Synthesis and photoluminescent behaviour of mixed ligand based beryllium complexes with 2-(2-hydroxyphenyl)benzoxazole (HPB) and 5-chloro-8-hydroxyquinoline (Clq) or 5,7-dichloro-8-hydroxyquinoline (Cl2q) or 2-methyl-8-hydroxyquinoline (Meq) or 8-hydroxyquinoline (q) are reported in this work. These complexes, that is, [BeHPB(Clq)], [BeHPB(Cl2q)], [BeHPB(Meq)], and [BeHPB(q)], were prepared and their structures were confirmed by elemental analysis, Fourier transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, and thermal analysis. The beryllium complexes exhibited good thermal stability up to ~300°C temperature. The photophysical properties of beryllium complexes were studied using ultraviolet-visible absorption and photoluminescence emission spectroscopy. The complexes showed absorption peaks due to
Small molecular metal complexes [
For color tuning tris(8-hydroxyquinolinato)aluminium (Alq3) [
Here we have tried quinolate and N,O donor ligand for tuning the color of the emissive metal chelates and therefore synthesized mixed ligand beryllium complexes with 2-(2-hydroxyphenyl)benzoxazole and 8-hydroxyquinoline as well as its substituted derivatives. The photophysical properties of these materials have also been investigated.
All the chemicals used to synthesize metal complexes were of analytical grade and purchased from Fluka. Solvents were of high purity and used as supplied.
Beryllium complexes were prepared by reacting two ligands, first as 2-(2-hydroxyphenyl)benzoxazole (HPB) and second as 5-chloro-8-hydroxyquinoline (Clq)/5,7-dichloro-8-hydroxyquinoline (Cl2q)/2-methyl-8-hydroxyquinoline (Meq)/8-hydroxyquinoline (q) separately with beryllium sulphate (ligands and metal) at 1 : 1 : 1 molar ratio in ethanol. A solution of HPB 0.5 g (1 mmol) was prepared in 20 mL pure ethanol and stirred on a magnetic stirrer at 60°C for 30 min and then a solution of Clq 0.36 g (1 mmol)/Cl2q 0.43 g (1 mmol)/Meq 0.32 g (1 mmol)/q 0.29 g (1 mmol) in 20 mL of pure ethanol was added to the reaction mixture. The reaction mixture was stirred at 60°C for 2 h; then a solution of beryllium sulphate (1 mmol) in 5 mL of deionized water was added dropwise. The yellowish precipitate of the complexes was formed, filtered, and dried at 100°C. The synthetic scheme is shown in Figure
Synthetic route of the beryllium complexes.
The elemental contents of carbon, hydrogen, and nitrogen were detected by Elemental Analyzer PerkinElmer 2400 CHN using combustion technique. PerkinElmer 2000 FTIR spectrometer using dry KBr was used to run IR spectral data in the range 4000–400 cm−1. 1H NMR analysis was performed by Bruker Avance 300 Proton NMR Spectrometer in CDCl3. Thermal gravimetric analysis (TGA) and differential thermal analysis (DTA) were carried out using Mettler Toledo TGA/SDTA851e instrument. Absorbances of these complexes (in methanol) and photoluminescence spectra (in solid and thin film) were recorded using spectrophotometer Horiba Jobin YVON Fluorolog Model FL-3-11 equipped with 450 W Xenon lamp as the excitation source. Optimized three-dimensional stable structures of metal complexes and their selected frontier molecular orbitals were obtained by density functional theory using Gaussian 03 package.
Yield: 65.2%; Anal. Calc. (C22H13N2O3BeCl) (found: C, 66.32; H, 3.29; N, 7.06; calc.: C, 66.41; H, 3.27; N, 7.04%); IR(KBr):
Yield: 63.8%; Anal. Calc. (C22H12N2O3BeCl2) (found: C, 61.06; H, 2.79; N, 6.51; calc.: C, 61.11; H, 2.77; N, 6.48%); IR(KBr):
Yield: 66.8%; Anal. Calc. (C23H16N2O3Be) (found: C, 73.16; H, 4.26; N, 7.45; calc.: C, 73.20; H, 4.24; N, 7.42%); IR(KBr):
Yield: 62.8%; Anal. Calc. (C22H14N2O3Be) (found: C, 72.75; H, 3.87; N, 7.74; calc.: C, 72.72; H, 3.85; N, 7.71%); IR(KBr):
The thermogravimetric analysis (TGA) and the differential thermal analysis (DTA) of beryllium complexes were carried out in nitrogen atmosphere with a heating rate of 10°C/min in temperature range of 0–500°C. The metal complexes exhibited high thermal stability and analogous thermal pattern of weight loss was observed at decomposition temperature for all complexes. The onset temperature of weight loss was 300°C, and temperature for nearly 10% weight loss was 350°C as shown in TGA plot of [BeHPB(Clq)] in Figure
Curve (A): TGA, and curve (B): DTA plots of [BeHPB(Clq)] complex.
The photophysical properties as absorption and emission characteristics of beryllium complexes were studied using spectroscopy techniques. Figure
UV-visible absorption spectra of beryllium complexes at 10−4 M concentration in methanol.
The optical band gap was calculated from absorption spectra; Figure
Energy versus absorption2 plot of beryllium complexes.
Upon excitation at absorption wavelengths the materials [BeHPB(Clq)], [BeHPB(Cl2q)], [BeHPB(Meq)], and [BeHPB(q)] fluoresced at 493, 496, 486, and 507 nm, respectively, in the visible spectra as shown in Figure
Photophysical properties of beryllium complexes.
Compound | Absorption |
Emission |
Optical band gap |
Thin film |
Emitted light | Color coordinates |
---|---|---|---|---|---|---|
BeHPB(Clq) | 287, 338 | 493 | 3.02 | 497 | Greenish blue |
|
BeHPB(Cl2q) | 278, 378 | 496 | 2.93 | 499 | Green |
|
BeHPB(Meq) | 246, 365 | 486 | 3.07 | 487 | Greenish blue |
|
BeHPB(q) | 282, 332 | 507 | 2.98 | 511 | Greenish blue |
|
Photoluminescence spectra of beryllium(II) complexes in solid state.
Photoluminescence spectra of beryllium(II) complexes in thin film form on glass substrate.
The Commission Internationale d’Eclairage (CIE) 1931 chromaticity color coordinates for emitted color of PL in solid powdered form were
CIE chromaticity diagram.
The minimal energy three-dimensional geometries and frontier molecular orbitals of prepared beryllium(II) complexes were computed with DFT/B3LYP/6-31G(d,p) method [
The optimized 3D structures and selected FMOs such as HOMO − 1, HOMO, LUMO, and LUMO + 1 are shown in Figure
Optimized 3D molecular geometry, molecular orbital surfaces (0.05 e au−3), and energies (in parenthesis, eV) of the selected frontier molecular orbitals of beryllium(II) complexes.
Beryllium complexes were synthesized and characterized by elemental analysis, 1HNMR, and FTIR techniques. The metal complexes had high thermal and chemical stability which were confirmed by TGA and DTA. The photophysical properties were investigated by absorption and emission spectroscopy. The emission maxima of the complexes were found in the greenish blue region having high intensity. The emission wavelength was 493 nm for [BeHPB(Clq)], 496 nm for [BeHPB(Cl2q)], 486 nm for [BeHPB(Meq)], and 507 nm for [BeHPB(q)] material. Ligand tuning can be useful for desirable light emission. The photoluminescent characterization confirmed the better luminescence properties of these complexes that could be efficiently used as emissive materials for display device applications.
The authors declare that there is no conflict of interests regarding the publication of this work.
Vandna Nishal and Devender Singh contributed equally to this work.
The authors gratefully recognize the financial support from the University Grant Commission (UGC) and Council of Scientific and Industrial Research (CSIR), New Delhi, India.