Due to its relatively low energy consumption and small environmental footprint, Organic Light-Emitting Diodes (OLEDs) are not only useful for fabrication of displays (TVs, cell-phone displays) hold a promise, for example, for solid-state lighting (SSL). The design of highly-efficient OLEDs requires matching of frontier orbital (HOMO-LUMO) energy of the different charge-transporting layers, high-charge mobility for the charge-transporting layers and a high quantum yield for the emissive material. The most efficient OLEDs are based on phosphorescent dyes. Phosphorescent emitters, however, also require the use of a matrix host in which the phosphorescent dye is dispersed. The hosts must possess excellent charge mobilities properties, preferably bipolar-behaviour, and triplet energy higher than the emissive compound. In the case of blue-emitting phosphorescent compounds, these requirements are even harder to obtain since the high-triplet energy needed can be achieved with compounds with short-conjugation.
An important part of our research is in synthesis and investigation of materials than can transport excitons (generated by electron-hole recombination) in electroluminescent devices. Exciton migration is a process of fundamental importance for a number of processes of high importance, including organic electroluminescence (EL), organic photovoltaics (OPV), photoconductors, and others. Example is the synthesis of Al(III) quinolinolates that act both as emitter and semiconductor hosts for other electroluminescent compounds.
Our work on photonic and electronic materials is yet another showcase of how organic synthesis and materials chemistry can address the large societal problems. An excellent example is shown below. In fact, researchers who mater organic chemistry are capable of solving scientific problems beyond their immediate research focus. Below we show a synthesis of several new Al(III) complexes. The goal of this research wa optimize the substituents on the quinolinolate (8-hydroxyquinoline) ligand to achieve a blue emission (fluorescence) for application in white-light emitting OLEDs. Note that the parent Al(III)quinolinolate (R1=R2=H) emits a green fluorescence (max 520 nm). Because the quinolinolate displays charge-transfer excited state where the electron-rich phenolate moiety provides a charge to the electron-poor moiety, we decided to introduce electron-withdrawing substituents (F, CF3, CN) on the phenolate to destabilize the HOMO and electron-donating methyl on the pyridine to destabilize the LUMO. The combination of two electronic effects that destabilize the charge transfer and make it more energy-demanding which results in a blue-shifted emission. This worked great - see the results below!
The Al (III) complexes were synthesized from the corresponding ligand by reaction with tris-isopropoxy aluminum(III). The synthesized semiconductor-emitters were then used for white-light emitting OLEDs (like the ones you would need for solid-state lighting (SSL). The electroluminescence spectra of the Al(III) compounds (A) and the photographs of the corresponding OLEDs are shown on the left (B). Co-deposition of the blue-emitting AL(III) complex with the red-emitting Ir(III) allows for singlet excitons to be emitted from the Al(III) complex while the diffusive triplet exciton migration results in a red emission (C). Combination of blue and red emission which generates a warm white light (D). For more information see Chem. Eur. J. 2011, 17, 9076-9082.
For other work on OLEDs and organic semiconductors, see ACS Appl. Electron. Mater. 2021, 3, 3365-3371; J. Mater. Chem. C. 2020, 8, 11988-11996; Advanced Optical Materials 2020, 8, 0191282.
Our work on organic semiconductors also find an application in OFETs. Our OTFT materials are used in the devices fabricated and tested by our collaborators in Japan. For example: Anal. Chem. 2016, 88, 1092-1095; Chem. Com. 2015, 51, 17666-17668; and Chem. Eur. J. 2014, 20, 11835-11846.