Crystal engineering approach to design colorimetric indicator array to discriminate positional isomers of aromatic organic molecules.

Discriminating by color: A 2D colorimetric indicator array has been designed by a crystal engineering approach (NH-O hydrogen bonding and charge-transfer complexation) involving solid-state co-grinding for the visual discrimination of positional isomers of dihydroxynaphthalene. Factors governing the close packing of pi planes, and hence color of the complex, were determined by single-crystal X-ray analysis, allowing fine tuning by crystal engineering. A 2D colorimetric indicator array to discriminate isomers of dihydroxynaphthalene has been designed by using a crystal engineering concept combined with solid-state co-grinding and charge-transfer complexation. The NH-O supramolecular synthon between the pyridyl N atom of the probe and hydroxy group of the analyte has been observed in all reported single-crystal X-ray structures of resultant CT complexes. Solid-state co-grinding with aromatic solids easily provided the highly concentrated solvent-free conditions required to promote the formation of charge-transfer complexes with brighter color changes. Probe D (N,N'-bis-(4-pyridyl)pyromellitic diimide) showed an excellent ability to discriminate eight isomers of dihydroxynaphthalene with a wide variety of colors of the resultant CT complexes.