A Novel Carbazolophane: A Comparison of the Performance of Two Planar Chiral CP-TADF Emitters

The prototypical example of a (cyclo)phane, [2.2]paracyclophane (PCP), has proven to be a versatile stereogenic moiety within the design of circularly polarized thermally activated delayed ﬂuorescence (CP-TADF) emitters; however, the exploration of other cyclophanes within CP-TADF emitter design has been largely neglected. Here, a comparative study of the photophysical and optoelectronic properties of two cyclophane emitters, (1,7) t BuCzpPhTrz and its isomer (1,4) t BuCzpPhTrz, is presented. The carbazolophane-triazine compound (1,7) t BuCzpPhTrz, obtained via an unprecedented intramolecular rearrangement, is the ﬁrst example of a planar chiral TADF emitter deviating from the PCP scaﬀold. Signiﬁcant geometrical change of the enclosed carbazole in (1,7) t BuCzp results in an attenuation of the donor strength, while the merits of rigidity and steric bulk remain. In particular, the full width at half maximum (FWHM) of the photoluminescence spectrum in toluene of (1,7) t BuCzpPhTrz is reduced by 34% and blue-shifted by 20 nm compared to that of (1,4) t BuCzpPhTrz. In doped ﬁlms, the compounds reach high photoluminescence quantum yields ( 𝚽 PL ) of 91 and 81%, respectively. The chiroptical properties reveal dissymmetry factors | g PL | of up to 5 × 10 − 4 . These results demonstrate the impact of the cyclophane for the development of CP-TADF materials and add to the currently limited scope of available planar chiral donors


Introduction
Compounds exhibiting thermally activated delayed fluorescence (TADF) show a dual emission where a prompt fluorescence originates from rapid radiative decay of singlet excitons and a delayed fluorescence follows the endothermic up-conversion of non-emissive triplet excitons to singlets.This reverse intersystem crossing (RISC) is exploited in organic light-emitting diodes (OLEDs), which can then reach an internal quantum efficiency (IQE) of up to 100%, making organic TADF materials desired targets for electroluminescent device applications. [1]TADF emitter design produces organic semiconductors that possess a suitably small energy gap, ΔE ST , between the first excited singlet (S 1 ) and triplet (T 1 ) excited states.1a] OLEDs face a trade-off between high contrast ratios, achieved by reducing reflection of incident light, and brightness loss, when ≈50% of the produced light from the emissive layer is absorbed by anti-glare filters that are present in most of these devices.These filters convert unpolarized light into circularly polarized (CP) light (CPL).Upon reflection, the polarization of the CPL is reversed, preventing it from passing the filter; however, besides the suppression of the reflected incident light, the generated light of the OLED is also trapped.A strategy to circumvent this loss channel is to use a material with inherent CPL, thus making anti-glare filters redundant.A quantification of the degree of CPL is the photoluminescence dissymmetry factor |g PL |, which can have a maximum value of 2. Most closed-shell organic emitters, however, have very low |g PL |, on the order of 10 −3 . [2]hus, to achieve optimal results in a device requires employing an emitter that simultaneously can harvest both singlet and triplet excitons to produce light, is bright (i.e., has a high photoluminescence quantum yield, Φ PL ), and that the light emanating be CP.
One strategy to design CPL-active materials is to incorporate a planar chiral motif within the emitter design. [3]Among such well-studied motifs are the (cyclo)phanes, a class of cyclic compounds featuring at least one bridged arene.The [2.2]paracyclophane (PCP), a prototypical example consisting of two closely stacked benzene rings, has been at the center of scientific attention, and has proven useful inter alia in catalysis, [4] -conjugated oligomers, [5] porous frameworks, [6] chemical vapour deposition (CVD) coatings, [7] and other material applications. [8]In 2018, we were the first to incorporate this three-dimensional scaffold within a CP-TADF emitter. [9]Decorating the decks with donor (D) and acceptor (A) units produced a material that possessed lowlying charge transfer (CT) excited states whose electronic communication was mediated through the -stacked core of the PCP.Both isomers, cis-and trans-Bz-PCP-TPA, showed blue TADF with photoluminescence maxima,  PL , at 480 nm and 465 nm, respectively in 15 wt% doped films in 1,3-bis(N-carbazolyl)benzene (mCP); however, their low Ф PL of 12% and 15% precluded their use as emitters in OLEDs (Figure 1).
The dominant molecular designs for PCP-containing TADF emitters are those with PCP-fused N-heterocyclic donors.The [2]paracyclo[2](1,4)carbazolophane (Czp) was introduced as a sterically more demanding and stronger donor that produces emitters with smaller ΔE ST compared to analogues using carbazole (Cz). [12]Compounds CzpPhTrz, CNCzpPhTrz and CF 3 CzpPhTrz emit at  PL of 482, 458, and 456 nm and all have high Ф PL of up to 70% in 10 wt% doped films in bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) or 2,8bis(diphenylphosphoryl)dibenzo [b,d]thiophene (PPT).12b,13] OLEDs with the latter two emitted at  EL of around 460 nm and showed EQE max of 7.4 and 12%, respectively.The devices suffered from severe efficiency roll-off resulting from the long  d of 135 and 158 μs, respectively, and possible degradation as a new emission band at longer wavelength was observed under electrical excitation.The device with CzpPhTrz emitted at  EL of 480 nm and reached an EQE max of 17%.The analogous phenoxazine-derived PCP, PXZp-Ph-Trz, has also been reported where the enantiomers emit CPL with |g PL | of 3.3 × 10 −3 . [14]The solution-processed CP-OLEDs emitted at  EL of 560 nm, showed an EQE max of 7.8% and an electroluminescence dissymmetry factor |g EL | of 4.6 × 10 −3 .The structurally related emitter PXZ-PT contains a PCP that bridges a phenoxazine (PXZ) donor and a triazine acceptor on the same deck.The respective enantiomers emit CPL with |g PL | of 1.9 × 10 −3 .The devices emitted at  EL of 557 nm and showed an EQE max of 20% and |g EL | of 1.5 × 10 −3 . [15]Recently, Czp-tBuCzB and Czp-POAB have been reported as the first examples of planar chiral multi-resonance TADF (MR-TADF) emitters, where the Czp donor decorates two known MR-TADF cores.Sky-blue and green CP-TADF devices emitted at  EL of 479 and 513 nm, and showed EQE max of 32 and 29% and |g EL | of 1.5 × 10 −3 and 1.3 × 10 −3 . [16]To the best of our knowledge, all planar chiral TADF emitters hitherto rely on the PCP scaffold, while the use of other phane-based moieties is scarce [17] and limited only to larger macrocyclic assemblies, where planar chirality is not present.

Structural Design and Synthesis
We re-visited earlier works [12,18] on the synthesis of planar chiral carbazole derivatives bearing PCP, and disclose the synthetic route towards (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz as outlined in Scheme 1a.Both carbazolophane isomers, (1,7)tBuCzp (3a) and (1,4)tBuCzp (4a), were obtained by Pd-catalyzed oxidative cyclization of 4-N-(4-(tertbutyl)phenyl)amino[2.2]paracyclophane (2a) under aerobic conditions.In the presence of palladium(II) acetate and pivalic acid in acetic acid (1,4)tBuCzp and (1,7)tBuCzp were successfully formed in an overall yield of 39% in a ratio of 5:2.The latter implicates an unexpected migration of the ethylene bridge to the aniline arene, which was confirmed by X-ray diffraction analysis.In contrast to the planar geometry of the carbazole moiety in (1,4)Czp, an angle of 38°between the two benzene planes of the carbazole was measured in (1,7)carbazolophane (for more details see Figure S84, Supporting Information).On the basis of previous CH-activation studies, [19] we propose a mechanism for the oxidative cyclization/rearrangement sequence (Scheme S1, Supporting Information).For the initial ortho palladation step, we consider two intermediates, the relevant for (1,7)tBuCzp formation being A (Scheme S2, Supporting Information).Consecutive CH insertion of both ortho palladation states followed by reductive elimination affords the expected carbazolophane (1,4)tBuCzp.The alternative pathway from A, involves palladation of the distorted C(sp 2 ) bridgehead atom and a [1,5]-alkyl shift, which should result in a release of ring strain.Reductive elimination of palladacycle C and [1,3]-H shift provides (1,7)tBuCzp and reforms the active palladium catalyst.Such an alteration of the PCP skeleton through C─C bond breakage has rarely been documented in the literature. [20]Compared to the conventional route of photo-deselenation to afford mixed carbazolophanes, [21] this finding not only highlights an efficient alternative approach to synthesize this new class of carbazolophane derivatives, but also facilitates their use as building blocks of elaborated structures.For example, the amino group in 3 needs no protection and can serve as a reactive site for further functionalization.
In order to explore the compatibility of this protocol, we studied a series of phenylamino[2.2]paracyclophanes of varying electronic and steric demand.As shown in Scheme 1b, the reaction proceeded in good-to-moderate yields for para-alkylated substrates (2a-c) with an overall yield of up to 52% and a ratio of 1:3 for the methylated cyclophanes 3c and 4c, respectively.The reaction yields of the (1,4)Czp derivatives (4b, 4c) are comparable to the previously reported studies; however, the formation of the (1,7)Czp derivatives (3b, 3c) have not as of yet been documented. [18]A decrease in yield was observed for meta-substituted amines (2d, 2e) due to the increase in steric hindrance.Consequently, only one selective regioisomer of 3d and 4d was formed.As determined by NMR spectroscopy analysis for both molecules, the carbazole C─C bond was formed at the less hindered position, confirming the observations of other Pd-catalyzed carbazole protocols based on CHactivation. [22]Electron-withdrawing groups in the para position are tolerated although slightly longer reaction times were required to achieve full conversion.No consumption was observed during the reaction of 2k, bearing electron-deficient groups meta to the amine.However, oxidative cyclization of nitro-substituted 2g yields solely the respective (1,4)carbazolophane 4g in 42%.In general, highly reactive functionalities (2h-j) were found to be incompatible under the studied reaction conditions and led to decomposition.
12b] The carbazole moiety of (1,7)tBuCzpPhTrz adopts a puckered conformation and the ethylene C-C bonds of the PCP are much shorter than those in (1,4)tBuCzpPhTrz (1.56 Å vs 1.58 Å).This brings the carbazole and phenylene rings closer together (2.31-3.33Å) than in (1,4)tBuCzpPhTrz (2.76-3.09Å).Due to the spatial orientation of the ethylene bridges in (1,7)tBuCzpPhTrz, which are positioned perpendicular to the triazine axis, the torsion angle between the pyrrole core of the carbazole to the grafted phenylene of the triazine in (1,7)tBuCzpPhTrz is shallower (33.4°) compared to (1,4)tBuCzpPhTrz (51.7°) and (1,4)CzpPhTrz (51.8°),where the steric hindrance of the cyclophane bridge becomes more pronounced.The compounds pack through - stacking of the triazine moieties, with the neighboring triazines facing towards each other in an anti-parallel fashion (Figure 2b,c).The intermolecular distance of these - stacked planes in the two crystal structures of the two compounds is in a similar range of around 3.3 Å.

Theoretical Calculations
We undertook a theoretical study to understand the impact of the connectivity of the cyclophane on the optoelectronics of the emitters.The ground-state optoelectronic properties of (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz were calculated using density functional theory (DFT) at the PBE0/6-31G(d,p) level in the gas phase.As shown in Figure 3, the torsion angles between the donor and acceptor moieties are around 55.5°f or (1,4)tBuCzpPhTrz and 36.2°for(1,7)tBuCzpPhTrz, which are in agreement to the angles found in the crystal structures (51.7°and 33.4°, respectively).The HOMO is mainly localized on the (1,4)tBuCzp or (1,7)tBuCzp donors with a small degree of electron density on the bridging phenylene, the HOMO energy levels are nearly identical at −5.46 and −5.42 eV, respectively.Similarly, the lowest unoccupied molecular orbital (LUMO) is located on the triazine acceptor and the bridge, with the LUMO of (1,7)tBuCzpPhTrz being slightly destabilized at −1.76 eV compared to that of (1,4)tBuCzpPhTrz at −1.86 eV.The HOMO-LUMO gap is thus essentially the same in the two compounds.
Time-dependent DFT (TD-DFT) calculations within the Tamm-Dancoff approximation (TDA-DFT), performed at the same level of theory, predict that both compounds have large ΔE ST of 0.30 and 0.37 eV for (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz, respectively.Both compounds have similar predicted S 1 levels to the parent (1,4)CzpPhTrz (S 1 level of 3.11 eV) and the S 1 state is predicted to show CT character with a high oscillator strength for the S 0 -S 1 transition.However, their triplet levels are more stabilized compared to the T 1 of (1,4)CzpPhTrz (2.81 eV), with predicted T 1 levels of 2.77 and 2.75 eV for (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz.There is a large spin-orbital coupling constant between the S 1 and T 1 states at their optimized ground-state geometries (0.33 cm −1 for (1,4)tBuCzpPhTrz and 0.31 cm −1 for (1,7)tBuCzpPhTrz).The NTO analysis of both compounds based on the optimized S 1 and T 1 geometries were also calculated (Figure S100, Supporting Information).For both compounds the S 1 and T 1 states showed similar location of particles mainly on the triazine group and holes mainly on the donor groups, indicating that both states possess CT character.Thus, on balance and despite the large predicted ΔE ST values, these two compounds are predicted to show TADF and emit in the blue.
and Table 2).The absorption spectra for both compounds have intense bands at around 385 nm, which are assigned by TDA-DFT calculations to the intramolecular charge transfer between the donor and the triazine acceptor.This assignment is also in line with that for (1,4)CzpPhTrz which has an ICT absorption band at 375 nm.The ICT band of (1,7)tBuCzpPhTrz has a higher molar absorptivity value (ɛ = 45 × 10 3 m −1 cm −1 ) than (1,4)tBuCzpPhTrz (ɛ = 29 × 10 3 m −1 cm −1 ), indicating a higher oscillator strength for the S 0 and S 1 transition, which is in line with the trend in predicted S 1 oscillator strengths.The steady-state photoluminescence (PL) spectra in toluene are broad and unstructured, as typically observed in donor-acceptor TADF compounds, with peak maxima,  PL , at 466 and 446 nm for (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz, respectively.Notably, the full width at half maxima (FWHMs) are different at 76 and 50 nm, respectively, suggesting that there is less geometric reorganization in the excited state of (1,7)tBuCzpPhTrz than (1,4)tBuCzpPhTrz.The emission of both compounds is blue-shifted compared to that of (1,4)CzpPhTrz ( PL of 470 nm in toluene).There is an observed positive solvatochromism in the PL of both compounds (Figure S102, Supporting Information), which corroborates the assignment of the S 1 state as having CT character.The optical gap (E g ), determined from the intersection of the normalized absorption and emission spectra, are 2.94 eV and 2.99 eV for (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz, respectively.The Φ PL values of (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz in degassed toluene solutions are 82 and 78%, respectively, which decrease to 68 and 72% upon exposure to oxygen.The time-resolved emission decays for both compounds have lifetimes,  PL , of 8.2 and 4.2 ns, no obvious delayed emission was detected.We next investigated their photophysical properties in both a wide bandgap polymer host, PMMA, and also a high triplet en-ergy small molecule host, DPEPO (T 1 = 3 eV [25] ), with a view to employing these compounds as emitters in OLEDs.The concentration of the doped films was chosen as 10 wt% as this is what was used for the previous study with (1,4)CzpPhTrz.
(1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz showed unstructured CT-based emission at  PL of 470 and 457 nm as in 10 wt% doped films in PMMA, respectively (Figure 4b).The FWHMs are 83 and 66 nm, respectively, which are slightly broader than FWHMs measured in toluene of 76 and 50 nm.These emission spectra are red-shifted to 480 and 473 nm in 10 wt% doped films in DPEPO, with FWHMs of 87 and 69 nm, reflecting the higher polarity of this host compared to PMMA.These  PL values are similar to that reported for (1,4)CzpPhTrz ( PL = 482 nm in 10 wt% doped films in DPEPO).The Φ PL values of the doped DPEPO films of (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz are 84 and 91%, respectively, which decrease to 78 and 81% under air, respectively.The Φ PL values in 10 wt% doped films of (1,7)tBuCzpPhTrz in DPEPO are similar to those of the 10 wt% doped films in mCP:PPT (1:1); the  PL values of 473 and 463 nm in doped mCP:PPT (1:1) films mirror those in PMMA (Table S9, Supporting Information).In 10 wt% doped films in mCP:PPT, the Φ PL of (1,4)tBuCzpPhTrz is slightly lower at 77% under N 2 and 68% under air.
The S 1 and T 1 energies for both compounds were extracted from the onsets of the steady-state PL and delayed emission spectra (1-10 ms) at 77 K in both 2-MeTHF glass and the 10 wt% doped films in DPEPO (Figure 5).The ΔE ST values of (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz are 0.11 and 0.23 eV in 2-MeTHF, while those in 10 wt% doped films in DPEPO are 0.18 and 0.25 eV, respectively.These are smaller than the calculated ΔE ST values and reflect the impact of the host in modulating the energies of the excited states.The larger ΔE ST for (1,7)tBuCzpPhTrz results from the smaller D-A torsion angle,   b) [nm] d) [μs] S 1 /T  b) Thin films were prepared by vacuum depositing at 10 wt% doping in DPEPO ( exc = 360 nm); c) The optical bandgaps were calculated from the intersection point of the normalized absorption and emission spectra; d) Average lifetime ( avg = ΣA i  i 2 /ΣA i  i 2 , where A i is the preexponential for lifetime  d ).Prompt and delayed emissions were measured by TCSPC and MCS, respectively ( exc = 379 nm); e) Determined from the onset of prompt and delayed spectra in 2-MeTHF, measured at 77 K.
as determined in the crystal structure analysis, that leads to a larger exchange integral.The time-resolved PL decays of the 10 wt% doped films of (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz in DPEPO at room temperature show a prompt fluorescence lifetime,  p , of 8.9 and 5.3 ns, and an average delayed fluorescence lifetime,  d , of 315 and 637 μs, respectively (Figure 6a,b).
The delayed emission is significantly quenched in the presence of oxygen while the prompt emission is invariant.The temperature-dependent time-resolved PL decays (Figure 6c,d) reveal the expected increase in the intensity of the delayed emission with increasing temperature that is emblematic of TADF.

Organic Light-Emitting Diodes
Having identified the potential of these compounds as emitters, vacuum-deposited OLEDs with (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz were fabricated.The first set of OLEDs was made using our previously reported stack, [12b] with DPEPO as the host.host had high turn-on voltages of 3.7 and 4.3 V, respectively.This indicated that DPEPO was not a favorable host, likely causing charge trapping on the host or charge accumulation at the EML interfaces. [27]Even though the devices with (1,4)tBuCzpPhTrz (Figure 8d) showed an EQE max of 18.5%, the efficiency roll-off was severe as the EQE 100 (EQE at 100 cd m −2 ) was 8% while the EQE 1000 (EQE at 1000 cd m −2 ) decreased to 2.2%.12b] The more pronounced efficiency roll-off in the devices with (1,4)tBuCzpPhTrz as compared to the reported device with (1,4)CzpPhTrz may in part be due to the longer t d (315 μs) of the former as compared to the t d (65 μs) of the latter.Emitters with longer t d typically correlate with devices showing more severe efficiency roll-off. [28]The device with (1,7)tBuCzpPhTrz achieved a comparable EQE max of 18%, but the efficiency roll-off was even more pronounced with the EQE 100 /EQE 1000 dropping to 4.8/1.1%.The electroluminescence (EL) spectra for these two devices were similar with emission peaks,  EL , at 486 and 490 nm for the devices with (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz, respectively (Figure 8e).The CIE coordinates for devices with both emitters were also similar at (0.19, 0.33) (Figure 8f).
In an effort to reduce the efficiency roll-off, a second set of devices was fabricated using a 1:1 mixture of mCP and PPT.The use of this ambipolar co-host system was expected to result in a wider recombination zone and improve the mobility of both charges.The device structure (Figure 8a) was thus modified to be: ITO/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HATCN, 10 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 25 nm)/TCTA (10 nm)/mCP (10 nm)/mCP:PPT (20 nm) with 10 wt% of (1,4)tBuCzpPhTrz or (1,7)tBuCzpPhTrz/PPT (10 nm)/TmPyPB (40 nm)/LiF (0.8 nm)/Al (100 nm).The thickness of the layers was optimized based on simulations for outcoupling efficiencies, and layer thicknesses were varied in increments of 5 nm to identify the best combination of optical and electrical properties (Figure S103, Supporting Information).Here, HATCN was used as the hole injection layer, TAPC as the hole transport layer, TCTA as electron blocking, mCP as the exciton blocking layer, PPT as the hole blocking layer, and TmPyPB for electron injection and transport.
The turn-on voltages were lower at 3.1 and 3.0 V for the devices with (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz, respectively, compared to those using DPEPO, indicating that the co-host system improved charge injection into the EML.This was also reflected in the higher current density and luminance as compared to the devices with DPEPO (Figure 8c).The EQE max of the OLEDs with (1,4)tBuCzpPhTrz decreased slightly to 17%, but there was a modest improvement in the efficiency roll-off where the EQE 100/ EQE 1000 were maintained at 11.2/4.8%.Similarly, the device with (1,7)tBuCzpPhTrz showed a lower EQE max of 13.5%; however, the EQE 1000 (3.2%) was slightly improved compared to the device using DPEPO.One factor that may contribute to the higher EQE for the device with (1,4)tBuCzpPhTrz compared to that with (1,7)tBuCzpPhTrz is the greater horizontal orientation of the transition dipole moment of the emitter in the former (Figure S104, Supporting Information).The  EL for the device with (1,4)tBuCzpPhTrz was 490 nm, which was similar to that of the device using DPEPO (486 nm).The  EL for the device with (1,7)tBuCzpPhTrz was 475 nm, which was blue-shifted compared to the device using DPEPO (490 nm), but is close to the  PL (467 nm in 1:1 mCP:PPT).These results point to a reduced propensity to aggregate in the co-host system compared to DPEPO for the vacuum-deposited films with (1,7)tBuCzpPhTrz.Indeed, aggregates were found to contribute to a red-shifting of the emission of (1,7)tBuCzpPhTrz in the 1:1 mCP:PPT cohost only at higher doping concentrations (Figure S105, Supporting Information).The CIE coordinates for the device with (1,4)tBuCzpPhTrz were (0.19, 0.38) while those for the device with (1,7)tBuCzpPhTrz were (0.16, 0.27) (Figure 8f,g) shows the photograph of the (1,7)tBuCzpPhTrz OLED in mCP:PPT cohost.

Figure 3 .
Figure 3. Theoretical modeling of a) the energies and electron density distributions of the HOMO/LUMO (ISO value = 0.02) and b) NTOs (particle and hole are represented by red and blue colors, respectively) and their associated vertical excitation energies of S 1 and T 1 states of (1,4)tBuCzpPhTrz and (1,7)tBuCzpPhTrz computed based on the ground-state optimized geometries, f is the oscillator strength.