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. 2025 Dec 12;21(1):e70485. doi: 10.1002/asia.70485

Modulating the Singlet–Triplet Energy Gap With Regio‐Specific Substitution on Dibenzo[a,c]phenazine to Realize Red TADF and Their OLEDs

Komal Vasant Barhate 1, Smruti Ranjan Nayak 1, Mohammad Amir Ahemad 2, Ksenija Glusac 3,4, Sangita Bose 2,✉, Neeraj Agarwal 1,✉
PMCID: PMC13394304  PMID: 41387262

ABSTRACT

Developing efficient orange‐red emitters based on donor–acceptor (D−A) π conjugates with thermally activated delayed fluorescence (TADF) remains a significant challenge. Herein, we designed a series of donor‐acceptor emitters (3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP) by incorporating twisted 10,11‐dihydro‐5H‐dibenzo[b,f]azepine (Az) as a donor alongside the rigid and planar dibenzo[a,c]phenazine (DBP) acceptor core molecule at different sites viz., 3,6‐, 2,7‐, and 2‐ positions. As a result of site‐specific substitution, considerable alteration in electronic properties was observed. For 3,6‐Az‐DBP, the energy gap between singlet and triplet states (ΔE ST) was found to be 0.42 eV, which is much higher than that for 2‐Az‐DBP (0.25 eV) and 2,7‐Az‐DBP (0.06 eV). Owing to miniscule ΔE ST, 2‐Az‐DBP and 2,7‐Az‐DBP showed orange and red TADF, respectively, in CBP blend. Both these compounds also showed red room temperature phosphorescence in powder samples having lifetimes in milliseconds time regime. Further, organic light emitting diode (OLED) of CBP doped 2‐Az‐DBP showed a high luminance of 6 × 103 Cd/m2 @ current density of 50 mA/cm2with EQEmax of about 12.3%. These studies provide a promising strategy, site selective substitution of donor on acceptor core, to improve the emission properties for red TADF emitters.

Keywords: positional isomers, room temperature phosphorescence, thermally activated delayed fluorescence, twisted donor

Site‐selective incorporation of a DBAze donor onto the DBP acceptor core effectively tunes molecular geometry and ΔEST, producing isomer‐dependent orange–red TADF and red RTP. This donor‐position engineering strategy delivers efficient red OLEDs, with 2‐Az‐DBP achieving an EQEmax of 12.3%.

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1. Introduction

Thermally activated delayed fluorescence (TADF) and metal‐free phosphorescent emitters have gained tremendous attention as they can harvest both singlet and triplet excitons in organic light‐emitting diodes (OLEDs) [1, 2, 3]. Since the pioneering work of Uoyama et al. in 2012, pure organic TADF materials have been greatly popularized among researchers owing to their capability of achieving 100% internal quantum efficiency [4]. In TADF materials, under electrical excitation populated triplet is converted to singlet excitons, with the aid of thermal energy, by reverse intersystem crossing (RISC) [5, 6, 7, 8]. A number of blue‐ and green‐emitting TADF and phosphorescent materials have been developed in the recent past [9, 10]. On the contrary, the emergence of orange‐red TADF materials is slow due to their low efficiency in OLEDs [11, 12, 13, 14, 15, 16]. Orange‐red emitters tend to decay non‐radiatively due to a smaller gap between S1 and S0 [12, 17, 18, 19]. Also, the radiative decay rate is decreased due to smaller overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) [20, 21, 22]. Most orange‐red TADF materials require extended π‐conjugation, strong donor (D)−acceptor (A) moieties, and small ΔE ST [23, 24, 25]. Rigidity in the molecules is generally prescribed to maximize the emission yield [26, 27]. However, the flaw in the rigid emitter is its tendency to attain planar geometry and thus to aggregate [28]. Sufficiently strong intramolecular charge transfer (CT) in D–A systems can also show redshifted emission with retaining orthogonality in their geometry. Such materials are potential red TADF candidates [11, 21].

In 2012, Uoyama et al. reported D–A red emitter by changing the number of donors on the strong accepting core like TPN [4]. Later on, based on this strategy many red TADF emitters have been developed by having different numbers of donors on the acceptor core or by utilizing the strong withdrawing acceptor core [29, 30]. Recently, a red TADF emitter with an internal host component was synthesized to give an undoped red TADF emitter. Hua et al. designed TADF emitters on trinapshtho[3,3,3]propellane (TNP) core. The stacking architecture of TNP helped minimize the concentration quenching and triplet–triplet annihilation (TTA) of triplet excitons. This design strategy resulted in an efficient orange‐red emitter with a high quantum yield of ∼70% and provides valuable information on the role of molecular packing on high‐performance TADF emitters [15]. Figure 1 shows some examples of recently reported red TADF materials. Toward the development of TADF materials, it is realized that twisted electron donating unit may also have a significant impact on CT character and thus affecting the TADF properties [31]. D–A materials with twisted geometry or with limited rotation on D–A bond are found to produce efficient TADF character [23, 32, 33]. Another very recent example of twisted geometry emitter is phenoxazine substituted deuterated dibenzo[g.p]chrysenes. Near orthogonal geometry in it assisted the efficient triplet generation at room temperature [34].

FIGURE 1.

FIGURE 1

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(a) Recent examples of orange‐red emitters, (b) schematic representation of TADF mechanism; S0, S1 are the ground and the excited singlet states, respectively; T1 is the triplet state; ISC is the intersystem crossing and RISC the reverse intersystem crossing processes; F and P are the fluorescence and phosphorescence, respectively; NR is the nonradiative decay; and (c) chemical structures of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP.

It is well understood that structural isomers are different compounds sharing the same molecular formula, although their atoms are connected through covalent bonds in unique configurations, which greatly influence their chemical and physical properties [35, 36, 37, 38]. Modification in isomer structures can lead to alterations in the geometric and electronic configurations of molecules [38, 39, 40, 41, 42, 43]. Ye et al. reported four isomeric DBP‐based emitters (DPyPzTPA, TPAPzDPy, PyPzDTPA, and DTPAPzPy) with mCBP‐hosted green to yellow OLEDs showing EQEmax values in the range 8.3%–16.6% [44]. In this article, we designed and synthesized dibenzophenazine (DBP) derivatives substituted with dibenzoazepine (3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP) at different positions, as shown in Figure 1. The electron‐accepting DBP core is suitable for TADF emitters and has been used earlier [45, 46, 47]. Especially for red to NIR emitters, DBP has been proven to be efficient in molecular design. Recently, Naz et al. reported PyBP‐based deep‐red/NIR emitters (DMACPyBP, PXZPyBP, and DPACzPyBP) having PLQY of about 62% demonstrating how acceptor extension can push emission into the NIR while highlighting the trade‐offs with PLQY and nonradiative loss [48]. Dibenzoazepine is chosen as an electron donor as it has a flexible ethylene bridge and bulky benzo groups in it. Also, we believe that aryl‐fused azepine, i.e., dibenzoazepine will have twisted as well as hindered rotation on the C–N bond. Barhate et al. presented how a flexible fragment in dibenzoazepine enabled to activate and enhance the triplet channel. This observation was drawn on multiple molecular conformations, twisted structure, and larger dihedral angles in dibenzoazepine [49, 50, 51]. By constructing such D–A materials, we expect TADF properties and efficient OLED properties in these isomers. Site‐specific substitution on dibenzophenazine core resulted in a vast variation in electronic properties. In toluene, 2,7‐Az‐DBP shows emission at 574 nm, which is nearly 100 nm red‐shifted compared to 3,6‐Az‐DBP, which emits at 475 nm. Lowering of ΔE ST could be achieved upon altering the site of substitution on DBP core, i.e., for 3,6‐derivative (3,6‐Az‐DBP) ΔE ST was found to be 0.46 eV which is far larger than for 2,7‐derivative (0.04 eV). Owing to minuscule ΔEST, 2,7‐Az‐DBP showed TADF in CBP blend and room temperature phosphorescence (RTP) in powder sample. The mono derivative, 2‐Az‐DBP, which emits at 547 nm and has an ΔE ST of 0.26 eV, exhibited TADF in doped films and RTP in its powdered form.

2. Result and Discussion

2.1. Design and Synthesis

The donor–acceptor materials having non‐planarity have been studied widely for TADF properties [30, 52, 53]. Orthogonal geometry between donor and acceptor minimizes the overlap between the HOMO and LUMO, which is one of the essential requirements for modulation of types of emission in emitters. Design strategy of these D–A materials is based on di‐substitution of dibenzo[a,c]phenazine (electron acceptor) with non‐planar and slightly flexible dibenzoazepine (electron donor) at C3/C6‐ or C2/C7‐ positions resulting in positional isomers (3,6‐Az‐DBP and 2,7‐Az‐DBP) as shown in Figure 1. Similarly, mono dibenzoazepine substitution on dibenzo[a,c]phenazine core at C2 position was achieved to get 2‐Az‐DBP. Due to difference in electron density at C3/C6‐ or C2/C7‐ positions, the excited state levels of these D–A emitters can be controlled [47, 54, 55]. Electronic properties of these derivatives are expected to change considerably as these are largely dependent on electron density at different atoms, conjugation, and planarity of the molecule. Charge transfer properties are also expected in these D–A materials which are widely studied to modulate the ΔE ST and type of emission in organics. Further, we expect substantial effect of substituent on the solid‐state arrangement in 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP and their photophysical properties in solid state.

Donor‐acceptor systems (3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP) were synthesized as shown in Scheme 1 given in Supporting Information. Bromination of phenanthrene‐9,10‐dione with bromine in nitrobenzene gave 3,6‐dibromo‐phenanthrenequinone (3,6‐Br2PhenQ) while with NBS in conc. H2SO4 it afforded 2,7‐dibromo‐phenanthrenequinone (2,7‐Br2PhenQ) along with minor 2‐bromo‐phenanthrenequinone (2‐Br2PhenQ). Condensation reactions of 3,6‐Br2PhenQ, 2,7‐Br2PhenQ, or 2‐Br2PhenQ with o‐phenylenediamine were carried out in methanol at room temperature to get the corresponding bromo derivatives of dibenzo[a,c]phenazine, i.e., 3,6‐Br2‐DBP, 2,7‐Br2‐DBP and 2‐Br‐DBP. Subsequently, dibenzoazepine was incorporated onto dibenzo[a,c]phenazine core by employing Pd catalyzed Buchwald–Hartwig coupling to afford 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP in good to very good yields (65%–85%). Detailed characterizations of these derivatives were carried out by different spectroscopic techniques such as 1H NMR, 13C NMR, FTIR, and HRMS and the related spectra are provided in Supporting Information. These derivatives are photostable under ambient light and are soluble in common organic solvents. In addition, thermal properties of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were studied using thermogravimetric analysis.

2.2. Theoretical Studies

In order to comprehend the characteristics of electronic transitions, we utilized calculations based on density functional theory (DFT) with the ORCA electronic structural program [56]. Details are provided in the Supporting Information. Ground state geometries of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were optimized at the PBE0‐D3(BJ)/def2‐TZVP level with tight convergence criteria and the RIJCOSX approximation [57, 58, 59], and molecular structures are given in Figure 2. It can be observed that the donor Az and the central acceptor DBP core are in different planes with a slightly twisted geometry. Also, the ethylene fragment in Az is twisted in nature resulting in nonplanar donor unit. Further excited‐state properties were computed using TDDFT at the TPSSh/def2‐TZVP level, including D3(BJ) dispersion correction [60, 61]. Frontier orbitals HOMO, HOMO – 1, LUMO + 1, and LUMO of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP are shown in Figure 2. It can be seen for 2‐Az‐DBP and 2,7‐Az‐DBP, HOMO is partially distributed on both acceptor and donor unit whereas for 3,6‐Az‐DBP it is localized on both donor and acceptor unit. LUMO on the other hand can be found on the acceptor for all three isomers. Overlaying the HOMO and LUMO distributions for each isomer reveals a slight overlap, thus meeting the main criterion for TADF molecular design. Moreover, a significant difference could be observed in orbital distribution for HOMO‐1. In 2,7‐Az‐DBP, HOMO‐1 is distributed over the donor and the lower part of the acceptor moiety while in 3,6‐Az‐DBP, it can be found distributed largely on the acceptor with some distribution of donor units as well. However, in 2‐Az‐DBP, HOMO‐1 to a large extent is localized only on the acceptor unit.

FIGURE 2.

FIGURE 2

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(a) Optimized molecular geometries and (b) orbital spatial orientation (HOMO, HOMO‐1, LUMO, and LUMO+1) for 2‐Az‐DBP, 2,7‐Az‐DBP, and 3,6‐Az‐DBP, respectively.

Likewise, the TDDFT calculations were computed to elucidate energy, orbital distribution, etc., of the excited states. The transition from the HOMO to the LUMO characterizes the first excited state of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP. The calculated oscillator strengths and maximum wavelengths (λ) along with their dominant orbital transitions are summarized in Tables S8–S10. The magnitude of spin–orbit coupling (SOC) between singlet and triplet states can be derived from the computed SOC matrix elements. A schematic illustration of energy levels and relevant SOC is shown in Figure S19.

2.3. Thermal Properties

The thermal properties of the 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were determined by thermogravimetric analysis (TGA) and the thermograms are shown in Figures S8–S10. The TGA thermogram for 3,6‐Az‐DBP indicates an initial minor weight reduction of approximately 5% up to around 380°C. This is followed by a substantial decrease of about 58.7% commencing at 425°C, which is attributed to the loss of two Az groups. In the case of 2,7‐Az‐DBP and 2‐Az‐DBP, a significant weight loss of 56.3% and 80.3%, respectively, was observed, which is attributed to the loss of Az. The decomposition temperatures (T d) were determined as 447, 451, and 425°C for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP, respectively. TGA studies showed the good thermal stability of all the three emitters.

2.4. X‐Ray Diffraction (XRD) Analysis

The ground state structure of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP was investigated by single x‐ray diffraction (SCXRD) analyses as shown in Figure 3. Single crystals of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were grown by solvent vaporization method using dichloromethane (DCM) and hexane mixture. Yellow plate like crystals were obtained for 3,6‐Az‐DBP and 2‐Az‐DBP while red colored plates were obtained for 2,7‐Az‐DBP. 3,6‐Az‐DBP and 2,7‐Az‐DBP showed disordered structure at sp3 hybridized carbons of Az donor units which could not be resolved further. This conformational change maybe due to slight flexibility in ethylene bridge [62]. Multiple C … H interactions (intramolecular) ranging from 2.733 to 2.961 Å as shown in Figure 4 were observed within 3,6‐Az‐DBP. In the two close‐by molecules of 3,6‐Az‐DBP, the orientation of dibenzoazepine can be seen in opposite alignment thus providing no room for π–π interaction. Crystal packing of 2‐Az‐DBP shows π–π interaction in the order of 3.394–3.531 Å and parallel orientation of two core groups with short intermolecular contacts with adjacent molecules. For 2,7‐Az‐DBP, the crystal packing along with π–π interaction in the order of 3.607–3.388 Å shows C … H interactions ranging from 2.804 to 2.941 Å. Thus, much closer and rigid packing was observed for 2,7‐Az‐DBP. The role of π–π interaction has been widely explored in providing multilayer channels facilitating ISC and lowering the singlet–triplet energy gap [63, 64, 65].

FIGURE 3.

FIGURE 3

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Ngle crystal structures of 3,6‐Az‐DBP (a), 2‐Az‐DBP (b), and 2,7‐Az‐DBP (c), and crystal packing and intermolecular interactions of 3,6‐Az‐DBP (d), 2‐Az‐DBP (e), and 2,7‐Az‐DBP (f).

FIGURE 4.

FIGURE 4

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Absorption (a), emission (b), and lifetime (c) profiles of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP in toluene at room temperature.

2.5. Photophysical Properties

2.5.1. Absorption and Emission Studies at Room Temperature (RT) in Solution

To investigate the electronic properties of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP, UV–vis absorption and photoluminescence studies were carried out in solvents of different polarity. Absorption and emission spectra for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP in toluene are shown in Figure 4 while those in other different solvents are given in Supporting Information. Photophysical data for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP are summarized in Table 1. The absorption spectra of these emitters displayed prominent bands in the range of 297–380 nm, which are associated with n–π* and π–π* transitions. This high‐energy transition is characteristic of dibenzo[a,c]phenazine core. Another, low energy transition for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP was found to be at 430, 352, and 395 nm, respectively, in toluene. A bathochromic shift of 78 nm is observed in 3,6‐Az‐DBP as compared to 2,7‐Az‐DBP. Interestingly, 2‐Az‐DBP, which is having one substituent at 2 position, showed bathochromic shift of 43 nm as compared to 2,7‐Az‐DBP. It suggests that electron density at 3,6 and 2,7 positions are quite different. 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP show absorption peak red shifted with increasing polarity of solvent from hexane to dimethyl sulfoxide followed by gradual decrease in absorption coefficients (ε) as tabulated in Tables S1, S3, and S5 implying slightly polar nature of their ground state. Emission maxima for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP is observed at 475, 619, and 547 nm, respectively. To further elucidate the nature of excited state, emission was recorded in different polarity of solvents. With increasing solvent polarity red shift in emission maxima was observed which is assigned to charge transfer (CT) feature [66]. Furthermore structureless broad emission in polar solvents confirms the charge transfer character [67, 68]. It is important to mention that 2,7‐Az‐DBP shows drastic bathochromic shift (∼140 nm) in emission compared to its positional isomer, 3,6‐Az‐DBP, as shown in Figure 4. Also, the Stokes shift for 3,6‐Az‐DBP was estimated as 2159 cm−1, which is much smaller than for 2,7‐Az‐DBP and 2‐Az‐DBP found to be 3414 and 3940 cm−1, respectively (see Supporting Information). Emission quantum yields were calculated for all three compounds in solution using Coumarin 153 as standard. Quantum yields of 3,6‐Az‐DBP and 2,7‐Az‐DBP were found to be 67% and 39%, respectively, in hexane, whereas, for 2‐Az‐DBP it is found to be 14%. Absorption and emission studies suggest the considerable alteration of electronic properties on substituting the dibenzoazepine at different positions.

TABLE 1.

Nonphysical parameters of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP in toluene at room temperature, in MeTHF, and powder at 77 K.

Comp. λabs a (nm) (log ε) λFl a (nm) ϕ a (%) τavg a (ns) λph b (nm) λph c (nm) τ c (A%) (ms) τ b (A%) (ms) ΔE ST (eV)
3,6‐Az‐DBP 430 (4.49), 453 (4.59) 474 57 2.61 595, 637 658 25.9 ± 1.60 (22%), 192.7 ± 3.96 (78%) 522.4 ± 2.5 (100%) 0.46 b 0.41 c
2,7‐Az‐DBP 352 (4.89), 511 (3.37) 619 8 7.32 651 663 35.0 ± 1.23 (26%), 251.3 ± 8.84 (74%) 48.5 ± 1.9 (12%), 310.9 ± 2.3 (88%) 0.04 b 0.04 c
2‐Az‐DBP 395 (4.04), 450 (3.19) 547 12 23.60 600, 637 656 26.9 ± 1.43 (35%), 134.1 ± 5.08 (65%) 661.6 ± 2.5 (100%) 0.26 b 0.19 c
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a

In toluene at 298 K.

b

In MeTHF at 77 K.

c

Powder at 77 K.

The excited state lifetimes of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were recorded in different polarity solvents and data are summarized in Tables S1, S3, and S5. Decay transients in different solvents at 298 K are found to be mostly mono‐exponential and are shown in Figures S12–S14. The excited state lifetime of 3,6‐Az‐DBP is observed to be shorter, ∼2 ns, while the compounds 2,7‐Az‐DBP and 2‐Az‐DBP exhibited comparatively longer lifetimes of 7 and 23 ns, respectively.

2.5.2. Emission Studies at Low Temperature

Emission of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP was measured at 298 and 77 K in Me‐THF (Figure 5). 3,6‐Az‐DBP shows emission peak at 486 nm, which is assigned to singlet relaxation, while at 77 K it showed additional vibrationally resolved peaks at 595 and 637 nm. To find the origin of these red shifted peaks at 77 K, we further recorded the gated emission with a delay of about 3 ms. The gated emission of 3,6‐Az‐DBP showed peaks at 595 and 637 nm and thus has been designated as phosphorescence. Similar studies were carried out for 2,7‐Az‐DBP and 2‐Az‐DBP, and their phosphorescence emission were found at 651 and 600 nm, respectively (Figure 5b,c). The knowledge of fluorescence (S1) and phosphorescence (T1) emission of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP enabled us to find the energy gap between singlet and triplet states (ΔE ST). The ΔE ST is found to be 0.46 eV for 3,6‐Az‐DBP, 0.04 eV for 2,7‐Az‐DBP, and 0.26 eV for 2‐Az‐DBP (Table 1). Considerable decrease in the ΔE ST of 2,7‐Az‐DBP and 2‐Az‐DBP as compared to 3,6‐Az‐DBP is a result of alteration in electronic properties upon site‐specific substitution of dibenzoazepine on DBP core.

FIGURE 5.

FIGURE 5

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Emission and decay profiles in MeTHF of 3,6‐Az‐DBP (a, d), 2,7‐Az‐DBP (b, e), and 2‐Az‐DBP (c, f) at 77 K.

Excited state decay of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were also studied at 77 K in MeTHF (Figure 5d–f). 3,6‐Az‐DBP showed emission at 595 and 637 nm, which decay with a lifetime of 522 ms much longer than its lifetime at room temperature. 2,7‐Az‐DBP showed bi‐exponential decay with lifetimes of 48 and 310 ms, while 2‐Az‐DBP showed mono‐exponential decay with excited state lifetime of 661 ms. Longer life‐times in milliseconds timescale are assigned for the triplet emission of these compounds.

2.5.3. Emission Studies in Powder and Thin Film

For solid‐state applications, it is essential to study the photophysical properties in powder and thin films. Emission of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP showed broad emission in powder (Figures S13, S15, and S17). 3,6‐Az‐DBP showed a peak at 568 nm and long tail beyond 600–750 nm. Gated emission with 300 µs delay time quenches the peak at 570 nm, while it showed a weak emission at about 700 nm which could be due to its triplet emission in powder form at 298 K. Excited state decay time for this peak is found to be 634 µs at 680 nm which confirms its phosphorescence nature. 2,7‐Az‐DBP shows broad emission in powder which is peaking at 635 nm and has long lifetime of 115 µs. Similarly, 2‐Az‐DBP shows emission at 585 nm in powder which has longer lifetime of 325 µs. These emission features of 2,7‐Az‐DBP and 2‐Az‐DBP at RTP were observed even in the gated emission. Longer lifetimes of emission in powder form for 2,7‐Az‐DBP and 2‐Az‐DBP confirm the phosphorescence in the solid state at room temperature. In ground powder and crystalline form, lifetimes were observed to be 250 and 325 µs, respectively. Longer lifetime in crystalline form as compared to ground powder suggests the effect of crystallinity on triplet stabilization. Molecular interactions and crystal packing as seen in single crystal structure are discussed under single crystal studies (vide supra).

From the low temperature fluorescence and phosphorescence spectra (see Figures S13, S15, and S17), the ∆E ST was calculated from the onset of S 1 and T 1. For 2,7‐Az‐DBP and 2‐Az‐DBP, ∆E ST was found to be 0.04 and 0.19 eV while for 3,6‐Az‐DBP about 0.41 eV was estimated. Calculated ΔE ST for 2,7‐Az‐DBP and 2‐Az‐DBP was found to be comparatively smaller and in alignment with the requirement of TADF. Therefore, we studied the temperature dependent emission of these compounds. From the emission temperature map, it was found that emission intensity is inversely proportional to the temperature in powder (Figure S19) which indicates no TADF in powder form. This could be due to triplet quenching in neat powder as expected. To reduce the triplet quenching often TADF materials are doped in suitable host matrix. One primary criterion in selecting a host material is its triplet energy, which should exceed the triplet state energy of the emitter. This prevents undesirable quenching of the emitter's triplet states by the host, thereby avoiding the suppression of delayed emission [45, 46]. Here, 4,4′‐Bis (N‐carbazolyl)‐1,1′‐biphenyl (CBP) was selected as the host for all three compounds. The optimum doping concentration for all three emitters was found to be 5 wt%, which showed maximum enhancement in the intensity of emission (see Figure S18). CBP doped film (5 wt%) of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP showed emission peak at 530, 570, and 640 nm, respectively, at 298 K. Upon lowering the temperature to 80 K gated emission with a delay of about 200–500 µs gave a red shifted emission at 600, 608 and 651 nm for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP CBP doped films, respectively (see Figure 6a–c). From the low temperature fluorescence and phosphorescence spectra, ∆E ST was estimated as 0.42, 0.25, and 0.06 eV for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP CBP doped films, respectively. Further to study the effect of temperature on the emission temperature mapping was carried out as shown in Figure 6d–f. In all the three emitters the emission intensity was found to be directly proportional to temperature indicating presence of TADF. Further the decay profiles were collected at prompt, i.e., 530, 570, and 640 nm for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP CBP doped films, respectively. At 300 K average lifetimes were found to be 8.8, 246.8, and 655.3 µs for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP CBP doped films, respectively, which significantly decreased with lowering in temperature as shown in Figure 6g–i. These studies confirmed the presence of TADF in these emitters. Photophysical data obtained in CBP doped films is summarized in Table 2.

FIGURE 6.

FIGURE 6

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Emission and decay profiles in CBP doped films (5 wt%) of 3,6‐Az‐DBP (a), 2,7‐Az‐DBP (b), and 2‐Az‐DBP (c) at 80 K, emission and decay temperature maps in CBP doped films (5 wt%) of 3,6‐Az‐DBP (d, g), 2,7‐Az‐DBP (e, h), and 2‐Az‐DBP (f, i).

TABLE 2.

Photophysical parameters in CBP‐doped film (5 wt%) of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP.

Comp. λ Fl (nm) ϕ F (%) λ Ph (nm) τ Flavg (ns) τ Phavg (µs) τ DF (A%) at 300 K (µs) τ DF (A%) at 100 K (µs)
3,6‐Az‐DBP 530 15 601, 648 2.3 8.8 20.8 ± 1.4 (24%), 409.8 ± 12.8 (76%) 24.2 ± 2.2 (30.8%), 322 ± 20.2 (69.2%)
2,7‐Az‐DBP 637 21 651 18.7 247 24.6 ± 1.1 (9.7%), 347.4 ± 3.5 (90.3%) 13.02 ± 0.57 (31.4%), 212.8 ± 7.2 (68.6%)
2‐Az‐DBP 570 12 608 37.8 655 401 ± 12.7 (41.2%), 1237 ± 31.7 (58.8%) 118.7 ± 14.1 (39.3%), 895.5 ± 183 (60.7%)
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2.6. Electrochemical Properties

Electrochemical properties of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were studied using cyclic voltammetry and cyclic voltammograms are shown in Figure S11. Both 3,6‐Az‐DBP and 2,7‐Az‐DBP display two oxidation potentials while 2‐Az‐DBP showed only one oxidation potential. Oxidation processes in all three emitters, 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP are reversible indicating their stable radical cation. A reversible first oxidation peak was found at 0.82, 0.47, and 0.85 V for 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP which was used to find out the energy level of highest occupied molecular orbitals (E HOMO), taking Fc/Fc+ as a standard [69]. The energy gap (E g) obtained from the intersection of absorption and emission spectra (S00) along with E HOMO was used to determine the energy level of lowest unoccupied molecular orbitals (E LUMO) HOMO/LUMO energy levels of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP −5.72/−3.13, −5.41/−3.14, and −5.77/−3.34 eV, respectively.

2.7. Electroluminescence Studies in Different OLED Geometries

Given that the photophysical investigations of 2,7‐Az‐DBP and 2‐Az‐DBP revealed red TADF characteristics, multilayer OLEDs with an optimized device configuration of [ITO/PEDOT:PSS/NPD/2‐Az‐DBP or 2,7‐Az‐DBP/BPhen/LiF‐Al] were fabricated. In addition, devices for CBP doped emitters (5 wt%) were also fabricated with the configuration of ITO/PEDOT: PSS/NPD/2‐Az‐DBP or 2,7‐Az‐DBP + CBP /BPhen/LiF‐Al. Optimized device geometries are shown in Figures 7a and 8a. In these devices, poly(3,4‐ethylenedioxythiophene)‐poly(styrenesulfonate) (PEDOT:PSS) was taken as the hole injecting layer (HIL), N,N′‐Di(1‐naphthyl)‐N,N′‐diphenyl‐(1,1′‐biphenyl)‐4,4′‐diamine (NPD) was taken as the hole transporting layer (HTL), Bathophenanthroline (BPhen) was chosen as the hole blocking layer (HBL) and LiF/Al was taken as the cathode. 4,4′‐Bis(N‐carbazolyl)‐1,1′‐biphenyl (CBP) was used as the host for the host–guest doped devices. Device parameters are tabulated in Table 3. From electro‐luminescence (EL) spectra of CBP doped and undoped 2‐Az‐DBP devices emission maxima was observed at ∼580 nm. In the case of di substituted 2,7‐Az‐DBP EL spectra showed emission peak at 638 and 630 nm in doped and undoped devices, respectively. 2‐Az‐DBP device emitted amber emission whereas 2,7‐Az‐DBP showed red emission, as confirmed by the images of the glowing devices (insets of Figures 7b and 8b). The CIE co‐ordinates for the four devices is given in Table 3. The turn‐on voltage V ON was measured to be 2.6 V for undoped 2‐Az‐DBP and 4.1 V for CBP doped 2‐Az‐DBP. Whereas, for the 2,7‐Az‐DBP emitter, V ON changed marginally from 2.9 to 3.4 V with doping. The 2‐Az‐DBP device shows a significant rise in EQEmax, increasing from 6.2% in undoped to 12.3% in 5 wt% CBP doped device, effectively doubling its efficiency. Similarly, 2,7‐Az‐DBP exhibits a comparable pattern, with its EQEmax increasing from 2.5% to 5.5% when doped in CBP (5 wt%). CBP doped 2‐Az‐DBP (5 wt%) OLEDs showed a high luminance of 6 × 103 Cd/m2 at a current density (J) of 50 mA/cm2 while the CBP doped 2,7‐Az‐DBP (5 wt%) device showed a luminance of 1.6 × 103 Cd/m2 at the same J, indicating higher performance of the 2‐Az‐DBP based devices. Photophysical and electroluminescence properties of a few dibenzophenazine derivatives have been summarized in Table S11. It is observed that EQE of 2‐Az‐DBP (12%) is moderate to high in comparison to other derivatives.

FIGURE 7.

FIGURE 7

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(a) Energy band diagram of the OLED device with CBP+2‐Az‐DBP (5 wt%), (b) electroluminescence spectra of 2‐Az‐DBP and CBP+2‐Az‐DBP (5 wt%) OLEDs (inset, shows picture of the glowing device), (c) current density (J), and luminance (L) versus voltage for both 2‐Az‐DBP and CBP+2‐Az‐DBP (5 wt%) devices. (The scale to the left is for J and that to the right is for L.) (d) EQE versus luminance for both the devices.

FIGURE 8.

FIGURE 8

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(a) Energy band diagram of the OLED device with CBP+2,7‐Az‐DBP (5 wt%), (b) electroluminescence spectra of 2,7‐Az‐DBP and CBP+2,7‐Az‐DBP (5 wt%) OLEDs (inset, shows picture of the glowing device), (c) current density (J), and luminance (L) versus voltage for both 2,7‐Az‐DBP and CBP+2,7‐Az‐DBP (5 wt%) devices. (The scale to the left is for J and that to the right is for L.) (d) EQE versus luminance for both the devices.

TABLE 3.

Device parameters for 2‐Az‐DBP, 2,7‐Az‐DBP and doped system of 2‐Az‐DBP and 2,7‐Az‐DBP OLEDs in CBP.

Emissive layer V ON (V) λ maxEL (nm) EQEmax (%) CIE co‐ordinates
2‐Az‐DBP 2.6 582 6.2 (0.53, 0.46)
CBP+2‐Az‐DBP (5 wt%) 4.1 579 12.3 (0.49, 0.49)
2,7‐Az‐DBP 2.9 638 2.5 (0.67,0.33)
CBP+2,7‐Az‐DBP (5 wt%) 3.4 630 5.5 (0.63,0.35)
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3. Conclusion

In this work, we report a series of donor–acceptor (D–A) emitters based on twisted dibenzoazepine donor and dibenzo[a,c]phenazine as a planar acceptor core molecule. 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP were synthesized. To explore the relation between structure and luminescence properties their detailed photophysical and electrochemical studies were carried out in different solvents at different temperatures. The ΔE ST values were determined from the low temperature fluorescence and phosphorescence spectra and found to be 0.41, 0.04, and 0.19 eV in CBP‐doped films of 3,6‐Az‐DBP, 2,7‐Az‐DBP, and 2‐Az‐DBP, respectively. From the temperature dependent emission and lifetime studies TADF was confirmed in 2,7‐Az‐DBP and 2‐Az‐DBP. Maximum EQE of about 12.3% and 5.5% was achieved in CBP doped devices of 2,7‐Az‐DBP and 2‐Az‐DBP, respectively. CBP doped 2‐Az‐DBP showed a high luminance of 6 × 103 Cd/m2 at 50 mA/cm2 while it is found to be 1.6 × 103 Cd/m2 at the same current density for 2,7‐Az‐DBP, indicating higher performance of the 2‐Az‐DBP based devices. These studies provide a promising strategy, site selective substitution of donor on acceptor core, to improve the emission properties for red TADF emitters.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting file: asia70485‐sup‐0001‐SuppMat.docx

ASIA-21-e70485-s001.docx (2MB, docx)

Supporting file: asia70485‐sup‐0002‐Data.zip

ASIA-21-e70485-s002.zip (387.6KB, zip)

Acknowledgments

The authors thank Mr. Mayuresh Mestry of CEBS for solving the single crystal structure. A.A. and S.B. acknowledges SERB for financial support (CRG/2021/001120). K.G. acknowledges the U.S. Department of Energy, Office of Science, Office of Basic Energy Science, Division of Chemical Sciences, Geosciences, and Biosciences, through Argonne National Laboratory under Contract DE‐AC02‐06CH11357.

Contributor Information

Sangita Bose, Email: sangita@cbs.ac.in.

Neeraj Agarwal, Email: na@cbs.ac.in.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Supporting file: asia70485‐sup‐0001‐SuppMat.docx

ASIA-21-e70485-s001.docx (2MB, docx)

Supporting file: asia70485‐sup‐0002‐Data.zip

ASIA-21-e70485-s002.zip (387.6KB, zip)

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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