New isomeric ancillary ligands and their EuIII complexes: a single component white light emissive phosphor and their applications in red/white smart LEDs, electronic noses, and temperature sensing

Abstract

Two isomeric ligands were designed and synthesized for the Eu(III) complexation. The designed ancillary ligands show greenish-yellow emission in the solution as well as in the solid state. However, the corresponding mCF₃ functionalized Eu-complex showed single component white light emission with balanced color purity; in contrast, the pCF₃ functionalized Eu-complex showed pure red emission. Extremely narrow band red emission with full width at half maximum (FWHM) of 5–6 nm (asymmetric ratio (AR) 12–14, color purity 92–93%) was observed with 345 nm light excitation (blue ∼445 nm for ligands) and could be a potential candidate for solid state lighting. The systematic solvatochromism study reveals that Eu(TTA)₃-TPA-DPA-mCF₃ shows multi-color/tunable emission with variable CIE color coordinates; in contrast, Eu(TTA)₃-TPA-DPA-pCF₃ shows pure red emission. The mechanism suggests partial and complete energy transfer from the ligand to the Eu(III) ion, which is supported by theoretical as well as experimental studies. Both the complexes have been applied as a potential phosphor for red/white LED fabrication, and particularly Eu(TTA)₃-TPA-DPA-mCF₃ is applied for white LED fabrication in conjugation with blue LED. The white LED produced superior white emission (CIE = 0.35, 0.34) with CRI = 86%) and CCT = 4645 K, whereas near UV conjugated white LED showed a CRI of 78%. In addition, to illustrate the prospective use of the scheme in sensor applications, the red fabricated LED can be applied for visual detection of acidic and basic vapors (act as electronic noses) from the environment due to emission swapping characteristics. Dual emission behavior of the complex led to the study of the thermometric property in higher temperature ranges (RT to 180 °C) and the sensitivity (S_r) was calculated to be 6.74%, which is the highest among those reported.