Controlled Energy Transfer from a Ligand to an Eu III Ion: A Unique Strategy To Obtain Bright-White-Light Emission and Its Versatile Applications Rajamouli Boddula, Kasturi Singh, Santanab Giri, and Sivakumar Vaidyanathan* Department of Chemistry, National Institute of Technology, Rourkela, Odisha 769008, India * S Supporting Information ABSTRACT: A new diphenylamine-functionalized ancil- lary-ligand-coordinated europium(III) β-diketonate com- plex showed incomplete photoexcitation energy transfer from a ligand to a Eu III ion. A solvatochromism study led to a balancing of the primary colors to obtain single- molecule white-light emission. Thermal-sensing analysis of the europium complex was executed. The europium complex, conjugated with a near-UV-light-emitting diode (395 nm), showed appropriate white-light-emission CIE color coordinates (x = 0.34 and y = 0.33) with a 5152 K correlated color temperature. T he new single-organic-molecule- or molecular-complex- based white-light-emitting sources are attractive owing to their potential applications in full-color smart displays and lighting sources [including white-organic-light-emitting diodes and solid-state lighting (SSL)]. 1 In general, white light can be generated by mixing three primary colors, to cover the entire visible spectrum. At present, several organic fluorophores having the capacity of emitting individual red-blue-green (RGB) color are known. However, white light generated by a single molecule (a single-component approach) has several advantages over that of simple RGB mixing (multicomponent emitters). 2 The benefits include improved stability, stable Commission International de I’Eclairage (CIE) color coordinates, and a simple fabrication process. 3 Stable white-light photo- and electroluminescence released in a single-molecule dyad have been documented. 4 White-light creation by aggregation-induced emission in a single organic molecule has also been reported. 5 An iridium-based molecular complex, which emits white light (from 440 to 800 nm in the spectral window), is known. 6 When the sensitizing/ energy-harvesting capability of a Ir III ion to lanthanides is used, an iridium-europium dyad has been used to release white light (bluish-green emission from the Ir III emissive center and red emission from the Eu III metal center). 7 However, generating white-light emission from a single-molecular complex is still a stimulating research manifold. The ever-increasing demand of the cumulative global energy crisis is reducing energy sources, and it can be overcome by highly energy-efficient lighting systems (SSL) that can help to conserve energy and reduce the overall lighting costs. 8 Recently, ligand-based incomplete/partial energy transfer (ET) to a Eu III metal center leading to white-light generation has become an attractive research task. 9,1d However, white-light emission from a single lanthanide complex is limited in its solution phase, and solid-state white-light emission is quite rare. Very recently, we have explored triphenylamine (TPA)- functionalized imidazole-phenanthroline based, a new bipolar ligand for a monochromatic red-light-emitting europium(III) complex. 10 Further, efforts have been made to obtain monochromatic red emission as well as investigate the effect of functionalization [extended the TPA moiety with diphenylamine (DPA)] on the luminescence properties of the complex in detail, where the ET process from a ligand to a Eu III metal ion plays a vital role. The DPA-decorated phenanthroline-fluorene-TPA ligand (Phen-Fl-TPA-DPA) and its corresponding europium- (III) β-diketonate complex [Eu(TTA) 3 -Phen-Fl-TPA-DPA, where TTA = thenoyltrifluoroacetone] have been synthesized. The fluorene moiety was used to design the europium complex because its strong π-π* absorption will improve the morphological properties and photostability of the complex. The presence of DPA moieties in the ligand widen the absorption outline and can act as light-harvesting units. TTA can act as an antenna, and the presence of fluorine in the ligand can decrease the vibrational quenching and increase the decay time. 11 In the presently studied europium(III) complex, ET to a central metal ion from a ligand and an antenna, is expected (Figure 1). The ligand can act as both sensitizer and a yellow-emitting source. The detailed experimental procedure for the synthesis of the ligand and corresponding europium(III) complex is given in Scheme S1 and Figures S1-S10, and their thermal stabilities were investigated (Figure S12). The amorphous nature of the complex was confirmed by X-ray diffraction (Figure S11). The UV-visible absorption spectra of the ligand and complex were carried out in solution (CHCl 3 , 1.0 × 10 -4 mol L -1 ), thin film, and solid state (Figure S13). The absorption spectrum of the ligand shows absorption ranging from 240 to 450 nm with λ max values at 306 and 280 nm (attributed to the π → π* transitions of Received: May 16, 2017 Figure 1. Chemical structure of europium(III) complex. Communication pubs.acs.org/IC © XXXX American Chemical Society A DOI: 10.1021/acs.inorgchem.7b01255 Inorg. Chem. XXXX, XXX, XXX-XXX