Modular Nitrogen‐Doped Concave Polycyclic Aromatic Hydrocarbons for High‐Performance Organic Light‐Emitting Diodes with Tunable Emission Mechanisms

Abstract Although bowl‐shaped N‐pyrrolic polycyclic aromatic hydrocarbons (PAHs) can achieve excellent electron‐donating ability, their application for optoelectronics is hampered by typically low photoluminescence quantum yields (PLQYs). To address this issue, we report the synthesis and characterization of a series of curved and fully conjugated nitrogen‐doped PAHs. Through structural modifications to the electron‐accepting moiety, we are able to switch the mechanism of luminescence between thermally activated delayed fluorescence (TADF) and room‐temperature phosphorescence (RTP), and to tune the overall PLQY in the range from 9 % to 86 %. As a proof of concept, we constructed solid‐state organic light‐emitting diode (OLED) devices, which has not been explored to date in the context of concave N‐doped systems being TADF/RTP emitters. The best‐performing dye, possessing a peripheral trifluoromethyl group adjacent to the phenazine acceptor, exhibits yellow to orange emission with a maximum external quantum efficiency (EQE) of 12 %, which is the highest EQE in a curved D‐A embedded N‐PAH to date.


SI-1 Materials and methods
General Remarks. All reagents and solvents were purchased from commercial sources and were used as received unless otherwise noted. Reagent grade solvents (CH2Cl2, hexane, ethyl acetate) were distilled prior to use. For water-sensitive reactions solvents were dried using Solvent Purification System from MBraun (https://www.mbraun.com/us/). Transformations with moisture and oxygen sensitive compounds were performed under a stream of argon. The reaction progress was monitored by means of thin layer chromatography (TLC), which was performed on aluminium foil plates, covered with Silica gel 60 F254 (Merck) or Aluminium oxide 60 F254 (neutral, Merck). Products purification was done by means of column chromatography with Kieselgel 60 (Merck) or Aluminium oxide (Fluka). The identity and purity of prepared compounds were proved by 1 H NMR and 13 C NMR spectrometry as well as by MS spectrometry (via EI-MS) and IR spectroscopy. The solubility of some compounds was too poorly to record 13 C NMR spectra. In these cases, it was possible to record 2D NMR spectra instead and select carbon signals. NMR spectra were measured on Bruker AM 500 MHz, Bruker AM 600 MHz, Varian 600 MHz or Varian 400 MHz instruments with TMS as internal standard.
Thermogravimetric Analyses (TGA) were performed using a Mettler-Toledo TGA/DSC 3+ thermal gravimetric analyzer. The measurements were conducted in nitrogen atmosphere, from 50°C to 500°C at the heating rate of 5°C/min. The temperature of 5 wt% and 10wt% of mass loss were determined.
Steady-state emission spectra were recorded on a HAMAMATSU C11347-01 spectrometer with an integrating sphere and Jobin Yvon Horiba Fluorolog 3, with solvent studies performed in clean 1 cm path-length photoluminescence cuvettes (Arieka Cells) and temperature dependent film photoluminescence films studies performed on within a liquid N2 cooled cryostat (Janis Research). Photoluminescence spectra were calibrated for detector efficiency using company supplied, instrument specific calibration files. The emitter materials was also degassed in toluene solvent using a custom made 1 cm path-length degassed cell stoppered with a Young tap and degassed using 5 freeze/thaw/pump cycles. The photoluminescence quantum yield (PLQY) of emitters in toluene solvent was determined using the reference method against 9,10-diphenylanthracene (DPA). Solid-state samples were prepared as 1% w/w ratio emitters in Zeonex ® polymer host on clean/dry sapphire disc substrates. Phosphorescence, prompt fluorescence (PF), and delayed fluorescence (DF) spectra and decays were recorded using nanosecond gated luminescence and lifetime measurements (from 400 ps to 1 s) using either third harmonics of a high energy pulsed DPSS laser emitting at 355 nm (Q-Spark A50-TH-RE).
Emission was focused onto a spectrograph and detected on a sensitive gated iCCD camera (Stanford Computer Optics) having a sub-nanosecond resolution. PF/DF time-resolved measurements were performed by exponentially increasing gate and integration times.
Temperature-dependent experiments were conducted using an helium cryostat (Janis Research) under a vacuum. Time-resolved measurements were performed by exponentially increasing the gate and delay times of iCCD Stanford Computer Optics 4Picos camera from laser excitation.
The delay and integration times are set at a time longer than the previous sum of delay and integration time to avoid overlap. As the next step, the curve is corrected by integrating the measured spectra to obtain proper luminescence decay profile. Every point represents the collected emission spectra of respective emitting specie.
TPBi 2,2',2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) was introduced as an Electron Transport Layer (ETL). Lithium fluoride (LiF) and aluminium were used as the cathode. Organic semiconductors and aluminium were deposited at a rate of 1 Ås-1, and the LiF layer was deposited at 0.1 Ås-1. CBP 4,4′-bis(N-carbazolyl)-1,1′-biphenyl was used as hosts for all emitters. All materials were purchased from Sigma Aldrich or Lumtec and were purified by temperature-gradient sublimation in a vacuum. OLEDs have been fabricated on precleaned, patterned indium-tin-oxide (ITO) coated glass substrates with a sheet resistance of 20 Ω/sq and ITO thickness of 100 nm. All small molecules and cathode layers were thermally evaporated in a Kurt J. Lesker SuperSpectros 200 evaporation system under pressure of 10 -7 mbar without breaking the vacuum. The sizes of pixels were 4 mm 2 , 8 mm 2 and 16 mm 2 . Each emitting layer has been formed by co-deposition of dopant and host at the specific rate to obtain 10% content of the emitter. The characteristics of the devices were recorded using a 6-inch integrating sphere (Labsphere) inside the glovebox connected to a Source Meter Unit and Ocean Optics USB4000 spectrometer.
Quantum-chemical calculations. The ground-state equilibrium geometries of 10a-10n were optimized with the use of the Møller-Plesset perturbation method of second order (MP2).
Afterwards, their excited electronic states were computed with the second-order algebraic diagrammatic construction (ADC(2)) method. [1,2] The spin-component scaling [3,4,5] (SCS) procedure was imposed at all times in both the MP2 and ADC(2) calculations. Thus, the methods are labelled as SCS-MP2 and SCS-ADC(2), respectively. The scaling factors were set to the values proposed in Ref. [3] for use with the MP2 method: cOS = 6/5 for the opposite-spin contributions, and cSS = 1/3 for the same-spin contributions. The S1 and T1 states of the molecules under study were also optimized using SCS-ADC(2) method.
The SCS-MP2 and SCS-ADC(2) calculations were performed with the program Turbomole, version 6.3.1., [6] taking advantage of the frozen core and resolution of the identity [7,8,9,10] approximations. A restricted Hartree-Fock (RHF) reference determinant was used. The cc-pVDZ basis set [11] was employed in combination with the default auxiliary basis set. [12] The DFT calculations were performed with the computational chemistry software package Gaussian 16. Revision A.03. [13] The B3LYP functional [14] was employed in combination with the def2-SVP basis set. [15] The calculated energies and gradients were corrected for dispersion effects via the D3 semiempirical correction scheme of Grimme and coworkers with Becke-Johnson damping. [16] The calculation of the IMS was performed with the with the gauge-independent atomic orbital (GIAO) method. [17,18,19,20,21] The ACID of the S0 state of 10l was computed using the program of Geuenich and coworkers, [22] which contains an interface to Gaussian 16.
On the technical side, all SCS-MP2 and SCS-ADC(2) geometry optimizations were carried out by interfacing Turbomole to Gaussian 16. In this setup, Gaussian acts as a 'wrapper' around Turbomole, and carries out the geometry optimization by calling this latter program for the energy and gradient. As per the default settings in Gaussian 16, geometries were optimized with the use of the Berny algorithm in redundant internal coordinates. Table S1. Vertical excitation energies (ΔE) and associated oscillator strengths (f) of the representative compound 10l calculated at the SCS-ADC(2) level of theory. Calculations were performed at the ground-state equilibrium geometry optimized at the SCS-MP2 level. 3.447 0.0000 Figure S1. Illustration of frontier molecular orbitals distribution for compound 10a.             2,6-dichloroiodobenzene (2.00 g, 7.32 mmol, 1.1 equiv.), 5Hdibenz[b,f]azepine (1.29 g, 6.66 mmol, 1.0 equiv.) and NaOtBu (1.28 g, 13.32 mmol, 2.0 equiv.) were added to an oven-dried and argon flushed pressure tube equipped with magnetic stir bar, following by dry toluene (40 mL). The mixture was purged with argon for 15 minutes. Afterwards, Pd(OAc)2 (44.8 mg, 0.2 mmol, 0.03 equiv.) and P(tBu)3HBF4 (116.0 mg, 0.4 mmol, 0.06 equiv.) were added at once and the reaction mixture was purged with argon an additional two minutes and the tube was tightly sealed. The reaction mixture was refluxed overnight under inert atmosphere. After cooling the reaction mixture to the room temperature, brine was addended and the mixture was extracted 3 times with DCM. The organic layers were combined and dried over anhydrous Na2SO4. Organic solvents were removed under vacuum and the crude mixture was purified by a column chromatography separation on silica gel (hexane) affording compound 11 (2.85 g, 84%) as a yellow solid. Rf=0.45 (SiO2, hexane/ethyl aetate, 19:1). Importantly, purification is also possible without column chromatography, through crystallization from hot hexane. 1  General procedure for the synthesis of 12: Compound 11 (2.00 g, 5.92 mmol, 1.0 equiv.), DBU (4 mL) and dry DMAc (10 mL) were added to a reaction vessel equipped with magnetic stir bar. The mixture was purged with argon for 10 minutes. Subsequently, Pd(PCy3)2Cl2 (221.6 mg, 0.30 mmol, 0.05 equiv.) was added to the vessel and the reaction mixture was purged with argon additional two minutes. The tube was placed in microwave reactor for 50 minutes (150W, 180°C). After cooling it, the reaction was quenched with water and the mixture was extracted 3 times with DCM (notably, extraction was repeated until removal of DMAc from the mixture). The combined organic layers were dried over anhydrous Na2SO4. Solvents were evaporated and the crude product was purified by a column chromatography separation on silica gel (hexane) affording the desired product 12 (1.37 g, 87%) as an orange solid. The target product was washed with hexane to get rid of residual starting material. Rf= 0.50 (SiO2, hexane/ethyl acetate, 19:1). 1  General procedure for the synthesis of dione (13): Procedure 1: To a solution of 12 (1.38 g, 5.21 mmol) in CH2Cl2 (28 mL) and THF (28 mL) were added RuCl3·xH2O (146.6 mg, 0.52 mmol, 0.1 equiv.), Nmethylimidazole (3 drops), and H2O (28 mL). NaIO4 (4.90 g, 22.9 mmol, 4.4 equiv.) was added in ca. 15 small portions over 30 min. The resulting mixture was stirred at room temperature overnight. DCM and sodium sulfite saturated solution were added, and the layers were separated. The aqueous phase was extracted with DCM 3 times, and the combined organic layers were washed with H2O 2 times, dried over Na2SO4. The filtrate was evaporated and purified by a column chromatography on silica gel (hexane) affording 13 (292 mg, 19%) as an dark yellow solid. Final product was washed with heaxane to remove substrate traces. Rf= 0.50 (SiO2, hexane/ethyl aetate, 19:1).

Procedure 2:
A 100 mL two-necked round-bottomed flask was charged with 12 (1.02 g, 3.84 mmol) and dry chlorobenzene (38 mL). The solution was purged with argon for 15 minutes. Afterwards, benzeneseleninic anhydride (BSA) (1.52g, 4.22 mmol, 1.1 eqiuv.) was added to the flask. The reaction mixture was refluxed (140°C) overnight with reflux condenser under argon atmosphere. After cooling it to the room temperature, the precipitate was formed and filtered off. Next, it was washed with DCM three times affording pure compound 13 (884 mg, 78%) as the dark yellow solid. Rf=0.80 (SiO2, DCM:MeOH, 19:1). 1  General procedure for the synthesis of phenazines (10a-n): A pressure tube was charged with 13 (50 mg, 0.17 mmol) and diamine (0.17 mmol), following by glacial acetic acid (5 mL) and anhydrous ethanol 99.8% (5 mL). The mixture was argon purged for 10 minutes and reaction was stirred overnight at 120°C. After cooling the mixture, precipitate was filtered off and washed with methanol (3x4mL) and ethyl acetate (3x4mL). 10d and 10f were recrystallized from hot DMF. 10d Dark purple solid, yield: 61%. The solubility of this compounds was too poorly to record 13 C NMR spectra. Carbon signals were selected from HSQC spectra. 1  10f Red solid, yield: 76%. The solubility of this compounds was too poorly to record 13 C NMR spectra. Carbon signals were selected from HSQC spectra. 1

SI-5 Photophysics
Two matrices were chosen to avoid the possible aggregation, polymer Zeonex as the stage between solution and solid-state and 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) as the host for OLED. It could be noticed that the fluorescence emission (maximum peak) is similar in the Zeonex matrix as it was in the solutions ( Figure S17). On the other hand, a solid matrix allows us to observe strong delay emission. Depending on the substitution, the thermally activated delayed fluorescence (TADF), room-temperature phosphorescence (RTP), or dual TADF & RTP was observed. Theoretically, to obtain efficient TADF emission, our compound should have as close as possible the singlet and triplet excited states. There are several ways to estimate singlet and triplet excited states, the onset of the respective emission or the maxima of the emissions are used. In our case, to allow for the comparison with theoretical calculations, we decide to use emission maxima. In our case, the lowest ΔEST gaps were observed for compounds with additional benzene ring (10b), thiophene ring (10g) and double bromine group (10i). In the two compounds (10b, 10i), the low ΔEST gap was connected with the same effect like in solution (an increase of conjugation, decrease the charge transfer energy). The groups affect the 1 CT energy state, where the triplet localized energy state remains ( 3 LE). In the case of the derivative with thiophene unit (10g), it seems that the group is affecting the triplet energy state, which is one of the highest ( Figure S17g). To proof, which actual emission process we observe, the emission of the compounds at different delayed times were collected and compared ( Figure   S18). Moreover, the emission spectra were obtained at different temperatures during the timeresolved photoluminescence measurement to estimate where is the phosphorescence and the point when the thermal activation occur. To decide if the compound exhibit emission through the TADF or RTP process, we need to compare ns range emission with at least μs one at 300 K and the phosphorescence emission. In our case, most of the compounds in the Zeonex matrix exhibit TADF emission with certain deviations. Compounds 10d,e,g,h,n had pure TADF emission, where delayed emission (80 ms, 300 K) matches the prompt ns emission. In the case   of 10a,b,c,f,i,l, we have mixed TADF&RTP emission where RTP contribution is higher than 1% and for compounds 10j,k, the RTP contribution is lower than 1%. Only compound 10m with a single bromine group had pure RTP emission at 80 ms delay time ( Figure S17m  Nevertheless, the most interesting behavior was observed for 10d and 10f. If we look closely, we observe inversion of singlet and triplet energy states in both cases. In the case of 10d, it may not be so obvious, but the prompt fluorescence emission is a set of two signals, one with maxima at 494 nm and the second at 590 nm. The signal at 494 disappear around 10 ns delay time, and a single 590 nm remain. That means our lowest singlet state is 590 nm (2.10 eV) and our real ΔEST=-0.25 eV ( Figure S17b). Similar but more visible inversion is observed for 10f, the triplet energy state is higher by 0.34 eV from the lowest singlet excited state ( Figure S17f). That's not all, in the case of 10d, as delayed emission, we observe TADF but not from S1 energy state but higher one (S2). In the case of the 10f compound, we observe mixed emissions based on TADF and RTP processes. In most of the cases, the emission developed at μs regime and (ISC/rISC) cycle till ms times. Nevertheless, if we would look closely to the RTP emitter (10m), we can notice that at μs the emission if due to TADF process not RTP, the emission start shifting at around 0.2 ms and pure RTP is observed ( Figure S18m). Similar observation could be found in 10f compound, in the μs delay time we observe only TADF and mixed TADF&RTP in ms delay time ( Figure S17f). Figure S18. Photophysical properties of 10a-n in Zeonex at 300K, insets are the respective spectra for particular delay time.
To evaluate the emissive properties of the compounds as the materials for OLEDs, the photophysics of the emitters in the CBP matrix were conducted ( Figure S19, S20). Analyzing the emission in CBP matric, different properties can be noticed in comparison to Zeonex.
Comparing emission at ms delay time, compounds 10a,e,f,h,i,j,k,l,n show pure TADF   emission, 10c,m shows mixed emission and 10b,d,g have RTP. The matrix significantly affects the 10d with anthraquinone and shifts from mixed emission to RTP one, but what is really interesting, in both cases (Zeonex, CBP), the emission from the lowest state is not visible and delay times. That means rISC process is blocked from passing to lowest state but the excited state follow to higher singlet state (Zeonex) or stay on lowest triplet energy state (CBP). As for the 10g, the pure TADF in Zeonex changes to pure RTP in CBP. If we look at respective energy levels (Table S3), for 10g, both singlet and triplet energies are lowered, but the triplet state is more affected, increasing the ΔEST up to 0.20 eV. This is enough for the TADF process to be turned off and RTP promoted. The process in 10b is a little bit different, if we look at the emission spectra development over delay time ( Figure S18b, S20b), TADF emission starts to be visible at μs range and circles over the ISC/rISC process to ms delay times where the RTP process is involved, for compound 10b, the shift from TADF to pure RTP is around 0.2 ms delay time ( Figure S20b). Because of the long-lived triplet state, probably in 10b based OLEDs, we would observe only TADF emission.
For the other RTP emitters, 10d and 10g, the RTP is observed already at μs delay times, suggesting that it should work as RTP emitters in OLEDs. Similarly, to the Zeonex matrix, we could observe singlet-triplet inversion in the same two compounds (10d and 10f), but the resulting effect is the opposite. In the case of 10d in Zeonex, we observe TADF character, where in CBP, the emission shift to RTP. In the 10f, where in Zeonex we had dual TADF/RTP emission, in CBP, we observe pure TADF even at longest delay times. For all compounds which had ΔEST in Zeonex close to 0, the gap significantly rose by at least 0.15 eV. For the others, the change of the gap was much smaller in the range of 0.05 eV to lower values. Figure S20. Photophysical properties of 10a-n in CBP at 300K, insets are the respective spectra for particular delay time.

SI-9 X-Ray crystallography analysis
The X-ray measurement of compound 10l was performed at 130.0(5) K on a Bruker D8 Venture PhotonII diffractometer equipped with a TRIUMPH monochromator and a MoKα fine focus sealed tube (λ = 0.71073 Å). A total of 2960 frames were collected with Bruker APEX3 program [23]. The frames were integrated with the Bruker SAINT software package [24] Table S4.
All non-hydrogen atoms were refined anisotropically. All hydrogen atoms were placed in calculated positions and refined within the riding model. The temperature factors of hydrogen atoms were not refined and were set to be 1.2 or 1.5 times larger than Ueq of the corresponding heavy atom. The atomic scattering factors were taken from the International Tables [28].
Molecular graphics was prepared using program Mercury 4.1 [29]. Thermal ellipsoids parameters are presented at 50% probability level in Figure S39.  Figure S39. Thermal ellipsoid plot at 50% probability level together with numbering scheme of atoms in the structure of 10l. Figure S40. Bond length (Å) of compound 10l determined from the X-ray structure (convex face)

SI-10 Aromaticity
The extended system of π-bonds present in the examined compounds prompted us to study aromatic properties of 10l in great details as a representative example of the full set. For the purposes of this analysis, the equilibrium geometries of the S0 and the T1 states of 10l were reoptimized at the density functional theory (DFT) level of theory imposing the C2v symmetry.
Constraining the molecule to C2v symmetry is not strictly necessary, but it does simplify the subsequent analysis by ensuring that the heavy atom skeleton adopts a planar geometry.
Afterwards, the aromaticity of 10l was characterized with the use of two criteria based on magnetic properties: the isotropic magnetic shielding [30,31] (IMS), and the anisotropy of the current (induced) density (ACID). [32,33] IMS Analysis The use of the IMS as a diagnostic of aromaticity relies on the fact that significantly positive (i.e. magnetically shielded) IMS values above or below the plane of a ring indicate the presence of induced diatropic ring currents, which can be associated with local aromaticity, whereas appreciably negative (deshielded) IMS values denote paratropic ring currents, and local antiaromaticity. The IMS is closely related to another aromaticity index, the nucleusindependent chemical shift (NICS) index [34,35] more specifically, the NICS index is defined as the negative of the IMS value calculated at an appropriately chosen point in space in the vicinity of the given ring.
To obtain a visual representation of the topography of the IMS, values of the IMS were computed on a plane located 1 Å above the plane on which the heavy atom skeleton of the molecule lies. The resulting IMS maps are shown in Figure S41 (a) and (b) for the S0 and T1 states, respectively. We focus first on the S0 state. In the regions above the six-membered rings (which are labeled I, IV, V, VII, and VIII), the IMS takes significantly positive values in the range from 10 to 15 ppm, showing that all these rings are aromatic. The two five-membered rings, II and III, appear to display less local aromatic character than the six-membered rings, as evidenced by lower, but still positive, IMS values. Lastly, in the region above the sevenmembered ring VI, the IMS takes near-zero to negative values, indicating that this ring is antiaromatic.
In the T1 state, the antiaromatic character of the seven-membered ring VI is lowered relative to the S0 state. Moreover, rings IV and V to either side of ring VI become less aromatic than in the S0 state. On the other hand, rings I, VII and VIII remain markedly aromatic.

ACID analysis
The ACID provides complementary measure of local aromaticity based on the magnetic properties of the molecule. In this approach, electron delocalization (a concept which includes aromaticity as a special case) is associated with appreciably large values of the ACID scalar field. Planar conjugated systems are considered aromatic if they exhibit strong diatropic ring currents in the π-bonding region.
As usual, the total ACID was separated into contributions from the σ and π orbitals, and here we discuss only the contribution from the π orbitals. The ACID of the π-bonding system is shown in Figure S42. The π-bonding system of 10l consists of three separate conjugated subsystems, of which one consists of only ring I, another of rings IV and V along with the intervening nitrogen atom, and the third of rings VII and VIII. The inspection of the directions of the ring currents further confirms that the six-membered rings I, IV, V, VII and VIII are all aromatic, while the seven-membered ring VI is antiaromatic.