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.