[2.2]Paracyclophane-Substituted Chiral Multiresonant Thermally Activated Delayed Fluorescence Emitters for Eﬃcient Organic Light-Emitting Diodes

The study reports two pairs of chiral multi-resonant thermally activated delayed ﬂuorescence (MR-TADF) materials PCP-DiKTa and Czp-DiKTa by decorating a known MR-TADF core, DiKTa, with diﬀerent [2.2]paracyclophane (PCP) based planar chiral groups. PCP-DiKTa shows narrow sky-blue emission with a full width at half maximum (FWHM) of 44 nm, while the emission of Czp-DiKTa is slightly broader with a FWHM of 66 nm and redshifted. Both emitters show high photoluminescence quantum yields of 93 and 99% for PCP-DiKTa and Czp-DiKTa, respectively. Enantiomerically pure samples of both compounds show chiroptical properties in the ground state while only Czp-DiKTa exhibits chiroptical activity in the excited state, with dissymmetry factors (|g PL |) of 4 × 10 − 4 . Organic light-emitting diodes (OLEDs) with PCP-DiKTa and Czp-DiKTa show maximum external quantum eﬃciencies (EQE max ) of 25.7 and 29.2%, with 𝝀 EL of 489 and 518 nm, and FWHMs of 53 and 69 nm, respectively. These EQE max values are higher than those of other reported devices employing PCP-based D-A type emitters. This work demonstrates that the PCP moiety is not only a powerful building block to develop planar chiral emitters but one that is compatible with the fabrication of high eﬃciency devices.


Introduction
Organic light-emitting diode (OLED) research has witnessed remarkable growth and dynamism since Tang and Van Slyke first reported an operational OLED at low turn-on voltage back in 1987. [1]OLED displays are now widely found in smartphones, smartwatches, TVs, and automotive screens, offering a panoply of advantages over other display technologies like displaying pure black, having a slim profile, being more energy efficient, and being able to be fabricated on many different substrates including those that are flexible and transparent. [2]Current OLED displays contain anti-glare filters for high-contrast viewing, resulting in 50% efficiency loss after passing through a linear polarizer and a quarter-wave plate.One solution to address this issue is to migrate to circularly polarized OLEDs (CP-OLEDs), implying the use of chiral emitters that emit circularly polarized luminescence (CPL). [3]Generally, the extent of CPL in a chiral emitter is quantified by the luminescence dissymmetry factor, g PL , where g PL = 2 (I L − I R )/(I L + I R ) = 4 (|μ e | ⋅ |μm| ⋅ cos)/(|μ e | 2 + |μm| 2 ), where I L and I R are the left-/right-handed emission intensities, μ e and μm are the electric and magnetic transition dipole moment vectors, respectively, and  is the angle between the two.For small organic molecules, the μm is typically much smaller than the μ e , so the relationship between g PL and the transition dipole moments can be simplified as g PL = 4⋅ μm ⋅ cos/μ e . [4]This equation implies that the maximum g PL is ±2, and that materials with magnetic dipole-allowed and electric dipole-forbidden transitions would be beneficial to maximize the g PL .In optimized CP-OLEDs, the CPL materials must also be able to harvest efficiently both singlet and triplet excitons, which means that they should emit either by phosphorescence or thermally activated delayed fluorescence (TADF).Thus, there has recently been an intense interest in the development of circularly polarized thermally activated delayed fluorescence (CP-TADF) emitters. [5]here are two main strategies employed for the design of CP-TADF molecules.One involves designing molecules with an intrinsically chiral TADF skeleton (utilizing point, axial, or planar chirality), while the other entails attaching chiral units to achiral TADF moieties.However, the emission is broad in donoracceptor CP-TADF compounds as there is large structural relaxation in the excited state.The color purity is insufficient for high-definition display applications hence, narrowband emitters are required.Recently, Hatakeyama and co-workers introduced multiresonant TADF (MR-TADF) compounds that are rigid pand n-doped polycyclic aromatic hydrocarbon compounds that show narrowband emission. [6]The electron difference densities of the S 1 and T 1 excited states compared to the ground state in these compounds show a pattern of alternating increasing and decreasing electron density that is responsible for the characteristic short-range charge transfer excited state and the suitably small singlet-triplet energy gap, ΔE ST , to enable TADF.Consequently, what is required within this materials space are highperformance CP-MR-TADF emitters.However, examples of CP-MR-TADF emitters remain limited (Figure S1, Supporting Information), especially compared to the thousands of reported TADF emitters reported to date.Devices based on these emitters using a single host often suffer from serious efficiency rolloff owing to the too long exciton lifetimes.One approach to optimize the device performance is to enhance the efficiency of the reverse intersystem crossing (RISC) process by having lowlying excited states of mixed short-range charge-transfer (SRCT) and long-range charge transfer (LRCT) character in the CP-MR-TADF emitters.Our prior work demonstrated that decorating an MR-TADF core with various electron-donating groups can modulate the interplay between the relative energies of the SRCT and LRCT states and their impact on the emission profile. [7]Therefore, the choice of decoration of MR-TADF cores with chiral groups to achieve CPL must also be mindful to retain the desired narrowband emission and to aid in enhancing the RISC kinetics.
[2.2]Paracyclophane (PCP) and its derivatives have emerged as a useful planar chiral skeleton, including in the construction of CP-TADF emitters. [8]Most of the examples of PCP-containing CP-TADF emitters are based on a D-A design.For example, our group integrated a PCP with a nitrogen-heterocycle to produce the bulky carbazolophane (Czp) donor and employed it within the CP-TADF emitter CzpPhTRZ (Figure 1). [9]The increased steric bulk of the Czp unit compared to carbazole (Cz) induced an increased torsion angle between the donor and the phenylene bridge, resulting in a relatively smaller ΔE ST of 0.16 eV.(R)-CzpPhTrz emits at  PL of 470 nm and has a g PL value of 1.2 × 10 −3 in toluene, while rac-CzpPhTrz has a Φ PL of 69% in 10 wt% doped films in DPEPO, which translated into OLEDs that achieved an EQE of 17% at an  EL at 480 nm; no OLEDs were fabricated using enantiopure samples of the emitter and thus no g EL was measured.Zheng, Ye, Liao et al. subsequently reported a structurally related CP-TADF molecule, PXZp-Ph-TRZ, using the same chiral paracyclophane-extended donor strategy as with CzpPhTrz but replacing Cz with a stronger phenoxazine (PXZ) donor. [10]The yellow emitter ( PL = 527 nm) has a much smaller ΔE ST of 0.03 eV than CzpPhTrz and a Φ PL of 60% in 10 wt% doped films in CBP.The solution-processed CP-TADF OLEDs showed an EQE max of 7.8%, and using enantiopure emitters the CP-OLEDs displayed a g EL of 4.6 × 10 −3 .With a similar skeleton, Chen, Li, Zhang et al. reported a pair of D-chiral -A type TADF emitter (R/S)-PXZ-PT, with the PCP acting as bridging unit. [11](R/S)-PXZ-PT emits at  PL of 565 nm in toluene, has a moderate ΔE ST of 0.19 eV in 2Me-THF, and a high Φ PL of 78% in 10 wt% doped films in CBP; additionally, the g PL was measured to be ±1.9 × 10 −3 in toluene.The vacuum-deposited CP-OLEDs with (R)-PXZ-PT exhibited a yellow emission [( EL of 557 nm, CIE of (0.44, 0.55)] with an EQE max of 20.1%.In these examples, the emission is broad owing to radiative decay from a LRCT excited state.Recently, Zheng, Liao, Pu et al. attached Czp to two different known MR-TADF cores and reported two CP-MR-TADF emitters. [12]Czp-tBuCzB, Czp-POAB emit at  PL of 478 and 498 nm and show narrowband emissions with FWHM of 23 and 36 nm, respectively, in toluene.Czp-tBuCzB and Czp-POAB have small ΔE ST of 0.09 and 0.13 eV in toluene, and high Φ PL of 98 and 96% in 5 and 8% doped films in 26DCzPPy, respectively.The sky-blue CP-MR-TADF OLEDs showed EQE max of 32.1 and 28.7%, respectively.For the OLED device with Czp-tBuCzB, the efficiency roll-off was small of 3.7% at 1000 cd m −2 ; however, the device with Czp-POAB showed a much more severe efficiency roll-off of 28.9% at 1000 cd m −2 , which the authors rationalized as due to the planar structure and longer  d than that of Czp-tBuCzB.
Here, we report two pairs of chiral MR-TADF enantiomers, PCP-DiKTa and Czp-DiKTa by decorating a known MR-TADF core, DiKTa, with different PCP-based planar chiral groups.Owing to the different donor strength of the two chiral groups, the energy of the LRCT state from these donors to the DiKTa acceptor will change and this will impact the photophysical and chiroptical properties of the emitters.PCP-DiKTa emits at  PL of 477 nm and has a narrow profile (FWHM of 44 nm), while the emission of Czp-DiKTa is red-shifted at  PL of 501 nm and is broader (FWHM of 66 nm).This implies that Czp is a stronger donor than PCP.Enantiopure Czp-DiKTa emitters displayed CPL activity with g PL of ±4 × 10  4.41 × 10 4 s −1 , respectively). [13]The OLEDs with PCP-DiKTa and Czp-DiKTa showed EQE max of 25.7 and 29.2%.The OLEDs with Czp-DiKTa showed a less pronounced efficiency roll-off with EQEs at 100/1000 cd m −2 of 25.9/17.1%.This work demonstrates the impact that the electronics of the chiral group has on the photophysics of the emitter and how this ultimately affects the device performance.

Molecular Design and Synthesis
The syntheses of Czp-DiKTa and PCP-DiKTa are outlined in Scheme S1 (Supporting Information).Czp-DiKTa was obtained in 66% yield as yellow solid following a palladium-catalyzed Buchwald-Hartwig C-N coupling between Czp and DiKTa-Br, and PCP-DiKTa was synthesized in 48% yield as a yellow solid following a sequence of Miyaura borylation of PCP-Br and Suzuki−Miyaura cross-coupling with DiKTa-Bpin.The molecular structures and purity were validated using a combination of 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy, high-resolution mass spectrometry (HRMS), melting point determination, elemental analysis (EA), and high-performance liquid chromatography (HPLC) (Figures S2-S14, Supporting Information).The thermal stability of the two compounds was investigated by thermogravimetric analysis (TGA) and different scanning calorimetry (DSC) (Figure S15, Supporting Information).Both compounds are thermally stable, with a 5% mass loss (T d ) occurring at 390 °C for PCP-DiKTa and 362 °C for Czp-DiKTa.

Theoretical Calculations
The ground-state properties of Czp-DiKTa and PCP-DiKTa were calculated using density functional theory (DFT) at the PBE0/6-31G(d,p) level in the gas phase starting from a geometry generated using Chem3D, [14] while the excited-state calculations were performed using the spin component scaling second-order approximate coupled-cluster (SCS-CC2) method in tandem with the cc-pVDZ basis set, which we have previously shown to be a sufficient level of theory to accurately predict ΔE ST in MR-TADF emitters. [15]In the ground state, the dihedral angle between the stereogenic donor and the DiKTa group are 47.6°f or PCP-DiKTa and 57.5°for Czp-DiKTa, which is similar to the dihedral angle of 58°found in the optimized structure of CzpPhTRZ. [9]For PCP-DiKTa, the HOMO is distributed across the entire molecule while the LUMO is mainly localized on the DiKTa fragment (Figure S16, Supporting Information).In contrast, the HOMO of Czp-DiKTa is mainly located on Czp donor, with a small degree of electron density on the adjacent arene of the DiKTa, while the LUMO is localized on the DiKTa fragment.The difference in the HOMO distributions reflects the electron-donating strength of the chiral donor groups.Compared to DiKTa (HOMO/LUMO = −6.20/−2.23 eV), [7a] PCP-DiKTa (−5.94/−2.24eV) has a similar LUMO level and a shallower HOMO level while Czp-DiKTa (−5.57/−2.36eV) has a much shallower HOMO level and a deeper LUMO, leading to a much smaller HOMO-LUMO gap.The calculated S 1 /T 1 energies are 3.38/3.11eV for PCP-DiKTa and 3.30/3.06eV for Czp-DiKTa, with corresponding ΔE ST values of 0.27 and 0.24 eV, respectively.These values are similar to the calculated S 1 /T 1 energies (3.46/3.20 eV) and ΔE ST value (0.26 eV) of DiKTa. [13]The oscillator strengths for the S 0 -S 1 transitions for PCP-DiKTa (0.24) and Czp-DiKTa (0.22) are larger than that of DiKTa (0.20), implying that these two emitters will be brighter.The difference density plots for the S 1 , S 1 , T 1 , and T 2 excited states are shown in Figure 2. The first singlet and triplet excited states show a pattern of alternating increasing and decreasing electron density that is typical of excited states of shortrange charge transfer (SRCT) character; there is a small contribution to the S 1 difference density plot originating from the chiral donors.The S 2 state for both molecules has n-* character, while the electron density of the T 2 is located mainly on the donor moieties.The different orbitals involved in the S 2 /T 2 states compared to the S 1 /T 1 states contribute to the spin-orbit coupling (SOC).The SOC matrix elements (SOCME) at the optimized T 1 geometry between S 1 and T 2 are 4.15 and 0.45 cm −1 for PCP-DiKTa and Czp-DiKTa, respectively, which are significantly larger than that between S 1 and T 1 (0.27 and 0.26 cm −1 , respectively, Figure S16, Supporting Information).

Optoelectronic Properties
The electrochemical properties of the two emitters were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in deaerated DCM with 0.1 m tetra-nbutylammonium hexafluorophosphate as the supporting electrolyte (Figure S17, Table S1, Supporting Information).Both compounds show irreversible oxidation waves, which are assigned to the oxidation of either the whole skeleton for PCP-DiKTa or the Czp for Czp-DiKTa according to an analysis of the HOMO distribution from DFT calculation.7b] The E ox /E red values were determined from the peaks of the DPVs.Compared to DiKTa (E ox /E red = 1.78/−1.34V), both PCP-DiKTa (1.58/−1.29 V) and Czp-DiKTa (1.14/−1.27V) have similar, albeit slightly anodically shifted reduction potentials while their oxidation potentials are cathodically shifted.The corresponding HOMO-LUMO gaps are 2.87 and 2.41 eV, respectively, and the trends in the electrochemical data largely mirror those in the DFT study.
The UV-vis absorption and steady-state photoluminescence (PL) spectra recorded at room temperature in dilute toluene are shown in Figure 3a.There is a similar band at 445 nm in the absorption spectra, to the SRCT transitions centered on the DiKTa core.Compared to DiKTa ( abs = 433 nm), [13] the SRCT band of PCP-DiKTa and Czp-DiKTa is slightly red-shifted and broader, indicating that this transition has an admixture of LRCT and SRCT character.The lower molar extinction coefficient () of 7.56 × 10 3 M −1 cm −1 for this band in Czp-DiKTa is due to the more twisted geometry between the Czp donor and DiKTa compared to PCP-DiKTa ( of 28.43 × 10 3 M −1 cm −1 ), which itself is similar in magnitude to the SRCT band in DiKTa ( of 22.06 × 10 3 M −1 cm −1 ). [16]This is analogous behavior to that observed in Cz-DiKTa and DMAC-DiKTa, both of which display a similarly twisted structure to Czp-DiKTa ( of 16.92 × 10 3 and 12.50 × 10 3 M −1 cm −1 , respectively), [7a] while the CT band in CzpPhTrz has a lower  of 4.65 × 10 3 M −1 cm −1 as the Czp group is more poorly electronically coupled to the TRZ acceptor.
Both compounds have unstructured PL spectra in toluene with  PL of 469 nm for PCP-DiKTa and 505 nm for Czp-DiKTa.Compared to the PL spectra of DiKTa, Cz-DiKTa, DMAC-DiKTa, and CzpPhTrz (FWHMs of 27, 54, 94, and 80 nm, respectively), [7a] that of PCP-DiKTa (FWHM of 37 nm) is slightly broader than that of DiKTa, while that of Czp-DiKTa (FWHM of 66 nm) is slightly broader than that Cz-DiKTa, and narrower than that DMAC-DiKTa and CzpPhTrz.The PL spectra of PCP-DiKTa show a small degree of positive solvatochromism (Figure S18, Supporting Information), which is consistent with an excited state of SRCT character that is emblematic of MR-TADF emitters.By contrast, there is a much stronger positive solvatochromism in the PL spectra of Czp-DiKTa, reflecting an excited state with significant LRCT character.Indeed, in tetrahydrofuran and ethyl acetate, Czp-DiKTa shows dual emission emanating from the LRCT and SRCT states.The S 1 and T 1 energies of both compounds were determined from the onsets of the steady-state PL and phosphorescence spectra at 77 K in toluene (Figure 3c,d).These are 2.67/2.51eV for PCP-DiKTa and 2.57/2.44 eV for Czp-DiKTa, resulting in ΔE ST of 0.16 and 0.13 eV for PCP-DiKTa and Czp-DiKTa, respectively.These values are smaller than those of DiKTa (0.22 eV), Cz-DiKTa (0.20 eV) and DMAC-DiKTa (0.21 eV).The Φ PL of PCP-DiKTa and Czp-DiKTa in degassed toluene solutions are 36 and 37%, respectively, which are higher than that of DiKTa in toluene (26%); these decrease to 32 and 30% upon exposure to oxygen.The time-resolved emission decays do not show any delayed fluorescence, with lifetimes,  PL , of 6.2 and 13.9 ns (Figure S19, Supporting Information).In contrast, DiKTa shows multiexponential kinetics containing both prompt and delayed PL lifetimes in toluene, with  p and  d of 5.1 ns and 23 μs, respectively. [16]e then explored the photophysical properties of the two emitters in an OLED-relevant host, 2,6-bis(3-(carbazol-9yl)phenyl)pyridine (26DCzPPy), which is one that is bipolar and thus can mediate both the transport of holes and electrons within the emissive layer of the device; [17] additionally, it has suitably high triplet energy of 2.71 eV to confine the excitons onto the emitter. [18]We studied the photophysical prop-erties of these films as a function of emitter doping concentrations from 2 to 20 wt% (Figure S20, Supporting Information).Both compounds displayed only modest red-shifting of their PL in this doping range, which implies little aggregation.Both emitters showed the highest Φ PL at 2 wt% doped films at 93% for PCP-DiKTa and 99% for Czp-DiKTa under N 2 , values which then reduced to 66 and 73% in air, respectively (Tables S2 and S3, Supporting Information).Therefore, we used 2 wt% emitter doped 26DCzPPy films for the subsequent photophysical measurements.The steady-state emissions of PCP-DiKTa and Czp-DiKTa peak at  PL at 477 and 501 nm, with FWHMs of 44 and 66 nm, respectively (Figure 3b, Table 1).Multiexponential decay kinetics under vacuum were observed, with prompt fluorescence lifetimes,  P , for PCP-DiKTa and Czp-DiKTa of 6.4 and 11.7 ns, respectively, and delayed fluorescence lifetimes,  d , of 179 and 140 μs, respectively (Figure 4).The k RISC of PCP-DiKTa and Czp-DiKTa are 3.01 × 10 4 and 4.41 × 10 4 s −1 , respectively, which are faster than that of DiKTa (2.52 × 10 4 s −1 ). [13]Full kinetics parameters are summarized in Table S4 (Supporting Information).Temperature-dependent time-resolved PL decay behavior revealed the expected increase in the contribution from the delayed emission with increasing temperature that defines TADF (Figure 4).The ΔE ST values estimated from the difference in energy between the onsets of In toluene at 298 K; b) spin-coated 2 wt% thin films in 26DCzPPy ( exc = 343 nm); c) Average lifetime ( avg = ΣA i  i 2 /ΣA i  i 2 , where A i is preexponential for lifetime  d ).Prompt and delayed emission were measured by TCSPC ( exc = 379 nm) and MCS ( exc = 343 nm), respectively; d) Determined from the onset of the SS PL and phosphorescence spectra in toluene at 77 K.
the steady-state PL and phosphorescence spectra at 77 K are 0.19 eV for PCP-DiKTa and 0.17 eV for Czp-DiKTa, which are slightly larger than those in toluene (Figure S21, Supporting Information).
The circularly polarized luminescence (CPL) was then examined.The CPL spectra were recorded in dilute solutions of toluene at r.t and are shown in Figure 5c for (1st)/(2nd)-Czp-DiKTa enantiomers.These were found to display mirror-image CPL signals at 507 nm with photoluminescence dissymmetry factors g PL of ≈±4 × 10 −4 and with signs similar to the low-energy ECD-active bands, i.e., positive for (2nd)-Czp-DiKTa (see g PL plots in Figure 5d).However, no clear CPL signals could be measured for (1st)/(2nd)-PCP-DiKTa.Since the emission process mirrors the low-energy absorption one, the absence of CPL signal may be explained by the fact that for PCP-DiKTa the band at 445 nm, corresponding to the HOMO-LUMO transition, does not display ECD activity (Figure 5a).The magnitude of the g PL of Czp-DiKTa is similar to those reported for other CP-MR-TADF emitters; for example, (R)/(S)-Czp-tBuCzB and (R)/(S)-Czp-POAB have g PL of

Organic Light-Emitting Diodes (OLEDs)
We next fabricated vacuum-deposited OLEDs.The OLED device stack and the chemical structures of the organic layers are shown in Figure 6a,b, respectively.The OLEDs consist of indium tin oxide (ITO)/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN, 5 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 45 nm)/1,3-bis(N-carbazolyl)benzene (mCP, 5 nm)/26DCzPPy as host (20 nm) with PCP-DiKTa, and Czp-DiKTa as emitter dopants/1,3,5-tris(3-pyridyl-3-phenyl)benzene (TmPyPB, 45 nm)/lithium fluoride (LiF, 1 nm)/aluminum (Al, 100 nm).Here, HATCN was used as the hole injection layer, TAPC as the hole transport layer, mCP as an exciton blocking layer, TmPyPB as electron transport layer, and LiF to reduce the work function of the top Al electrode.The devices were made with 2, 5, 10, and 15 wt% doping of PCP-DiKTa or Czp-DiKTa, with the lower concentrations of emitter giving higher efficiency, helped by higher Φ PL (Table S3, Supporting Information).The current density-voltage-luminance (JVL) characteristics of the OLEDs with 2 and 5 wt% doping of PCP-DiKTa and Czp-DiKTa are shown in Figure 6c and Table 2.All devices had turn-on voltages of between 3.8 and 4.1 V.For the OLED with PCP-DiKTa, the 2 wt% device showed the highest EQE max of 25.7% in accordance with the high Φ PL for this concentration.The EQE decreased to 19.1% at 100 cd m −2 and to 8.7% at 1000 cd m −2 (Figure 6d).The OLED emitted at  EL at 489 nm (Figure 6e) with a FWHM of 53 nm, which is close to the PL ( PL of 477 nm; FWHM of 44 nm) of the emitter molecule.The CIE coordinates are (0.162, 0.450), Figure 6f.With an increase in the doping concentration of the emitter to 5 wt%, the EQE max of the device dropped to 20.5%, which is consistent with the drop in Φ PL from 93% (for the 2 wt% doped film) to 83% (for the 5 wt% doped film).The devices with higher concentrations of the emitter are shown in Figure S22 (Supporting Information).
The OLEDs with Czp-DiKTa (5 wt%) showed a very high EQE max of 29.2%, which decreased to 25.9% at 100 cd/m 2 and a high EQE of 17.1% at 1000 cd/m 2 as shown in Figure 6d and Table 2. Thus, compared to the device with PCP-DiKTa, the EQE max was both higher and the efficiency roll-off less severe in the device with Czp-DiKTa.The device emitted at  EL at 518 nm (FWHM of 69 nm), Figure 6e, which was slightly redshifted compared to the PL ( PL = 501 nm; FWHM = 66 nm).The corresponding CIE coordinates are (0.256, 0.610).With 2 wt% of the emitter concentration, a slightly lower EQE max of 27.8% was recorded despite the higher Φ PL (99 and 97% for the 2 and 5 wt% doped films, respectively).The EQE 100 /EQE 1000 were 23.1/15.0%,reflecting a more severe efficiency roll-off in this device.Overall, the efficiency roll-off for Czp-DiKTa devices at all concentrations (Figure S22, Supporting Information) was less pronounced.Photographs of the OLEDs with PCP-DiKTa and Czp-DiKTa are shown in Figure 6e, respectively.This study serves as an illustrative example of how a CP-TADF emitter can be tailored to achieve narrow emission and superior device performance, mitigating efficiency roll-off concerns.

Conclusion
−4  while no CPL was detected for PCP-DiKTa enantiomers.Both compounds have high Φ PL of 93 and 99% as 2 wt% doped films in 26DCzPPy.The shorter delayed lifetimes,  d , of PCP-DiKTa and Czp-DiKTa than that of the parent DiKTa ( d = 242 μs) is associated with faster reverse intersystem crossing (RISC) (k RISC of 3.01 × 10 4 s −1 and

Figure 2 .
Figure 2. Difference density plots and energies for the S 1 , S 2 , T 1 , and T 2 of PCP-DiKTa and Czp-DiKTa calculated at SCS-CC2/cc-pVDZ in the gas phase (ISO value = 0.02).The blue color represents an area of decreasing electron density, and the yellow color represents an increased electron density between the ground and excited states.

2402036 (7 of 9 )Figure 6 .
Figure 6.a) Schematic of the OLED stack; b) Chemical structures of the materials used in the device; c) Current density-voltage-luminescence (JVL) characteristics; d) External quantum efficiency (EQE) versus luminescence; e) Electroluminescence spectra, inset figures are photographs of the OLEDs fabricated with PCP-DiKTa 2 wt% and Czp-DiKTa 2 wt%; f) CIE coordinates of the devices.

Table 1 .
Photophysical properties of PCP