An Exceptionally Stable, Porphyrinic Zr Metal-Organic Framework Exhibiting pH-Dependent Fluorescence Hai-Long Jiang, †,‡,§ Dawei Feng, †,§ Kecheng Wang, † Zhi-Yuan Gu, † Zhangwen Wei, † Ying-Pin Chen, † and Hong-Cai Zhou* ,† † Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States ‡ Hefei National Laboratory for Physical Sciences at the Microscale, Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China * S Supporting Information ABSTRACT: A reaction between a Zr IV salt and a porphyrinic tetracarboxylic acid leads to a metal-organic framework (MOF) with two types of open channels, representing a MOF featuring a (4,8)-connected sqc net. The MOF remains intact in both boiling water and aqueous solutions with pH ranging from 1 to 11, a remarkably extensive pH range that a MOF can sustain. Given its exceptional stability and pH- dependent fluorescent intensity, the MOF can potentially be applied in fluorescent pH sensing. ■ INTRODUCTION In recent two decades, metal-organic frameworks (MOFs) have received tremendous attention due to their powerful attributes on structural and chemical versatility and tailor- ability. 1 The rational design in their construction promises these porous materials tailored to speci fic functional applications, such as gas storage/separation, catalysis, sensor, and drug delivery. 2-5 Despite this, stability has being recognized as a crucial issue on the way to practical applications of MOFs. Thermally, MOFs usually stabilize up to 250 °C and the best record demonstrates that the framework is able to maintain until even higher than 500 °C, which are robust enough as porous functional materials. 6 However, most MOFs are more or less sensitive to moisture, which could be one of the key limitations to meet the requirements of various applications. In addition, seldom were MOFs reported to be chemically resistant in an acidic or basic medium. On the basis of the previous reports, MOFs constructed by imidazole or pyrazolate derivatives are inclined to survive under alkaline conditions, 6d,7 while carboxylate-coordinated MOFs would withstand an acidic environment. 8 To target the MOFs with both thermal and chemical stabilities, our group has aimed at zirconium(IV) carboxylates. According to “soft and hard acids and bases” theory, zirconium(IV) cation and carboxylate anion belong to hard acid and hard base, respectively, which makes the coordination bonds between zirconium and carboxylic oxygens very strong and potentially tolerable to attack of water, acid, and even base. However, the inert coordination bonds between Zr 4+ cations and carboxylate anions make ligand exchange reactions extremely slow, which is unfavorable for defect repair in the process of crystal growth. Therefore, it is especially difficult to obtain single crystals of zirconium carboxylate MOFs and only a couple of them have been structurally characterized to date. 6a,8b,9 During our persistent efforts in this research line, we have reported PCN-56 to -59 with UiO type structure that remains intact in dilute acid and base (2 < pH < 11) 9f and PCN-222 survives in concentrated HCl solution 8b recently. In this work, the assembly of zirconium(IV) and tetracarboxylic acids based on porphyrin center that could play multi- functionalities (such as light-harvesting, oxygen transportation, and catalysis, etc. 10 ), leads to exceptionally stable Zr-MOFs, which are not only thermally stable (≥350 °C) but also retain frameworks in the aqueous solutions with pH ranges from 1 to 11. To our knowledge, this is the broadest pH range that a porphyrinic MOF can survive thus far. Remarkably, by perfectly combining the wide-pH-range stability and the dye character- istic of involved porphyrin center, our preliminary results have demonstrated that the resultant MOF could present pH- dependent fluorescence. ■ MATERIALS AND METHODS Materials. Methyl 4-formylbenzoate was purchased from Oakwood Products, Inc. Pyrrole, propionic acid, N,N-dimethylformamide (DMF), N, N-diethylformamide (DEF), benzoic acid, acetone, zirconium(IV) chloride, glacial acetic acid, and zinc(II) chloride were purchased from Alfa Aesar. The tetrakis(4-carboxyphenyl)- porphyrin (H 2 TCPP) and [5,10,15,20-tetrakis(4-carboxyphenyl)- porphyrinato]-Zn(II) (Zn-TCPP) ligands were synthesized according to previous reports. 8b All commercial chemicals were used without further purification unless otherwise mentioned. Instrumentation. Powder X-ray diffraction (PXRD) was carried out with a BRUKER D8-Focus Bragg-Brentano X-ray Powder Diffractometer equipped with a Cu sealed tube (λ = 1.54178) at 40 kV and 40 mA. Elemental analyses (C, H, and N) were performed by Atlantic Microlab, Inc. (Norcross, Georgia). Thermogravimetric Received: July 10, 2013 Published: August 28, 2013 Article pubs.acs.org/JACS © 2013 American Chemical Society 13934 dx.doi.org/10.1021/ja406844r | J. Am. Chem. Soc. 2013, 135, 13934-13938