Versatile Luminescent Europium(III)-β-Diketonate-imidazo-bipyridyl Complexes Intended for White LEDs: A Detailed Photophysical and Theoretical Study Kasturi Singh, Rajamouli Boddula, and Sivakumar Vaidyanathan* Department of Chemistry, National Institute of Technology Rourkela, Rourkela-769 008, Odisha, India * S Supporting Information ABSTRACT: Three ancillary ligands based on imidazo-bipyridyl with phenyl (Ph), naphthyl (Np), and triphenylamine (TPA) substitution were synthesized and secondhand to formulate the consistent europium(III) ternary complexes using thenoyltrifluoroacetone as an anionic ligand. The complete investigation of spectroscopic, photophysical, and electrochemical properties was carried out. The attained results for all the ancillary ligands and their corresponding Eu complexes were compared with one another. All the Eu complexes reveal a broad excitation band ranging from the near-UV to blue region, along with high intense emission and apposite color purity. To further understand the ligand-to-metal energy transfer (ET) process, the geometry of the ligand was optimized and the energy level location (singlet and triplet) was calculated by using DFT and TD-DFT calculations. On the basis of the theoretical calculation, the ET mechanism was proposed. From PL emission spectra in the solid state, complete ET occurs from Ph, Np based ancillary ligands to the Eu 3+ ion, which yields a pure red emission, whereas the TPA functionalized based Eu complex shows incomplete ET. Fortunately, white emission was observed in the TPA based Eu complex in the solid state. The white LED was fabricated by using a white emitting complex integrated with 395 nm emitted LED (InGaN) chips under 20 mA forward-bias current. The excitation source from LED was fully observed by the complex shown for 3Eu and showed yellowish emission in different concentrations (the similar observation also reflected in solid). However, in the case of 1Eu and 2Eu complexes, they showed close to white emission. The Commission International de I’Eclairage (CIE) chromaticity coordinates are close to the National Television Standard Committee standard value for white emission, and in addition, the complex 3Eu coated with the blue LED chip (460 nm) by PMMA (1:10) showed bright white emission with CIE x, y values of 0.30, 0.33, respectively. ■ INTRODUCTION Lanthanide based molecular complexes have received a great deal of attention due to their brilliant applications in different provinces, such as biomedical diagnostics, biological labeling, photonic devices, and solar energy conversion. 1-4 Lanthanide ions are known to emit light in the visible or else near-infrared (IR) spectral region under UV or IR excitation by either a down- or up-conversion process. Lanthanides have unique spectral properties, atomic-like or line-like emission bands, and long-lived excited states (microseconds to milliseconds). The 4f electrons can be arranged liberally in any other seven 4f orbitals, resulting in them being able to absorb or emit various wavelengths ranging from the ultraviolet and visible to IR spectral region in the electromagnetic spectrum, excluding for when the 4f sublevels of Ln 3+ are completely empty or full (La 3+ and Lu 3+ ). By choosing the organic ligand, one can obtain highly luminescent lanthanide complexes by means of the “antenna effect” and populate the emitting levels of the Ln 3+ ions, as the 4f-4f electronic transition is a parity forbidden one. In other words, accumulating the ligating moieties could shield the central metal ion against the solvent in order to avoid nonradiative deactivation processes. The molecular design of the apt organic antennas/ligands which can efficiently transfer their excited energy to the lanthanide ions is a stimulating research task. It also plays a vital role in tailoring the photophysical properties of the lanthanide molecular complexes for specific applications. β-Diketones based ligands were considered as an important class of energy harvesters for the Ln 3+ ions, 5-8 as well as they give very volatile, thermodynami- cally stable, and highly luminescent complexes, which are essential conditions for optoelectronic or lighting applica- tions. 9,10 Tris-β-diketonate lanthanide complexes are coordina- tively unsaturated and usually remain solvated. The solvent molecules contain high energy O-H/C-H oscillators, and these oscillators lead to nonradiative deactivation which has a detrimental impact on Ln luminescence. 11 The energy deactivation via a nonradiative process could be prohibited by introducing neutral ancillary ligands, such as phenanthroline (phen), 2,2′ -bipyridine (bpy), 12 bis(pyrazolyl)methane (bpm), 13 and pyrazole, 14 or fully fluorinated β-diketones. 15 These bidentate ancillary ligands replace the solvated water molecules and lead to a coordinately saturated Ln 3+ complex. 16 It Received: June 20, 2017 Published: July 26, 2017 Article pubs.acs.org/IC © 2017 American Chemical Society 9376 DOI: 10.1021/acs.inorgchem.7b01565 Inorg. Chem. 2017, 56, 9376-9390