Effect of a twin-emitter design strategy on a previously reported thermally activated delayed fluorescence organic light-emitting diode

In this work we showcase the emitter DICzTRZ in which we employed a twin-emitter design of our previously reported material, ICzTRZ. This new system presented a red-shifted emission at 488 nm compared to that of ICzTRZ at 475 nm and showed a comparable photoluminescence quantum yield of 57.1% in a 20 wt % CzSi film versus 63.3% for ICzTRZ. The emitter was then incorporated within a solution-processed organic light-emitting diode that showed a maximum external quantum efficiency of 8.4%, with Commission Internationale de l’Éclairage coordinate of (0.22, 0.47), at 1 mA cm−2.


General methods
NMR spectra were recorded using the following devices: 1 H NMR: Bruker Avance 400 (400 MHz), 13 C NMR: Bruker AM 400 (100 MHz). Chloroform-d1 from Eurisotop was used as solvent for NMR. Chemical shifts δ were expressed in parts per million (ppm) and referenced to chloroform ( 1 H: δ = 7.26 ppm, 13 C: δ = 77.16 ppm). The signal structure is described as follows: s = singlet, d = doublet, t = triplet, q = quartet, quin = quintet, b = broad singlet, m = multiplet, dd = doublet of doublet, dt = doublet of triplet. The spectra were analysed according to first order. All coupling constants are absolute values and expressed in Hertz (Hz).
Matrix-assisted laser desorption ionization-time of flight (MALDI-TOF) mass spectra were recorded on a BRUKER Biflex IV spectrometer with a pulsed ultraviolet nitrogen laser (200 µJ at 337 nm) and a time-of-flight mass analyzer with a 125 cm linear flight path.
The infrared spectra of solid samples were recorded on Bruker IFS 88 and measured by attenuated total reflection (ATR method). Absorption is given in wave numbers ῡ [cm −1 ].
Analytical thin layer chromatography (TLC) was carried out on Merck silica gel coated aluminum plates (silica gel 60, F254), detected under UV-light at 254 nm or stained with "Seebach staining solution" (mixture of molybdato phosphoric acid, cerium(IV) sulfate tetrahydrate, sulfuric acid and water) or basic potassium permanganate solution. Solvent mixtures are understood as volume/volume. Spectroscopic grade solvents were either purchased from Fisher Scientific or Sigma Aldrich (<50 ppm H2O). Other solvents were acquired from Sigma Aldrich, Carl Roth or Fisher Scientific. Unless otherwise stated, all solvents and reagents were used without further purification.

Electrochemistry measurements. (In a manner analogous to [1]) Cyclic Voltammetry (CV) and
Differential Pulse Voltammetry analyses were performed on an Electrochemical Analyzer potentiostat model 620D from CH Instruments. Sample of DICzTRZ were prepared in DCM that was degassed by sparging with DCM-saturated nitrogen gas for 10 minutes before measurements. All measurements were performed in 0.1 M DCM solution of tetrabutylammonium hexafluorophosphate, which was used as the supporting electrolyte. An Ag/Ag + electrode was used as the reference electrode while a glassy carbon electrode and a platinum wire were used as the working electrode and counter electrode, respectively. The redox potentials are reported relative to a saturated calomel electrode (SCE) with a S3 ferrocene/ferrocenium (Fc/Fc + ) redox couple as the internal standard (0.46 V vs SCE for DCM [2]). For emission studies, aerated solutions were bubbled with compressed air for 5 minutes and spectra were taken using the same cuvette as for the absorption analysis. Degassed solutions were prepared via three freeze-pump-thaw cycles and spectra were taken using a home-made Schlenk quartz cuvette. Steady-state emission, excitation spectra and time-resolved emission spectra were recorded at 298 K using an Edinburgh Instruments F980. Samples were excited at 340 nm for steady-state measurements and at 378 nm for time-resolved measurements.
Photoluminescence quantum yields for solutions were determined using the optically dilute method [3] in which four sample solutions with absorbances of ca. 0.104, 0.087, 0.065 and 0.047, at 360 nm were used. The Beer-Lambert law was found to remain linear at the concentrations of the solutions. For each sample, linearity between absorption and emission intensity was verified through linear regression analysis with the Pearson regression factor (R 2 ) for the linear fit of the data set surpassing 0.9. Individual relative quantum yield values were calculated for each solution and the values reported represent the slope obtained from the linear fit of these results. The equation Φs = Φr(Ar/As)(Is/Ir)(ns/nr) 2 was used to calculate the relative quantum yield of the sample, where (Φr) is the absolute quantum yield of the external reference quinine sulfate (Φr = 54.6% in 1 N H2SO4) [4], A stands for the absorbance at the excitation wavelength, I is the integrated area under the corrected emission curve and n is the refractive index of the solvent. The subscripts "s" and "r" representing sample and reference, respectively.
The experimental uncertainty in the emission quantum yields is conservatively estimated to be 10%, though we have found that statistically, we can reproduce ΦPL values to 3% relative error.
Thin-film ΦPL measurements were performed using an integrating sphere in a Hamamatsu C9920-02 system. A xenon lamp coupled to a monochromator enabled excitation selectivity, chosen here to be 340 nm. The output was then fed into the integrating sphere via a fibre, S4 exciting the sample. PL spectra were collected with a multimode fibre and detected with a backthinned CCD. Doped thin films were prepared by mixing sample (3 wt %) and host material in toluene solution, followed by spin-casting on a quartz substrate. The ΦPL of the films were then  The average lifetime was then calculated using the following:  Two exponential decay model: with weight defined as 1 = where A1 and A2 are the preexponential-factors of each component.
Theoretical calculations: All ground state optimizations have been carried out at the Density Functional Theory (DFT) level with Gaussian09 [5] using the PBE0 [6] functional and the 6-31G(d,p) basis set [7]. Excited state calculations have been performed at Time-Dependent DFT (TD-DFT) within the Tamm-Dancoff approximation (TDA [8,9]) using the same functional and S5 basis set as for ground state geometry optimization. This methodology has been demonstrated to show a quantitative estimate of EST in comparison to experiment [10].
Angular dependent photoluminescence spectroscopy. Thin films of emitter and host were spin-coated on a pre-cleaned quartz glass substrate. This substrate was then glued with an index matching fluid on a fused-silica prism which was mounted on the rotating stage.
The organic film was then irradiated with a UV laser (Kimmon, HeCd laser, λ = 325 nm) under vertical incidence and was rotated from −90° to +90° with respect to the substrate normal. The luminescence was recorded with a grating spectrograph coupled to a liquidnitrogen cooled charge-coupled device (Princeton Instruments Acton 2300i with PyLoN detector) in s and p polarization mode. P-polarized signal was then subjected to numerical simulation to calculate the orientation factor (a): Where Ʃ 2 is the sum of the power emitted by vertically oriented dipoles and Ʃ 2 is the sum of the power emitted by all emitting dipoles [11]. The parameter a (or synonymously v = <cos²>) denotes the second moment of the TDM's angular distribution around the surface normal of the film, where  is the angle between the molecule's TDM vector and the said direction. The details about the method can be further found in the reference [11,12].