Inclusion crystals as vapochromic chemosensors: Fabrication of a mini-sensor array for discrimination of small aromatic molecules based on side-chain engineering of naphthalenediimide derivatives

Vapochromism, chromism induced by vapor desorption/absorption, has received considerable attention in recent years due to its potential applications in chemical sensors for toxic gases and vapors. In this paper, a set of naphthalene diimide (NDI) derivatives with various bulky groups is investigated for chromogenic and fluorogenic detection of small aromatic molecules in the vapor phase. Among the studied compounds, NDI with a 2-benzophenone unit in its structure exhibited a unique vapochromic/vapoluminescent behavior toward toluene, p-xylene, 4-fluorotoluene, and anisole. Detection limits in the parts-per-million (ppm) range are accomplished with some compounds. Optical measurements and X-ray crystal structure analyses clearly suggested that intermolecular charge-transfer interactions between the electron-deficient NDI and the electron-rich aromatic guest molecules as well as the inherent porosity of the sensor materials contribute to their unique sensing abilities. Discrimination of xylene isomers was demonstrated by taking advantage of selective inclusion phenomena. Two binary sensor ensembles, including our previously reported NDI derivative, enable the construction of a mini-sensor array that can be used to discriminate benzene derivatives - including xylene isomers - with the naked eye. The present simple, low-cost, metal-free, and effective sensor materials may promote practical applications with advantages relative to other known sensor materials.