Novel organic thermally activated delayed fluorescence (TADF) dyes as electrochemiluminescent (ECL) emitters in solution and solid state

• Applicant: Dr. Fabio Rizzo
• Subject Area: Organic Molecular Chemistry - Synthesis and Characterisation
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464509280

Electrochemiluminescence (ECL) is the electrochemical activation of suitable molecules or ions to generate excited state species (emitters) that cause luminescence, upon sufficiently exergonic electron transfer reactions. Since its early development, ECL found particular use as an analytical tool because of the low background and proportionality of the light intensity to the concentration of the emitting species. There is an increasing demand for the synthesis, characterization and application of new efficient ECL materials, such as organic dyes, metal complexes, and nanoparticles, that could emit at wavelengths other than those typical of ruthenium complexes, which are the most frequently used ECL emitters.Arguably, organic compounds could offer a more general answer to this challenge particularly because of their chemical versatility and, therefore, the possibility of modulating electronic and photophysical properties precisely. ECL emission arises from an excited species R* produced through ion-ion annihilation of electrogenerated radical anions and cations. The ratio of the lowest excited singlet (S1) and triplet (T1) states of R* formed by ion-ion annihilation is considered to be 1:3 following spin statistics, indicating a maximum ECL efficiency of 25% for fluorescent molecules. The limitation of efficiency can be overcome by spin up-conversion induced by thermal activation through reverse intersystem crossing (RISC) process. Because the rate constant of RISC is smaller than that for the radiative transition S1-S0, delayed fluorescence can be observed. Efficient thermally activated delayed fluorescence (TADF) can occur when the gap S1-T1 (ΔEST) is sufficiently small. Many TADF organic molecules composed of electron donor and acceptor groups with small ΔEST have been developed in the last few years for application in organic light-emitting diodes (OLEDs). However, the use of TADF for ECL purposes is still very rare with only three publications about this subject reported so far.This project aims to prepare innovative organic TADF molecules that will be used as ECL emitters. Besides the synthetic investigation to introduce different donor-acceptor pairs to tune the emission properties, the correlation between molecular structure and TADF and ECL properties will be deeply investigated to achieve a deeper understanding of the phenomena. Additionally, the properties of these compounds will be translated from solution to solid state through the formation of self-assembled monolayers and thin films. The combination of TADF and ECL in solid state is completely unexplored, and will offer important features for future applications in optoelectronics and as analytical tools.

• DFG Programme: Research Grants