Highly Efficient Electroluminescent Materials with High Color Purity Based on Strong Acceptor Attachment onto B–N-Containing Multiple Resonance Frameworks

Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Highly Efficient Electroluminescent Materials with High Color Purity Based on Strong Acceptor Attachment onto B–N-Containing Multiple Resonance Frameworks Yincai Xu, Chenglong Li, Zhiqiang Li, Jiaxuan Wang, Jianan Xue, Qingyang Wang, Xinliang Cai and Yue Wang Yincai Xu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Chenglong Li *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Zhiqiang Li Jihua Laboratory, Foshan 528200, Guangdong Province, Jiaxuan Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Jianan Xue State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Qingyang Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012, Xinliang Cai State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 and Yue Wang *Corresponding authors: E-mail Address: email protected E-mail Address: email protected State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Jihua Laboratory, Foshan 528200, Guangdong Province https: //doi. org/10. 31635/ccschem. 021. 202101033 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The development and enrichment of organic materials with narrowband emission in longer wavelength regions beyond 515 nm still remains a great challenge. Herein, a synthetic methodology for narrowband emission materials has been proposed to functionalize multiple resonance (MR) skeletons and generate a universal building block, namely, the key intermediate DtCzB-Bpin, which can be utilized to construct multifarious thermally activated delayed fluorescence (TADF) materials with high color purity through a simple one-step Suzuki coupling reaction. Based on this unique synthetic strategy, a series of efficient narrowband green TADF emitters has been constructed by localized attachment of 1, 3, 5-triazine and pyrimidine derivatives-based acceptors onto B–N-containing MR frameworks with 1, 3-bis (3, 6-di-tert-butylcarbazol-9-yl) benzene (DtCz) as the ligand. The precise modulation of the acceptor is an intelligent approach to achieve bathochromic shift and narrowband emission simultaneously. The DtCzB-TPTRZ-based organic light-emitting diode (OLED) exhibits pure green emission with Commission Internationale de L'Eclairage (CIE) coordinates of (0. 23, 0. 68), a maximum external quantum efficiency (EQE) of 30. 6%, and relatively low efficiency roll-off. Download figure Download PowerPoint Introduction Purely organic thermally activated delayed fluorescence (TADF) materials have been established as one of the most promising emitters for organic light-emitting diodes (OLEDs), which can capture electro-generated dark triplet excitons for light emission and achieve highly efficient conversion of electric energy to light via an endothermally assisted reverse intersystem crossing (RISC) process from the lowest triplet (T1) state to the lowest singlet (S1) state. 1–4 According to Boltzmann statistics, a sufficiently small singlet–triplet energy splitting (ΔEST) between the S1 and T1 is indispensable in guaranteeing an effective RISC process, which can be achieved by diminishing the overlap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). 5–9 A commonly adopted strategy is to apply intramolecular charge transfer (ICT) configuration with the aid of a donor–acceptor (D–A) molecular skeleton, which inevitably induces a Stokes shift. Meanwhile, broad emission spectra are generated from vibronic coupling between the ground state and singlet excited state as well as structural relaxation of the S1 state. 10 As for display applications, broadband emission lacks high color purity and cannot achieve a wide color gamut display, which are both important for accurate regeneration of authentic colors of the image content. 11, 12 Although excellent color purity can be obtained by cutting off the margin region of the original broadband electroluminescence (EL) with color filters or optical microcavities, the drawback is that these treatments sacrifice the actual efficiency value of OLEDs. 13, 14 Encouragingly, the most highlighted advantage of the D–A structure is the extreme flexibility of emission maximum regulation spanning, which is wide enough within the visible spectral region due to the ICT characteristics. Recently, multiple resonance (MR) type TADF materials based on B–N-containing conjugated molecules composed of rigid skeletons with alternate arrangements of HOMO and LUMO, have shown considerable potential in fabricating highly efficient OLEDs with extraordinary color purity (Scheme 1a). 15–24 MR-type TADF molecules demonstrate unique excitonic features of narrow full-width at half-maximum (FWHM), giant oscillator strength (f), large extinction coefficient, and near-unity photoluminescence quantum yield (PLQY). Although it is relatively easy to synthesize MR-type TADF emitters with high efficiency and narrowband emission in blue and sky-blue regions, narrowband TADF emitters in longer wavelength regions with emission maximum over 515 nm remain scarce. To construct a wide color gamut full-color display, it requires not only narrowband deep blue emitters but also ultrapure green and red emitters that comply with the Commission Internationale de L'Eclairage (CIE) coordinates requirements defined by National Television System Committee (NTSC). Therefore, it is critical to continue to explore emitters with narrowband emission in longer wavelength regions for both academic research and commercial applications. Scheme 1 | (a) MR framework. (b) Diagram sketch of attaching acceptors onto the MR framework. Download figure Download PowerPoint Only a few MR-type TADF molecules emitting in longer wavelength regions have been reported until now. 19–23 The MR molecules were synthesized via a tandem lithiation–borylation–annulation reaction or one-shot electrophilic C–H borylation reaction. Therefore, the adopted synthesis method for target molecules was only based on the borylation of aromatic amine ligands previously synthesized, which severely restricts the expansion of the MR molecular family. The reported synthetic method for MR molecules suffers several major disadvantages. First, the low yield of the borylation reaction involving large aromatic amine ligands with complex structures leads to a complex final reaction mixture, which contains the product molecule, amine ligands, borylation isomers, and other by-products with very similar polarities and similar or very near molecular weights, imposing difficulties in the separation and purification. Second, for the ligands with stronger electron acceptor moieties, such as 1, 3, 5-triazine, pyridazine, pyrimidine, and pyrazine groups, the borylation on the para-carbon position of the acceptor-substituted phenyl ring is not conducive. In other words, this class of ligands cannot directly coordinate with boron and form a MR framework (Supporting Information Figure S1). These electron withdrawing groups (EWGs) can reduce the electron density of para-carbon atoms and thus, suppress the C–H borylation reaction with boron tribromide/triiodide. 24 Even if alkyllithium is employed as a dehydrogenation/dehalogenation reagent for the ligands with the most EWGs, such as cyano or carbonyl groups, the side chemical reactions can take place at high temperature (ca. 60 °C) and transform cyano or carbonyl groups into undesired or unforeseen groups. 22 In principle, the introduction of strong acceptors into the para-carbon position of B-substituted phenyl rings can remarkably depress the LUMO energy level (Scheme 1b), and slightly change that of HOMO, resulting in an obvious decrease of the band gap and red-shift emission. 19, 20 A precisely localized introduction of a strong acceptor should be an efficient approach to shift the emission to a longer wavelength region. Therefore, developing a modified methodology of MR framework with strong auxiliary acceptors is a significant challenge for achieving organic emitters with narrowband emission in longer wavelength regions. Herein, we present a straightforward synthetic methodology to functionalize the MR skeleton that can be attached onto strong acceptors. In this method, the parent molecule DtCzB was selected as the original skeleton (Figure 1), which could be converted into the key intermediate DtCzB-Bpin with high yield with catalytic amounts of di-mu-methoxobis (1, 5-cyclooctadiene) diiridium (I) (Ir (COD) (OCH3) 2) and 4, 4′-di-tert-butyl-2, 2′-bipyridine (dtbpy) (Figure 1b). Then, a wide variety of functional groups can be introduced by a one-step uncomplicated Pd-mediated Suzuki coupling reaction, resulting in diverse efficient emitters with high color purity. Moreover, it can be inferred that this synthetic method of functionalizing DtCzB can be extended to its carbazole-based analogues. Herein, four representative molecules are presented and employed to demonstrate the outstanding performance of this synthetic approach (Figure 1a). Importantly, the resultant 2, 4, 6-triphenyl-1, 3, 5-triazine 1, 3-bis (3, 6-di-tert-butyl-9H-carbazol-9-yl) benzene boron (DtCzB-TPTRZ) -based OLED exhibits pure green emission with CIE coordinates of (0. 23, 0. 68), a remarkable maximum external quantum efficiency (EQE) of 30. 6%, and relatively low efficiency roll-off. Figure 1 | (a) Molecular structures of the investigated compounds. (b) Synthetic procedures: (1) B2Pin, Ir (COD) (OCH3) 2, dtbpy, THF, reflux. (2) Ar-X (X = Cl or Br), K2CO3, Pd (PPh3) 4, THF, water, reflux. Download figure Download PowerPoint Experimental Methods Synthesis of materials All reagents were purchased from Energy Chemical Co. (Shanghai, China) and J H, 7. 89; N, 3. 65. Found: C, 81. 48; H, 7. 93; N, 3. 68. Synthesis of DtCzB-DPTRZ 2-chloro-4, 6-diphenyl-1, 3, 5-triazine (160. 6 mg, 0. 6 mmol), DtCzB-Bpin (383. 3 mg, 0. 5 mmol), and potassium carbonate (K2CO3) (138 mg, 1 mmol) were added with water (2 mL) and THF (16 mL). The mixture was bubbled with nitrogen for another 5 min, and tetrakis (triphenylphosphine) palladium (0) (Pd (PPh3) 4) (28. 9 mg, 0. 025 mmol) was added under high flow nitrogen. Then the mixture was heated to reflux and stirred for 12 h. After cooling to room temperature, the reaction mixture was extracted with dichloromethane and water, the combined organic layer was condensed in vacuum, and then the crude product was purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (2∶1) to afford a yellow solid (239. 8 mg). Yield: 55%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9. 57 (s, 2H), 8. 94 (s, 2H), 8. 79 (d, J = 7. 4 Hz, 4H), 8. 56 (d, J = 8. 7 Hz, 2H), 8. 34 (d, J = 1. 8 Hz, 2H), 8. 14 (d, J = 2. 0 Hz, 2H), 7. 67 (t, J = 7. 1 Hz, 2H), 7. 61 (dd, J = 8. 4, 6. 2 Hz, 6H), 1. 67 (s, 18H), 1. 56 (s, 18H). The signal of 13C was not detected due to the poor solubility of the target compound. ESI-MS (M) m/z: 870. 73 M+ (calcd: 871. 48). Anal. Calcd for C61H58BN5: C, 84. 02; H, 6. 70; N, 8. 03. Found: C, 84. 08; H, 6. 71; N, 8. 09. Synthesis of DtCzB-TPTRZ, DtCzB-PPm and DtCzB-CNPm Each were synthesized in the same way as 2, 4-diphenyl-1, 3, 5-triazine 1, 3-bis (3, 6-di-tert-butyl-9H-carbazol-9-yl) benzene boron (DtCzB-DPTRZ), but 2-chloro-4, 6-diphenyl-1, 3, 5-triazine was replaced with equivalent stoichiometric amounts of 2- (4-bromophenyl) -4, 6-diphenyl-1, 3, 5-triazine, 5-bromo-2-phenylpyrimidine, and 5-bromo-2-cyanopyrimidine, respectively. DtCzB-TPTRZ Yellow solid (293. 9 mg). Yield: 62%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9. 10 (d, J = 19. 0 Hz, 2H), 9. 04–8. 95 (m, 2H), 8. 86–8. 71 (m, 4H), 8. 64–8. 54 (m, 2H), 8. 51–8. 41 (m, 4H), 8. 26 (d, J = 9. 6 Hz, 2H), 8. 18–8. 07 (m, 2H), 7. 71 (dt, J = 8. 9, 1. 9 Hz, 2H), 7. 59 (dq, J = 15. 8, 8. 2, 7. 7 Hz, 6H), 1. 68 (s, 18H), 1. 55 (s, 18H). 13C1H NMR (151 MHz, chloroform-d) δ/ppm: 171. 65, 171. 22, 145. 60, 145. 44, 145. 02, 144. 74, 141. 76, 138. 31, 136. 24, 136. 12, 132. 53, 129. 83, 129. 01, 128. 63, 128. 00, 127. 21, 124. 52, 123. 67, 122. 53, 121. 72, 120. 72, 117. 37, 114. 17, 107. 01, 35. 19, 34. 83, 32. 21, 31. 85. ESI-MS (M) m/z: 946. 57 M+ (calcd: 947. 51). Anal. Calcd for C67H62BN5: C, 84. 88; H, 6. 59; N, 7. 39. Found: C, 84. 89; H, 6. 66; N, 7. 38. DtCzB-PPm Yellow solid (238. 5 mg). Yield: 60%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9. 27–9. 15 (m, 2H), 8. 96 (s, 2H), 8. 65 (dd, J = 6. 6, 3. 7 Hz, 2H), 8. 39 (dt, J = 5. 3, 2. 1 Hz, 2H), 8. 31–8. 15 (m, 6H), 7. 62 (dt, J = 6. 7, 2. 6 Hz, 5H), 1. 65 (s, 18H), 1. 53 (s, 18H). 13C1H NMR (151 MHz, chloroform-d) δ/ppm: 155. 53, 145. 53, 144. 81, 141. 55, 138. 22, 138. 04, 136. 87, 131. 86, 131. 21, 129. 64, 128. 81, 128. 40, 127. 16, 124. 55, 123. 66, 122. 71, 121. 54, 120. 94, 117. 42, 114. 12, 105. 58, 35. 17, 34. 80, 32. 15, 31. 80. ESI-MS (M) m/z: 793. 80 M+ (calcd: 794. 45). Anal. Calcd for C56H55BN4: C, 84. 62; H, 6. 97; N, 7. 05. Found: C, 84. 69; H, 6. 99; N, 7. 10. DtCzB-CNPm Orange solid (238. 0 mg). Yield: 64%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 8. 16 (s, 4H), 7. 96 (d, J = 9. 0 Hz, 2H), 7. 82–7. 77 (m, 2H), 7. 59 (s, 2H), 7. 48 (d, J = 8. 4 Hz, 2H), 7. 21 (s, 2H), 1. 54 (s, 18H), 1. 49 (s, 18H). 13C1H NMR (151 MHz, chloroform-d) δ/ppm: 155. 86, 146. 04, 145. 11, 144. 78, 143. 92, 141. 38, 137. 81, 135. 88, 129. 58, 127. 17, 124. 69, 123. 64, 121. 20, 117. 57, 116. 99, 115. 84, 113. 97, 105. 60, 32. 10, 31. 77. ESI-MS (M) m/z: 742. 84 M+ (calcd: 743. 42). Anal. Calcd for C51H50BN5: C, 82. 35; H, 6. 78; N, 9. 42. Found: C, 82. 40; H, 6. 82; N, 9. 48. Synthesis of 9- (2-bromophenyl) -9H-3, 9′-bicarbazole The synthesis procedure is presented in Scheme 2. 120 mL solution of anhydrous N, N-dimethylformamide (DMF) containing 9H-3, 9′-bicarbazole (10. 3 g, 31. 0 mmol) was slowly added into a mixture of cesium carbonate (Cs2CO3) (14. 7 g, 45. 0 mmol) and 60 mL anhydrous DMF, and then 25 mL anhydrous DMF solution containing 2-bromofluorobenzene (5. 2 g, 30. 0 mmol) was injected. The mixture was heated and stirred at 140 °C for 12 h. After cooling to room temperature, the reaction mixture was poured into ice water (1000 g). The white powder solid was filtered out and dried in vacuum, and then further purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (2: 5) to afford a white solid (13. 1 g). Yield: 90 %. 1H NMR (500 MHz, CD2Cl2) δ/ppm: 8. 31 (d, J = 1. 9 Hz, 1H), 8. 17 (d, J = 7. 7 Hz, 2H), 8. 13 (d, J = 7. 8 Hz, 1H), 7. 93 (d, J = 8. 0 Hz, 1H), 7. 60 (dd, J = 7. 2, 1. 7 Hz, 2H), 7. 54 (dd, J = 8. 5, 2. 0 Hz, 1H), 7. 50–7. 38 (m, 6H), 7. 32–7. 25 (m, 4H), 7. 13 (d, J = 8. 2 Hz, 1H). ESI-MS (M) m/z: 487. 85 M+ (calcd: 487. 40). Anal. Calcd for C66H74BrN3: C, 73. 93; H, 3. 93; N, 5. 75. Found: C, 74. 13; H, 3. 99; N, 5. 81. Scheme 2 | Synthetic procedures of PhCzBCz. Download figure Download PowerPoint Synthesis of 9- (2- (9-phenyl-9H-carbazol-3-yl) phenyl) -9H-3, 9′-bicarbazole 9- (2-bromophenyl) -9H-3, 9′-bicarbazole (PhBCzBr) (13. 0 g, 26. 7 mmol), 9-phenyl-9H-carbazol-3-ylboronic acid (8. 4 g, 29. 4 mmol), and K2CO3 (7. 4 g, 53. 4 mmol) were added with water (50 mL) and THF (200 mL). The mixture was bubbled with nitrogen for another 5 min, and Pd (PPh3) 4 (1. 5 g, 1. 3 mmol) was added under high flow nitrogen. Then the mixture was heated to reflux and stirred for 12 h. After cooling to room temperature, the reaction mixture was extracted with dichloromethane and water, the combined organic layer was condensed in vacuum, and the crude product was purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (4∶1) to afford a white solid (12. 1 g). Yield: 70%. 1H NMR (500 MHz, CD2Cl2), δ/ppm: 8. 12 (d, J = 8. 2 Hz, 3H), 7. 98 (d, J = 7. 8 Hz, 1H), 7. 85 (d, J = 7. 6 Hz, 1H), 7. 82 (d, J = 1. 6 Hz, 1H), 7. 74 (d, J = 7. 8 Hz, 1H), 7. 71–7. 61 (m, 3H), 7. 50 (t, J = 7. 7 Hz, 2H), 7. 41–7. 29 (m, 9H), 7. 28–7. 14 (m, 7H), 7. 11 (dd, J = 8. 6, 1. 7 Hz, 1H), 7. 04 (d, J = 8. 5 Hz, 2H). 13C1H NMR (151 MHz, CD2Cl2) δ/ppm: 142. 69, 142. 18, 141. 42, 140. 96, 140. 42, 137. 71, 134. 97, 132. 38, 130. 81, 130. 32, 130. 17, 129. 65, 128. 83, 127. 79, 127. 18, 126. 84, 126. 42, 126. 32, 126. 19, 125. 48, 124. 34, 123. 49, 123. 31, 123. 04, 120. 72, 120. 44, 120. 32, 120. 30, 119. 99, 119. 84, 119. 50, 116. 66, 110. 72, 110. 17, 109. 71. ESI-MS (M) m/z: 650. 42 M+ (calcd: 649. 80). Anal. Calcd for C61H58BN5: C, 88. 72; H, 4. 81; N, 6. 47. Found: C, 88. 92; H, 4. 89; N, 6. 57. Theoretical calculation method A B3LYP method, including Grimme's dispersion correction with a 6-31G (d, p) basis set, was used to fully optimize the geometries of the ground state in a gas state by Gaussian 09 software package. The properties of the excited state were calculated by time-dependent density functional theory (TDDFT) with the same theory level as DFT. The HOMO and LUMO were visualized with Gaussview 5. 0. A detailed calculation of reorganization energies can be found in the Supporting Information. Results and Discussion Synthesis and characterization Starting from the parent molecule DtCzB, four target molecules were successfully prepared through two easy handling steps. The key step in obtaining the target compounds was the successful synthesis of the DtCzB-Bpin precursor. In the presence of catalytic amounts of Ir (COD) (OCH3) 2 (1% molar stoichiometric ratio) and dtbpy (2% molar stoichiometric ratio), which are commercially available at low prices, the intermediate DtCzB-Bpin can be easily transformed via DtCzB and B2Pin with high yield, 25 and then employed as a building block for the construction of versatile compounds through a one-step Suzuki coupling reaction. The preparation processes are robust enough to manufacture target compounds at a commercial scale. Detailed synthetic procedures are shown in the Experimental Methods, and the NMR spectra of all compounds are shown in Supporting Information Figures S2–S6. Computational simulations and photophysical properties To determine the and of with to and ground state geometries were initially by and the of S1 were The energy band oscillator and potential are in Figure 2. The of four compounds are to that of the parent molecule, which on the nitrogen atoms and atoms at in the DtCzB The are localized on the boron and the atoms at its in the of the DtCzB and extended to the to by the LUMO energy of the four compounds are by the potential by EWGs, which can the ICT narrow the and generate red-shift The in oscillator of the investigated as to the parent molecule, is to the ICT the oscillator strength still at high high directly to the DtCzB the intramolecular are and the and DtCzB are (Supporting Information Figure the molecule DtCzB-DPTRZ a structure based on the intramolecular between the and DtCzB The other molecules 1, 3-bis (3, 6-di-tert-butyl-9H-carbazol-9-yl) benzene boron and 1, 3-bis (3, 6-di-tert-butyl-9H-carbazol-9-yl) benzene boron without intramolecular molecular and large of between and DtCzB in the ground the DtCzB-DPTRZ molecule, the and S1 have very similar and the are to structural between and S1 the other and S1 and relatively significant structural between and S1 Figure 2 | HOMO and LUMO oscillator and molecular in geometries of the investigated compounds red and blue and Download figure Download PowerPoint To reorganization energies were calculated to the which is to the and emission processes and have a significant on the emission spectral After the the conversion and relaxation to the of the molecule and then to the emission which the be from the singlet or triplet excited of the molecules with and of In principle, the reorganization energies are in small Stokes and narrowband As in Figures the reorganization energies were calculated to be and for DtCzB-TPTRZ, and which are that of DtCzB (Supporting Information Figure but still relatively The that the have on the parent skeleton in of the molecular and reorganization Therefore, it is to that the compounds should have narrowband The four molecules attached with acceptors the original features of DtCzB and the MR According to the of the reorganization energies from S1 to (Supporting Information Figure the representative with large to reorganization energies are in the region for DtCzB-TPTRZ, and The large reorganization energies from the of the molecular skeleton and the of the (Supporting Information Figure and the between and DtCzB (Supporting Information Figure and vibronic coupling are to narrow the representative with large to reorganization energies are in regions. the reorganization energies for the of DtCzB-DPTRZ have small is the major between and S1 of DtCzB-DPTRZ is only the between the DtCzB and ring with small in of and in In this the reorganization energies of DtCzB-DPTRZ are not that of the parent molecule DtCzB the in the to the of a emission in the emission to the of by Based on the it can be that the reorganization energies of the and emission processes can be achieved by the structural between the ground and excited which small Stokes and narrowband to TADF Figure | and S1 and reorganization energies of DtCzB-DPTRZ DtCzB-TPTRZ DtCzB-PPm and DtCzB-CNPm Download figure Download PowerPoint The photophysical properties of four compounds of and spectra were in solution As in Figures and the spectra display an band at and nm for DtCzB-TPTRZ, and which to ICT all green fluorescence with emission at and 515 small Stokes of and and narrow of and respectively. and spectral ICT (Supporting Information Figure and which are with the calculated (Supporting Information the solubility is very which can be by the strong intramolecular by the large molecular conjugated which the of intramolecular between the and DtCzB The of the molecular structure leads to and ICT the emission in with the emission exhibits small with an that has significant on color which from the vibronic coupling to the large molecular conjugated To the the phenyl was introduced into DtCzB-TPTRZ to the between the and DtCzB which the intramolecular the of DtCzB-TPTRZ was and the of the molecular skeleton to emission As the emission is which is to the