Spectroscopic studies of excited states in carbene-metal-amide emitters

• Main Author: Reponen, Antti-Pekka
• Format: Dissertation
• Language: English
• Published: University of Cambridge 2022
• Subjects: OLED; organic semiconductors; organometallic; spectroscopy; transient absorption
• DOI: 10.17863/CAM.99738

Organic semiconductors are finding increased applications in modern technologies over their inorganic counterparts and hold several advantages from flexibility to better processability. In particular, organic light-emitting diodes (OLEDs) are already available commercially and familiar in consumer applications. Unfortunately, lack of stable and efficient blue-emitting materials has been a long-standing problem which limits the general usefulness of current OLED technologies. As such, development of new materials for and approaches to light emission in OLEDs is a very active area of research. Carbene-Metal-Amides (CMAs) are a recent class of organometallic semiconductors with proposed applications in high-performance OLEDs. Fast triplet harvesting in CMAs occurs through thermally activated delayed fluorescence (TADF). In contrast to usual TADF emitters, CMAs incorporate a metal centre which enhances spin-orbit coupling effects. However, the in-depth emission mechanism in CMAs has been subject to some uncertainty, and high-performing blue CMAs have remained elusive. In this thesis, we use steady-state and time-resolved spectroscopic methods to study excited-state properties in a range of CMA materials. First, using single-atom donor substitutions, we show that energies of multiple states can be simultaneously shifted. We find that these shifts can be qualitatively rationalized from a perturbation theory approach, resulting in a design principle for blueshifting emission while avoiding donor-localized states previously linked to slow emission in blue CMAs. Next, we perform extensive transient absorption characterization of a coinage metal CMA series, using environmental effects to adjust excited-state energies. We find the first direct linkage of excited-state absorption features to specific state characters for CMAs. Results establish groundwork for investigating the population and role of non-emitting ligand-centered states, which have so far remained a source of uncertainty. Finally, we investigate unusual donor fluorescence in CMAs. We evaluate possible sources, such as free donor units and orthogonal conformers. We find several pathways for apparent free donor formation, from degradation to dilution. These results inform us about material stability considerations and advice caution in solution studies of copper CMAs especially.