Drastic Fluorescence Enhancement of Metal-Organic Framework for Ultra-Efficient Detection of Trace Benzene Vapor

Abstract Indoor detection of volatile organic compound (VOC, such as benzene and toluene vapors) concentration is necessary due to the serious toxicity hazards even at trace level. However, physisorbents usually exhibit weak interactions between sorbate and sorbent, thus exhibiting poor responsive signal especially in the presence of trace concentrations of VOCs. Herein, we report a new flexible metal-organic framework [Sr2(BINDI)(DMF)(H2O)]∙G (H4BINDI = N,N′-bis(5-isophthalic acid)naphthalenediimide, DMF = N,N-dimethylformamide, G = 0.6DMF) that exhibits interesting pore-opening behavior after immersing in H2O, hence allowing aromatic hydrocarbon molecules to diffuse into the pores. Interestingly, the pore-opening phase [Sr2(BINDI)(H2O)2] shows significant (∼116 fold) and extremely fast (< 1 minute) fluorescence "turn-on" behavior after exposed to saturated benzene vapor. The limit of detection concentration for benzene vapor can be calculated as 77 ppb. Thus this material represents the first MOF as fluorescent sensor achieving visual detection of trace benzene vapor by naked eyes. Theoretical calculations and single-crystal structure of [Sr2(BINDI)(H2O)2]⊃benzene reveal that the special "bilateral π-π stacking" interactions between electron-rich aromatic hydrocarbon and electron-deficient NDI cores facilitate electron transfer, and greatly enhance the intensity of fluorescence. Furthermore, this material can be fabricated into simple test strips, which enable visual detection of trace benzene vapor (3 ppm) under UV light.