Modulation of Weak Interactions in Structural Isomers: Positional Isomeric Effects on Crystal Packing and Physical Properties and Solid-State Thin-Film Fabrication

Selective formation of positional isomers and accordingly tuning the physicochemical properties of small conjugated organic molecules through structural isomers is an effective crystal engineering for a fascinating successful delivery of thermally stable and photophysically exciting compounds. By small structural skeleton changes, the single crystal of the naphthalenemaleonitrile isomers is found to exhibit a drastic change in crystal packing array, which in turn is found to tune the thermal and physicochemical properties. The α-isomer (A) forms the “herringbone packing” (HP) due to peri-interaction-sensitive C–H···(Ar)π (Ar = naphthalene ring) interactions, and the β-isomer (B) forms the “bricklayer packing” (BP) due to π(C≡N)···π(Ar) stacking interactions. These two positional isomers have revealed insight of molecular packing-dependent structure–property relationship. In this report, we show that a simple modification of relatively less common weak interactions, such as C–H···π(Ar) ↔ π(C≡N)···π(Ar), through the preparation of isomers, can lead to a drastic change in crystal packing (HP ↔ BP). Also, this report demonstrates that by a small structural diversity, one can obtain significant changes in the physicochemical properties like melting behavior, enthalpy, entropy, and electrical properties in the solid state. Therefore, it transpires from this study that structural isomer provides a useful complement to intermolecular nonbonding interactions as a tool to design new promising materials.