From ACQ to AIE: The CN(π)-(π)Ar interaction driven structural and photophysical properties of aromatic ring conjugated novel diaminomaleonitrile derivatives

• A new series of aromatic ring conjugated diaminomaleonitrile derivatives have been synthesized, and their structural and photophysical studies have been reported here'. • A molecular level correlation between solid-state packing and emission responses is presented. • The determining role of weak interaction, namely CN(π)-(π)Ar interaction towards the fluorescence emission, is unique. This interaction guides whether the molecule will be in Herringbone packing (HP), or Brick-layered packing (BP), or Ladder-type packing (LP). The type of packing is found to dictate the photophysical response in solid state. • In addition, this weak interaction can be modulated through the change in aromatic ring substitution. This weak interaction is found to make an impact on aggregation-induced fluorescence. • It is shown that one can make an ACQ (aggregation cause quenching) molecule to demonstrate AIE (aggregation-induced emission) effect by suitably replacing the aromatic ring with better conjugation. Studying solid and solution state photoluminescence properties are exclusive to understanding Aggregation Caused Quenching (ACQ) and Aggregation Induced Emission (AIE) behaviour of organic molecules to explore the mechanism of molecular luminescence in greater detail. A unique hydrogen-bonded dimer with imperative interaction (CN)π---π(Ar) was designed and synthesized by selecting diaminomaleonitrile ( DMN ) as stator and decorated with different aromatic rotors (phenyl to pyrene). Depending on the intermolecular interaction present in the molecular system, (AIE) or (ACQ) has been observed and found to be tunable. The single-crystal x-ray diffraction analysis of these derivatives shows that AIE characteristics could be seen where to (CN)π---π(Ar) intermolecular interactions between the aromatic rotors and stator, coupled with NH–-–NC intermolecular hydrogen bonding is present, while the absence of these interaction reflected in ACQ observation. The significant impacts of steric, conjugation and electronic effects on the ACQ / AIE properties are presented based on crystallography analysis, optical spectra measurements and theoretical calculations.