Four-membered N-heterocyclic carbenes in carbene metal amide emitters: a quantum chemical view

Using computational chemistry, we have scanned a set of four-membered N-heterocyclic carbenes with bulky substituents for their ability to form carbene metal amides (CMAs) with excellent thermally activated delayed fluorescence (TADF) properties. In comparison to the properties of their well-known five- and six-membered analogs, the transition dipole moments of the first excited singlet states of the corresponding Cu^(I)carbazolide (Cz) complexes increase. For CMAs of the most promising four-membered carbene, a lactam-based carbene (4LAC), detailed investigations of the TADF properties have been performed using advanced quantum chemical methods. Due to the small energy separation between its singlet and triplet ligand-to-ligand charge-transfer (LLCT) states, 4LAC-Ag^(I)-Cz exhibits the best ratio between reverse intersystem crossing (rISC) and intersystem crossing in the coinage metal triad for a coplanar orientation of the ligands. The TADF properties of the corresponding Cu^(I)and Au^(I)complexes benefit from twisted ligand-ligand alignments, achieved by using tetrafluorocarbazolide (4FCz) as donor ligand. The moderate reduction of the fluorescence rate constant upon twisting by about 45-50° is overcompensated by a decrease of the singlet-triplet energy gap, thus improving the TADF performance. Overall, with fluorescence rate constants of the order of 10⁷s^-1and rISC rate constants between 10⁹and 10¹⁰s^-1, TADF should have competitive advantage over common triplet deactivation processes such as triplet-triplet annihilation. Like in other CMAs, full excited-state geometry relaxation in liquid solution is detrimental for the emission properties. In the solid state, where the formation of a perpendicular ligand-ligand alignment is sterically hindered by the environment, 4LAC-M-Cz and 4LAC-M-4FCz are predicted to be efficient TADF compounds with red to orange emission.