Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes

Abstract

Open AccessCCS ChemistryCOMMUNICATION7 Nov 2022Simple Double Hetero[5]helicenes Realize Highly Efficient and Narrowband Circularly Polarized Organic Light-Emitting Diodes Wei Yang†, Nengquan Li†, Jingsheng Miao, Lisi Zhan, Shaolong Gong, Zhongyan Huang and Chuluo Yang Wei Yang† Department of Chemistry, Renmin Hospital of Wuhan University, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072 College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 †W. Yang and N. Li contributed equally to this work.Google Scholar More articles by this author , Nengquan Li† College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 †W. Yang and N. Li contributed equally to this work.Google Scholar More articles by this author , Jingsheng Miao College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 Google Scholar More articles by this author , Lisi Zhan Department of Chemistry, Renmin Hospital of Wuhan University, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072 Google Scholar More articles by this author , Shaolong Gong Department of Chemistry, Renmin Hospital of Wuhan University, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072 Google Scholar More articles by this author , Zhongyan Huang College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 Google Scholar More articles by this author and Chuluo Yang *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Renmin Hospital of Wuhan University, Hubei Key Lab on Organic and Polymeric Optoelectronic Materials, Wuhan University, Wuhan 430072 College of Materials Science and Engineering, Shenzhen University, Shenzhen 518060 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.022.202101661 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Helicene-based emitters with unique inherent circularly polarized luminescence (CPL) are promising yet remain a formidable challenge for highly efficient circularly polarized organic light-emitting diodes (CP-OLEDs), ascribed to their tough synthesis, low emission efficiency, and easy racemization in the thermal deposition process. Herein, a pair of helicene-based enantiomers, namely (P)- helicene-BN and (M)- helicene-BN, were developed, which merge helical chirality and the B/N/S inserted polycyclic aromatic framework to concurrently feature CPL and narrow thermally activated delayed fluorescence (TADF) characteristics. Benefiting from the excellent thermal/photophysical/chiroptical properties, the narrowband green CP-OLEDs based on enantiomers achieved maximum external quantum efficiencies (EQEmax) of up to 31.5%, and dissymmetry factor (|gEL|) of 2.2 × 10−3. This work reveals the great potential of helicene-based emitters in CP-OLEDs. Download figure Download PowerPoint Introduction Helicenes with polycyclic aromatic hydrocarbon structure have attracted increasing research interests due to their promising applications in asymmetric catalysis,1 nonlinear optics,2 molecular machines,3,4 molecular recognition,5 and so on. Since the first two aza[5]helicenes were reported in 1903 by Meisenheimer and Witte,6 much effort has been devoted to helicene chemistry. However, the optoelectronic application of helicenes has rarely been explored. The helical shape with ortho-fused aromatic rings endow helicenes with unique helical chirality and chiroptical features, for example, circularly polarized luminescence (CPL).7,8 In recent years, circularly polarized organic light-emitting diodes (CP-OLEDs) have become an active research topic due to their promising applications in three-dimensional displays.9–13 In this context, the helical chirality of helicenes are highly promising candidates for CP-OLEDs. However, there is a dilemma, that is, small-molecule helicenes (with less than six aromatic rings) are prone to racemization due to the low energy barrier, while helicenes with large molecular weights can suffer from difficulty in vacuum deposition and hinder device fabrication. At an early stage, helicenes were adopted as the chiral dopant for conventional fluorescent emitters, which suffered from low internal quantum efficiencies (IQE) of only 25% in electroluminescence (EL), despite the high EL dissymmetry factor (gEL).14 In terms of improving the quantum efficiencies, helicenes with phosphorescence feature are promising, which can theoretically enable an IQE of 100%. One successful example of phosphorescent CPL emitter is the Pt complex (Scheme 1) reported by Fuchter and co-workers,15 which adopted helicene as the ligand to induce chirality and achieved a high gEL. As an alternative, helicenes featuring thermally activated delayed fluorescence (TADF) are attractive, because they can achieve an IQE of 100%.16 However, it is a formidable challenge to construct helicene-based TADF emitters with a typical donor–acceptor (D–A) structure. The fused aromatic rings of helicenes usually induce large π-conjugation, which can significantly reduce the triplet energy level of the molecule and make it impossible to achieve a small singlet-triplet energy gap (ΔEST) for TADF. Nevertheless, the multiple-resonance (MR)-induced TADF (MR-TADF) effect with the B/N doped polycyclic fused aromatic structure provides the possibility of constructing helicene-based purely organic TADF emitters.17–25 On the one hand, the complementary resonance effects of the electron-deficient B and electron-rich N/O atoms can effectively separate the frontier molecular orbitals, which induces the short-range charge transfer (CT) to achieve a small ΔEST for efficient TADF.26–28 On the other hand, from the perspective of chemical structure, the inherently fused aromatic structures of MR-TADF effect represent a perfect platform for constructing heterohelicenes, which excludes the use of D/A units. Importantly, the synthesis of such heterohelicenes can be easily accessed because of the significant progress in the synthetic chemistry of the MR-TADF emitters based on the B/N fused structures. Therefore, heterohelicenes merging the CPL, MR effect, and TADF characteristics for highly efficient CP-OLEDs could be anticipated. Scheme 1 | Representative helicene-based emitters in CP-OLEDs. Download figure Download PowerPoint In this communication, MR-TADF heterohelicenes for highly efficient CP-OLEDs are targeted. As a proof of concept, we designed a helical M-shaped double hetero[5]helicene, namely helicene-BN, characterized by the B/N/S embedded polycyclic fused aromatic skeleton, which was conveniently synthesized by a two-step protocol. Expectedly, the resolution of helicene-BN afforded two helically chiral enantiomers, namely (P)- helicene-BN with a right-handed helix and (M)- helicene-BN with a left-handed helix, which concurrently exhibited CPL properties, and MR-TADF features. CP-OLEDs based on these enantiomers achieved very high maximum external quantum efficiencies (EQEmax) over 30% and high luminance over 70,000 cd/m2, accompanied by pure green emission with a narrow full width at half maximum (FWHM) of 49 nm. Meanwhile, the CP-OLED based on (M)- helicene-BN exhibited a |gEL| value as high as 2.2 × 10−3, which is among the best values of the reported TADF emitters. This study presents novel helically chiral MR-TADF emitters for CP-OLEDs. Results and Discussion The chemical structure and synthetic route of the designed racemic molecule (rac)- helicene-BN are depicted in Figure 1, which involved palladium-catalyzed Buchwald–Hartwig coupling reaction between 12H-benzo[a]phenothiazine and aryl bromide, followed by t-butyl lithium-mediated borylation reactions. The final product was fully characterized by 1H NMR, 13C NMR, and high-resolution mass spectrometry. The optically pure enantiomers of (P/M)- helicene-BN were then separated by a chiral high-performance liquid chromatography column with enantiomeric excesses (ee) >98%. The single crystals of the racemate, P and M enantiomers, were obtained to gain insight into the absolute configurations and molecular packing models. (P)- helicene-BN and (M)- helicene-BN exhibit dextrorotatory and levorotatory characteristics (Figures 2a and 2b), respectively. Only one type of dimer exists in the crystals of the enantiomers. Besides, the stacking patterns of (P)- helicene-BN and (M)- helicene-BN are in line with the sense of the helix-shape arrangements in the opposite direction. With respect to the (rac)- helicene-BN crystal composed of P and M configurations with a ratio of 1∶1 (Figure 2c), two kinds of dimers were involved, further forming a two-dimensional (2D) supramolecular sheet framework. Figure 1 | Synthetic routes and molecular structures. Download figure Download PowerPoint Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations based on single-crystal data were performed to gain further insight into the molecular electronic features of (M)- helicene-BN ( Supporting Information Figure S1). The lowest unoccupied molecular orbital was predominantly localized on the boron atom and the ortho/para positions of triphenyl boron core, while the highest occupied molecular orbital (HOMO) was dispersed on several segments involving nitrogen atoms, the sulfur atoms, the meta positions of boron atom, and the peripheral phenyl units outside the triphenyl boron skeleton. This distribution profile was coincident with MR-dominated frontier orbital characteristic and also implied the involvement of the double hetero[5]helicene emission core, which contributed to the helically-chiral CPL activity. The calculated natural transition orbitals for the S1 state of the molecule further demonstrated that the MR-type S1 transition was well preserved, which was conducive to maintaining a narrow FWHM. Additionally, the calculated energy barrier of the isomerization process from M to P-configuration was as high as 317.16 kcal mol−1 ( Supporting Information Figure S2), indicating excellent chiral stability. Figure 2 | The absolute configurations, intermolecular interactions in dimers, and helical packing arrangements along the a axis of (P)-helicene-BN crystal (a) and (M)-helicene-BN crystal (b). The single-crystal structure, intermolecular interactions in dimers, and 2D-sheet stacking mode along the c axis of (rac)-helicene-BN crystal (c). Hydrogen atoms and solvent molecules are omitted for clarity. CCDC numbers: 2092273, 2092274, and 2092275. Download figure Download PowerPoint The HOMO energy level of the emitter was calculated to be −5.18 eV from the oxidation potential ( Supporting Information Figure S3), which matched well with the theoretical simulation. The emitter also exhibited great thermal stability with a decomposition temperature (Td) of 414 °C, enabling easy vacuum thermal deposition process for OLED fabrication. The basic photophysical properties of (M)- helicene-BN were studied in dilute toluene. As depicted in Supporting Information Figure S4, the intense absorption bands peaking at around 483 nm were assigned to the CT transition. The emitter showed green emission with a peak at 520 nm. The small Stokes shift of only 37 nm indicated a slight change of the molecular conformation in excited states owing to the rigid structure, which in turn led to the narrow FWHM of 46 nm. Moreover, the emitter exhibited a small ΔEST value of 0.18 eV calculated from the prompt fluorescence and phosphorescent spectra at 77 K in toluene, with singlet (S1) and triplet (T1) energy levels of 2.56 and 2.38 eV, respectively (Figure 3a). The luminescent properties of (M)- helicene-BN doped in 1,3-dihydro-1,1-dimethyl-3-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)indeno-[2,1-b]carbazole (DMIC-TRZ) host film were also investigated. As displayed in Supporting Information Figure S5, the 1 wt % (M)- helicene-BN doped film showed narrowband green emission peaking at 525 nm with a FWHM of 48 nm. The ΔEST value estimated from the fluorescence and phosphorescence spectra of the doped film was 0.15 eV, benefiting the efficient reverse intersystem crossing (RISC) process from T1 to S1 state. The delayed fluorescence feature was manifested by the second-order exponential transient photoluminescence (PL) decay profiles composed of nanosecond-scale and microsecond-scale components under degassed conditions (Figure 3b and Table 1). The fitted lifetimes of prompt (τp)/delayed (τd) fluorescence were 5 ns/71.8 μs. The delayed component was reduced under aerated conditions, implying delayed fluorescence originates from the triplet state. In addition, the temperature-dependent transient PL spectra of (M)- helicene-BN were measured ( Supporting Information Figure S5b), and it could be seen that the intensity of the delay component gradually increased with increasing temperature from 77 to 300 K. These results clearly confirm the TADF characteristics of the emitter. Figure 3 | (a) Normalized prompt fluorescence (77 K) and phosphorescence (77 K) spectra of (M)-helicene-BN in toluene (1 × 10−5 M). (b) Transient PL spectra of 1 wt % (M)-helicene-BN doped into DMIC-TRZ host film under aerated and degassed conditions. IRF: the instrument response functions of the light source. Download figure Download PowerPoint Table 1 | Photoluminescence Properties of (M)-Helicene-BN Doped in the DMIC-TRZ Film at Room Temperature Emitters τpa (ns) τda (μs) ФPLb (%) ФPc (%) Фdc (%) kPd (×108 s−1) kdd (×105 s−1) kISCe (×108 s−1) kRISCe (×104 s−1) kr,se (×107 s−1) (M)- helicene-BN 5 71.8 98 29.4 68.6 2 0.14 1.4 4.6 5.9 aThe lifetimes of prompt and delayed fluorescence. bThe total fluorescence PLQY under oxygen-free conditions. cThe prompt and delayed fluorescence PLQY under oxygen-free conditions. dThe rate constants of prompt and delayed radiative. eThe rate constants of ISC, RISC, and radiation transition process. Excitation wavelength for the transient PL experiment: 377 nm. The PLQY was obtained using absolute PL quantum yield spectrometer (C13534, Hamamatsu Photonics) equipped with a calibrated integrating sphere and excited at 330 nm. The film was prepared by vacuum deposition with a thickness of 100 nm, and the emitter concentration is 1 wt %. The chiroptical properties of the enantiomers in the ground and excited states were investigated by circular dichroism (CD) and CPL measurements, respectively. As depicted in Supporting Information Figure S7, the enantiomers exhibited mirror-image CD spectra with clear Cotton effects at around 484, 381, and 325 nm, manifesting the chiroptical activity of the enantiomers in the ground state, which indicates the presence of the intramolecular chiral exciton coupling within the helical environment. Mirror-image CPL spectra were also measured for the two enantiomers with positive signal for (P)- helicene-BN and negative signal for (M)- helicene-BN in both toluene solution and vacuum thermal deposition films (Figures 4a–4d). In toluene, the luminescence dissymmetry factors (gPL) are +2.0 × 10−3 for (P)- helicene-BN and −2.1 × 10−3 for (M)- helicene-BN. In doped films, the gPL values are +1.3 × 10−3 for (P)- helicene-BN and −2.0 × 10−3 for (M)- helicene-BN, suggesting that no racemization occurred during the thermal deposition process due to the large energy barrier for racemization, which should benefit from the large steric hindrance surrounding the polycyclic architecture. These results are among the best values of the reported chiral MR-induced TADF materials, demonstrating superior chiroptical properties of the newly designed enantiomers. Figure 4 | Circularly polarized photoluminescence spectra and gPL values of (P/M)-helicene-BN measured in (a and b) toluene solution (1 × 10−5 M) and (c and d) films doped in DMIC-TRZ host. Download figure Download PowerPoint To evaluate the EL performance of (P/M)- helicene-BN, OLEDs with a device configuration of indium tin oxide (ITO)/dipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(HATCN) (5 nm)/1,1-bis((di-4-tolylamino)phenyl)-cyclohexane (TAPC) (30 nm)/tris(4-carbazoyl-9-ylphenyl)amine (TCTA) (15 nm)/3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) (10 nm)/emitting layers (EML) (50 nm)/1,3,5-triazine-2,4,6triyl)tris(benzene-3,1-diyl)(tris(diphenylphosphine oxide)) (POT2T) (20 nm)/1-(4-(10-([1,1′-biphenyl]-4-yl)anthracen-9-yl)phenyl)-2-ethyl-1H-benzo[d]imidazole (ANT-BIZ) (30 nm)/Liq (2 nm)/Al (100 nm) were fabricated. Here, HATCN acted as a hole injection layer; TAPC and ANT-BIZ were used as hole- and electron-transport layers, respectively; while mCBP and POT2T served as exciton-blocking layers, respectively; (P)- helicene-BN and (M)- helicene-BN were doped into DMIC-TRZ host with the doping concentration of 1 wt % to serve as the EML, corresponding to device A and device B, respectively. The current density–voltage–luminance (J–V–L), current efficiency (CE)/power efficiency (PE)/EQE versus luminance characteristics, and EL spectra are presented in Figure 5 and Supporting Information Figure S8, and the key device parameters are summarized in Table 2. Figure 5 | (a) External quantum efficiency versus luminance characteristics of (P/M)-helicene-BN based devices. Inset: EL spectra at 4 V, the corresponding CIE coordinates and photograph of the devices. (b) J–V–L characteristics. (c) CPEL spectra and (d) the gEL values versus wavelength curves of (P/M)-helicene-BN based CP-OLEDs. Download figure Download PowerPoint Table 2 | Summary of the Device Performances of (P)-Helicene-BN and (M)-Helicene-BN Device Emitter Vona (V) EQEb (%) CEmaxc (cd/A) PEmaxd (Im/W) Lmaxe (cd/m2) ELpeakf (nm) CIEg (x, y) gELh (×10−3) FWHM (nm) A (P)- helicene-BN 2.4 31.5/29.6/18.7 117.5 153.8 71,483 523 (0.26, 0.66) +1.2 49 B (M)- helicene-BN 2.4 30.7/28.3/17.9 117.8 154.2 77,168 524 (0.26, 0.66) −2.2 50 aTurn on voltage at 1 cd/m2. bExternal quantum efficiency at a maximum, 100, and 1000 nits. cThe maximum value of current efficiency. dThe maximum value of power efficiency. eThe maximum luminance. fThe peak wavelength of the EL spectrum. gCIE1931 coordinates. hThe gEL values of (P)- helicene-BN or (M)- helicene-BN-based device. As shown in Figures 5a and 5b, the two devices were turned on at a low voltage of 2.4 V. Devices A and B exhibited almost identical pure green EL peaking at 523 and 524 nm with Commission Internationale de l’Eclairage (CIE) coordinates of (0.26, 0.66) and FWHMs of 49 and 50 nm, respectively, which are comparable with the corresponding PL spectra of (P)- helicene-BN and (M)- helicene-BN. These results demonstrate that the EL emission only originated from the emitters, indicating the complete energy transfer from DMIC-TRZ host to the dopants. Notably, excellent performances were achieved for the two devices. Device A based on the (P)- helicene-BN emitter achieved the EQEmax, maximum current efficiency (CEmax), maximum power efficiency (PEmax) and maximum luminance (Lmax) of 31.5%, 117.5 cd/A, 153.7 lm/W, and 71483 cd/m2, respectively. Impressively, the device also showed relatively low efficiency roll-offs. For example, the EQE values of device A were maintained as high as 29.6% (100 cd/m2) and 18.7% (1000 cd/m2), corresponding to roll-offs of only 6.0% and 40.7%, respectively. Due to the same photophysical properties of the two enantiomers, device B based on (M)- helicene-BN also secured outstanding performances similar to those of device A, with EQEmax, CEmax, PEmax, and Lmax of 30.7%, 117.8 cd/A, 154.2 lm/W, and 77168 cd/m2, respectively. These results rank (M)- helicene-BN and (P)- helicene-BN among the most efficient MR-TADF emitters ever reported. Subsequently, the circularly polarized electroluminescence (CPEL) characteristics of the two enantiomers were investigated. As presented in Figure 5c, the corresponding CP-OLEDs exhibited mirror-image CPEL spectra with the positive signal for (P)- helicene-BN and negative signal for (M)- helicene-BN. It is stressed that the (M)- helicene-BN-based device achieved a |gEL| value of 2.2 × 10−3, which is one of the highest values for CP-OLEDs based on purely organic TADF emitters. The gEL value was +1.2 × 10−3 for the P enantiomer (Figure 5d). These results clearly verified that the pronounced CPEL feature of (P)- helicene-BN and (M)- helicene-BN enantiomers inherited from the helical chirality of the helicene-BN skeleton. Combining the high device efficiencies, narrowband emission, and pronounced CPEL properties of the two enantiomers, it is concluded that the helicene-BN skeleton with the helical chirality is a promising building block for CP-MR-TADF emitters towards highly efficient CP-OLEDs. Conclusions We have demonstrated novel helically chiral CP-MR-TADF enantiomers, namely (P)- helicene-BN and (M)- helicene-BN, based on simple double hetero[5]helicene architecture for highly efficient CP-OLEDs. The newly developed emitters exhibited a small ΔEST of 0.15 eV, high photoluminescence quantum yield (PLQY) of 100%, and strong CPL characteristics. The CP-OLEDs based on helicene-BN showed pure green emission peaking at 524 nm with a FWHM of 49 nm, and achieved EQEmax values of 31.5%. Significantly, intense CPEL signal and a |gEL| value up to 2.2 × 10−3 were achieved for the CP-OLEDs, revealing the great potential of helical chirality in high-performance CP-OLEDs. This study provides a good example for feasible applications of helicene-based compounds in optoelectronics. Supporting Information Supporting Information is available and includes synthesis and characterization, general information, absorption spectra, PL and time-resolved spectra, thermogravimetric analysis curve, crystal data and theoretical calculation data. Conflict of Interest There is no conflict of interest to report. Funding Information This work was supported by the National Natural Science Foundation of China (grant nos. 52130308, 51903160, and 52022071), Shenzhen Science and Technology Program (grant no. KQTD20170330110107046), and the Shenzhen Technology and Innovation Commission (grant no. JCYJ20180507182244027). Acknowledgments The numerical calculations in this paper have been done on the supercomputing system in the Supercomputing Center of Wuhan University.