Regiochemistry of Donor Dendrons Controls the Performance of Thermally Activated Delayed Fluorescence Dendrimer Emitters for High Efficiency Solution‐Processed Organic Light‐Emitting Diodes

Abstract The potential of dendrimers exhibiting thermally activated delayed fluorescence (TADF) as emitters in solution‐processed organic light‐emitting diodes (OLEDs) has to date not yet been realized. This in part is due to a poor understanding of the structure–property relationship in dendrimers where reports of detailed photophysical characterization and mechanism studies are lacking. In this report, using absorption and solvatochromic photoluminescence studies in solution, the origin and character of the lowest excited electronic states in dendrimers with multiple dendritic electron‐donating moieties connected to a central electron‐withdrawing core via a para‐ or a meta‐phenylene bridge is probed. Characterization of host‐free OLEDs reveals the superiority of meta‐linked dendrimers as compared to the already reported para‐analogue. Comparative temperature‐dependent time‐resolved solid‐state photoluminescence measurements and quantum chemical studies explore the effect of the substitution mode on the TADF properties and the reverse intersystem crossing (RISC) mechanism, respectively. For TADF dendrimers with similarly small ∆E ST, it is observed that RISC can be enhanced by the regiochemistry of the donor dendrons due to control of the reorganization energies, which is a heretofore unexploited strategy that is distinct from the involvement of intermediate triplet states through a nonadiabatic (vibronic) coupling with the lowest singlet charge transfer state.


OLED fabrication and characterization
The OLED devices were fabricated using a bottom-emitting architecture. A pre-patterned indium tin oxide (ITO) glass substrate with a sheet resistance of 15 Ω square -1 was pre-cleaned carefully with detergent and deionized water and then exposed to UV-ozone for 15 min.
PEDOT:PSS was spin-coated onto the clean ITO substrate as the hole-injection layer, followed by thermal treatment under 120 o C for 30 min. Then a 10 mg/mL chlorobenzene solution of our dendrimers was spin-coated to form a 35-45 nm thick emissive layer (EML) and annealed at 120 °C for 10 min to remove residual solvent before transfer to the vacuum chamber. A 40 nm-thick electron-transporting layer (ETL) of Tm3PyPB was then vacuum deposited at a rate of 1 Å/s, which was controlled in situ using quartz crystal monitors. The electron injection layer LiF was deposited at a rate of 0.1 Å/s while the Al cathode was deposited at a rate of 10 Å/s through the shadow mask defining the top electrode. The spatial overlap of the anode and cathode electrodes determined the active area of the OLED, which was estimated to be 4 mm 2 .
Electroluminescence (EL), CIE color coordinates, and spectra were obtained via a Spectrascan PR655 photometer, and the luminance-current-voltage characteristics were determined with a computer-controlled Keithley 2400 Source meter. EQE was calculated from the current density, luminance, and EL spectrum, assuming Lambertian emission distribution.    Energy (eV) S17 Figure S7. Temperature dependent PL decays of dendrimer neat films.

Determination of photophysical rate constants
• The rate constants were determined according to the method described in literature. [14] • Prompt lifetime ( &' ) and delayed lifetime ( (' ) were determined from the monoexponential fits of the prompt and delayed components of the PL decay at RT.

(' &'
⁄ was determined from the ratio of the corresponding integrals in the PL decay curves.
• Nonradiative triplet decay rate, k 98 : = ; .()* Figure S16. Marcus' parabolic free energy curves for the description of the reorganization energy. Table S5. Reverse intersystem crossing rates determined from experiment and computation.