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

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 N⋯H-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.