Linker-Induced Structural Diversity and Photophysical Property of MOFs for Selective and Sensitive Detection of Nitroaromatics

The linker [1,1':3',1″-terphenyl]-4,4',4″,6'-tetracarboxylic acid (H4L) was used to construct two three-dimensional (3D) metal-organic frameworks (MOFs), namely, {[Cd2(L)(L1)(DMF)(H2O)](2DMF)(3H2O)}n (1) and {[Cd4(L)2(L2)3(H2O)2](8DMF)(8H2O)}n (2) (DMF = N,N'-dimethylformamide) in the presence of colinkers 4,4'-bipyridine (L1) and 2-amino-4,4'-bipyridine (L2), respectively, under solvothermal condition. A small change in the colinker leads to significant differences in the overall structure of the MOFs. Topological analysis reveals that the framework 1 exhibits 6,4-connected forbidden sub-configuration (FSC) topology, while the framework 2 exhibits twofold interpenetrated and (3,4,4)-connected new network topology with Schläfli point symbol {4.6(2)}{4.6(4).8}{4(2).6(2).8(2)}. The crystallographic investigation reveals the framework 2 having single helix structure, which is further coiled through noncovalent interaction, afforded a double-helix structure similar to DNA. These double helices are further connected through the colinker L2 to form an overall 3D structure. Besides framework 2 exhibits remarkable fluorescence intensity compared to 1. Framework 2 displayed a strong emission at 457 nm when a sample of 2 was dispersed in ethanol and excited at 334 nm. This emission is selectively and completely quenched in the presence of 2,4,6-trinitrophenol (TNP) allowing its detection in the presence of other nitroaromatic compounds. The quenching constant for TNP was found to be 3.89 × 10(4) M(-1), which is 26 times higher than that of TNT demonstrating greater and selective quenching ability. The emission is restored to its original value when the sample after collected by filtration is dispersed in fresh ethanol for 1 d. Interestingly, when solid 2 is exposed to different nitroaromatic compounds, its emission is quenched selectively in the presence of nitrobenzene. In this case, the emission is restored upon heating the sample to 150 °C for 2 h.