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White Light Emitting Lanthanide Metal Complexes for Electroluminescent Materials

Periodic Reporting for period 1 - WHITELIGHT (White Light Emitting Lanthanide Metal Complexes for Electroluminescent Materials)

Reporting period: 2018-01-02 to 2020-01-01

The need to reduce energy consumption has prompted a considerable research effort for developing new energy-efficient lighting systems to replace conventional light sources (i.e. incandescent and fluorescent lamps). At present, both inorganic, i.e. based on silicon semiconductor technology, or organic light emitting diodes (OLEDs), stand out as the best alternative lighting devices, in term of efficiency and duration.
White light production for general lighting applications requires the generation and the intensity control of the three fundamental (red, green and blue) or two complementary (blue and yellow) colors. Desirable properties for a candidate emitting material are high luminescence intensity, color purity, easy synthesis, good chemical and electrical stability, and low cost production.
The wider availability of white emitting materials will foster their utilization in the next generation of lighting solution, reinforcing the traditional leadership of the European industries in the field. Addressing and providing innovative solutions to the important scientific challenges surrounding the materials for lighting solutions is expected to impact on the everyday quality of life in the EU and beyond.
The main scientific objective of this project is the development of white light emissive materials that are highly efficient, thermally, photo- and electrochemically stable and possess electron and hole transport character. These properties make them suitable for the development of white OLEDs (WOLEDs), or other electroluminescent devices, such as Light Electrochemical Cells (LECs).
To address the two strategies outlined above for white light generation, one blue emitting bridging ligand and three series of europium (Eu, red emitter), terbium (Tb, green emitter) and samarium (Sm, yellow emitter) chelates have been synthesized using β-diketones ligands. Their structural and functional properties have been tested by a set of advanced characterization techniques available at CNR.
Unfortunately, the intensity of the three RGB components was not well balanced in the materials prepared, and a pure white emission has not been achieved. Anyway, interesting results were obtained for an Eu,Tb dyad. Its optical properties were analyzed at different temperature, showing a change – visible to the naked eye – from red (room temperature) to yellow (low temperature) of the overall emission color of the system. This suggest the use of such materials as sensor of temperature.
The best performing Eu-based complexes were used as active components in the fabrication of bright (435 cd/m2) red emitting OLED devices, demonstrating the suitability of the materials synthesized in the project for optoelectronic applications.
The results obtained so far are encouraging and have demonstrated the possibility to prepare multiple emitting single molecules, and they have also proved to be suitable as active materials in OLED devices. This allow to envisage their use of multiple lanthanide-based materials in the fabrication of white light emitting optoelectronic devices. Besides, as some of the materials prepared inside the project have displayed an unexpected temperature sensitive emission color, they could find application as temperature sensors.
Working OLED based on an Europium red emitting complex
CIE 1931 spectral chromaticity coordinates of Eu,Tb dyad at different temperatures