Conformational Engineering of Two-Coordinate Gold(I) Complexes: Regulation of Excited-State Dynamics for Efficient Delayed Fluorescence

Abstract

Carbene−Au−amide (CMA) type complexes, in which the amide and carbene ligands act as an electron donor (D) and acceptor (A), respectively, can exhibit strong delayed fluorescence (DF) from a ligand to ligand charge transfer (LLCT) excited state. Although the coplanar donor−acceptor (D-A) conformation has been suggested to be a crucial factor favoring radiative decay of the charge-transfer excited state, the geometric structural factor underpinning the excited-state mechanism of CMA complexes remains an open question. We herein develop a new class of carbene−Au−carbazolate complexes by introducing large aromatic substituents onto the carbazolate ligand, the presence of which are conceived to restrict the rotation of the Au−N bond and thus confine a twisted D-A conformation in both ground and excited states. A highly twisted D-A orientation is found for the complexes in their crystal structures. Photophysical studies reveal that the twisted conformation induces a decrease in the gap (ΔE ST ) between the lowest singlet excited state (S 1 ) and the triplet manifold (T 1 ) and thus a faster reverse intersystem crossing (RISC) from T 1 to S 1 at the expense of oscillator strength for an S 1 radiative transition. In comparison with the coplanar analogue, the twisted complexes exhibit comparable or improved DF with quantum yields of up to 94% and short emission lifetimes down to sub-microseconds. The tuning of excited-state dynamics has been well interpreted by density functional theory (DFT) and time-dependent DFT (TDDFT) calculations, which unveil much faster RISC rates for twisted complexes. Solution-processed organic light-emitting diodes (OLEDs) based on the new CMA complexes show promising performances with almost negligible efficiency rolloff at a brightness of 1000 cd m −2 . This work implies that neither a coplanar ground-state D-A conformation nor a dynamic rotation of the M−N bond is the key to the realization of efficient DF for CMA complexes.

Au⋅⋅⋅H−C Interactions Support a Robust Thermally Activated Delayed Fluorescence (TADF) Gold(I) Complex for OLEDs with Little Efficiency Roll‐Off and Good Stability

“…These structural features of IPzIDCz provide a shielding to the metal center, as shown in its space‐filling structure (Figure S1). The buried volume (% V bur ) around gold atom of IPzIDCz was calculated to be 70.5 % which is close to that for CMA complexes containing a large aromatic group on the carbazole ligand (Figure 2c) [8g] . In contrast, the Au atom in IPzTPA is exposed to a greater degree with a much smaller % V bur of 55.6 % (Figures 2d and S1).…”

Two-Coordinate Coinage Metal Complexes as Solar Photosensitizers

“…It has been shown that coplanar ligand orientation is important for achieving appreciable oscillator strength of the S0 → S1 transition. 22,23,33 Systems with orthogonal ligands have notably low oscillator strength because the ᴨ systems of the ligand become spatially orthogonal. Au BCz MAC crystallized with a disorder of the C=O and the two methyl groups as shown in Figure 2.…”

“…LEDs, but have never been directly investigated for solar photoredox. [17][18][19][20][21][22][23][24][25][26][27] It is valid to question whether or not two-coordinate complexes would work at all. These molecules are coordinatively unsaturated, which could allow for deleterious non-radiative decay of the excited state via solvent coordination.…”

A Configurationally Confined Thermally Activated Delayed Fluorescent Two‐Coordinate CuI Complex for Efficient Blue Electroluminescence

“…However, recent researches indicate τ TADF is determined by not only$${\ {k}_{{\rm r},{{\rm S}}_{1}}}$$ but Δ E ST as well [29] . Several TADF CMAs with orthogonal geometries also presented efficient PL properties due to extremely small Δ E ST at a sacrifice of $${{k}_{{\rm r},{{\rm S}}_{1}}}$$ [30–32] . It is inferred that a balance between Δ E ST and $${{k}_{{\rm r},{{\rm S}}_{1}}}$$ can be obtained for short τ TADF if a twist configuration can be confined to a certain degree in the middle (Figure 1a), but no such configurationally confined complex is reported to the best of our knowledge.…”