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P. M, M. Sinha and A. Poduval, CrystEngComm, 2015, DOI: 10.1039/C5CE01886B.
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The Structural Diversity, Band Gap Energy and
Photoluminescence Properties of
Thiophenedicarboxylate based Coordination Polymers
Prabu M†a, K. S. Asha†a, Mekhola Sinhaa, Anamika Poduvala, and Sukhendu Mandal*a
A series of 2,5-thiophenedicarboxylic acid (TDC) ligand based coordination polymers were solvothermally synthesized, namely, 1, [Mg3(OH)2(H2O)4(TDC)2]; 2, [NH2(CH3)2]2[Mg3(TDC)4]; 3, [Mg2(TDC)2(DMF)2]; 4, [Zn2(TDC)2(bpe)] (solvent)X ; and 5, [(CH3)2NH2][In(TDC)], where DMF =N,
N’-dimethylformamide, bpe = 1,2-Bis(4-pyridyl)ethane. The compound 1 was synthesized in MeOH-H2O solvent mixture and it has a two dimensional layered structure. The compound 2 was obtained using DMFH2O mixture which has a three dimensional structure, It contains Mg3O16 trimeric units and which linked by TDC anion to form the three-dimensional connectivity. Compounds 3 and 5 were synthesized using only DMF as a solvent, Compound 4 was synthesized using DMF-MeOH mixture. Compound 3 is also three-dimensional structure where Mg4O20 cluster linked through TDC anions. Compounds 4 and 5 are three-dimensional structure and are having 2-fold interpenetration. All the compounds are well characterized through powder X-ray diffraction, IR spectroscopy, photoluminescence spectroscopy and thermogravimetric analysis. Interestingly compound 5 crystallizes in chiral space group which have been confirmed by solid state circular dichroism (CD) spectroscopy. We have measured the optical band gap energy for all the compounds and results showed the band gap energy varies from 3.44 eV to 3.86 eV. This variation in band gap energy may be due to the difference in structural arrangement.
Enormous attempts have been made to the synthesis of metal-organic frameworks (MOFs) or porous coordination polymers (PCPs) for the new structure with novel topology and potential applications in catalysis, gas/solvent storage and separation, magnetism, ion exchange, luminescence, etc.1-5 The controlled synthesis of MOFs or
PCPs for target materials is still a great challenge, as several factors are associated with its synthesis. It has been proved that several factors like solvent,6 pH,7 counter anion,8 concentration9 and temperature10 play a crucial role for the determination of overall structure. Thus, control over solvothermally obtained products is very difficult. For synthesizing target materials it is very important to understand the role of the above said factors. Temperature and solvents are the important factors, which governs the overall structure of the materials. Pioneer work by Cheetham et al.10e and Natarajan et al.10g,h have shown how thermal desolvation and subsequent changes in the coordination environment lead to different dimensionality. Due to their variation in physical and chemical properties, different solvents influence the crystal growth rate, and the crystal morphology.6 Solvents also play an important role via solute–solvent interactions towards the synthesis of the hierarchy of structures. Cheng and co-workers have shown that how the volume ratio of H2O in the DMF-H2O mixture influenced the shapes and sizes of the crystals.11 Recently we have studied the modulation of crystal growth and morphology by tuning the ratio of solvent in the solvent mixture during the reaction in Cd based MOF.12
In the recent times, interest has been directed towards the incorporation of s-block metals as the component of these extended networks instead of transition metals. This is so because the construction of MOFs from smaller s-block elements, like Li and Mg is appealing as they have good potential to form lightweight materials.13-20 Mg2+ ion offers a dual advantage of using a lightweight metal and a metal ion that behaves in similar ways to the well described three-dimensional coordination networks of Zn2+. Both of these metal ions, Zn2+ and Mg2+ have comparable ionic size (0.72 pm for Mg2+; 0.74 pm for Zn2+), and have similar affinity towards rigid aromatic carboxylate linkers. Both are commonly observed to adopt octahedral geometry.21
It has been reported that indium based frameworks exhibit interesting structural features. The 8-coordination mode of the indium metal ion with four equivalent carboxylate ligands gave rise to topologically interesting structures.22
Rigid aromatic linkers impart structural strength and are frequently chosen as building blocks for the structures requiring high thermal stability.23-25 2, 5-Thiophene dicarboxylic acid (TDC) ligand being an aromatic rigid molecule ensures stability of the network. The sulphur atom of the thiophen ring contains lone pairs of electrons which can be easily delocalised within the thiophen ring. This types of ligands exhibit unique physical and chemical properties due to good chargetransfer abilities.