Aryl-Substituted Acridine Donor Derivatives Modulate the Transition Dipole Moment Orientation and Exciton Harvesting Properties of Donor–Acceptor TADF Emitters

Thermally activated delayed fluorescence (TADF) compounds are highly attractive as sensitizing and emitting materials for organic light-emitting diodes (OLEDs). The efficiency of the OLED depends on multiple parameters, most of which rely on the properties of the emitter including those that govern the internal quantum and outcoupling efficiencies. Herein, we investigate a series of aryl substituted acridine donor derivatives of the donor–acceptor TADF emitter DMAC-TRZ, with the objective of correlating their properties, such as triplet harvesting efficiency and transition dipole moment orientation, with their corresponding device efficiency. The decoration of the DMAC donor with substituted aryl groups not only modifies the molecular weight and length of the emitter but also affects the emission color and the capacity for the emitters to efficiently harvest triplet excitons. The presence of electron-withdrawing 4-cyanophenyl and 4-trifluoromethylphenyl groups in, respectively, CNPh-DMAC-TRZ and CF3Ph-DMAC-TRZ, blue-shifts the emission spectrum but slows down the reverse intersystem crossing rate constant (kRISC), while the opposite occurs in the presence of electron-donating groups in tBuPh-DMAC-TRZ and OMePh-DMAC-TRZ (red-shifted emission spectrum and faster kRISC). In contrast to our expectations, the OLED performance of the five DMAC-TRZ derivatives does not scale with their degree of horizontal emitter orientation but follows the kRISC rates. This, in turn, demonstrates that triplet harvesting (and not horizontal emitter orientation) is the dominant effect for device efficiency using this family of emitters. Nonetheless, highly efficient OLEDs were fabricated with tBuPh-DMAC-TRZ and OMePh-DMAC-TRZ as emitters, with improved EQEmax (∼28%) compared to the reference DMAC-TRZ devices.


Light outcoupling efficiency simulations
The light outcoupling for the device stack reported in the manuscript has been simulated using the commercial software SETFOS from Fluxim. 23A mode analysis for the six OLEDs (with their individual PL emission spectra considered) was performed while sweeping the transition dipole orientation a from horizontal (a=0 to vertical a=1).The experimentally measured orientation is marked by a black square for each emitter and is given together with the simulated light outcoupling in the table below.

Figure S38 .
Figure S38.Time-resolved fluorescence decay at a) room temperature; b) 80 K of the extended derivatives in 10 wt% doped films in mCP (lexc= 355 nm).

Figure S40 .
Figure S40.HOMO and LUMO electron density distributions and energy levels, excited-state energy levels of DMAC and its five aryl decorated derivatives (Obtained via DFT and TD-DFT at the PBE0/6-31G(d,p) level, Isovalues: MO=0.02).

Figure
Figure S41.a) OLED stack; b) pictures of the operating OLEDs.

Figure
Figure S42.a) EQE vs Luminance curves taken above 500 cd m -2 and b) Electroluminescence spectra (at 1000 cd m -2 ) of the t BuPh-DMAC-TRZ OLEDs doped at different concentrations.

Mass found: 387
.Spectral data matches those previously reported in the literature.19

HPLC retention time: 10
Calculated C: 83.54, H: 4.77, N: 11.69; Found C: 82.63, H: 4.78, N: 11.55.The carbon content is not within 0.4% error; however, the remaining characterization is consistent and demonstrates the identity and purity of the compound.

Table S2 .
Properties of the excited states involved in the UV-vis absorption transitions of

Table S3 .
Calculated optoelectronic properties of DMAC-TRZ and derivatives.