Phenoxazine-Dibenzothiophene Sulfoximine Emitters Featuring Both Thermally Activated Delayed Fluorescence and Aggregation Induced Emission

In this work, we demonstrate dibenzothiophene sulfoximine derivatives as building blocks for constructing emitters featuring both thermally activated delayed fluorescent (TADF) and aggregation-induced emission (AIE) properties, with multiple advantages including high chemical and thermal stability, facile functionalization, as well as tunable electron-accepting ability. A series of phenoxazine-dibenzothiophene sulfoximine structured TADF emitters were successfully synthesized and their photophysical and electroluminescent properties were evaluated. The electroluminescence devices based on these emitters displayed diverse emissions from yellow to orange and reached external quantum efficiencies (EQEs) of 5.8% with 16.7% efficiency roll-off at a high brightness of 1000 cd·m−2.

Sulfoximines are monoaza analogs of sulfones featuring high stability and versatile chemistry that have been widely studied in the pharmaceutical field [41,42]. Apart from that, the mild basic nitrogen atom in sulfoximine provides the possibility for facile chemical functionalization [43]. In this way, the photophysical properties could be easily adjusted by tuning the electron-withdrawing ability of the functionalized sulfoximines using appropriate N-substitutes [44]. In this work, we report three TADF fluorophores with aggregation-induced emission (AIE) character using dibenzothiophene sulfoximine with different N-functional units as an acceptor and 10H-phenoxazine (PXZ) as a donor. These emitters were denoted as PXZ-SFIP, PXZ-SFIC, PXZ-SFIS, with the NH site substituted by diphenylphosphoryl, benzoyl, and benzenesulfonyl groups, respectively, as shown in Scheme 1. The facile metal-free functionalization of dibenzothiophene sulfoximine provides delicate control of electron deficiency of the acceptors and regulates the overall photophysical performance. OLEDs fabricated with these TADF emitters as a dopant in emitting layer obtained diverse electroluminescence (EL) from yellow to orange, with a maximum external quantum efficiency (EQEmax) exceeding 5%, and insignificant efficiency rolloff of 16.7% at a high brightness of 1000 cdˑm −2 , demonstrating the phenoxazine-dibenzothiophene sulfoximine compounds as promising emitters for TADF-OLEDs. Scheme 1. Synthesis route of the TADF emitters with functionalized dibenzothiophene sulfoximine-type acceptor.

Materials
All starting chemicals and reagents were purchased from commercial suppliers and used without further purification. Scheme 1. Synthesis route of the TADF emitters with functionalized dibenzothiophene sulfoximine-type acceptor.

Materials
All starting chemicals and reagents were purchased from commercial suppliers and used without further purification.

General Information
The Fourier-transformed infrared spectra (FT-IR) were recorded on a Thermo Nicolet 6700 Spectrometer. The 1 H and 13 C nuclear magnetic resonance (NMR) spectra were collected in dilute CDCl 3 solution at 25 • C on a Bruker AVANCE III 500 superconducting-Molecules 2021, 26, 5243 3 of 12 magnet high-field NMR spectrometer at 500 MHz and 126 MHz, respectively, using tetramethylsilane (TMS) as an internal standard. Thermogravimetric analysis (TGA) was carried out with a TA instrument TGA Q50 under nitrogen flow (20 mL·min −1 ), and differential scanning calorimetry (DSC) was performed with a TA DSC Q200 under nitrogen. The glass transition temperature (T g ) was determined from the second heating scan. UV-Vis absorption spectra were recorded on a Shimadzu UV-2700 recording spectrophotometer. Photoluminescence (PL) spectra were recorded on a Hitachi F-4600 fluorescence spectrophotometer. Phosphorescence spectra of doped thin films were conducted at 77 K. The transient photoluminescence decay curves were measured by a single photon counting spectrometer from Edinburgh Instruments (FLS920) with a Picosecond Pulsed UV-LASTER (LASTER377) as the excitation source. The photoluminescence quantum efficiencies were measured using an absolute photoluminescence quantum yield measurement system (C9920-02, Hamamatsu Photonics).
Cyclic voltammetry (CV) was carried out in nitrogen-purged dichloromethane at room temperature with a CHI voltammetric analyzer. Tetrabutylammonium hexafluorophosphate (0.1 M) was used as the supporting electrolyte. The conventional three-electrode configuration consisted of a platinum working electrode, a platinum wire auxiliary electrode, and an Ag/Ag + reference electrode with ferrocenium ferrocene (Fc + /Fc) as the internal standard. Cyclic voltammograms were obtained at a scan rate of 100 mV/s. Formal potentials were calculated as the average of cyclic voltammetric anodic and cathodic peaks. The HOMO energy levels of the compounds were calculated according to the formula: −[4.8 + (E 1/2(ox/red) − E 1/2(Fc/Fc+ ))] eV. The LUMO energy levels of the compounds were calculated according to the HOMO values and the absorption on-set of the longer wavelength.

Fabrication and Characterization of Devices
The ITO coated glass substrates with a sheet resistance of 15 Ω square −1 were consecutively ultrasonicated with acetone/ethanol and dried with nitrogen gas flow, followed by 20 min ultraviolet light-ozone (UVO) treatment in a UV-ozone surface processor (PL16 series, Sen Lights Corporation). Then, the sample was transferred to the deposition system. All organic layers were deposited at a rate of 1 Å s −1 , and subsequently, Liq was deposited at 0.2 Å s −1 and then capped with Al (ca. 4 Å s −1 ) through a shadow mask in a vacuum of 2 × 10 −5 mbar. For all the OLEDs, the emitting areas were determined by the overlap of two electrodes as 0.09 cm 2 . The as-fabricated devices were measured in an ambient environment without any encapsulation. The EL spectra and Commission Internationale de l'Eclairage (CIE) coordinates were recorded with a Keithley 2400 source meter unit. The current density-voltage-luminance (J-V-L) curves of the devices were measured with a PHOTO RESEARCH SpectraScan PR 735 spectrometer. The EQE was calculated from the current density, luminance, and EL (electroluminescence) spectrum, assuming a Lambertian distribution.

Synthesis and Characterization
The Br-SFIP/Br-SFIC/Br-SFIS compounds were prepared according to the previously reported procedure [45]. The donor unit 10H-phenoxazine was connected to the Br-SFIX precursor via Buchwald-Hartwig coupling. The 1 H NMR and 13 C NMR spectra were in good agreement with the structure of the TADF molecules of PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS.

Thermal and Electrochemical Properties
Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were employed to investigate the thermal properties of the TADF compounds. High decomposition temperatures (T d s: corresponding to the temperature at 5% weight loss) of 350, 320, and 355 • C were recorded for PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS, respectively (see ESI, Figure S1). Meanwhile, the emitters presented sufficient high glass transition temperatures (T g s) to prevent crystallization upon operation of the devices, as indicated by DSC measurements, where PXZ-SFIC presented the highest T g value of 117 • C, comparing to that of 109 • C for PXZ-SFIP and 98 • C for PXZ-SFIS (see ESI, Figure S2). To study the electrochemical properties of the designed TADF compounds, cyclic voltammetry was performed ( Figure S3). The HOMO levels of the emitters were estimated to be~−5.19 eV for all cases, reflecting comparable oxidation potential arising from PXZ electron-donating units (Table 1).

Theoretical Calculation
Density functional theory (DFT) calculations at the B3LYP-D3(BJ)/def2-SVP level were carried out to reveal the optimized ground-state geometries and the electronic structures of the designed emitters. As shown in Figure 1, the dihedral angles between the PXZ donor planes and the adjacent dibenzothiophene sulfoximine moieties were measured to be 78.5 • , 85.6 • , and 85.4 • in PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS, respectively, leading to nearly complete separation of the frontier molecular orbitals (FMOs). The diphenylphosphoryl, benzoyl, and benzenesulfonyl substituted dibenzothiophene sulfoximines exhibited a trend of increased electron-withdrawing ability, demonstrated by gradually deepened LUMO levels of −2.23 eV for PXZ-SFIP, −2.39 eV for PXZ-SFIC, and −2.58 eV for PXZ-SFIS in sequence. The energy gaps between HOMO and LUMO of the three compounds were calculated to be 2.74 eV, 2.69 eV, and 2.68 eV, respectively, as depicted in Figure 1.

Photophysical Properties
The UV-Vis absorption spectra of the TADF compounds in dilute toluene solutions (10 −5 molˑL −1 ) are exhibited in Figure 2. The bands below 350 nm with high intensity were ascribed to π-π* absorption from the conjugated skeleton, while the broad structureless bands peaking at 400~430 nm represented characteristic intramolecular charge transfer (ICT) transition from the phenoxazine to dibenzothiophene sulfoximine unit. Upon excitation, the PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS compounds revealed fluorescence with singular Gaussian-shaped emission bands, peaking at 576 nm, 574 nm, and 611 nm, respectively. Amongst three emitters, PXZ-SFIS, acquiring more electron-deficient benzenesulfonyl substitution on imine, caused a larger Stocks shift with red-shifted emission, while the emission bands of PXZ-SFIP and PXZ-SFIC were nearly overlapping with each  (Table 1).
Meanwhile, the oscillator strengths (f ) from S 0 →S 1 were calculated to be 0.0177, 0.0028, and 0.0001 for PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS, respectively, predicting a higher singlet radiative transition rate (k r,S ) of PXZ-SFIP due to the presence of small yet adequate overlap between its FMOs. The corresponding information is summarized in Table S1.

Photophysical Properties
The UV-Vis absorption spectra of the TADF compounds in dilute toluene solutions (10 −5 mol·L −1 ) are exhibited in Figure 2. The bands below 350 nm with high intensity were ascribed to π-π* absorption from the conjugated skeleton, while the broad structureless bands peaking at 400~430 nm represented characteristic intramolecular charge transfer (ICT) transition from the phenoxazine to dibenzothiophene sulfoximine unit. Upon excitation, the PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS compounds revealed fluorescence with singular Gaussian-shaped emission bands, peaking at 576 nm, 574 nm, and 611 nm, respectively. Amongst three emitters, PXZ-SFIS, acquiring more electron-deficient benzenesulfonyl substitution on imine, caused a larger Stocks shift with red-shifted emission, while the emission bands of PXZ-SFIP and PXZ-SFIC were nearly overlapping with each other. Optical bandgaps of the emitters were 2.60 eV, 2.54 eV, and 2.49 eV, separately, as estimated from the onset of the absorption spectra from the longer wavelength. The decreasing optical bandgap corresponded well with increasing electron deficiency of the functioned dibenzothiophene sulfoximine, giving deepening LUMO levels of −2.59 eV for PXZ-SFIP, −2.64 eV for PXZ-SFIC, and −2.70 eV for PXZ-SFIS (Table 2).  AIE characteristics of the PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS compounds in THF/water mixed solvents with different water fractions (fW) were investigated, as shown in Figure 3 and Figure S4. In solutions with fW lower than 80%, the fully dissolved emitters were almost non-emissive where peripheral functional groups could freely rotate and dissipate energy. Further increasing fW led to a significant increase in PL intensities by the formation of aggregates, as explained by the restriction of intramolecular rotation (RIR)  AIE characteristics of the PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS compounds in THF/ water mixed solvents with different water fractions (f W ) were investigated, as shown in Figure 3 and Figure S4. In solutions with f W lower than 80%, the fully dissolved emitters were almost non-emissive where peripheral functional groups could freely rotate and dissipate energy. Further increasing f W led to a significant increase in PL intensities by the formation of aggregates, as explained by the restriction of intramolecular rotation (RIR) of the TADF emitter [45]. This distinct AIE behavior was beneficial for solid-state emission due to the suppression of undesirable aggregation-caused quenching (ACQ), which could potentially improve device performances and alleviate undesirable efficiency roll-off at high brightness in OLEDs. Experimental excited state energies were measured by fluorescent and phosphorescent spectroscopy in film state, as an imitation of the environment in emissive layers (EML) of OLEDs. Here, bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) was selected as host material due to its large energy band gap and high T1 level (~3.4 eV). As shown in Figure 4, S1 and T1 levels of the emitters were estimated from the onset of fluorescent and phosphorescent spectra, giving the values of 2.61/2.59 eV for PXZ-SFIP, 2.54/2.52 eV for PXZ-SFIC, and 2.51/2.48 eV for PXZ-SFIS. The small ΔESTs around 0.02~0.03 eV were consistent with the previous theoretical calculation results, assuring an effective up-conversion process from T1 to S1. To confirm their TADF character, the transient PL decay profiles of the three emitters were recorded in both solution and doped films. The samples in degassed toluene displayed obvious delayed components that could be largely quenched in aerated conditions due to the sensitivity of the triplet state excitons to oxygen ( Figure S5). Meanwhile, the transient PL curves of 10 wt% doped DPEPO films could also be fitted with biexponential functions, with prompt and delayed fluorescence lifetimes (τp/τd) of 30.5 ns/1.2 μs, 28.7 ns/1.3 μs, and 29.9 ns/1.2 μs for PXZ-SFIP, PXZ-SFIC, and PXZ-SFIS, separately, and the ratios of the delayed fluorescence were 64.4%, 62.3%, and 61.3%. The absolute photoluminescent quantum yield (ΦPL) of the DPEPO films doped with the TADF emitters was found to be 52%/41%/21% for PXZ-SFIP/PXZ-SFIC/PXZ-SFIS.
Subsequently, the S1→S0 radiative and nonradiative decay rate constants (kr,S, knr,S) together with the ISC (kISC) and RISC rate constants (kRISC) were extrapolated to provide more in-depth information on the TADF characteristics and PL efficiencies ( Table 3). The emitters shared comparable kISC and kRISC values due to similar ΔESTs, noting that all kRISCs were at the state-of-the-art level of ~10 6 s −1 to guarantee effective utilization of triplet excitons. On the contrary, the kr,S values decreased significantly from 6.1/5.4 × 10 6 s −1 of PXZ-SFIP/PXZ-SFIC to 2.7 × 10 6 s −1 of PXZ-SFIS, in line with the trend of theoretically predicted oscillator strengths. The PXZ-SFIS also possessed the largest knr,S in these emitters, with this value even outcompeting its kr,S, manifesting considerable nonradiative dissipation of
In 10 wt% doped devices, the EL spectra clearly originated from the TADF emitters, suggesting complete energy transfer from DPEPO host to dopants. The devices displayed yellow to orange emissions peaking at 553 nm for PXZ-SFIP, 572 nm for PXZ-SFIC, and 584 nm PXZ-SFIS, respectively. PXZ-SFIP with the highest ΦPL exhibited the best device performance with the maximum external quantum efficiency (EQEmax) of 5.75%, current efficiency (CEmax) of 17.06 cdˑA −1 , and power efficiency (PEmax) of 13.40 lmˑW −1 . Comparatively, devices based on PXZ-SFIC and PXZ-SFIS showed slightly inferior efficiencies with maximum EQE values at 3.17% and 1.30%, separately, which could be well explained by their suppressed singlet radiative transition and faster non-radiative internal conversion process. It is noteworthy that the devices did not come across serious efficiency rolloff at high brightness, attributing to the emitters' AIE character, i.e., the efficiency roll-off value of the PXZ-SFIP device was only 16.7% at 1000 cdˑm −2 . Additionally, the shortened τDFs and efficient RISC process may also suppress notorious triplet-triplet annihilation (TTA), singlet-triplet annihilation (STA), or triplet polaron exciton annihilation. The EL data for all the emitters are summarized in Table 4. Subsequently, the S 1 →S 0 radiative and nonradiative decay rate constants (k r,S , k nr,S ) together with the ISC (k ISC ) and RISC rate constants (k RISC ) were extrapolated to provide more in-depth information on the TADF characteristics and PL efficiencies ( Table 3). The emitters shared comparable k ISC and k RISC values due to similar ∆E ST s, noting that all k RISC s were at the state-of-the-art level of~10 6 s −1 to guarantee effective utilization of triplet excitons. On the contrary, the k r,S values decreased significantly from 6.1/5.4 × 10 6 s −1 of PXZ-SFIP/PXZ-SFIC to 2.7 × 10 6 s −1 of PXZ-SFIS, in line with the trend of theoretically predicted oscillator strengths. The PXZ-SFIS also possessed the largest k nr,S in these emitters, with this value even outcompeting its k r,S , manifesting considerable nonradiative dissipation of excitons through internal conversion with its more stabilized CT energy as governed by the energy-gap law.

Conclusions
In summary, a series of TADF emitters were constructed by 10H-phenoxazine and novel dibenzothiophene sulfoximine derivatives. By attaching diphenylphosphoryl, benzoyl, and benzenesulfonyl units with gradually increased electron deficiencies at the secondary amine in dibenzothiophene sulfoximine acceptors, bathochromic shifted fluorescence from yellow to orange were obtained with more stabilized CT energy. The orthogonal D-A conformation endowed these emitters nearly degenerate S1 and T1 state, high kRISCs at the level of ~10 6 s −1 , and efficient delayed fluorescence. At the same time, the emitters manifested distinct AIE properties with ΦPL up to 52% in film state. Subsequently, devices based on PXZ-SFIP as emitting material realized a maximum EQE of 5.8% with reduced efficiency roll-off. This work not only demonstrates dibenzothiophene sulfoximine as potential construction units for AIE-active TADF emitters, but also provides a meaningful strategy for designing color-managing acceptors.
Supplementary Materials: The following are available online, Figure S1: The thermogravimetric analysis (TGA) plots of PXZ-SFIS, PXZ-SFIC and PXZ-SFIP under N2 stream (flow rate: 20 mL min -1 ; heating rate: 10 °C min -1 ), Figure S2: DSC thermograms (second heating cycle) of PXZ-SFIS, PXZ- In 10 wt% doped devices, the EL spectra clearly originated from the TADF emitters, suggesting complete energy transfer from DPEPO host to dopants. The devices displayed yellow to orange emissions peaking at 553 nm for PXZ-SFIP, 572 nm for PXZ-SFIC, and 584 nm PXZ-SFIS, respectively. PXZ-SFIP with the highest Φ PL exhibited the best device performance with the maximum external quantum efficiency (EQE max ) of 5.75%, current efficiency (CE max ) of 17.06 cd·A −1 , and power efficiency (PE max ) of 13.40 lm·W −1 . Comparatively, devices based on PXZ-SFIC and PXZ-SFIS showed slightly inferior efficiencies with maximum EQE values at 3.17% and 1.30%, separately, which could be well explained by their suppressed singlet radiative transition and faster non-radiative internal conversion process. It is noteworthy that the devices did not come across serious efficiency roll-off at high brightness, attributing to the emitters' AIE character, i.e., the efficiency roll-off value of the PXZ-SFIP device was only 16.7% at 1000 cd·m −2 . Additionally, the shortened τ DF s and efficient RISC process may also suppress notorious triplet-triplet annihilation (TTA), singlet-triplet annihilation (STA), or triplet polaron exciton annihilation. The EL data for all the emitters are summarized in Table 4.

Conclusions
In summary, a series of TADF emitters were constructed by 10H-phenoxazine and novel dibenzothiophene sulfoximine derivatives. By attaching diphenylphosphoryl, benzoyl, and benzenesulfonyl units with gradually increased electron deficiencies at the secondary amine in dibenzothiophene sulfoximine acceptors, bathochromic shifted fluorescence from yellow to orange were obtained with more stabilized CT energy. The orthogonal D-A conformation endowed these emitters nearly degenerate S 1 and T 1 state, high k RISC s at the level of~10 6 s −1 , and efficient delayed fluorescence. At the same time, the emitters manifested distinct AIE properties with Φ PL up to 52% in film state. Subsequently, devices based on PXZ-SFIP as emitting material realized a maximum EQE of 5.8% with reduced efficiency roll-off. This work not only demonstrates dibenzothiophene sulfoximine as potential construction units for AIE-active TADF emitters, but also provides a meaningful strategy for designing color-managing acceptors.  Table S1: Natural transition orbitals (NTO) in the S 1 and T 1 states of the TADF emitters, Table S2: Thermal analysis of the three compounds.