Control of the dual emission from a thermally activated delayed fluorescence emitter containing phenothiazine units in organic light-emitting diodes

The development of single-component organic dual light-emitting molecules is of interest for a range of applications including white organic light-emitting diodes. Herein, a new thermally-activated delayed fluorescent molecule containing 4,6-bis-phenyl phenothiazine as donor units and 2-thiophene-1,3,5-triazine as acceptor unit was synthesized using a simple cost-effective method. This compound shows two stable molecular conformations due to the presence of the phenothiazine units in its molecular structure. These conformers exhibit different photophysical properties in both solution and thin films. The electroluminescence properties of this novel emitter were then examined in organic light-emitting diodes and the results provide useful insights into the influence of the device architecture on the dual emission characteristics. The experimental results were consistent with the optical simulations and the optimized architecture led to the fabrication of electroluminescent devices with an external quantum efficiency of 11.5% and a maximum luminance value of 10 370 cd m−2.


Introduction
Organic light-emitting diodes (OLEDs) are now used in commercially available display applications and are extremely promising for solid-state lighting sources. Since the demonstration of uorescent OLED in 1987 by Tang and VanSlyke, 1 great enhancement of the electroluminescence (EL) quantum efficiency has been achieved using phosphorescence, 2-4 triplet fusion 5,6 and thermally-activated delayed uorescence (TADF). [7][8][9][10][11][12][13][14][15] In particular, light-emitting molecules with TADF properties are considered now as the third generation of OLED materials and enable the harvesting of light from both singlet and triplet excitons without the use of any noble heavy metals. A large number of studies has already reported highperformance OLEDs using blue, green, red and near infrared TADF emissive dopants. [16][17][18][19][20] In most cases, to achieve such a high-efficiency TADF emission, a small singlet-triplet energy gap (DE ST ) is crucial to facilitate the upconversion from triplet to singlet excited states via thermally-activated reverse intersystem crossing (RISC) process. The most popular approach that have been used to develop TADF OLED emitters showing simultaneously small (DE ST ) and increased oscillator strength is based on twisted molecular donor-acceptor architectures with large dihedral angles between the electron-donating and accepting units, resulting in a small spatial overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) frontier orbitals. [21][22][23][24][25] Interestingly, among the large variety of TADF OLED emitters, some of them have been found to exhibit dual emission due to the existence of stable conformers. [26][27][28][29][30][31][32][33] This class of TADF emitters is evidently of strong interest for white TADF OLEDs and could be promising for other imaging or sensing applications beneting from a broad emission. Noticeably, while the rst example of single-component dual TADF emission was obtained using dyes containing phenothiazine units, 28 other moieties such as 9,9-dimethyl-9,10-dihydroacridine and phenoxazine have also been recently incorporated into lightemitting dyes with dual stable conformations leading to dual TADF emission during excitations. 26,34 In this context, the development of innovative TADF molecular architectures with dual conformations for highly efficient OLEDs and a deeper understanding of this dual emission phenomenon are still required.
In this study, we report on the synthesis of a novel TADF emitter ((T-TRZ)-PTZ) containing peripheral phenothiazine donor units and a central 2-thiophene-1,3,5-triazine acceptor unit using a cost-effective synthetic method. A schematic representation of the chemical structure of this dye is shown in Fig. 1. The 2-thiophene unit was herein chosen to shi the dual TADF emission toward longer wavelengths compared to the previously-reported phenyl analogue 29 and to determine its potential inuence on the electronic structure of the molecule. 35,36 In this context, quantum chemistry calculations as well as the characterization of the photophysical and EL properties of this novel compound are expected to provide insights into the dual emission of TADF molecules containing phenothiazine units. In addition, optical simulations of the EL device properties are carried out to demonstrate the role played by the OLED architecture on the dual emission properties.

Synthesis of (T-TRZ)-PTZ
Most of the reaction steps for the synthesis of emitters based on 1,3,5-triazine use expensive catalysts such as palladium. Recently, we reported for the rst time a cost-effective and simple synthetic method for the synthesis of TADF emitters incorporating 1,3,5-triazine moieties, which is based on the cobalt catalysed cross coupling of arylzinc with aryl halide. 29 The important benet from this approach is related to its production cost which is lower than using palladium catalyst. In the present work, we used this cobalt catalysed cross coupling method for the synthesis of (T-TRZ)-PTZ 3. The synthetic route is based on three main steps as described in Scheme 1. In rst place, the mono-substitution reaction of the 1,3,5-triazine with the thiophen-2-ylzinc(II) bromide catalysed by the cobalt crosscoupling was performed leading to the mono-thiophene triazine intermediate 1 containing two chloro functions that can be further substituted. This rst step is then followed by another cobalt cross-coupling between the triazine intermediate 1 and the 4-uorophenylzinc(II) bromide, previously prepared from commercially available 4-bromouorobenzene. This last coupling leads to the triazine intermediate 2 presenting two uoro functions. The nal product 3 was then obtained from 2 by a simple nucleophilic aromatic substitution of phenothiazine in presence of a strong base such as sodium hydride. Detailed chemical characterization of the intermediates and nal product, including NMR and HRMS, is provided in ESI (Fig. S1-S4 †).

Quantum chemistry calculations
All the quantum chemistry calculations were performed using Gaussian 09. 37 The electronic structures of (T-TRZ)-PTZ were examined using density functional theory (DFT) and timedependent density functional theory (TDDFT) with the Tamm-Dancoff approximation (TDA) 38 for avoiding triplet instabilities. 39 The ground-state geometries of (T-TRZ)-PTZ were optimized at the B3LYP/6-31G* level. Because of the presence of two phenothiazine units in ((T-TRZ)-PTZ), different stable conformations can be expected for this molecule. As shown in Fig. 2 and Table S1, † one axial (conformer A) and one equatorial (conformer E) congurations are evidenced by the calculations and are found to show minor energy differences (Table S2 †). Fig. 3 shows the distribution of the hole and electron natural transition orbitals (NTOs) of S 1 and T 1 calculated at the B3LYP/ 6-31G(d) level of theory based on the S 0 geometry for both conformers A and E using Multiwfn 3.4. 40 For each conformer, it is interesting to note that the donor moieties dominate the hole NTOs while the acceptor moieties determine the electron NTOs. Compared to conformer A, the almost identical hole and electron NTOs for the S 0 / S 1 and S 0 / T 1 charge transfer (CT) transitions in conformer E suggest a vanishing DE ST . 41 Thus a more facile exciton transformation channel via ISC and RISC may process between S 1 and T 1 . As shown in Table S3, † the two conformers are expected to exhibit different photophysical properties. For instance, calculations indicate that the conformer A should exhibit a strong absorption peak at about 378 nm (3.28 eV) while conformer E should display a atter and broad absorption band around 532 nm (2.33 eV). The theoretical results also conrm that (T-TRZ)-PTZ should exhibit a dual emission as a direct consequence of the presence of two phenothiazine units in its molecular structure. Importantly, the theoretical results show that the calculated DE ST values of the conformers A and E in their optimized ground-state geometries are 0.46 eV and 0.01 eV, respectively. This suggests that TADF activity should be signicantly more effective in the conformer E. Interestingly, these DE ST values are slightly different from those calculated in the previouslyreported phenyl analogue 29 (0.0079 and 0.71 eV for the E and A conformers, respectively).

Photophysical properties of (T-TRZ)-PTZ
The absorption and steady-state photoluminescence (PL) spectra of (T-TRZ)-PTZ measured at room temperature in degassed toluene solution are displayed in Fig. 4a. Concentration of the solution was 1 Â 10 À5 M. The dye (T-TRZ)-PTZ exhibits two main intense absorption bands at 330 and 364 nm, which are assigned to the 1 (p,p*) ) S 0 transition of the 1,3,5 triazine. In addition, the weak absorption tail at wavelengths longer than 400 nm is related to the intramolecular CT character of the quasi-equatorial conformer of the molecule. The steady-state PL spectrum of (T-TRZ)-PTZ in toluene shows a dual emission with two peaks at around 430 and 593 nm, respectively. The peak at 593 nm shows the highest intensity, a large Stokes shi and a full-width-at-half-maximum of 96 nm, which can be assigned to the emission from the singlet excited state of the quasi-equatorial conformation. The peak at 430 nm presents a signicantly lower PL intensity and is attributed to the emission from the singlet excited state of the quasi-axial conformer. Noticeably, the PL emission of (T-TRZ)-PTZ in toluene is about 18 nm red-shied compared to that of the previously-reported phenyl analogue. 29 The results indicate that the incorporation of the thiophene unit in (T-TRZ)-PTZ affects the singlet-triplet gap as evidenced by the quantum chemistry calculations and the steady-state PL emission of the molecule. Fig. 4b shows the absorption and steady-state PL spectra of (T-TRZ)-PTZ doping at 6 wt% a 3,3 0 -bis(N-carbazolyl)-1,1 0biphenyl (mCBP) host. The lm was thermally evaporated on top of a fused silica substrate. The mCBP host was selected due to its high triplet energy T 1 , which prevents back energy transfer from the guest to the host molecules and connes the triplets within the guest emitters. Because of the small doping concentration of the guest molecules, the absorption spectrum of the mCBP blend is strongly dominated by the absorption of the host molecules. In particular, the absorption bands observed at 330 and 344 nm are due to the presence of carbazole units in the mCBP molecule. 42 Noticeably, considering the excitation wavelength of 340 nm, the steady-state PL spectrum of the mCBP blend does not show any emission from the host.
This implies that an efficient energy transfer takes place from the mCBP host to the (T-TRZ)-PTZ doping molecules. [43][44][45] It can also be seen that the steady-state PL spectrum of the blend lm exhibits a dual emission with two peaks located around 423 and 523 nm, respectively. Similarly to the solution case, the low and high energy peaks are attributed to the quasi-equatorial and quasi-axial conformers, respectively. It is worth noting that the emission peak of the quasi-equatorial conformer is signicantly blue-shied in the blend, as compared to its PL spectrum in toluene. This is presumably due to the ICT character of (T-TRZ)-PTZ and the different polarity of the environment. 11,20,46,47 To gain further insights into the photophysics of (T-TRZ)-PTZ, the transient PL decay of the equatorial conformer was measured in degased and non-degased toluene solution at a low concentration around 10 À6 M. Excitation wavelength was 337 nm and the emission wavelength was 593 nm. The kinetics shown in Fig. 5 can be described by the sum of two exponential decay functions. We dene the slow and fast time constants characteristics of these exponential decay functions to the delayed and prompt uorescence lifetimes, respectively. While the prompt uorescence lifetime does not change with the presence or not of oxygen and is in the order of 10 ns, the delayed uorescence lifetime is found to increase from 50 ns to 570 ns aer degasing the toluene solution. These data indicate that the equatorial conformer of (T-TRZ)-PTZ shows some TADF properties, in accordance with the theoretical results obtained by quantum chemistry calculations. The observed behavior in presence of oxygen shows indeed that the triplet states involved in the TADF process are readily quenched by the oxygen triplet ground state and this fully supports the assignment of TADF. An important parameter which needs to be measured in order to evaluate the potential of (T-TRZ)-PTZ for OLEDs is its  photoluminescence quantum yield (PLQY) in thin lm. We found that the PLQY of the 6 wt% mCBP blend in nitrogen atmosphere yields a value of 43%.  Fig. 6a. a-NPD and mCP were chosen for the hole transport layers while DPEPO and TPBi were selected for the hole blocking and electron transport layers, respectively. Fig. 6b-d show the EL characteristics of this OLED. The device exhibits greenyellowish emission with a turn-on voltage of 4.6 V, a maximum current efficiency of 35.47 cd A À1 , a power efficiency of 11.1 lm W À1 , a maximum luminance up to 10 370 cd m À2 and a maximum EQE of 9.4% without any light outcoupling   enhancement. These data could be reproduced in several devices. The obtained EQE value is signicantly higher than the upper theoretical limit stated for conventional uorescent OLEDs. We attribute this result to an efficient TADF process enabling the effective upconversion of the triplets to the singlets via RISC, 7 which is in agreement with the quantum chemistry calculations and previous reports devoted to similar dyes.

Characterization of the OLEDs
Looking at the EL spectra in Fig. 6b, it can be seen that only the emission from the quasi-equatorial conformer is observed. Previous reports have provided evidence of the strong inuence of the OLED architecture on the EL spectra due to optical cavity effects. [48][49][50][51][52][53] In order to examine the potential impact of these optical cavity effects on the dual emission spectrum of (T-TRZ)-PTZ, we carried out optical simulations to see if these optical cavity effects could hide the emission band at 427 nm. These optical simulations were performed using SETFOS 4.2 soware in the wavelength range between 380 and 750 nm. The lightemitting process taking place in the emissive layer of the OLED is simulated based on the model of emissive dipoles embedded within a multilayer stack. 54 Since the thicknesses of these layers are comparable to the wavelength of the emitted light, internal reections at the different interfaces occur, leading to optical interference effects. To model these interference effects in the OLED, a transfer matrix method is used, 55 taking into account the optical indices of the emissive layer determined by variable angle spectroscopic ellipsometry. 56 It should be noted that the calculation was carried out by assuming randomly distributed transition dipole moment approximated by a Dirac distribution placed at the interface between the emissive layer and the mCBP one.
It is well established that the distance between the emitting zone and the metal cathode predominantly affects the interference conditions at the position of the emissive zone. In this context, we decided to vary the thickness of the TPBi layer in the (T-TRZ)-PTZ OLED and see how this affects the intensity of the band at 427 nm in the EL spectrum. Fig. 7 shows the calculated EL spectrum of the OLED for different thicknesses of the TPBi lm (varying from 40 to 70 nm). The results demonstrate that the EL spectrum is red-shied and the intensity of the band at 427 nm is decreased when the TPBi thickness increases. In other words, the fact that the emission peak at 427 nm was not   observed in the EL spectrum can be explained by these interference effects. This clearly demonstrates that the architecture of the OLED should be carefully designed to optimize the dual emission in the EL spectrum.
To verify experimentally these modelling results, we fabricated an OLED using the same multilayer structure but with a thickness of 35 nm for the TPBi layer. The electroluminescence data obtained with such a device architecture are reported in Fig. 8. Noticeably, in contrast to what was achieved with a TPBi lm thickness of 55 nm, the emission peak at 427 nm can be observed in the EL spectrum. This fully supports the results obtained from the optical simulations and demonstrates the importance of the optical cavity effects on the dual emission from TADF OLEDs. The J-V curve and the EQE of the TADF OLED were also measured as a function of the current density. The maximum EQE value and luminance measured for this OLED were found to be as high as 11.5% and 10 370 cd cm À2 , respectively, providing evidence that not only the dual emission but also the overall performances of the EL device was improved aer the optimization of the TPBi lm thickness.

Conclusions
In summary, we designed and synthesized the novel TADF molecule (T-TRZ)-PTZ, which contains 4,6-bis-phenyl phenothiazine as donor units and 2-thiophene-1,3,5-triazine as acceptor unit. A simple and low-cost synthetic method based on cobalt catalysed cross-coupling was used to produce this lightemitting compound. This method ts with the asset of reducing OLED cost thanks to the used low-cost reagents. The photoluminescence spectrum of (T-TRZ)-PTZ in solution and lm exhibits a dual emission which is due to the incorporation of phenothiazine unit in the molecular structure, resulting in the existence of two stable molecular conformations for (T-TRZ)-PTZ. The electroluminescence properties of (T-TRZ)-PTZ are then characterized in OLEDs and the best device exhibited maximum external quantum efficiency and luminance values of 11.5% and 10 370 cd m À2 , respectively. Based on experimental and optical simulations results, this study also provides new useful evidences that the OLED architecture must be carefully designed to optimize the dual emission in the electroluminescence spectrum. This latter result should be extremely relevant to further studies devoted to the development of high-efficiency white OLEDs based on single-component organic dual lightemitting molecules.

Photophysical measurements
For the photophysical measurements, the degassed toluene solution of (T-TRZ)-PTZ was placed in a quartz cuvette. In parallel, the organic thin lm was deposited by thermal evaporation onto a precleaned fused silica substrate. UVvisible absorption and steady-state PL spectra were measured using a UV-2550 spectrometer (Shimadzu) and a Fluoromax-4 spectrophotometer (Horiba Scientic), respectively. The transient PL decay was measured by a streak camera (Hamamatsu Photonics C4334). The samples were placed under vacuum (<4 Â 10 À1 Pa) and photoexcited by a nitrogen gas laser (Ken-X, Usho Optical Systems) delivering 500 ps pulses at a repetition rate of 20 Hz and emitting at 337 nm.

OLED fabrication and measurements
The devices were prepared using precleaned indium tin oxide (ITO) substrates. The different organic layers were sequentially thermally evaporated under vacuum lower than 3 Â 10 À4 Pa. Finally, the LiF/Al electrodes were deposited through a shadow mask on top of the multilayer structure. The device active area was 4 mm 2 . To avoid any degradation and emission quenching due to oxygen and moisture, the OLEDs were encapsulated in a glovebox lled with nitrogen. The J-V-L characteristics were collected using a source meter (Keithley 2400, Keithley Instruments Inc.) and an absolute external quantum efficiency measurement system (C9920-12, Hamamatsu Photonics). An optical ber connected to a spectrometer (PMA-12, Hamamatsu Photonics) was used to record the electroluminescence spectra.

Conflicts of interest
There are no conicts to declare.