Rational Design of the Platinahelicene Enantiomers for Deep-Red Circularly Polarized Organic Light-Emitting Diodes

[n]Helicene derivatives are most popular chiral structures to construct luminescent materials with circularly polarized (CP) light, which have revealed appealing application in chiral optoelectronics. Particularly, because of the unique phosphorescent emission, platinahelicene has great application prospects in CP organic light-emitting diode (CP-OLED). Herein, by decorating the pyridinyl-helicene ligand with trifluoromethyl (-CF3) unit in a specific position, a pair of platinahelicene enantiomers was prepared and separated with extremely twisted structure showing not only superior thermal and configurational stability but also good CP luminescence (CPL) property with dissymmetry factors (|gPL|) of 6 × 10−3. Moreover, the evaporated CP-OLEDs based on platinahelicene enantiomers exhibited the deep-red emission with the peak at 653 nm as well as obvious CP electroluminescence (CPEL) signals with the |gEL| in 10−3 order. Therefore, the design strategy provides an efficient way to improve the CPL properties of platinahelicene to cope with the future application in CP-OLEDs.


INTRODUCTION
In recent years, the exploration of chiral materials enabling circularly polarized (CP) light has attracted widespread attention in the field of chiral optoelectronics (Zhang et al., 2014). Compared with the traditional way of obtaining CP light (Grell et al., 2001) with polarizer, CP organic light-emitting diodes (CP-OLEDs) (Li et al., 2015(Li et al., , 2018 could emit CP light directly, which avoid brightness loss and complicated device structures. In term of current research (Han et al., 2018), it is still an important way to design chiral materials with excellent CPL properties for achieving high-performance CP-OLEDs. And the degree of circularly polarized photoluminescence (CPPL) or circularly polarized electroluminescence (CPEL) is characterized by the dissymmetry factor (g PL or g EL ) defined as g = 2× I /I =2 × (I L -I R )/ (I L + I R ). For example, Meijer and co-workers (Peeters et al., 1997) firstly obtained CP electroluminescence (CPEL) by using a chiral-substituted poly(p-phenylenevinylene) derivative as emitter. Since then, there has been some progress in chiral luminescent materials and efficient CP-OLEDs. Notably, although several reported chiral lanthanide complexes (Zinna et al., 2015 show excellent CPL properties with the |g PL | values over 1, the poor device performances make them difficult to be applied for practical OLEDs. On the contrary, chiral transition metal complexes [Ir(III), Pt(II)] and thermally activated delayed GRAPHICAL ABSTRACT | A pair of chiral deep-red platinahelicene enantiomers show good CPL property with |g PL | of 6.0 × 10 −3 . And the fabricated CP-OLEDs exhibit EQE max of 4.6% with the |g EL | at the degree of 10 −3 .
fluorescence materials (Feuillastre et al., 2016;Han et al., 2017;Wu et al., 2019) exhibit increasingly satisfactory results in term of the electroluminescence properties. Nevertheless, most of these materials used in efficient OLEDs exhibit poor CPL properties with the |g PL | value in the range of 10 −5 to 10 −3 (Han et al., 2018;Yan et al., 2019b) and mostly <5 × 10 −3 . Hence, it is important to provide a design strategy to improve their CPL properties.
Generally, introducing chiral units to luminescent materials is an efficient way to construct CPL materials (Sakai et al., 2015;Sánchez-Carnerero et al., 2015). Compared with chiral carbon centers, the special conjugated screw-shaped structure of [n]helicene derivatives makes it possess intrinsic chirality, which is one of the most effective chiral units (Dhbaibi et al., 2019;Zhao et al., 2019). Particularly, platinahelicene (Norel et al., 2010;Hellou et al., 2017) has not only the chirality of helicene but also the properties of phosphorescent emission, making it a potential choice for efficient CP-OLEDs. For example, Crassous and co-workers ( (Norel et al., 2010), (Shen et al., 2014)) developed a series of chiral platinahelicene exhibiting both phosphorescent and CP emission. Their elegant molecular design was based on the introduction of pyridinyl-helicene ligand into cyclometalated complexes. Later, a solution processed CP-OLED based on platinahelicene (Brandt et al., 2016) was reported by Fuchter group, achieving a very high |g EL | value of 0.38 with a maximum luminance (L max ) of 222 cd m −2 and a maximum current efficiency (η max ) of 0.25 cd A −1 , indicating the application of platinahelicene in CP-OLEDs. However, owing to the poor volatility, this kind of platinahelicene is difficult to be fabricated by vacuum deposition to gain better device performances. Subsequently, we modified the platinahelicene with the sterically hindered -CF 3 and -t Bu group to enable the material to be sublimated for the preparation of evaporated devices (Yan et al., 2019a), showing the |g EL | of (1.1-1.6) × 10 −3 with a L max of 11,590 cd m −2 and a η max of 22.52 cd A −1 , respectively. Meanwhile, the device performances are the highest values among the reported devices based on chiral phosphorescent Pt(II) complexes. Although the device performances were improved a lot, the |g PL | of 3.7 × 10 −3 still need to be promoted.
Herein, we created helicene core through a special structure design combining the -CF 3 group and helicene structure, and further improving the CPL property of platinahelicene. Compared with the platinahelicene reported by Crassous and Fuchter (Norel et al., 2010;Brandt et al., 2016), (RAC)-Pt possesses superior thermal stability and could be easily sublimated owing to the introduction of additional large hindered groups (-CF 3 , t Bu). Meanwhile, the special substitution position of the group not only makes the helicene structure more configurationally stable, but also endows this complex better CPL performance because of the more twisted structures. Therefore, based on our design strategy we successfully prepared a new kind of platinahelicene which was further prepared evaporated CP-OLED with good performance. In this way, P-Pt and M-Pt separated by (RAC)-Pt show higher g PL of −5.9 × 10 −3 /6.0 × 10 −3 . And both deep-red electroluminescence (λ max = 653 nm) and CP light activity were achieved by fabricating the evaporated OLEDs.

General Method
NMR measurements were conducted on a Bruker AM 400 spectrometer. The mass spectra were recorded by an electrospray ionization (ESI) mass spectrometer (LCQ fleet, Thermo Fisher Scientific) and Matrix Assisted Laser Desorption Ionization Time of Flight Mass Spectrometry (autoflex TOF/TOF, Bruker Daltonics), high-resolution mass spectra were recorded on a MICROTOF-Q III instrument. Ultraviolet-visible (UV-vis) absorption spectra were measured on a UV-3100 spectrophotometer and PL spectra were obtained from a Hitachi F-4600 photoluminescence spectrophotometer. The absolute photoluminescence quantum yields (Φ) and the decay lifetimes of the complex were measured by HORIBA FL-3 fluorescence spectrometer. Thermogravimetric analysis (TGA) was performed on a Pyris 1 DSC under nitrogen at a heating rate of 10 • C min −1 . (RAC)-Pt was separated by CHIRALPAK IE (IE00CD-RH008) column which was employed as stationary phase and the hexane/dichloromethane (80/20) was employed as eluent. Cyclic-voltammetry measurement system carried out at room temperature in deaerated CH 3 CN, employing a polished Pt plate as the working electrode, terta-n-butylammonium perchlorate (0.1 M) as the supporting electrolyte and Fc + /Fc used as the reference, with the scan rate of 0.1 V/s. The HOMO and LUMO levels were obtained by the following equation: , and E g was estimated from the UV-vis absorption spectra, E g = hc/λ abs . The circular dichroism (CD) and CPL spectra were measured in the same condition with UV-Vis absorption spectra and PL spectra. The CD spectra were measured on a Jasco J-810 circular dichroism spectrometer with "low" sensitivity. The circularly polarized photoluminescence (CPPL) and circularly polarized electroluminescence (CPEL) spectra were measured on a Jasco CPL-300 spectrophotometer based on "Continuous" scanning mode at 200 nm/min scan speed. The test mode adopts "Slit" mode with the E x and E m Slit width 3,000 µm and the digital integration time (D.I.T.) is 2.0 s with multiple accumulations (10 times or more).

Theoretical Calculation
All the DFT and TD-DFT calculations were carried out using Gaussian 16 software package. The initial structures were created according to the GaussView 6 based on crystal structures. The ground state geometry optimization with frequent calculations for all the complexes were performed using B3LYP exchangecorrelation functional. On the basis of the optimized structures, vertical transition energy calculations were carried out with B3LYP functional, too. For all the calculations, a combination of basis sets that Lanl2dz for platinum and 6-31G (d,p) for the others were employed and the solvent effect were considered by C-PCM model in CH 2 Cl 2 .

Device Fabrication
All the final products were purified by sublimation before applied to device fabrication. Moreover, the OLEDs with the emission area of 0.1 cm 2 were fabricated on the pre-patterned ITO-coated glass substrate with a sheet resistance of 15 Ω sq −1 . The ITO glass substrates were cleaned with ITO lotion and deionized water and then dried in an oven at 120 • C. The devices were fabricated by vacuum deposition of the materials at <10 −4 Pa with the deposition rate of 1-2 Å s −1 . The platinahelicene and host were co-evaporated from two separate sources. The LiF and Al were deposited with the rate of 0.1 and 3 Å s −1 , respectively. SCHEME 1 | Synthetic routes and molecular structure of (RAC)-Pt.

Preparation and Characterization of Platinahelicene
All experiments were performed under nitrogen atmosphere and the relevant procedures referred to the reported literature (Norel et al., 2010). The starting reactants and solvents were used as commercial grade without further purification. The synthetic routes of (RAC)-Pt are depicted in Scheme 1. The helicene ligand 4 and (C N )Pt(µ-Cl) chloride-bridged dimmer were prepared by the reported methods. Then the platinahelicene was readily obtained by mixing the µchloro-bridged dimer with 2,2,6,6-tetramethylheptane-3,5-dione sodium salt. The target platinahelicene was purified by column chromatography and sublimation. Due to the rapid racemization at room temperature, the helicene ligand is difficult to be separated into stable enantiomers. On the contrary, with the introduction of coordination bond, P-Pt and M-Pt are configurationally stable, which could be separated by (RAC)-Pt by using chiral HPLC with enantiomeric purity higher than 99%.
1-Fluoro-4-(6-Trifluoromethyl-2-Pyridyl)-Benzo[g]Phenanthrene (4) The mixture of cis and trans 3 (1.0 g, 2.55 mmol), I 2 (30 mg, 0.12 mmol) and 10 ml propylene oxide were dissolved in toluene (1,000 mL). The solution was irradiated for 4 h by 250 W mercury Frontiers in Chemistry | www.frontiersin.org  To a solution of 4 (400 mg, 1.02 mmol) in ethoxyethanol (30 mL) was added K 2 PtCl 4 (423 mg, 1.02 mmol). The suspension was gently warmed with stirring until all the platinum salt dissolved. The solution was then refluxed for 16 h to yield a dark green suspension. After cooled to room temperature, water (100 mL) was added. The precipitation was filtered and dried in air and then dark yellow solid (549 mg, 88%) was obtained.

2) Preparation of (RAC)-Pt:
To a solution of µ-chlorobridged dimer (549 mg, 0.44 mmol) in ethoxyethanol (25 mL) was added the 2,2,6,6-tetramethylheptane-3,5-dione sodium salt (217 mg, 0.97 mmol). The reaction mixture was refluxed overnight and then concentrated under reduced pressure. The single crystals of two enantiomers were obtained by vacuum sublimation and the single crystal diffraction analysis further confirmed the preconceived structure. As shown in Figures 1A, 5B, two symmetric enantiomers adopted twisted planar structures could be easily divided into P and M configurations. Interestingly, because the -CF 3 group is inserted into the helicene-like skeleton, the twist angle of pyridine and the coordinate five-membered rings is 17.3 • , bigger than the relevant report (Norel et al., 2010;Yan et al., 2019a), and the consecutive twist angles between the fused rings (10.6, 12.1, 12.3, and 9.9 • ) lead to a helical curvature (hc, the angle between the terminal helicene rings) value of 38.7 • in P-Pt and M-Pt, which is smaller than the conventional organic or heteroatomic [6]helicene system (Graule et al., 2009). Although the helicenebased ligand shows a large distortion, the central Pt(II) still adopts a planar coordination form with the dihedral angle 5 • between plane O 1 -Pt-O 2 and N 1 -Pt-C 1 . Moreover, the smaller hc value leads to relative strong π-π interactions, as a result, P-Pt and M-Pt pile up in a parallel face-to-face style. As shown in Figures 1C,D, combining with the help of the special chiral P or M helicene unit, these strong interactions can self-assemble P-Pt and M-Pt to form P and M spiral structures, which may be a reason of the good CPL properties of the platinahelicene.

Thermal and Configurational Stability
Thermal and configurational stability was investigated by TGA and CD spectra, respectively, which are vital parameters for chiral materials to fabricate CP-OLEDs. As shown in Figure S12, the decomposition temperature (T d , 5% loss of weight) of (RAC)-Pt is 321 • C, which benefits the application during the operation of CP-OLEDs. Moreover, due to the introduction of large sterically hindered groups, these enantiomers are endowed with a better configurational stability, which could preserve the original configuration (Figure S11) even at 240 o C under vacuum of 1 × 10 −4 Pa overnight.

Electrochemical Property and Theoretical Calculation
Cyclic voltammetry (CV) measurements and density functional theory (DFT) calculations were further conducted to investigate the electronic and structural features of P-Pt and M-Pt. As depicted in Figure 2A, both complexes have irreversible oxidation and reduction waves and the electrochemical data are summarized in Table S3. The oxidation onsets of these two enantiomers reference Fc + /Fc are the same with the value of 0.78 V. Moreover, both P-Pt and M-Pt display the first reduction potential at about −1.43 V. These results demonstrated the chiral configuration has ignored influence on the electrochemical properties of the enantiomers. Therefore, the calculated highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels for P-Pt and M-Pt are −5.58/−3.24 eV, respectively, which are helpful for the carriers' injection and transport in OLEDs.
To gain a better insight of the electron cloud distributions and structure features, the molecular simulations were further executed by density functional theory (DFT) preformed with Gaussian 09 software, and the accurate energy and location of HOMO/LUMO were calculated by QMForge program. As shown in Figure 2B, the HOMO/LUMO are mainly distributed in the helicene ligand (77.09/91.51%) together with d orbitals of Pt atom (18.11/6.64%), respectively. Obviously, the distributions of HOMO/LUMO on ancillary ligand are very small, indicating the little influence on the energy level of this Pt(II) complex.

Photophysical Property
To clarify the photophysical properties of (RAC)-Pt, room temperature UV-vis absorption spectrum in CH 2 Cl 2 is depicted in Figure 3 (middle). The short-wavelength absorption bands at 227 and 284 nm are mainly associated with π-π * transition, while the weak absorption bands peaking at 350-500 would be attributed to the mixed singlet and triplet metal-to-ligand charge transfer ( 1 MLCT and 3 MLCT) transitions because of the strong spin-orbital coupling effect of the Pt atom. The room temperature PL spectrum of (RAC)-Pt was further studied with the emission peak at 650 nm and the Commission Internationale de L'Eclairage (CIE) coordinates of (0.68, 0.32) demonstrating the deep-red emission. When measured at 77 K, the emission peak has a blue shift of 14 nm with the emission peak at 636 nm. Furthermore, emission lifetime (τ ) and absolute photoluminescence quantum yield (PLQY) were recorded for (RAC)-Pt in CH 2 Cl 2 under nitrogen atmosphere. The phosphorescence lifetime of (RAC)-Pt (τ = 4.6 µs) is in range of the similar reported Pt(II) complexes (Shen et al., 2014). Moreover, as shown in Figure S9, the linear fit curve on logarithmic scale indicates the phosphorescent emission is basically the only pathway. But due to the deep-red emission with tail to the near-infrared region, the PLQY of (RAC)-Pt is only about 4%.

Chiroptical Property
CD spectra were reported for both enantiomers P-Pt and M-Pt in CH 2 Cl 2 . As depicted in Figure 3 (top), P-Pt shows a strong negative cotton effect at 249 nm and positive cotton effects at 313, 363 and 446 nm, as well as corresponding to the UV-Vis absorption spectrum. The M-Pt reveals exactly the opposite CD spectrum further indicating that the two components are a pair of enantiomers. Furthermore, in the range of 350-500 nm, relatively weak cotton effect is observed, indicating the circular dichroism of the MLCT process.
CPL spectra of P-Pt and M-Pt were also preformed [Figure 3 (bottom)] to prove the chiroptical properties of the luminescent molecular upon excitations. The P-Pt shows negative CPL signals in 550-850 nm region while the M-Pt shows symmetrical signals in the same position with the |g PL | of about 6.0 × 10 −3 which is superior compared with the reported chiral Pt(II) complexes (Shen et al., 2014;Fu et al., 2019). In order to explore the application of materials in OLEDs, the CPL properties of P-Pt and M-Pt in doped films were also investigated. When the enantiomers were doped into the host material 26DCzPPy (2,6bis(3-(9H-carbazol-9-yl)phenyl)pyridine) at a concentration of 5 wt% by vacuum evaporation, the |g PL | factors are smaller than that in the solution state with the values about 4.0 × 10 −3 . Apparently, as for the CPL performance of P/M-Pt, the host material 26DCzPPy has a certain weakening effect. However, in order to obtain better device performance, 26DCzPPy was selected to further fabricate OLED owing to its unique bipolar characteristic.
Notably, as shown in Figure 5, the two different enantiomerbased OLEDs show obvious CPEL properties with opposite g EL of −1 × 10 −3 and 1 × 10 −3 at 653 nm for P-Pt and M-Pt, respectively, which demonstrate the successful preparation of CP-OLEDs and the device performances of P-SD, M-SD, P-DD, and M-DD are listed in Table 1 and Figure S13. Notably, although the device performance of P-SD shows better performance at low current density, P-DD with wider recombination area performs better at higher current density. However, it is obvious that the measured |g EL | factors are smaller than the chiral material in solution and film. The reason may be the reflected circularly polarized electroluminescence from metal electrode causing the spiral direction of CP light reverse which impair the intensity of CPEL Yan et al., 2019a). Anyway, successfully obtained CPEL signals indicate a feasible way to fabricate evaporated CP-OLEDs. Meanwhile, appropriate host material and device structure may also be required for better CPEL and EL performances.

CONCLUSION
In summary, a pair of chiral platinahelicene enantiomers with excellent thermal and configurational stability was designed and synthesized. Meanwhile, owing to the special structure design strategy, P-Pt and M-Pt show good CPL properties possessing |g PL | of 6.0 × 10 −3 in solution and 4.0 × 10 −3 in doped film, respectively. And the fabricated devices P-SD, M-SD, P-DD, and M-DD exhibited deep-red emission with the emission peak around 653 nm and the EQE max of 4.6% as well as the |g EL | on the order of 10 −3 .

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/Supplementary Material.

AUTHOR CONTRIBUTIONS
Z-PY designed and synthesized the platiahelicene complexes, carried out relevant measurement, and wrote the manuscript. X-FL helped in material synthesis and purification. KL helped in the theoretical calculation. Y-XZ and J-LZ designed the whole research.