.