Two (5,5)-connected isomeric frameworks as highly selective and sensitive photoluminescent probes of nitroaromatics

Abstract

Herein we report the first two (5,5)-connected isomeric frameworks, namely, (Me₂NH₂)[Zn₂L(H₂O)]·3.5DMF (1) and (Me₂NH₂) [Zn₂L(H₂O)]·6DMF·4H₂O (2) (DMF = dimethylformamide, H₅L = 5,5′-(6-(4-carboxyphenylamino)-1,3,5-triazine-2,4-diyldiimino) diisophthalic acid), obtained via assembly reactions between Zn²⁺ and a semi-rigid pentacarboxylate ligand (L⁵⁻). Single-crystal X-ray diffraction analyses reveal that both compounds are three dimensional metal–organic frameworks (MOFs) built of the same {Zn₂(CO₂)₅} molecular building blocks (MBBs) and L⁵⁻ ligands but have different topologies (point symbols of (4⁴·6⁶) and (4⁶·6⁴)(4⁶·6⁴) for 1 and 2, respectively). The mechanisms of the selective and efficient quenching of their photoluminescence (PL) by a series of nitroaromatic (NACs) solutions could be explained by electron transfer, long range energy transfer and/or electrostatic interactions. Remarkably, 1 and 2 can impressively detect the concentrations of dinoseb in solutions down to 0.09 and 0.11 ppm, respectively. Their PL could also be quenched by nitrobenzene (NB) and 4-nitrotoluene (4-NT) vapors, and the emission from 2 can be more quickly quenched than that from 1 possibly due to 2's larger pores and faster uptake of NAC vapors. 1 and 2 demonstrate significantly better performances than the two previously reported 4-connected MOFs using Zn²⁺ and a ligand isomeric to L⁵⁻ in detecting NACs in both suspension and vapour mainly due to the ligands' different LUMOs and arrangements of carboxylate groups (L. Di, J. J. Zhang, S. Q. Liu, J. Ni, H. Zhou and Y. J. Sun, Cryst. Growth Des., 2016, 16, 4539). This work sheds light on not only understanding of the formation of framework isomers but also the development of MOF-based NAC probes with better performances via judicious selection of suitable ligands.