Synthesis, Characterization, Optical and Luminescence Properties of Copper Based Metal Organic Frame Works

Herein, we report synthesis of two new copper metal organic frameworks. The organic linkers were terephthalic acid with 6-Dihydroimidazo[2,1-b]thiazole-2-carbaldehyde and terephthalic acid with 3-benzothiazol-2-yl-malonaldehyde used in the copper nano metal organic framework (MOF). Both the Cu-MOF’s were characterized by XRD, UV-vis spectroscopy and FTIR. XRD crystallographic studies revealed the presence of copper metal at 2θ at 18.4°. Tauc plots were simulated to calculate the band gap of both Cu-MOF’s and result indicated the band gap energy of Cu-MOF 1 at 3.31 eV and for Cu-MOF 2 was at 3.57 eV. The UV-Visible absorption studies indicated two bands for Cu-MOF 1 and Cu-MOF 2 at 326 nm. However, the second band in Cu MOF 1 at 509 nm was slightly shifted to higher wavelength at 516 nm in Cu-MOF 2 due to the extension of π-π* transition. The photoluminescent properties of both Cu-MOF’s indicated a strong band at 505 nm. Thus, the optical properties of both the Cu-MOF’s infers that these can be a promising semiconductor material for various electronic applications.


Experimental
Materials: All the chemicals and reagents procured from Avra Synthesis Private Ltd, Spectro Chem Ltd., Sigma Aldrich, India and used without any further purifications. 1,1,3,3-tetramethoxy propane 99% (Sigma Aldrich), Imidazolidinethione, 99% (Sigma Aldrich), 2-Mercaptobenzothiazole, 99% (Spectro Chem), Terephthalic acid 98% (Avra Synthesis Private Ltd), Cu (NO 3 ) 2 .3H 2 O 98% (Sigma-Aldrich Ltd). Instrumentation: The morphological analysis for the synthesized MOFs were caried out by using powder X ray diffractometer (Bruker, Germany) fitted with D8-fine focus ceramic X-ray tube, Cu-Kα source radiation (λ =1.5406 Å) at room temperature. The presence of various organic functional group in the MOFs were studied using Fourier transformed infrared spectrometer (Bruker-Alpha, Germany). The optical properties of MOFs were studied by recording the maximum absorption using UV-Visible absorption Spectrometer (Spector 210 plus). 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 3-benzothiazol-2-yl-malonaldehyde:
To a stirred solution of 2mercaptobenzothiazole (0.250 g, 0.0014 mol) in acetonitrile, 2-bromomalonaldehyde (0.225g, 0.0014 mol) was added drop wise for a period of 15 minutes. Kept for vigorous stirring at room temperature for an hour at room temperature and at 80 ° C for two hours in vacuum for the removal of solvent. Acetone was added and the pale colored solid was filtered, washed by acetone and further the compound obtained was dried in vacuum. Yield: 80% (0.283g). The synthesis of Cu-MOF's are as shown in Fig.1. Engineering Chemistry Vol. 3 Figure 2 indicates the UV-visible spectra of the synthesized Cu-MOF's. The Cu-MOF 1 showed two absorption bands at 326 nm and 509 nm whereas Cu-MOF 2 showed absorption bands at 326 nm and 516 nm. 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 [83] and apparently due to the optical transition of organic ligands to that of copper metal charge transfer. Increase in wavelength in Cu-MOF 2, attributed to the π-π* of phenyl ring thus decreasing the energy required between the two transition states [84].

XRD diffraction studies
The phase purity and crystallinity of synthesized Cu-MOFs were identified with the XRD diffraction studies as shown in Fig 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
where K is crystallite shape constant (0.94), β is full width at half maximum, λ is wavelength of Xray Cu-Kα radiation (1.5406 Å) and θ is glancing angle. The crystalline particle sizes were observed between 200-250 nm.

Optical properties
Band gap analysis: The UV-visible absorption spectra was carried out using 210 plus UV-Visible absorption Spectrometer. Tauc plots were obtained from simplifying equation 2 [88]. Graphically the bandgap energy was obtained by extrapolating the tangential line intersecting x axis at hγ = Eg as shown in Figure 6. The band gap energy for Cu-MOF 1 was recorded at 3.31 eV and that of Cu-MOF 2 at 3.57 eV. Structurally presence of phenyl ring didn't have impact on the band gap energy.

Conclusion
This work reports the synthesis of copper based nano metal organic frame works starting from starting from terephthalic acid with 6-Dihydroimidazo[2,1-b]thiazole-2-carbaldehyde and 3-benzothiazol-2yl-malonaldehyde. The synthesized Cu-MOF's were confirmed by a presence of sharp absorption band at 326 nm and broad shoulder band at 509 nm for Cu-MOF 1. A similar band was observed at 326 nm and 516 nm for Cu-MOF 2. An increase in 16 nm for the second band in Cu-MOF 2 was due to enhanced in π-π* transition. Further, the absence of Ald C-H stretching band and carbonyl stretching band in FT-IR spectroscopy of both Cu-MOF's resulted must be a co-ordination bond formed between the copper metal and the oxygen of aldehyde group. The XRD pattern of Cu-MOF's showed a sharp peak 2θ at 18. 4° for copper crystals with a cubic faced centered structure. The photoluminescent spectrum revealed a sharp band at 505 nm for both Cu-MOF's. Band gap energy of Cu-MOF's were calculated by using Tauc plots. The band gap energy for Cu-MOF 1 was calculated to be at 3.14 eV whereas for Cu-MOF 2 the band gap energy was 3.57 eV. This study results that the both Cu-MOF's are promising material for semiconductors, provided further optimization of band gap energy can be carried out to achieve a band gap energy of 1 to 0.5 eV for semiconductor applications. Further, to reduce the band gap, the synthesized Cu-MOF will be doped with metal oxides and screened for their sensory and photoluminescent applications.