Synthesis and Optical Properties of Copper Terephthalate Metal Organic Frame Works

Two new copper-based metal organic frame work (Cu-MOF21 and CU-MOF-22) was synthesized using bromo malonaldehyde and terephthalic an amino terephthalic acid. They synthesized CU-MOFs were characterized by FT-IR, UV-Visible spectroscopy. The XRD diffraction pattern indicated 2 θ at 17.3° and 26.8°. The Tauc’s method was employed to calculate the band gap of Cu-MOFs and was found that Cu-MOFS-21 exhibited 3.14 eV and Cu-MOF-22 with average bandgap energy at 3.61 eV attributed to the ligand-metal charge transfer. The results indicate that both Cu-MOFs can be further modified by suitable dopants to enhance the conductivity and reduce the band gap energy. Keywords: Metal organic frameworks, Copper metal, Bandgap energy, photoluminescence,

Preparation of 2-bromomalonaldehyde: Starting material 2-bromomalonaldehyde was prepared using the procedure given in literature. To a 100 ml of aqueous solution of 1, 1, 3, 3tetramethoxypropane (100g, 0.12M), concentrated HCl (4.3mL) was added and stirred until it forms homogeneous solution, wherein temperature of the reaction mixture was maintained below 35 °C and later bromine (0.15M) solution was added drop wise slowly and stirring was continued for another 30 minutes. Then, reaction mixture was concentrated under vacuum maintaining temperature below 50 ͦ C until thick slurry was obtained, and further washed using 200 mL cold water, 100 ml of cold dichloromethane and dried in vacuum. Yield: 65%, MP: 148 °C (Lit: 148 ͦ C).
Preparation of Cu-MOF, compound 1: Followed the solvothermal method to synthesis these compounds. An equimolar amount of terephthalic acid (0.724g, 0.004 mol) and Cu (NO3)2.3H2O (1.053g, 0.004 mol) were taken in 50 ml DMF and kept at 1200C in hot air oven for 48 hours. Nano polymer MOFs was kept for stirring by adding one equivalent of 2bromomalonaldehyde and continued for stirring for another two hours. Then centrifuged the reaction mixture (7000 rpm for 5 min) by repeatedly washing the compound formed with DMF, then with ethanol and dried the composite in vacuum oven at 150° C.
Preparation of Cu-MOF, compound 2: Synthesized via the solvothermal method where in same molar concentration of 2-amino terephthalic acid (0.724 g, 0.004 mol) and Cu (NO3)2.3H2O (1.053 g, 0.004 mol) in 50 mL of DMF and placed in hot air oven at 120 ͦ C for 48 hrs. Nano polymer MOFs was kept for stirring by adding one equivalent of 2-bromomalonaldehyde and continued for stirring for another two hours. Further, it was centrifuged (7000 rpm for 5 min) with repeated washing in DMF, then with ethanol and the composite was dried. Figure 1. depicts the synthesis of Cu-MOFs. Both the absorption bands recorded may corresponds to the n-π* and π-π* excitation attributing the interaction between the oxygen of the organic framework and apparently due to the optical transition of organic ligands to that of copper metal charge transfer [52].
FT-IR Spectral Analysis: The FT-IR spectrum of both CUMOFs were indicated in the Fig  No 3. The presence of the O-H stretching band at 3353 cm -1 were observed in the CU-MOF-1. Also, the band at 3024 cm -1 corresponds to Ar C-H str, 2890 cm -1 at Ald C-H stretching due to fermi resonance and band at 1041 cm -1 corresponding to C-O stretching.
Engineering Chemistry Vol. 2 XRD diffraction studies: The phase purity and crystallinity of synthesized Cu-MOFs were identified with the XRD diffraction studies as indicated in Figure 4. The A sharp arrow headed tripods with amorphous type of nature of peaks were observed due to the presence of pure copper with a cubic face centered structure. In both the Cu-MOFs the peaks values i,e 2θ at 17.3 ⁰ , 26.8 ⁰ , attributed to planes at (020), (040)respectively.. Many other peaks were observed between 2θ at 10° to 50°. Most of the peaks match with the literature reported as JCPDS card no. 89-2838 and 04-0836 for copper [53]. The particle size calculation was done with respect to FESEM as less intense peaks were obtained in the XRD pattern. The crystallite size of Cu-MOFs were calculated using Scherrer equation.

Photoluminescence features of Cu-MOFs
The PL emission characteristic of Cu-MOFs were shown in Fig

Conclusion
This work reports the synthesis of two new Cu-MOFs using bromo monaldehyde as organic linker and copper metal as conducting material. The synthesized CU-MOFs were characterized by FT-IR, XRD and UV-Visible spectrometer. The photoluminescence spectrum inferred the maximum excitation of both the Cu-MOFS at 516 nm. The synthesized Cu-MOF -21 showed 3.14 eV band gap Engineering Chemistry Vol. 2 energy whereas the band gap energy for Cu-MOF-22 was at 3.61 eV. Though the obtained band gap energy is more, the bandgap energy can be addressed by adding suitable dopants and this work is in progress by our team to enhance the optical property of the Cu-MOFs.