Illuminating Advances: Photochemistry and Photophysics of N-Heterocyclic Carbenes (NHCs) and Its Structural Correlation

Abstract

This chapter discusses the significant impact of N-heterocyclic carbene (NHC) ligands on the optical properties of metal complexes. It highlights the key role of NHC ligands in shifting the d−d transitions of these complexes to shorter wavelength regions, rendering them luminescent. The study focuses on NHC-based metal complexes involving copper, silver, platinum, iridium, and coinage metals, showcasing their luminescent properties and potential applications in organic light-emitting diodes (OLEDs). However, it also underscores a challenge: the quantum yields of these metal complexes may not be sufficient for practical applications. To address this issue, this chapter proposes modifying the NHC ligand with electron-rich substituents, aiming to reduce the HOMO-LUMO gap and shift d-d transitions into the UV region. Additionally, it introduces cyclic (alkyl)(amino)carbene (CAACs) as promising ligands for tuning the photophysical properties of metal complexes. CAACs are noted for their ability to decrease the HOMO-LUMO gap compared to NHCs and their electrophilic nature. Luminescent CAAC metal complexes are found to exhibit sub-microsecond lifetimes, making them suitable candidates for OLED applications. Ultimately, the research challenges researchers to better understand the structure, bonding, and properties of these complexes to create stable and highly luminescent materials using abundant and cost-effective metals like iron, copper, zinc, and silver.