Carbene–Metal–Amide Bond Deformation, Rather Than Ligand Rotation, Drives Delayed Fluorescence

Abstract

We report three characteristics of ideal thermally activated delayed fluorescence molecular systems apparent in carbene-metal-amides: (a) an exceptionally small singlet-triplet gap that effectively eliminates the thermal activation barrier to reverse intersystem crossing; (b) significant singlet oscillator strength promoting fluorescence in the region of this small barrier; and (c) enlarged spin-orbit coupling driving reverse intersystem crossing in this region. We carry out highly correlated quantum-chemical calculations to detail the relative energies of and spin-orbit couplings between the singlet and triplet states, finding that they fall closer together in energy and couple more strongly in going from the singlet ground-state to the triplet optimized geometry. This structural reorganization is defined not by rotation of the ligands but by a nontrivial bending of the carbene-metal-amide bond angle. This bending reduces carbene-metal-amide symmetry and enhances singlet-triplet interaction strength. We clarify that the reverse intersystem crossing triggering delayed fluorescence occurs around the coplanar triplet geometric optimum.

Efficient Vacuum‐Processed Light‐Emitting Diodes Based on Carbene–Metal–Amides

“…Subsequent emission must therefore come via the triplet state, such that PL and EL follow similar pathways. [4,6,8] The emission energy blueshifts at low temperature, consistent with previous reports, [4] which we interpret to mean that relaxation modes for the CT excitons are progressively frozen out. Emission is found to be thermally activated with an activation energy of 79 meV.…”

“…[4] CMA materials consist of a cyclic (alkyl)(amino)carbene (CAAC) linked via a metal (typically copper(I) or gold(I)) to a carbazolate or diphenylamide as the amide ligand. [4,[6][7][8] It is typically assumed that to achieve efficient emission, a compromise must be made between exchange energy and oscillator strength. [4,[6][7][8] It is typically assumed that to achieve efficient emission, a compromise must be made between exchange energy and oscillator strength.…”

“…[5] Both the exchange energy and the oscillator strength in CMA compounds are calculated to depend on the dihedral angle between the carbene and amide ligands about the CmetalN axis, and on deformation of the CmetalN bond. [4,[6][7][8] It is typically assumed that to achieve efficient emission, a compromise must be made between exchange energy and oscillator strength. The ability to control the distribution of rotational conformers, for example, through alternative processing methods, may result in emissive layers exhibiting different photo physical properties, which in turn will affect device performance.…”

Environmental Control of Triplet Emission in Donor–Bridge–Acceptor Organometallics

“…This work concluded that indirect spin‐orbit coupling (SOC) (i.e., S 1 –T n –T 1 ) mediated by torsional motion may influence the rate of triplet harvesting. Taffet et al in examining the structurally related Cu(I) analogue CMA2 concluded that intersystem crossing (ISC) was likely most effective in a sterically constrained coplanar configuration, relying on a breaking of planar symmetry by distortion of the CCuN central axis to allow coupling between S 1 and T 1 . The underlying process coupling the T 1 state to the singlet manifold is therefore unclear.…”

“…Taffet et al in examining the structurally related Cu(I) analogue CMA2 concluded that intersystem crossing (ISC) was likely most effective in a sterically constrained coplanar configuration, relying on a breaking of planar symmetry by distortion of the CCuN central axis to allow coupling between S 1 and T 1 . [15] The underlying process coupling the T 1 state to the singlet manifold is therefore unclear. We use the experimental results above to provide new insight to this question.…”