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. 2023 Jul 21;5(8):4174–4186. doi: 10.1021/acsaelm.3c00443

Derivatives of 2-Pyridone Exhibiting Hot-Exciton TADF for Sky-Blue and White OLEDs

Iryna Danyliv †, Khrystyna Ivaniuk †, Yan Danyliv †, Igor Helzhynskyy †, Viktorija Andruleviciene ‡, Dmytro Volyniuk ‡, Pavlo Stakhira †, Glib V Baryshnikov §,∥,*, Juozas V Grazulevicius ‡,*
PMCID: PMC10449007  PMID: 37637972

Abstract

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Development of emissive materials for utilization in organic light-emitting diodes (OLEDs) remains a highly relevant research field. One of the most important aspects in the development of efficient emitters for OLEDs is the efficiency of triplet-to-singlet exciton conversion. There are many concepts proposed for the transformation of triplet excitons to singlet excitons, among which thermally activated delayed fluorescence (TADF) is the most efficient and widespread. One of the variations of the TADF concept is the hot exciton approach according to which the process of exciton relaxation into the lowest energy electronic state (internal conversion as usual) is slower than intersystem crossing between high-lying singlets and triplets. In this paper, we present the donor–acceptor materials based on 2-pyridone acceptor coupled to the different donor moieties through the phenyl linker demonstrating good performance as components of sky-blue, green-yellow, and white OLEDs. Despite relatively low photoluminescence quantum yields, the compound containing 9,9-dimethyl-9,10-dihydroacridine donor demonstrated very good efficiency in sky-blue OLED with the single emissive layer, which showed an external quantum efficiency (EQE) of 3.7%. It also forms a green-yellow-emitting exciplex with 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine. The corresponding OLED showed an EQE of 6.9%. The white OLED combining both exciplex and single emitter layers demonstrated an EQE of 9.8% together with excellent current and power efficiencies of 16.1 cd A–1 and 6.9 lm W–1, respectively. Quantum-chemical calculations together with the analysis of photoluminescence decay curves confirm the ability of all of the studied compounds to exhibit TADF through the hot exciton pathway, but the limiting factor reducing the efficiency of OLEDs is the low photoluminescence quantum yields caused mainly by nonradiative intersystem crossing dominating over the radiative fluorescence pathway.

Keywords: pyridone, hot exciton, delayed fluorescence, exciplex, OLED

1. Introduction

Rapid development of science and technology requires development of new organic materials and devices based on them as components of solid-state lighting and display technologies, medicine, biotechnology, etc. Considerable research interest in the development of new organic semiconductors for organic light-emitting devices (OLEDs) is related to the applications of these devices as components of flat panel displays, mobile electronics, smartphones, and lighting devices.1−3 Despite the continuous efforts in the development of improved materials and devices, there is still room for their perfection.

A convenient way to improve device efficiency is the utilization of emitters exhibiting delayed fluorescence (DF). DF can be realized by the utilization of triplet excitons through the upconversion of nonradiative triplet states into radiative singlet states4 in two ways. Two excited triplet states (T1) during the upconversion process can produce one singlet (S1) excited state. This process is named triplet–triplet annihilation (TTA). In another way, the temperature-assisted reverse intersystem crossing (RISC) process results in 100% efficiency of triplet state harvesting.5 The process is known as thermally activated delayed fluorescence (TADF). The TADF mechanism appears by the conversion of harvested triplet excitons from the locally excited triplet state T1(LE) to the locally excited singlet state S1(LE) at the same spectral wavelength.6 Small singlet–triplet energy splitting (ΔES-T), spatial separation of highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) with a small overlap, and efficient RISC process are the main requirements for the realization of the TADF mechanism.7,8 RISC can occur not only between T1 and S1 but also between higher triplet (Tn, n ≥ 2) and singlet states (Sm, m ≥ 1).9 Compounds allowing triplet harvesting via upper-level triplet–singlet RISC are used as emitters and hosts in the fabrication of efficient host-free OLEDs with an external quantum efficiency (EQE) higher than 5%, which is the theoretical limit of EQE for electroluminescent devices based on emitters exhibiting prompt fluorescence.10−13 Consequently, one of the ways of further enhancement of efficiency of the OLEDs is related to the development of novel compounds, including those exhibiting TADF, which occurs via upper-level triplet–singlet RISC.

Electron-donating nitrogen-containing aromatic heterocyclic compounds such as 9H-carbazole, 9,9-dimethyl-9,10-dihydroacridine, 9H-phenothiazine, 9H-phenoxazine, etc. have been widely used in the synthesis of efficient TADF emitters.14−16 Their derivatives are characterized by high thermal and electrochemical stability, good hole-transporting properties, and high and stable triplet states.17−19 In turn, electron-withdrawing moieties, such as nitrile,20,21 triazine,22,23 pyrimidine,24,25 pyridine,26 etc. were widely applied in the design of bipolar TADF emitters. For the achievement of TADF, careful selection of the combination of donor (D) and acceptor (A) moieties in the molecules is required.

In this article, we report the synthesis and characterization of new D–A-type TADF emitters based on a small acceptor (pyridin-2(1H)-one) moiety combined with a series of common donor species (9H-carbazole, 9,9-dimethyl-10H-acridane, 9H-phenothiazine, and 9H-phenoxazine). The compound containing 9,9-dimethyl-10H-acridane moiety, which exhibited the highest photoluminescence quantum yield in the solid state, was tested in electroluminescent devices. The EQE values of 3.7, 6.9, and 9.8% were achieved for sky-blue OLED with the single emissive layer, for green-yellow OLED based on the emission of exciplex with 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (MTDATA), and for white OLED based on the combination of both exciplex and single layer emissions, respectively. In addition, this compound demonstrated the effect of “hot” exciton TADF.9

2. Results and Discussion

2.1. Molecular Design and Synthesis

It is well known that 2-hydroxypyridine is capable of exhibiting keto–enol tautomerization under specific conditions.27 Utilization of this property makes it possible to implement two consequent amination reactions, which are depicted in Scheme 1. 9H-Carbazole, 9,9-dimethyl-9,10-dihydroacridine, 9H-phenothiazine, and 9H-phenoxazine were selected for the molecular design of the target compounds. The first step in the synthesis was Ullmann condensation, which afforded 2-pyridone substituted with a bromophenyl moiety. The following Ullmann condensation (in case of 9H-carbazole aromatic amine) or Buchwald–Hartwig cross-coupling reaction (in case of 9,9-dimethyl-9,10-dihydroacridine, 9H-phenothiazine and 9H-phenoxazine aromatic amines) yielded the target compounds 1-(4-(9H-carbazol-9-yl)phenyl)pyridin-2(1H)-one (PyPhCz), 1-(4-(9,9-dimethylacridin-10(9H)-yl)phenyl)pyridin-2(1H)-one (PyPhDMAC), 1-(4-(10H-phenothiazin-10-yl)phenyl)pyridin-2(1H)-one (PyPhPTZ), and 1-(4-(10H-phenoxazin-10-yl)phenyl)pyridin-2(1H)-one (PyPhPXZ). The chemical structures of the obtained derivatives were confirmed by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis (Supporting Information).

Scheme 1. Synthetic Routes and Chemical Structures of Target Compounds.

Scheme 1

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2.2. Thermal Characterization

The thermal properties of the synthesized compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA and DSC curves are shown in Figures 1a,b and S1. The temperatures of the transitions are summarized in Table 1. All derivatives exhibited a relatively high temperature of 5% weight loss (TID) ranging from 295 to 320 °C. The single-stage TGA curves reflecting full weight loss show that most of the compounds are subjected to sublimation during TGA measurements. The 2-pyridone derivatives were obtained as crystalline substances after synthesis and purification. Therefore, the endothermic melting peaks were observed for all of the derivatives in the first DSC heating scans (Figures 1b and S1). In the following cooling scans, compounds PyPhCz and PyPhDMAC showed crystallization signals at 170 and 167 °C, respectively. The melting signals were observed in the subsequent heating scans at 185 and 247 °C (Figures 1b and S1, Table 1). Meanwhile, compounds PyPhPTZ and PyPhPXZ were found to be able to form molecular glasses with glass-transition temperatures (Tg) of >66 °C (Figure S1 and Table 1).

Figure 1.

Figure 1

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(a) TGA of the derivatives of 2-pyridone and (b) DSC curves of PyPhCz.

Table 1. Thermal Characteristics of the Derivatives of 2-Pyridone.

compound TIDa, °C Tmb, °C Tcrc, °C Tgd, °C
PyPhCz 320 185 170 –
PyPhDMAC 295 247 167 –
PyPhPTZ 318 206 – 66
PyPhPXZ 307 175 – 66
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a

Determined by TGA; determined by DSC from.

b

First heating scan.

c

Cooling.

d

Second heating scan.

2.3. Photophysical Properties

Before the investigation of the photophysical properties of the target compounds, UV–vis absorption and photoluminescence (PL) spectra of the tetrahydrofuran (THF) solution of acceptor 1-(4-bromophenyl)pyridin-2(1H)-one (BrPh2PY) were recorded (Figure 2a). In addition, PL and phosphorescence (phosphorus) spectra of the THF solution of BrPh2PY were recorded at 77 K (Figure 2b). The low-energy absorption band of the dilute THF solution of BrPh2PY peaked at 312 and 322 nm. The PL spectrum of BrPh2PY has a maximum at 379 nm. The relatively high lowest singlet (S1 = 3.74 eV) and triplet (T1 = 3.31 eV) energy levels were obtained for the selected acceptor (BrPh2PY). Typically, electron-accepting and electron-donating moieties with high triplet levels are utilized in the design of efficient TADF emitters.28 Analysis of the photophysical properties of BrPh2PY shows that it has great potential to be used in the design of TADF emitters with different emission colors, including deep-blue color.

Figure 2.

Figure 2

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Absorption and PL spectra (a) as well as PL and Phos. spectra (b) of the dilute THF solution of BrPh2PY recorded at 77 K. Absorption (dashed lines) and PL (solid lines) spectra (c) of the toluene (Tol) and THF solutions as well as of solid films (Film) of the derivatives of 2-pyridone and PL and Phos. spectra (d) of their dilute THF solutions recorded at 77 K.

UV–vis absorption and photoluminescence (PL) spectra of the solutions of the derivatives of 2-pyridone in the solvents of the different polarities and of the solid films are shown in Figure 2c,d. The wavelengths of low-energy absorption and PL maxima are collected in Table 2. The low-energy absorption bands of the dilute solutions were observed in the region of 287–340 nm. Taking into account the absorption spectrum of BrPh2PY, it can be presumed that the low-energy absorption bands of the studied derivatives of 2-pyridone (PyPhs) result from their donor–acceptor structure. At wavelengths longer than 350 nm, absorption spectra of PyPhs are characterized by shoulders with long tails reaching 400 nm. These tails are caused by the intramolecular charge-transfer (CT) states formed by the donor–acceptor interactions in the ground state. The low-energy absorption bands of the solid films were found to be slightly bathochromically shifted (304–380 nm) in comparison with those of the corresponding solutions of the derivatives. The shifts are probably caused by the intermolecular interactions in the solid state.

Table 2. Photophysical Characteristics of the Compounds.

compound λabsa,b,c, nm λPLa,b,c, nm ΦPLa ΦPLb ΦPLc S1, eV T1, eV ΔEST, eV
PyPhCz 340/340/380 399/407/411 0.04 0.02 0.02 3.77 3.12 0.65
PyPhDMAC 290/287/304 429/482/441 0.05 0.045 0.04 3.63 3.24 0.39
28PyPhPTZ 322/319/336 468/550/480 0.01 0.01 <0.01 3.19 2.67 0.52
PyPhPXZ 324/322/333 472/545/480 0.07 0.03 <0.01 3.38 2.82 0.56
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a

Toluene solution.

b

THF solution.

c

Solid state. λabs is the wavelength of absorption maximum; λPL is the wavelength of photoluminescence maximum; Φ is photoluminescence quantum yield. The concentration of solutions was 10–5 M.

The solutions in toluene and thin films of 2-pyridone derivatives demonstrated PL in the violet-blue range of the visible spectrum with emission intensity maxima in the regions of 399–472 and 411–480 nm, respectively. The bathochromic shifts of emission bands were observed for the THF solutions relative to those of the toluene solutions of the derivatives, with the wavelength maximum emission intensities varying from 407 to 550 nm (Table 2). PL and phosphorescence spectra were recorded at 77 K (Figure 2d). The lowest singlet (S1) and triplet (T1) energy levels and ΔEST values were determined from the high-energy onsets of the corresponding spectra (Table 2). Large ΔEST values for PyPhs (>0.39 eV) do not support the possibility of the reverse intersystem crossing between the lowest triplet and singlet states. The photoluminescence efficiencies were found to be rather low for both, toluene solutions and solid films. The highest photoluminescence quantum yields (PLQY) were observed for the solid sample of PyPhDMAC and for a toluene solution of PyPhPXZ. Taking into account these observations, we decided to focus on compound PyPhDMAC as an emissive component of OLEDs. Surprisingly, PyPhDMAC-based OLED demonstrated a quite high EQE of 3.7% (more details are given in the next section), which is close to the theoretical limit of 5% for fluorescent OLEDs. We assume that compound PyPhDMAC might exhibit TADF. As follows from our quantum-chemical calculations, PyPhDMAC sustains a three-state model (Tables 3 and 4) typical for emitters demonstrating TADF of type I through the spin–vibronic coupling mechanism (following nomenclature by Monkman and coauthors).29 One should note that the S1 state of PyPhDMAC can be assigned to the charge-transfer (1CT) state (HOMO is localized on the donor part, while LUMO is on the acceptor part, Figure S2), while the T1 state is localized on the acceptor part only (i.e., is the 3LE state). Thus, spin–orbit coupling matrix element (SOCME) between these states is considerable (0.64 cm–1, Table 4) in line with earlier reported conclusions by Brédas et al.30 and is also consistent with the classical El-Sayed rule for ISC rates. The electronic configuration of the T2 state is the same as for the S1 state (i.e., T2 is the 3CT state), and thus it is quasi-degenerated with 1CT. Therefore, the mechanism of TADF exhibited by PyPhDMAC can be considered since 3LE is coupled with 3CT by vibronic coupling and 3CT-to-1CT rISC is thus possible (SOCME is small, 0.04 cm–1, but nonzero). However, based on the T1 state geometry of PyPhDMAC, our calculations predict very large S1-T1 (1.12 eV) and T2-T1 (1.11 eV) gaps that unbales efficient reverse internal conversion between 3LE and 3CT states. Thus, vibronically assisted TADF is not the case for PyPhDMAC. Another idea of how TADF can be realized in PyPhDMAC is the hot exciton concept.9 Accounting for the fact that S1 and T2 states are quasi-degenerated and both correspond to HOMO–LUMO electronic configuration, the corresponding 3CT (T2) and 1CT (S1) excitons can be populated initially through electron–hole recombination (OLED is fabricated following the closest energy matching between injected electrons and holes vs LUMO and HOMO energy levels, respectively). Taking into account the very large T2–T1 separation, the internal conversion from T2 to T1 should be slower than T2–S1 reverse ISC (S1–T2 gap is only 0.01 eV, SOCME is small but nonzero, 0.04 cm–1), which converts T2 excitons to emissive S1 excitons. Thus, the TADF channel for T2 “hot” excitons should dominate over nonradiative quenching of T2 excitons, which results in the OLED heterostructure demonstrating an external quantum efficiency of around 3.7%. For compound PyPhCz, four triplet states are lower in energy than the S1 state, which makes a triplet yield high and thus quenches the S1 state fluorescence. The rISC channel for compound PyPhCz is also possibly accounting for the close-lying T4 and S1 states of the same HOMO–LUMO configuration coupled by SOC with ⟨S1|ĤSO|Tn⟩ = 0.20 cm–1 (at T1 geometry). Also the T5–S2 channel is a possible way to get rISC in PyPhCz. The gap is less than 0.1 eV, and the SOCME is 0.23 cm–1.

Table 3. Excited State Energies for the Derivatives of 2-Pyridonea.

compound E(S1)vert E(S1)S1 E(Tn)S1 E(T1)vert E(S1)T1 E(Tn)T1
PyPhCz 3.77 (0.36) [3.65] 3.28 (0.38) [3.11] 2.30 (n = 1) 2.73 3.67 2.59 (n = 1)
3.00 (n = 2) 2.73 (n = 2)
3.24 (n = 3) 3.07 (n = 3)
3.43 (n = 4)
PyPhDMAC 3.36 (10–4) 2.92 (10–3) [2.89] 2.30 (n = 1) 2.73 3.11 1.99 (n = 1)
2.91 (n = 2) 3.10 (n = 2)
PyPhPTZ 3.41 (5 × 10–4) 2.654 (4 × 10–4) [2.65] 2.29 (n = 1) 2.73 2.58 1.94 (n = 1)
2.41 (n = 2) 2.73 (n = 2)
2.648 (n = 3) 2.98 (n = 3)
PyPhPXZ 3.09 (2 × 10–4) 2.579 (4 × 10–4) [2.63] 2.30 (n = 1) 2.73 2.84 1.99 (n = 1)
2.54 (n = 2) 2.77 (n = 2)
2.577 (n = 3) 2.82 (n = 3)
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a

Calculated by the TDDFT/LC-ωPBE/6-31G(d) method (ω = 0.14; PCM model was used for accounting of toluene solvent effect). Experimental references are presented in parentheses.

Table 4. Orbital Nature of Low-Lying Excited States for the Derivatives of 2-Pyridone Together with the Values of Spin–Orbit Coupling Matrix Elements (SOCMEs) ⟨S1|ĤSO|Tn⟩ Calculated at S1 and T1 State Geometries.

compound ⟨S1|ĤSO|Tn⟩S1, cm–1 ⟨S1|ĤSO|Tn⟩T1, cm–1 assignmentS1 assignmentT1
PyPhCz 0.30 (n = 1) 0.04 (n = 1) S1: H → L (96%) S1: H → L (89%)
0.75 (n = 2) 0.53 (n = 2) T1: H-2 → L (78%) T1: H → L+1 (73%)
0.30 (n = 3) 0.08 (n = 3) T2: H → L (61%) T2: H-2 → L (72%)
0.20 (n = 4) T3: H-1 → L (88%) T3: H-1 → L+1 (72%)
T4: H → L (65%)
PyPhDMAC 0.93 (n = 1) 0.64 (n = 1) S1: H → L (97%) S1: H → L (97%)
0.10 (n = 2) 0.04 (n = 2) T1: H-1 → L (86%) T1: H-1 → L (90%)
T2: H → L (97%) T2: H → L (96%)
PyPhPTZ 0.69 (n = 1) 3.21 (n = 1) S1: H → L (98%) S1: H → L+1 (93%)
1.22 (n = 2) 1.75 (n = 2) T1: H-1 → L (75%) S2: H → L (70%)
0.10 (n = 3) 2.96 (n = 3) T2: H → L+1 (69%) T1: H → L+1 (88%)
T3: H → L (95%) T2: H-1 → L (62%)
T3: H → L (68%)
PyPhPXZ 0.75 (n = 1) 0.58 (n = 1) S1: H → L (98%) S1: H → L (97%)
1.35 (n = 2) 1.39 (n = 2) T1: H-1 → L (85%) T1: H-1 → L (90%)
0.13 (n = 3) 0.09 (n = 3) T2: H → L+3 (49%) T2: H → L+3 (58%)
T3: H → L (94%) T3: H → L (96%)
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Similarly, compound PyPhPTZ can show TADF through the rISC between hot excitons, mainly within the T2–S1 and T3–S2 pairs. It is important to note that the T2–S1 rISC channel is thermodynamically allowed (T2 is higher in energy than S1 by 0.15 eV; thus, it does not require thermal activation). At the same time, the gaps T2–T1 and T3–T1 are relatively large (0.79 and 1.04 eV, respectively), preventing internal conversion within these pairs. For the related compound PyPhPXZ, the hot exciton rISC mechanism is similar to that of PyPhDMAC, but an additional T2 state is very close energetically to S1 (0.07 eV lower) and T3 (0.05 eV lower) states. This makes possible equilibration and mixing between T2 and T3 states through vibronic coupling, thus facilitating the rISC process. Similarly to PyPhPTZ and PyPhDMAC, the T1 state is very low-lying for PyPhPXZ. This allows us to predict slow T2–T1 and T3–T1 nonradiative deactivations. Summarizing, all of the studied compounds may be able to exhibit TADF through the rISC involving hot excitons, but deactivation of the S1 state through the ISC still dominates over the radiative decay of the S1 state. This results in low quantum yields of photoluminescence (PLQYs of the solid samples of PyPhPXZ and PyPhPTZ are less than 1%, PLQY of the layer of PyPhCz is just 2% and that of the layer PyPhDMAC is 4%). This, to a great extent, limits the applicability of the synthesized compounds for the preparation of active layers of OLEDs. Modification of the linking group and chemical tuning of donor moieties can be considered as potential ways to enhance radiative rate constants.

Both the solutions and the solid samples of the compounds demonstrate similar trends of PL decay (Figure 3). The common trend for all of the cases is the presence of a very fast decay component in the range of 0.35–1.37 ns (Table 5). Accounting for the very weak fluorescence quantum yield observed for all of the compounds, this fast component can be assigned to nonradiative quenching of the S1 state most likely through the ISC channel. Indeed, for the S-containing compound PyPhPTZ sustaining internal heavy atom effect on the ISC process, fast decay component (τ1) is the fastest one in the whole series (just 0.34 ns for the solution in toluene). The second slower component of PL decay can be assigned to combined deactivation channels including ISC between S1 and particular low-lying Tn states, prompt fluorescence, and TADF via a hot exciton channel. For example, one can assume that for PyPhPXZ, the toluene solution of which demonstrates the highest PLQY, τ2 corresponds to a large amount of prompt fluorescence combined with S1–T2 and S1–T1 ISC, but in the case of PyPhPTZ, τ2 lifetime corresponds mainly to S1–T2 and S1–T1 ISC accounting for negligible PLQY (Table 2).

Figure 3.

Figure 3

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PL decay curves of the toluene solutions (left) and thin solid films (right) of the compounds recorded at room temperature.

Table 5. Fitting Results for PL Decay Curves of the Toluene Solutions and Thin Solid Films of the 2-Pyridone Derivatives at Room Temperature.

  toluene
 films
compound τ1, ns/Rel1, %a τ2, ns/Rel2, %a χ2 τ1, ns/Rel1, %a τ2, ns/Rel2, %a χ2
PyPhCz 0.42/94.76 6.75/5.24 1.279 0.55/97.15 7.57/2.85 1.293
PyPhDMAC 1.03/89.43 6.56/10.57 1.286 1.05/57.61 6.35/42.39 1.125
PyPhPTZ 0.35/42.50 2.65/57.50 1.151 0.57/95.71 6.54/4.29 1.278
PyPhPXZ 1.37/42.37 3.16/57.63 1.085 0.58/90.90 7.95/9.10 1.240
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a

Rel1 and Rel2 (in %) are contribution ratios of lifetimes τ1 and τ2, respectively

For the solid films, both τ1 and τ2 are generally lower than for toluene and the contribution of τ1 significantly dominates over τ2 (Table 5). Only for PyPhDMAC, the contribution of τ2 is equivalent to that of τ1. This observation we interpret in terms of hot exciton TADF combined with prompt fluorescence in line with conclusions by Kuehne31 and Ma9,10 that upper-level rISC in hot exciton TADF materials is fast and practically indistinguishable from fluorescence. In summary, the results of photoluminescence decay measurements confirm the presence of hot exciton TADF of PyPhDMAC and explain the unusually high efficiency of the corresponding sky-blue OLED.

2.4. Electrochemical and Photoelectrical Properties

Cyclic voltammetry (CV) measurements were used to estimate the ionization potentials (IPCV) and electron affinities (EACV), which are the key parameters of the materials used for the fabrication of the OLEDs (Table 6). CV curves of the solutions of the compounds in anhydrous dimethylformamide (DMF) are shown in Figure 4. The positive and negative voltages were applied for the investigation of the redox behavior of the synthesized compounds and the estimation of the electronic energy levels. Due to the observation of both reduction and oxidation processes from CV curves of tested compounds, CV measurements of the solutions of the compounds revealed bipolar behavior. PyPhDMAC, PyPhPTZ, and PyPhPXZ containing 9,9-dimethyl-10H-acridine, 9H-phenothiazine, and 9H-phenoxazine moieties, respectively, were characterized by the quasi-reversible oxidation process, while compound PyPhCz with 9H-carbazole fragment showed the irreversible oxidation waves during the first scan, probably due to the participation of unsubstituted C-3 and C-6 positions of 9H-carbazole.32 The oxidation peaks of 2-pyridone derivatives were observed in the range of 0.28–0.88 V. Irreversible reduction processes with analogous shapes were observed for all of the studied compounds, due to the similar electron-deficient fragment. The reduction peaks were observed in the region of −2.52 to – 2.56 V. To estimate the IPCV and EACV values, the onsets of oxidation and reduction potentials were used, respectively. The EA values were found to be quite similar for all of the investigated compounds (2.24–2.28 eV). The highest IPCV value of 5.68 eV was observed for the compound PyPhCz containing the 9H-carbazole fragment. The lower IPCV values (5.34, 5.12, and 5.08 eV) were obtained for other 2-pyridone-based derivatives. This observation can apparently be attributed to the stronger electron-donating effect of 9H-phenothiazine, 9H-phenoxazine, and 9,9-dimethyl-10H-acridane units.

Table 6. Electrochemical and Photoelectrical Characteristics of 2-Pyridone Derivativesa.

compound Eox vs Fc+/Fconset Ered vs Fc+/Fconset IPcv., eV EACV, eV
PyPhCz 0.88 –2.53 5.68 2.27
PyPhDMAC 0.54 –2.52 5.34 2.28
PyPhPTZ 0.32 –2.55 5.12 2.25
PyPhPXZ 0.28 –2.56 5.08 2.24
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a

Eox vs Fc+/Fconset, Ered vs Fc+/Fconset—onsets of oxidation and redaction potentials respectively; IP—ionization potential, IPCV = Eoxonset + 4.8; EA—electron affinity, EACV = 4.8 – |Eredonset|.

Figure 4.

Figure 4

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CV curves of the 2-pyridone-based derivatives.

2.5. Electroluminescent Properties

Taking into account the highest solid fluorescence quantum yield and TADF effect, we utilized compound PyPhDMAC for the fabrication of nondoped and exciplex emission-based OLEDs. The nondoped device A has the following structure:

2.5.

Devices A–C were fabricated by step-by-step deposition of hole- and electron transport layers, organic emissive layers, and metal electrodes onto precleaned ITO-coated glass substrate under a vacuum of 10–5 Torr. CuI33 and TPD34 (N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) were used as materials for hole-transporting layers. The TPBi35 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) and TSPO1 (diphenyl[4-(triphenylsilyl)phenyl]phosphine oxide)36 were used as electron-transporting and hole-blocking layers, respectively. The hole transport layer (HTL) and the electron transport layer (ETL) were chosen to confine holes and electrons in the emitting layer that improves the performance of the device. The Ca layer topped with a 200 nm aluminum (Al) layer was used as the cathode.37,38 The active area of the obtained devices was 2*3 mm2. The density–voltage and luminance–voltage dependences were recorded using a semiconductor parameter analyzer HP4145A. Electroluminescence (EL) spectra were recorded with an Ocean Optics USB2000 spectrometer.

As shown in Figure S4, the emission maxima wavelengths of device A were located in the range of 440–480 nm, which was close to the PL spectrum of the solid film of PyPhDMAC. This observation confirms exciton recombination in the emitting layer. The EL spectra were stable in the wide range of driving voltages, confirming the good color stability of the device (Figure S5a). Device A was characterized by relatively low-efficiency roll-off and good stability of efficiency of the device in all of the range of current density and by a maximum EQE value of 3.7% (Table 7).

Table 7. EL Characteristics of Devices A–C.

device Von, V brightnessmax, Cd m–2 ηcmax, cd A–1 ηpmax, lm W–1 EQEmax, % λmaxEL, nm CIE1976 (u,v)
A 6 9900 6.1 2.3 3.7 482 (0.134, 0.391)
B 5.8 31 200 15.7 6.5 6.9 517 (0.121, 0.478)
C 5.7 35 370 16.1 6.9 9.8 425/480/519 (0.151, 0.441)
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Von,—torn-on voltage, ηcmax—maximum current efficiency, ηpmax—maximum power efficiency, EQEmax,—maximum value of external quantum efficiency, λmaxEL—EL maximum, CIE1976 (u, v)—chromaticity coordinates.

The exciplex emission-based OLED was prepared taking into account the results of the investigation of the photophysical properties of the PyPhDMAC. The 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine (m-MTDATA) was used as the donor counterpart for PyPhDMAC to induce exciplex emission. The energy difference of the HOMOs of m-MTDATA and PyPhDMAC is 0.24 eV, while that for the LUMO levels is 0.28 eV (Figure 5a). The structure of device B was as follows:

2.5.

Figure 5.

Figure 5

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Schematic energy diagram of device C (a) and chromaticity diagram of devices A–C (b).

Figure S3b shows the relaxation dynamics of the exciplex emission of PyPhDMAC and m-MTDATA, which take longer times than the relaxation dynamics of fluorescence of solid films of PyPhDMAC or m-MTDATA. Because of the mismatch in the LUMO and HOMO energy levels, cross-coupling of the charge carriers occurred at the interface of the layers of m-MTDATA and PyPhDMAC and as a result, intense broad exciplex emission was observed in the region of 500–750 nm (Figure S3a). The spectrum of the exciplex of the mixture of m-MTDATA and PyPhDMAC is broader and red-shifted in comparison to the spectra of the solid films of PyPhDMAC or m-MTDATA (Figure S3a).

In order to fabricate the efficient WOLED, the technique of combining basic colors was used. The three-color WOLED was realized by simultaneously combining the intrinsic emission and exciplex emissions of the same materials (Figure 5a) similarly to previously reported results.39,40 The principal scheme of the fabricated WOLED based on exciplex enhanced TADF looks as follows:

2.5.

Since PyPhDMAC exhibited good performance in nondoped sky-blue OLEDs, it was anticipated that using PyPhDMAC as the blue-emitting compound, high-performance white OLED (WOLED) can be obtained. To accomplish this goal, the efficient yellow-green exciplex emission was introduced into the device to design full-color WOLED. Additionally, the thin layer of m-MTDATA was used to get blue emission. The layers of PyPhDMAC and m-MTDATA were separated by the layer of TSPO1 in the device to disturb the injection of carriers. The second TSPO1 interlayer with a thickness of 4 nm could block energy transfer between m-MTDATA and TPBi and modify the recombination zone. As a result, the designed structure of the device C allowed us to obtain exciton recombination zones from three emitters, i.e., deep-blue m-MTDATA, sky-blue PyPhDMAC, and yellowish-green interface exciplex emitters. The resulting WOLED presented a combined emission from different excited states, i.e., blue emission from the layer of m-MTDATA thin layer, exciplex emission from the interface of the layer of m-MTDATA and PyPhDMAC and sky-blue exciton emission from PyPhDMAC (Figure 5b).

The lighting parameters are presented in Figures 6 and S6. Device B, based on exciplex emission of the interface of m-MTDATA and PyPhDMAC turned on at 5.8 V and showed high CE, PE, and EQE of 15.7 cd A –1, 6.5 lm W –1, and 6.9%, respectively (Table 7).

Figure 6.

Figure 6

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Current density and brightness versus voltage characteristics and photo of devices A–C (a, c, and e, respectively) and current efficiency-brightness-external quantum efficiency (b, d, and f, respectively) and power efficiency–voltage (inside) curves.

The graphs of the dependence of current efficiency and external quantum efficiency on brightness are shown in Figure 6. The external quantum efficiency was stable over the full scale of brightness and varied by 0.4% at luminance from 1000 to 10 000 Cd m–2. For device B, the value of quantum efficiency dropped by 1.7%, and for device C, it was two times smaller at a brightness of 35 000 Cd m–2 compared to those at 100 Cd m–2. On the one hand, these values are high for device C, but if we talk about operating voltages (1–10 V) and operating brightness (100–1000 Cd m–2), the decrease in performance is also within 0.5–2%. On the other hand, in the present work, the performance parameters of devices at critical values of voltages and current densities were studied. In device C (Figures 6 and S8), the current density is very high at maximum brightness, which causes thermal degradation effects in the device. This is due to several reasons: the thickness of the device, energy barriers between the emissive and functional layers, and the presence of nonradiative transitions.

The WOLED (device C) based on the combination of three deep-blue, sky-blue, and green-yellow emitters with CIE coordinates (u, v) of 0.151; 0.441 showed a turn-on voltage of 5.7 V, EQE exceeding 9.8%, and CE of more than 16 Cd A –1.

3. Conclusions

The new series of donor–acceptor compounds based on 2-pyridone were synthesized, and their properties were investigated experimentally and theoretically. It was determined that compounds containing 9H-carbazole or 9,9-dimethyl-9,10-dihydroacridine moiety are crystalline materials while derivatives of 2-pyridone with 9H-phenothiazine or 9H-phenoxazine fragment are capable to form amorphous layers. The compounds exhibit emission in the violet and blue regions. The derivative of 2-pyridone and 9-dimethyl-9,10-dihydro-acridine is characterized by “hot” exciton TADF. Based on the theoretical calculations, it is assumed that a very large gap between T2 and T1 results in a slow internal conversion from T2 to T1. Therefore, the RISC process from T2 to S1 with a gap of only 0.01 eV is possible. Emissive TADF channel from T2 to S1 dominates over nonradiative quenching of “hot” excitons resulting in the external quantum efficiency of sky-blue OLED of 3.7%. The derivative containing a 9,9-dimethyl-9,10-dihydroacridine moiety also demonstrates exciplex-forming properties. The film of the compound and 4,4′,4″-tris[phenyl(m-tolyl)amino]triphenylamine shows an exciplex emission at 520 nm. The studied 2-pyridone derivative was tested in exciplex-based OLEDs exhibiting green-yellow and white emissions. The best white emitting device showed maximum current efficiency of 16.1 cd A–1, power efficiency of 16.9 lm W–1, and external quantum efficiency of 9.8%.

4. Experimental Section

4.1. Instrumentation

1H and 13C nuclear magnetic resonance (NMR) spectra were reordered on a Bruker DRX 400 P (400 MHz (1H), 100 MHz (13C)) spectrometer at room temperature in a deuterated chloroform (CDCl3) solution. The data are given as chemical shifts in δ (ppm), and tetramethylsilane (TMS) was used as an internal standard. Mass spectra were obtained by the electrospray ionization mass spectrometry (ESI-MS) method on an Esquire-LC 00084 mass spectrometer. Elemental analysis was performed with EuroEA Elemental Analyzer.

UV/vis spectra were recorded in quartz cells on an AvaSpec-USB2 spectrophotometer for 10–5 M solutions of the compounds. Photoluminescence (PL) spectra of 10–5 M solutions of the compounds were performed on an Edinburgh Instruments’ FLS980 fluorescence spectrometer. Thin solid films for recording UV/vis and PL spectra were prepared by spin-coating technique utilizing SPS-Europe Spin150 Spin processor using 2.5 mg/mL solutions of the compounds in THF on the precleaned quartz substrates. Photoluminescence quantum yields of the solutions and of the solid films were performed using the integrated sphere (Edinburgh Instruments) coupled to the FLS980 spectrometer, calibrated with two standards: quinine sulfate in 0.1 H2SO4 and rhodamine 6G in ethanol.41 Fluorescence decay curves of the samples were measured using a time-correlated single photon counting technique utilizing the PicoQuant PDL 820 ps diode laser as an excitation source (wavelength of 374 nm).

Differential scanning calorimetry (DSC) measurements were made on a TA Instruments “DSC Q100” calorimeter. The samples were heated at a scan rate of 10 °C/min under a nitrogen atmosphere. Thermogravimetric analysis (TGA) was performed on a “Mettler TGA/SDTA851e/LF/1100” at a heating rate of 20 °C/min under a nitrogen atmosphere. Electrochemical measurements were done using μAutolab Type III (EcoChemie, Netherlands) potentiostat, in a three-electrode cell using platinum rod as the counter electrode, glassy carbon as the working electrode (diameter 2 mm), and Ag/AgNO3 as the reference electrode with a scan rate of 2.5 mV/s with concentration of compounds 1.0 × 10–4 mol/dm3. The measurements were calibrated using internal standard ferrocene/ferrocenium (Fc/Fc+). Cyclic voltammetry (CV) experiments were conducted in the dry solvent solution containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as the electrolyte at room temperature under a nitrogen atmosphere. Deaeration of the solution was achieved by a nitrogen bubbling for ca. 10 min before measurement.

One of the synthesized derivatives was studied in electroluminescence devices. Devices were fabricated by vacuum deposition of organic semiconductor layers and metal electrodes onto the precleaned indium tin oxide (ITO)-coated glass substrate under vacuum of 10–6 Torr. ITO was used as the anode, which has great electric conductivity and light transmittance. Ca and Al are used as cathode. The HOMO energy level of CuI matches the structure with the weak potential barrier at the interface, which is conducive to hole injection and reducing the turn-on voltage, respectively. A hole-blocking layer (HBL) in the devices using TSPO1 is added between the emitting layer (m-MTDATA) and the electron-transporting layer, which blocks m-MTDATA and TPBi from forming exciplex at the interface. The density–voltage and luminance–voltage characteristics were measured by using a Keithley 6517 Binair instrument without passivation immediately after the preparation of the device. The brightness measurements were carried out by using a calibrated photodiode. The electroluminescence spectra were recorded with an Avaspec-2048L spectrometer. Device efficiencies were calculated from the luminance, current density, and EL spectrum.

4.2. Materials and Synthesis

The starting compounds, i.e., 2-hydroxypyridine, 1-bromo-4-iodobenzene, 9,10-dihydroacridine, 1,10-phenanthroline, palladium acetate (Pd(CH3COO)2), tritert-butylphosphine (P(t-Bu)3), copper, copper(I) chloride (CuCl), cesium carbonate (Cs2CO3), potassium carbonate (K2CO3), and sodium chloride (NaCl), were purchased from Sigma-Aldrich and used as received.

4.2.1. 1-(4-Bromophenyl)pyridin-2(1H)-one (BrPh2PY)

BrPh2PY was synthesized by Ullmann condensation.42 1-Bromo-4-iodobenzene (7.45 g, 0.026 mol), 2-hydroxypyridine (2.5, 0.026 mol), copper (0.168 g, 2.6 mmol), copper(I) chloride (0.26 g, 2.6 mmol), potassium carbonate (7.26 g, 0.52 mol), and 1,10-phenanthroline (0.974 g, 3.4 mmol) were dissolved in dimethyl sulfoxide (DMSO) under argon atmosphere. The reaction mixture was heated at 140 °C for 24 h. After cooling, the reaction mixture was filtered through a 2 cm layer of Celite and washed with ethyl acetate. The solvent was removed under reduced pressure, and the product was recrystallized from isopropanol and vacuum-dried to afford light-brown crystals. 1H NMR (400 MHz, CDCl3) δ 7.55 (d, J = 8.3 Hz, 2H), 7.38–7.30 (m, 1H), 7.22 (t, J = 8.9 Hz, 3H), 6.62 (d, J = 9.3 Hz, 1H), 6.20 (t, J = 6.7 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ: 162.21, 140.18, 139.78, 138.55, 137.50, 132.57, 128.33, 121.95, 106.44. Mp 100 °C.

4.2.2. 1-(4-(9H-Carbazol-9-yl)phenyl)pyridin-2(1H)-one (PyPhCz)

PyPhCz was prepared by Ullmann condensation.42 A mixture of BrPh2PY (0.5 g, 2.00 mmol), 9H-carbazole (0.4 g, 2.39 mmol), Cu (0.01 g, 0.20 mmol), CuCl (0.02 g, 0.20 mmol), K2CO3 (0.55 g, 3.98 mmol), and 1,10-phenanthroline (0.07 g, 0.40 mmol) were dissolved in o-DCB (10 mL) under an argon atmosphere. The reaction mixture was heated at 170 °C for 24 h. After cooling down, the reaction mixture was filtrated through a 2 cm layer of Celite and washed with dichloromethane (DCM). The solvent was removed under reduced pressure, and the crude product was purified by flash column chromatography on silica gel using an eluent system EtOAc/HEX = 1.5/1, crystallized from isopropanol and vacuum-dried to afford yellow crystals (0.58 g, 87% yield). 1H NMR (400 MHz, CDCl3) δ 8.08 (d, J = 7.8 Hz, 2H), 7.64 (d, J = 8.6 Hz, 2H), 7.57 (d, J = 8.6 Hz, 2H), 7.44–7.33 (m, 6H), 7.24 (t, J = 7.4 Hz, 2H), 6.65 (d, J = 8.9 Hz, 1H), 6.25 (t, J = 6.7 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ: 162.40, 140.58, 140.08, 139.55, 137.82, 128.14, 127.67, 126.13, 123.59, 122.15, 120.36, 109.78, 106.32. Elemental analysis calcd for C23H16N2O (%): C 82.12, H 4.79, N 8.33, O 4.76; found (%): C 82.06, H 4.77, N 8.39. m/z: 336.13.

4.2.3. 1-(4-(9,9-Dimethylacridin-10(9H)-yl)phenyl)pyridin-2(1H)-one (PyPhDMAC)

PyPhDMAC was prepared by the Buchwald–Hartwig cross-coupling reaction.43 A mixture of appropriate BrPh2PY (0.5 g, 2.00 mmol), 9,9-dimethyl-10H-acridane (0.5 g, 2.39 mmol), and Cs2CO3 (1.30 g, 4.00 mmol) was dissolved under argon in dry toluene (10 mL) and was stirred for 10 min at room temperature. Then, Pd(CH3COO)2 (0.09 g, 0.40 mmol) and P(t-Bu)3 (0.04 g, 0.19 mmol) under argon were added. The reaction mixture was heated at 100 °C for 24 h. After cooling down, the reaction mixture was filtrated through a 2 cm layer of Celite and washed with DCM. After that, the solvent was removed under reduced pressure. The crude product was purified by flash column chromatography on silica gel using eluent system EtOAc/HEX = 1.5/1, crystallized from isopropanol, and vacuum-dried to afford olive crystals (0.33 g, 45% yield). 1H NMR (400 MHz, CDCl3) δ 7.61 (d, J = 8.5 Hz, 2H), 7.45–7.34 (m, 6H), 6.96–6.84 (m, 4H), 6.65 (d, J = 9.2 Hz, 1H), 6.25 (dd, J = 11.0, 7.4 Hz, 3H), 1.62 (s, 6H). 13C NMR (100 MHz, CDCl3) δ: 162.29, 141.29, 140.60, 140.05, 137.79, 132.29, 130.14, 129.08, 126.45, 125.31, 122.18, 120.84, 114.17, 106.27, 35.99, 31.25. Elemental analysis calcd for C26H22N2O (%): C 82.51, H 5.86, N 7.40, O 4.23; found (%): C 82.46, H 5.91, N 7.45. m/z: 378.17.

4.2.4. 1-(4-(10H-Phenothiazin-10-yl)phenyl)pyridin-2(1H)-one (PyPhPTZ)

PyPhPTZ was prepared by the same method as PyPhDMAC, starting from BrPh2PY (0.5 g, 2.00 mmol), phenothiazine (0.29 g, 2.44 mmol), Cs2CO3 (1.30 g, 4.00 mmol), Pd(CH3COO)2 (0.09 g, 0.40 mmol), and P(t-Bu)3 (0.04 g, 0.19 mmol) in dry toluene (10 mL). The crude product was purified by flash column chromatography on silica gel using an eluent system EtOAc/HEX = 1.5/1, crystallized from isopropanol, and vacuum-dried to afford white crystals (0.31 g, 43% yield). 1H NMR (400 MHz, CDCl3) δ 7.51 (d, J = 8.6 Hz, 2H), 7.39 (d, J = 8.7 Hz, 2H), 7.37–7.33 (m, 2H), 7.03 (dd, J = 7.4, 1.4 Hz, 2H), 6.85 (dt, J = 24.7, 7.2 Hz, 4H), 6.65–6.60 (m, 1H), 6.39 (d, J = 8.0 Hz, 2H), 6.22 (t, J = 6.7 Hz, 1H). 13C NMR (100 MHz, CDCl3) δ: 162.33, 140.02, 139.46, 137.78, 129.34, 128.66, 127.09, 123.29, 122.63, 122.11, 117.97, 106.25. Elemental analysis calcd for C23H16N2OS (%): C 74.98, H 4.38, N 7.60, O 4.34, S 8.70; found (%): C 74.92, H 4.33, N 7.66, S 8.75. m/z: 368.10.

4.2.5. 1-(4-(10H-Phenoxazin-10-yl)phenyl)pyridin-2(1H)-one (PyPhPXZ)

PyPhPXZ was prepared by the same method as PyPhDMAC, starting from BrPh2PY (0.5 g, 2.00 mmol), phenoxazine (0.44 g, 2.40 mmol), Cs2CO3 (1.30 g, 4.00 mmol), Pd(CH3COO)2 (0.09 g, 0.40 mmol), and P(t-Bu)3 (0.04 g, 0.19 mmol) in dry toluene (10 mL). The crude product was purified by flash column chromatography on silica gel using eluent system EtOAc/HEX = 1.5/1, crystallized from isopropanol, and vacuum-dried to afford light-brown crystals (0.31 g, 45% yield). 1H NMR (400 MHz, CDCl3) δ 7.57 (d, J = 8.5 Hz, 2H), 7.44–7.33 (4H), 6.66–6.52 (m, 8H), 5.94 (dd, J = 7.8, 1.3 Hz, 2H). 13C NMR (100 MHz, CDCl3) 162.23, 143.94, 140.70, 140.10, 139.08, 137.62, 134.00, 131.86, 129.29, 123.31, 122.17, 121.66, 115.58, 113.44, 106.35. Elemental analysis calcd for C23H16N2O2 (%): C 78.39, H 4.58, N 7.95, O 9.08; found (%): C 78.44, H 4.04, N 7.89. m/z: 352.12.

4.3. Computational Details

Ground singlet state (S0) of PyPhCz, PyPhDMAC, PyPhPTZ, and PyPhPXZ molecules were optimized at the B3LYP/6-31G(d)44−47 level of density functional theory (DFT) using Grimme’s empirical dispersion correction (GD3).48 Based on optimized S0 state geometries, the first singlet excited state (S1) geometry was optimized by time-dependent (TD) DFT method49 employing the same GD3-B3LYP/6-31G(d) approach, while the first triplet excited state (T1) was optimized by spin-unrestricted UB3LYP/6-31G(d) method. The polarizable continuum model (PCM) was used during all geometry optimization procedures.50 By using optimized S1 and T1 geometries, the energies of singlet and triplet excited states were clarified by range-separated LC-ωPBE51 with manually tuned ω value equal to 0.14 that gives the best agreement with experimentally observed fluorescence wavelength. All of these calculations were performed within Gaussian16 software.52 The spin–orbit coupling matrix elements ⟨S1|ĤSO|Tn⟩ were calculated by using zeroth-order regular approximation (ZORA)53 for ĤSO operator at TDDFT/PBE0/TZP54,55 level of theory with accounting of solvent effect within COSMO model56 similarly to the methodology proposed by Brédas et al.57 SOC calculations were performed within ADF2021 software.58

Acknowledgments

This work was supported by the Ministry of Education and Science of Ukraine (project nos. 0123U101690 and 0121U107533). The quantum-chemical calculations were performed with computational resources provided by National Academic Infrastructure for Supercomputing in Sweden (NAISS 2023/5-77) at the National Supercomputer Centre (NSC) at Linköping University partially funded by the Swedish Research Council through grant agreement no. 2022-06725. G.B. thanks the support by the Swedish Research Council through starting grant no. 2020-04600. This project also received funding from European Social Fund (project no. 09.3.3-LMT-K-712-23-0125) under grant agreement with the Research Council of Lithuania (LMTLT).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsaelm.3c00443.

  • Thermal properties (DSC curves), photophysical properties of the studied materials and electroluminescence properties of the fabricated devices, and 1H and 13C spectra of synthesized materials (PDF)

The authors declare no competing financial interest.

Supplementary Material

el3c00443_si_001.pdf (719.3KB, pdf)

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