Non-,Micro-,and Mesoporous Metal-Organic Framework Isomers:Reversible Transformation,Fluorescence Sensing,and Large Molecule Separation Hai-Long Jiang,Yoshiro Tatsu,Zhang-Hui Lu,and Qiang Xu* National Institute of Ad V anced Industrial Science and Technology(AIST),Ikeda,Osaka563-8577,Japan
Received February23,2010;E-mail:q.jp
Over the past few years,metal-organic frameworks(MOFs)have attracted tremendous attention owing to their intriguing structural topologies and wide potential applications as functional materials.1,2 The porosity of MOFs plays crucial roles in gas storage and separation, transport of organic substrates and products in catalysis,etc.1b,2 Significant challenges remain although some strategies have been developed for achieving large porosity in MOFs in recent years.1b,c,2c So far,in the limited mesoporous MOFs,mesoporous cages are included in most cases3whereas MOFs with1D mesoporous channels are particularly rare.4Introducing longer bridging ligands as a main strategy,nevertheless,reduced porosity imposed by interpenetration, which is almost unavoidable.5It is still elusive,although framework interpenetration was suppressed in a few MOFs.4a,5a,6The only case of controlling interpenetration with the same reactants in one-pot synthesis was reported very recently.6b Yet,the systematic modulation of pore size,currently,mostly depends on size-alterable organic linkers.1b,2c However,to the best of our kno
wledge,no MOFs with continuously tunable pore sizes based on the same ligands and metal centers/SBUs were reported to date.Herein,for thefirst time,we prepared nonporous to microporous MOFs by interpenetration control from the same reactants and further enlarged micropores to mesopores by simply decreasing reactant concentrations and reaction time.All three MOFs are isomers based on the same dicadmium(II)SBU and ligands.Strikingly,the microporous and mesoporous MOFs have been demonstrated to be reversibly transformed.In addition,the microporous MOF could be a promising luminescent probe and the mesoporous MOF has been evaluated as a chromatographic stationary phase for large molecule(dye)separation.
Solvothermal reactions of Cd(NO3)2·4H2O,4,4′-bipyridine(bpy), and2-amino-1,4-benzenedicarboxylic acid(L)in DMF yielded Cd(L)(bpy)(1,C2/m),Cd(L)(bpy)·4H2O·2.5DMF(2,Pbam),and Cd(L)(bpy)·4.5H2O·3DMF(3,P6/m)with the same framework formula but different isolated solvents based on single crystal X-ray structural,thermogravimetric(TG),and elemental analyses.7 The asymmetric unit of nonporous1contains one Cd atom and two half L ligands,having a half occupancy factor,and one-half bpy.The Cd atom coordinates to two nitrogens from two bpy ligands andfive carboxylate oxygens from three different L ligands, two L ligands adopt different coordination modes,and bpy bridges two Cd atoms(Figure1a).Such connectivity leads to a3D2-fold interpenetrated network of1,in which each netw
ork has a pillared-layer structure with bpy as a pillar and a planar channel size of1.3×1.7nm2surrounded by Cd(II)and L ligands(Figure1b).The channels nearly disappear with a free volume of only3.4%upon interpenetration.7,8Further examinations reveal that1adopts two interpenetrating6-connected R-Po net with the Schla¨fli symbol of (41263)(Figure1c).
By merely lowering the reaction temperature,microporous2was obtained.It adopts a similar framework structure with the single network of1,but slight distortion to the structure and pores inevitably occurs to meet the systematic stability of2,in which interpenetration is suppressed.There is one Cd atom,one L ligand with a half occupancy factor,and a half bpy in the asymmetric unit.The coordination environments of Cd(II)and bpy in1and2 are the same.The unique L ligand in2has the same coordination functions with the two independent L in1(Figure1a,1d),7leading to the same6-connected topology of1and2(Figure1c,1f).The resultant pillared-layer framework with bpy as the pillar has a planar channel size of1.1×1.9nm2surrounded by Cd(II)and L(Figure 1e).7PLATON calculation gives a free volume in2of61.2%upon interpenetration control,much higher than that of1(3.4%).8
In contrast to2,upon decreasing the concentrations of the reactants and reaction time,mesoporous3was successfully prepared.In the asymmetric units of2and3,all are the same except that a180°planar turning happens to one of the three L ligands connected to Cd(II),7 leading to two typ
es of channels in3along the c-axis(Figure1d-h). In addition to the triangular channel,the larger hexagonal open-channel has a mesoporous size of1.8×2.3nm2parallel to the ab plane.As in the case of1and2,the bpy linker still acts as the pillar in3to connect Cd-L layers,resulting in a3D open framework with enormous open channels and a very high free volume of68.2%.8Its structure can be simplified to a rarely reported6-connected kag net with a Schla¨fli symbol of(32·48·65)(Figure1i).9a,b
It is very interesting to note that the reversible transformation occurs for compounds2and3with different sizes of channels. Compound3undergoes spontaneous transformation to2in air upon losing isolated solvents.The framework of3remains over2months when it is immersed in mother liquor/DMF,showing its stability. The transformation starts once it is taken out,and the process can befinished in4days in air along with the
decrease/disappearance Figure1.Coordination environments of Cd and ligands in(a)1,(d)2, and(g)3.(b)View of the single network in1and(c)topological view of 1down the b-axis.(e)View and(f)topological view of2down the c-axis.
(h)View and(i)topological view of3down the c-axis.For clarity,the only position was kept for the disordered N atom in amino
group.
Published on Web04/02/2010
10.1021/ja101541s 2010American Chemical Society 55869J.AM.CHEM.SOC.2010,132,5586–5587
of the characteristic peak at ∼3.9°(Figure 2a).7It is proposed that the transformation is triggered by solvent departure,which results in an unstable framework of 3with too large and bare pores.A theoretical calculation carried out with a VASP program showed the energy of 3was slightly higher than that of 2,which re-vealed the feasibility of reversible transformation between 2and 3.7,9c With this in mind,our efforts of stretching the pores of 2with more solvents at higher temperature have successfully converted 2to 3under a pressure estimated to be ∼265kPa (Figure 2a).7It is assumed that the flexibility of L ligand benefits its turning,finally leading to the reversible transformation between 2and 3.
TG analyses in a He stream show that 1starts to lose weight and decompose above 260°C,whereas heating 2and 3results in the loss of H 2O and DMF from room temperature to ∼300°C and ∼215°C (corresponding to weight losses of 35.1%for 2and 40.5%for 3),respectively,followed by framework combustion.7The solid-state photoluminescence (PL)spectra of as-synthesized 1-3were investigated under λex )362nm.Compound 1exhibited weak bands at ∼435and ∼525nm,which could be attributed to the ligand-to-metal charge transfer (LMCT)and intraligand fluorescent emission,respectively.10Compounds 2and 3displayed strong emission at ∼435nm assigned to LMCT.10Interestingly,the PL of desolvated 2(denoted as 2a )was almost quenched upon desolvation and presented similar emissions as those in 1,possibly due to the distortion of framework.Strikingly,the reaction between pvp and amino
PL spectra of 2a in different solvent emulsions exhibited excellent fluorescence sensing for small molecules.As shown in Figure 2b,the PL intensity was strongly dependent on the solvent molecule.When 2a was dispersed in acetonitrile,the fluorescence intensity gradually increased with increasing amounts of H 2O (Figure 2c),where the system rapidly reached the equilibrium state.It is assumed that the restoration of distorted framework 2a in different solvents is responsible for the fluorescence enhancement.The encouraging result reveals 2a could be a promising luminescent probe for detecting small molecules.11Significantly,3was approved to be effective for size-exclusion dominant liquid chromatographic (LC)separation of Rhodamine 6G (larger than the pore of 2whereas smaller than that of 3)and Brilliant Blue R-250dyes (larger than pores of 2and 3)whereas 2with smaller pores was not (Figure 2d).7As far as we know,this is the first report on a mesoporous
MOF as an LC stationary phase for large dye separation,although limited MOFs for small molecule LC separations were reported.12The study could open up a new avenue to large molecule separation.In conclusion,for the first time,three MOF isomers based on the same structural units but hierarchical pores were prepared,in which not only interpenetration control was realized but also the micro-and meso-porous MOFs can be transformed reversibly.Compounds 2and 3could be a potential luminescent probe and an LC stationary phase,respectively.In addition,the available amino groups and
hierarchical pores in these MOF isomers make them good candidates for postsynthetic covalent modification for further applications;13efforts to realize these are underway.
Acknowledgment.We gratefully thank the reviewers for their valuable suggestions,Prof.Banglin Chen at Univ.Texas San Antonio for his helpful discussions,and AIST and JSPS for financial support.H.-L.J.thanks JSPS for a postdoctoral fellowship.
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JA101541S
Figure 2.(a)Powder XRD patterns for the reversible transformation
between 2and 3.(b)PL intensities of 2a introduced into various pure solvents and (c)PL spectra of 2a acetonitrile emulsion in the presence of various amounts of H 2O under λex )362nm.(d)Separation of R250and Rhodamine 6G achieved using 3as the stationary phase (the flow rate of mobile phase (DMF):0.5mL/min;detected wavelength:540nm).
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