Excited-State Lifetime Modulation by Twisted and Tilted Molecular Design in Carbene-Metal-Amide Photoemitters

Carbene–metal–amides (CMAs) are an emerging class of photoemitters based on a linear donor–linker–acceptor arrangement. They exhibit high flexibility about the carbene–metal and metal–amide bonds, leading to a conformational freedom which has a strong influence on their photophysical properties. Herein we report CMA complexes with (1) nearly coplanar, (2) twisted, (3) tilted, and (4) tilt-twisted orientations between donor and acceptor ligands and illustrate the influence of preferred ground-state conformations on both the luminescence quantum yields and excited-state lifetimes. The performance is found to be optimum for structures with partially twisted and/or tilted conformations, resulting in radiative rates exceeding 1 × 106 s–1. Although the metal atoms make only small contributions to HOMOs and LUMOs, they provide sufficient spin–orbit coupling between the low-lying excited states to reduce the excited-state lifetimes down to 500 ns. At the same time, high photoluminescence quantum yields are maintained for a strongly tilted emitter in a host matrix. Proof-of-concept organic light-emitting diodes (OLEDs) based on these new emitter designs were fabricated, with a maximum external quantum efficiency (EQE) of 19.1% with low device roll-off efficiency. Transient electroluminescence studies indicate that molecular design concepts for new CMA emitters can be successfully translated into the OLED device.

TGA curves for the complexes p. S18 Electrochemistry p. S19 X-ray Crystallography p. S19 Photophysical characterisation p. S26 OLED device fabrication p. S38 Computational details p. S47 Table S1. Values for the rate equations for complexes 1-4. p. S34 Table S2. Additional photophysical characterisation for complex 3 and 4. p. S38 Table S3. OLED performance summary of complex 1 p. S41 Table S4. OLED performance summary of complex 2 p. S42 Table S5. OLED performance summary of complex 3 p. S44 Table S6. OLED performance summary of complex 4 p. S45 Table S7. Energy of the HOMO and LUMO (in eV) and their overlap (in %). p. S55 Table S8. Coordinates for ground and excited states (S1 and T1) geometries p. S56 3 NMR spectroscopy.

X-Ray Crystallography.
Complex 1 crystallizes with two independent molecules in the unit cell. The atoms C3, C26 and C27 were disordered over two half-populated positions for complex 1. The unit cell for monoclinic polymorph of complex 2 contains two independent molecules whereas triclinic polymorph of 2 contains three independent molecules and one 2-methyl-pentane molecules as a solvate. Further analysis of the residual electron density for the triclinic polymorth of 2 indicates the presence of two additional and severely disordered 2-methylpentane molecules which 20 contribution was removed from the diffraction data with PLATON/SQUEEZE for the final refinement. 1,2 The unit cell for of complex 3·Benzene contains two independent molecules of 3 and two co-crystallized benzene molecules. The unit cell for of complex 3·CH 2 Cl 2 contains one independent molecule of 3 and one co-crystallized CH 2 Cl 2 molecule. Crystals were mounted in oil on glass fiber and fixed on the diffractometer in a cold nitrogen stream. Data were collected using an Oxford Diffraction Xcalibur-3/Sapphire3-CCD diffractometer with graphite monochromated Mo K α radiation (λ = 0.71073 Å) at 140 K. Data were processed using the CrystAlisPro-CCD and -RED software. 3 The structure was solved by direct methods and refined by the full-matrix least-squares against F 2 in an anisotropic (for non-hydrogen atoms) approximation. All hydrogen atom positions were refined in isotropic approximation in a "riding" model with the U iso (H) parameters equal to 1.2 U eq (C i ), for methyl groups equal to 1.5 U eq (C ii ), where U(C i ) and U(C ii ) are respectively the equivalent thermal parameters of the carbon atoms to which the corresponding H atoms are bonded. All calculations were performed using the SHELXTL software. 4 The principal crystallographic data and refinement parameters: Crystals suitable for X-ray diffraction study were obtained by layering the CH 2 Cl 2 solution of complex 1 hexanes at room tempearature followed by cooling at -20 °C. CCDC number 1956198, C 39 H 45 AuF 2 N 2 , Orthorhombic, space group P2 1

Thin-film preparation
The solid-state samples for steady-state UV-Vis, photoluminescence (PL) and time-resolved PL measurements were prepared on the pre-cleaned Quartz substrates. Solution-processed complex 3 and 4 were spin-coated inside a solvent glovebox from anhydrous chlorobenzene solutions in either pristine or blended conditions. The samples were then annealed in the hot plate at 90°C for 10 minutes to remove the remaining solvent. Vacuum-deposited pristine or doped thin films for complex 1 and 2 were prepared by thermal evaporation under high vacuum (10 -7 torr). The thickness of all the thin-films is ca. 80-100 nm.

Photophysical Characterisation
Solution UV-visible absorption spectra were recorded using a Perkin-Elmer Lambda 35 Liquid helium was used for sample cooling and a temperature-controlled cryostat was used for temperature regulation.
28 Figure S9. Normalized UV-vis and emission spectra in toluene, THF and CH 2 Cl 2 for complexes 1 (a, b),    Fitting and calculation of the activation energy for reverse intersystem crosing (rISC), radiative ( ) and non-radiative decay rate ( ) As the non-radiative decay is no longer negligible for twisted/tilted complexes, we assume it follows the same rate equation of activation energy 7 (S1) Where and are the activation energy for radiative decay and non-radiative decay ∆ , ∆ , respectively. is the activation energy for reverse intersystem crosing (rISC) from triplets to ∆ , singlets. Both decay rates are related to the total PL intensity ( ) and the absolute decay rate ( ) with the following relationships: is the temperature dependent total PL intensities calculated by integrating the timedependent decays over the complete measurement window and is the reciprocal of the corresponded decay time when the normalized PL integral equals to . 1 The equations S3 and S4 were adopted as the constraints to get all the values ( , , , 1 2 ∆ , , and ) in equations S1 and S2. Because the non-radiative decay process becomes ∆ , 0 1 significant only at high temperature, below that temperature T 1 , and increase with temperature. Consequently, the value of can be calculated by fitting the curve of VS. ∆ , Temperature from 10 K to K 1 .
We take complex 3 as the example to demonstrate the fitting process: Figure S12. (a) Total PL intensity (black dots) and absolute decay rate (red dots) obtained from temperature-dependent time-resolved PL spectra by using an electrically gated ICCD, the red dash line is the fitting to the decay rate from 10 K to 175 K by using the rate equation; (b) experimental absolute decay rate and fitted using equation S3; (c) experimental total PL intensity and fitted using equation S4.
The total PL intensity begins to drop from 175 K, so we only fit the data of and below 175 K by using the rate equation (S1). The calculated average activation energy is 42.2 meV and we substitute this value as in equation S1. By fitting the and among the ∆ , whole measurement window (10K to 100K) and applying equations S3 and S4, we obtained the optimised values for other constants in equations S1 and S2 and collected in Table S1.

Solution-processing device fabrication
Devices were carried out by solution processing with a forward configuration as follows:

OLED Characterisation.
The forward-viewing current-voltage-luminance characteristics of these OLED devices were measured using a Keithley 2400 source meter, Keithley 2000 multimeter and a calibrated Si photodiode (from RS components), which was placed at a distance of 4 cm from the devices.
External quantum efficiencies were calculated from on-axis irradiance assuming a Lambertian emission profile and accounting for photodiode quantum efficiency across the electroluminescence spectrum. The electroluminescence spectra were obtained by a fibre spectrometer (Flame-S-VIS-NIR-ES, Ocean Optics). All the measurements were carried out at room temperature under ambient conditions.   (c) and 4 (d).    Figure S27. The difference of electronic density associated with the singlet and triplet excited states in the ground state geometry for complex 2.    Figure S29. The difference of electronic density associated with the singlet and triplet excited states in the ground state geometry for complex 4.