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

Abstract 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 10 7 s −1 and rISC rate constants between 10 9 and 10 10 s −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.