Alternating Donor – Acceptor π -Conjugated Macrocycle Exhibiting Ef ﬁ cient Thermally Activated Delayed Fluorescence and Spontaneous Horizontal Molecular Orientation

which combines three 5-(N-carbazolyl)-phenylen-1,3-diyl as D units and three 6-phenyl-1,3,5-triazin-2,4-diyl as A units, possesses a small singlet – triplet energy gap and hence can emit ef ﬁ cient green TADF both in solution and doped thin ﬁ lms. Comparative experimental and computational investigations of the electronic and photophysical properties of the macrocycle with its analogous noncyclic compound reveal key advantages of the cyclic molecular con ﬁ guration for actual emitters. Organic light-emitting diodes incorporating the TADF π -conjugated macrocycle as an emitter dem-onstrate high external electroluminescence quantum ef ﬁ ciencies of up to 15.7%, outperforming the devices based on the noncyclic emitter. Herein, the impor-tance of geometric design for producing novel organic emitters with fascinating optoelectronic and morphological characteristics is highlighted.


Introduction
Macrocycles (cyclic macromolecules) involving π-conjugated electronic systems continue to attract significant research interest because of their unique structural features, as well as their potential applicability to optoelectronic functional materials. [1] Over the past decade, the macrocyclization of (hetero)arenes via crosscoupling protocols has afforded a variety of cyclic π-conjugated molecules, typified by cyclo-para-phenylenes (CPPs), [2] cyclometa-phenylenes (CMPs), [3] and cyclothiophenes. [4] These contributions have paved the way for new research into molecular nanocarbons.
Although the first synthesis of pure CMP hydrocarbons was achieved by Staab and Binnig in the 1960s, [5] it was only recently that CMPs and their homologs were demonstrated to function as organic semiconductor materials. [3c-e,6] After the recent advent of effective synthetic methods of synthesizing strained CPP architectures, [2] a variety of shapepersistent macrocycles consisting of representative π-conjugated cores, such as naphthalene, [7] pyrene, [8] fluorene, [9] and stilbene, [10] have been developed. Unlike their conventional linear counterparts, such cyclic π-conjugated molecules typically exhibit unique size-dependent photoluminescence (PL) and electronic properties. Nevertheless, thus far, there are only a few reports on π-conjugated macrocycles that can serve as actual emitters in organic light-emitting diodes (OLEDs). [6,9b,10] Although their aromatic constituents can undoubtedly be utilized for optoelectronic applications, the PL and electroluminescence (EL) characteristics of most reported macrocyclic entities tend to be inferior to those of conventional noncyclic fluorophores.
One of the major hurdles for future optoelectronic applications of π-conjugated macrocycles is the limited tunability (or variability) of their electronic structures, stemming from their uniform and symmetrical π-systems. An attractive approach is to introduce electronic donor-acceptor (D-A) motifs [11][12][13] or bridging heteroatoms (e.g., B, N) [14] into cyclic π-conjugated scaffolds to modulate their optoelectronic properties. In this regard, D-A-containing macrocycles based on CPPs [11] and CMPs [12] have been synthesized; however, these molecules have not been DOI: 10.1002/adpr.202100021 A versatile design of thermally activated delayed fluorescence (TADF) π-conjugated macrocycles incorporating electron-donor (D) and acceptor (A) units into a cyclo-meta-phenylene motif with an alternating pattern is presented. The new π-conjugated macrocycle, which combines three 5-(N-carbazolyl)phenylen-1,3-diyl as D units and three 6-phenyl-1,3,5-triazin-2,4-diyl as A units, possesses a small singlet-triplet energy gap and hence can emit efficient green TADF both in solution and doped thin films. Comparative experimental and computational investigations of the electronic and photophysical properties of the macrocycle with its analogous noncyclic compound reveal key advantages of the cyclic molecular configuration for actual emitters. Organic light-emitting diodes incorporating the TADF π-conjugated macrocycle as an emitter demonstrate high external electroluminescence quantum efficiencies of up to 15.7%, outperforming the devices based on the noncyclic emitter. Herein, the importance of geometric design for producing novel organic emitters with fascinating optoelectronic and morphological characteristics is highlighted.
tested in actual devices. Recently, π-conjugated macrocycles formed by alternating bithiophene and perylene diimide D-A moieties have been reported to serve as nonfullerene n-type organic semiconductors, and outperform acyclic analogues in organic solar cells. [13] In 2020, a macrocycle composed of alternating p-phenylene diamine and dibenzo[a,j]phenazine D-A moieties was developed, [15] which exhibited thermally activated delayed fluorescence (TADF), [16,17] and was used to produce efficient OLEDs. These recent reports provide insight for the further exploration of attractive π-conjugated macrocycles based on electronic D-A motifs for extensive device applications.
Herein, we report a rational strategy for the design of π-conjugated macrocycles with TADF ability, for high-efficiency OLEDs. To this end, we designed a CMP-analogous macrocycle, MC-C3T3 (Figure 1), by connecting three carbazolyl-mphenylene (C) and three phenyltriazine (T) units in an alternating fashion. Compared with the acyclic trimeric compound CTC, this novel π-conjugated macrocycle was found to exhibit superior PL and EL characteristics featuring TADF. OLEDs incorporating MC-C3T3 afforded a maximum external EL quantum efficiency (η ext ) as high as 15.7%, as well as a high maximum luminance exceeding 3000 cd m À2 . Comparison of the π-conjugated cyclic and acyclic D-A entities provides useful insights into the effects of the unconventional molecular geometry and cyclic conjugation on the photophysical and optoelectronic properties of these molecules.

Synthesis and Characterization
The approach for synthesizing MC-C3T3 is shown in Figure 2. First, we attempted the one-pot macrocyclization (Route A) via sixfold Suzuki-Miyaura coupling reactions between the equimolar amounts of meta-difunctionalized monomers 1 and 2 using [Pd(PPh 3 ) 4 ] as the catalyst and Cs 2 CO 3 as the base under dilute and anhydrous conditions. [3b,12a] Here, tert-butyl groups were introduced at the 3,6-positions of carbazole to enhance its solubility. After partial purification, the matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass analysis of the obtained emissive crude mixture suggested the presence of the desired hexameric macrocycle (m/z ¼ 1525.67, assigned to its protonated ion peak [M þ H] þ ), in addition to the major noncyclic oligomeric products. However, this reaction yielded only a trace amount of MC-C3T3, which hampered its isolation and further investigation. Hence, we altered the synthesis route to assemble MC-C3T3 in a [3þ3] manner (Route B), in which the possible cyclization products are essentially limited to the hexamer (MC-C3T3), dodecamer, etc., without any intermediate ring sizes. This route greatly facilitated the production and isolation of MC-C3T3. As a key precursor, trimeric dichloride 3 was readily obtained via twofold cross-coupling between 1 and 2. The complementary diboronate precursor 6 was prepared by the nucleophilic substitution of 4 with two equivalents of 3,6-di-tert-butylcarbazole, followed by Pd-catalyzed borylation using bis(pinacolato)diboron. In the final step, macrocyclization was accomplished via the Suzuki-Miyaura cross-coupling between the equimolar amounts of 3 and 6 under the same conditions as Route A to furnish the desired MC-C3T3. After successive purification by silica gel chromatography and preparative gel permeation chromatography, MC-C3T3 was isolated as a yellow powdery solid in 18% yield. However, no larger macrocycles were obtained as by-products under these reaction conditions.
The chemical structures of MC-C3T3 and the separately synthesized acyclic CTC were ascertained by 1 H and 13 C NMR spectroscopy, MALDI-TOF mass spectrometry, and elemental analysis (Supporting Information). Consistent with its symmetric cyclic structure, two singlets were observed in the 1 H NMR spectrum of MC-C3T3 in a low magnetic field (10.72 and 9.13 ppm in CDCl 3 ), originating from the cyclized meta-phenylene units, together with a distinct set of signals of the exocyclic phenyl and N-carbazolyl (Cz) groups in the typical aromatic region (8.5-7.2 ppm). The protonated ion peak (m/z ¼ 1525.68. [MþH] þ ) in the MALDI-TOF mass spectrum confirmed the chemical composition of MC-C3T3, as shown in Figure 2. Figure 3 shows a comparison of the electronic absorption and PL properties of MC-C3T3 and the corresponding fragmented CTC in deoxygenated toluene solutions, and Table 1 shows the key photophysical data. The profiles of the absorption spectra of both D-A-structured molecules were similar, featuring two major absorption bands. The stronger higher-energy absorptions assigned to the π-π* transitions appeared at the same positions below 350 nm. Moreover, weaker lower-energy absorptions originating from the intramolecular charge transfer (ICT) were observed in the range of 350-450 nm, with molar absorption coefficients (ε) in the order of 10 3 M À1 cm À1 . The ICT absorption of MC-C3T3 was red-shifted compared with that of CTC, and this trend correlated with the PL peak wavelengths (λ PL ). Upon www.advancedsciencenews.com www.adpr-journal.com photoexcitation, CTC emitted deep-blue PL with λ PL at 468 nm, whereas MC-C3T3 emitted green PL with a bathochromically shifted λ PL at 496 nm ( Figure 3a ,b). In addition, the full width at half-maximum of the PL spectrum (E fwhm ) and associated reorganization energy (λ S ) [18] of MC-C3T3 were smaller than those of CTC (Table 1) because of the rigid macrocyclic structure of the former. Notably, the absolute PL quantum yield (Φ PL ) of MC-C3T3 was as high as 77% in toluene, surpassing the Φ PL of CTC (49%).

Photophysical Properties in Solutions and Thin Films
More importantly, in contrast with the monoexponential nanosecond-scale PL decay of CTC, the transient PL curve of MC-C3T3 revealed two distinct exponential decay components in deoxygenated toluene ( Figure 3c). While the prompt decay component of MC-C3T3 with a lifetime (τ p ) of 38 ns corresponds to normal fluorescence (i.e., singlet excited state, S 1 ! ground state, S 0 ), the delayed decay component with a longer lifetime (τ d ) of 24 μs is attributed to TADF involving intersystem crossing (ISC) and reverse intersystem crossing (RISC) www.advancedsciencenews.com www.adpr-journal.com processes (i.e., S 1 ! triplet excited state, T 1 ! S 1 ! S 0 ). [16,17] The delayed decay component gradually intensified with increasing temperature (Supporting information), testifying the typical TADF feature. In aerated solution, however, the delayed decay component of MC-C3T3 completely vanished and the Φ PL value decreased to 21% as a consequence of transfer of the excited energy to triplet molecular oxygen. Thus, the S 1 state of MC-C3T3 can be effectively populated by up-conversion from its T 1 state via RISC at ambient temperature (300 K) under oxygen-free conditions. The S 1 -T 1 energy gaps (ΔE ST ) were experimentally determined to be 0.13 and 0.24 eV for MC-C3T3 and CTC, respectively, in toluene (Table 1, Supporting  Information). Therefore, we propose that the slight difference in ΔE ST for MC-C3T3 and CTC is the main factor determining the presence or absence of the TADF capability. We then evaluated the steady-state and transient PL characteristics of these two species in doped thin films at a concentration of 10 wt% using 4,4 0 -bis(9-carbazolyl)-1,1 0 -biphenyl (CBP) as the host. The extracted photophysical data are shown in Table 1 (see the Supporting Information for details). The doped films of MC-C3T3 and CTC exhibited bright green and blue PL, with λ PL at 489 and 458 nm and Φ PL of 65% and 44%, respectively, under N 2 atmosphere. While the CTC-doped film merely exhibited normal fluorescence with τ p of 55 ns, the MC-C3T3-doped film showed obvious TADF characteristics consisting of two emission components with τ p and τ d of 27 ns and 28 μs, respectively, similar to the cases in solution. Based on the S 1 and T 1 energies estimated from the onset wavelengths of the timeresolved fluorescence and phosphorescence spectra at 15 K (Supporting Information), ΔE ST for MC-C3T3 in the doped film was calculated to be 0.15 eV, which is indeed half of that of CTC and is favorable for effective RISC. This trend is consistent with the photophysical data measured in their solutions.

Excitonic Properties Based on Computational Simulations
To understand the origin of TADF, as well as the difference in the electronic structures of the cyclic MC-C3T3 and acyclic CTC, density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations were carried out at the PBE0/6-31 G(d) level. As schematically shown in Figure 4, for MC-C3T3, the highest occupied molecular orbital (HOMO) and its almost degenerated HOMO À 1 and HOMO À 2 are mostly localized on the three electron-donating Cz units, whereas the lowest unoccupied molecular orbital (LUMO) and upper-lying LUMO þ 1 are delocalized over the entire macrocyclic π-backbone, with negligible   Measured in oxygen-free toluene solution (10 À5 M) at 300 K; b) Measured for 10 wt%-doped thin film in CBP host matrix at 300 K; c) PL emission maximum; d) Absolute PL quantum yield evaluated using an integrating sphere under N 2 ; e) Fractional quantum yields for prompt fluorescence (Φ p ) and delayed fluorescence (Φ d ): Φ p þ Φ d ¼ Φ PL ; f ) PL lifetimes for prompt fluorescence (τ p ) and delayed fluorescence (τ d ) components measured under N 2; g) Not determined; h) Full width at half-maximum of the PL spectrum, given in wavelength; i) Reorganization energy along the S 1 ! S 0 ICT transition, estimated from each PL spectrum; j) Rate constant of fluorescence radiative decay (S 1 ! S 0 ): k r % Φ p /τ p ; k) Rate constant of nonradiative internal conversion; k nr % k r ·(1 À Φ PL )/Φ PL ; l) Rate constant of ISC (S 1 ! T 1 ): k ISC % k r ·(1/Φ p À 1/Φ PL ); m) Rate constant of RISC (T 1 ! S 1 ) estimated by assuming that most of the T 1 excitons can upconvert to S 1 excitons through RISC, and that non-radiative exciton losses primarily occur in the S 1 state: k RISC % Φ PL /(k r · τ p · τ d ); n) Singlet-triplet energy gap estimated from onset wavelengths of the fluorescence and phosphorescence spectra recorded at 77 K for the solutions and at 15 K for the doped films (Supporting Information).
contributions from the exocyclic Cz units. As often seen in highly symmetric π-conjugated macrocycles, the HOMO ! LUMO and HOMO ! LUMO þ 1 transitions in MC-C3T3 (denoted by orange and purple arrows in Figure 4) seem to be forbidden, with zero oscillator strengths ( f ) in theory, due to cancellation of the transition dipole moments. Instead, the ICT transitions assigned to HOMO À1, À2 ! LUMO, and HOMO À1, À2 ! LUMO þ 1 (denoted by red and green arrows) correspond to the experimentally observed low-energy absorption bands. The frontier molecular orbital distributions of MC-C3T3 stand in contrast to those of CTC, for which the LUMO and LUMO þ 1 are concentrated on the central triazine and adjacent phenylene rings. The calculated LUMO energy level of MC-C3T3 (À2.31 eV) was much deeper (or energetically stabilized) than that of CTC (À1.94 eV), while retaining very similar HOMO and HOMO À 1 energy levels. Consequently, the bandgap energy of MC-C3T3 was calculated to be smaller (by 0.26 eV) than that of CTC, consistent with the red-shifted PL of MC-C3T3.
To gain a more in-depth understanding into the nature of the excited states with different spin multiplicities, natural transition orbital (NTO) [19] analyses were carried out for MC-C3T3 and CTC using TD-DFT at the same level ( Figure 5; the NTOs of CTC are available in the Supporting Information). For MC-C3T3, the NTO "hole" wavefunction in the optimized S 1 state resided solely on one Cz unit (represented as Cz-1 in Figure 5), and its paired "electron" wavefunction tended to be delocalized onto the adjoining half-arc (not whole) of the macrocyclic π-backbone. The formation of this completely charge-separated (CT-dominated) S 1 state is accompanied by a large fragmental rotation of Cz-1, [20] with a change in its dihedral angle (θ 1 ) from 49 in the initial ground state (S 0 ) to 65 in the S 1 state, minimizing the holeelectron overlap integral, hΨ h Ψ e i. Unlike Cz-1, the dihedral angles of the two other Cz units (Cz-2 and Cz-3) remained almost unchanged (θ 2 ¼ θ 3 ¼ 50 ) upon S 1 excitation. Furthermore, the ISC process was found to occur in tandem with rotational relaxation of the Cz-1 unit, with a decrease in θ 1 to 48 in the resulting T 1 states. In this case, the NTO "hole" wavefunction is extended to the nearby phenylene ring of the macrocycle in the T 1 state, and hence largely overlaps with the paired "electron" wavefunction, resulting in an excited state featuring the hybridized local and charge-transfer (HLCT). The contribution of such a localized excitation (LE) component in the T 1 state can facilitate a change in the orbital angular momentum, correlated with spin-orbit coupling (SOC), allowing the spin-flipping RISC into the S 1 state possessing pure CT nature. Given that the TD-DFT calculations afforded a moderate SOC matrix element hS 1 jĤ SOC jT 1 i of 0.11 cm À1 between the T 1 and S 1 states (Supporting Information), we envision that the RISC for MC-C3T3 primarily occurs between the T 1 ( 3 HLCT) and S 1 ( 1 CT) states. For MC-C3T3, the T 1 state (2.35 eV) is the only state that is lower in energy than the S 1 state (2.45 eV); the T 2 state (2.60 eV) is energetically higher than the S 1 state, which is consistent with the foregoing notion. In addition, the calculated adiabatic ΔE ST values for MC-C3T3 and CTC (0.10 and 0.18 eV, respectively; Figure 5) are in reasonable accordance with the experimental results (Table 1).  Hole and electron distributions of natural transition orbitals (NTOs) and excitation energies for the lowest-excited singlet and triplet (S 1 and T 1 ) states of MC-C3T3, calculated at the PBE0/6-31G (d) level. E V : vertical excitation energy from S 0 to S 1 /T 1 ; E A : adiabatic excitation energy from S 0 to S 1 /T 1 ; www.advancedsciencenews.com www.adpr-journal.com
device B based on fluorescent CTC (η ext ¼ 4.2%, η c ¼ 9.1 cd A À1 , and η p ¼ 7.2 lm W À1 ). We also note that the emission patterns of both devices were basically Lambertian (Figure 6d). To the best of our knowledge, the EL performance of MC-C3T3 is the highest among those reported for π-conjugated macrocycles. [6,9b,10,15] By assuming that the holes and electrons are fully balanced and recombined to generate excitons within the emission layer (EML), the theoretical internal EL quantum efficiency (η int ) and η ext of the TADF-OLEDs can be represented by the following equations [16] where η S and η T denote the rates of exciton production in the singlet and triplet states (25% and 75%, respectively), Φ p and Φ d are the fractional quantum efficiencies of the prompt and delayed components, respectively (see Table 1), Φ ISC is the intersystem crossing efficiency (Φ ISC ¼ 1 À Φ p ), and η out is the light outcoupling efficiency. Based on Equation (1), the theoretical maximum η int for device A was estimated to be %50%. According to Equation (2), considering the experimental maximum η ext of 15.7%, the η out for device A was deduced to be %32%, which is 1.6 times higher than the typical value (η out %20%) [24] for conventional OLEDs without additional optics for improving the light outcoupling.

Molecular Orientation Behavior and Molecular Dynamics
To unveil the origin of the enhancement of η out and thereby η ext in the OLED incorporating MC-C3T3, we further verified the molecular orientation behavior of MC-C3T3 in the doped film using angle-dependent p-polarized PL spectroscopy. [25] Figure 7a shows the normalized PL intensities as a function of the emission angle for MC-C3T3 and CTC doped into the CBP host matrices, which were used to quantify the emitting dipole orientations within the thin films. Here, the dipole orientation ratio (Θ) is defined as where p ‖ and p ⊥ represent the horizontal and vertical components of the emitting dipoles, respectively (i.e., Θ ¼ 100% for fully horizontal dipoles; Θ ¼ 67% for isotropic random dipoles; Θ ¼ 0% for fully vertical dipoles). Notably, for MC-C3T3, Θ was as high as 89%, indicating that in the doped film, the emitting dipoles of the cyclic MC-C3T3 molecules have a strong tendency to adopt horizontal orientations relative to the substrate. As the transition dipole moment of MC-C3T3 lies nearly parallel to the macrocycle plane (yet with a slight tilt, Figure 7b), the rigid disklike geometry of the π-conjugated macrocycle is anticipated to induce such a spontaneous anisotropic molecular orientation. A similar tendency to adopt horizontal dipole orientations with high Θ values of over 80% has been observed for linear or planarshaped organic fluorophores [25a,26] and TADF emitters, [25b,27] as well as spherically shaped organometallic phosphors, [28] in the corresponding vacuum-deposited doped films. The EL emitted from the horizontally oriented dipoles preferentially propagates in the vertical direction relative to the substrate, thus enhancing η out and the resulting EL efficiency of the OLEDs. In contrast, for CTC, Θ was much lower (58%), suggesting that CTC adopted preferential isotropic (or rather slightly vertical) dipole orientations in the doped film. This propensity can be attributed to the conformational flexibility and diversity of the acyclic CTC, which causes frequent fragmental rotation, rearrangement, and randomization on the substrate surface during the vacuum-deposition process.
To gain insight into the origin of the molecular orientations, we further carried out all-atom molecular dynamic (MD) simulations of the vacuum deposition of MC-C3T3 and CTC as a guest dopant (10 wt%) with CBP as the host (90 wt%). Figure 8a,b shows the MD snapshots representing the doped thin films deposited on hydroxylated SiO 2 substrates, which were generated by successively releasing individual molecules of either CBP or MC-C3T3 (or CTC) above the surface at 0.2 ns intervals, at 300 K (see the Supporting Information for details). The resultant simulated systems contained 1680 CBP and 60 MC-C3T3 molecules for the 10 wt%-MC-C3T3:CBP blend and 1600 CBP and 100 CTC molecules for the 10 wt%-CTC:CBP blend. It was found that MC-C3T3 and CTC were molecularly dispersed within the CBP host matrix without forming clusters or aggregates. To quantitatively evaluate the molecular orientation of the Figure 7. a) Angle-dependent p-polarized PL spectra of MC-C3T3 (red circles) and CTC (black circles) in the 10 wt%-emitter:CBP doped films, measured at the respective λ PL wavelengths, and schematic illustration of the measurement setup. The black and gray lines represent the simulated patterns with the dipole orientation ratios (Θ) of 100% (fully horizontal dipole orientation) and 67% (isotropic random dipole orientation), respectively. b) Simulated molecular structures and transition dipole moments for MC-C3T3 (left) and CTC (right), calculated at the PBE0/ 6-31 G(d) level. For CTC, the most energetically favorable conformer is depicted.
www.advancedsciencenews.com www.adpr-journal.com dopants, we here adopted the orientation-order parameter (S) given by the following equation where θ denotes the angle between the normal direction of the substrate surface (z-axis) and the vector of the molecule defined by connecting two atoms within each molecule (shown in Figure 8a,b), and the angled bracket represents the ensemble average. Accordingly, S ¼ À0.5 when the molecules are completely parallel to the substrate surface, S ¼ 0 when they are randomly oriented, and S ¼ 1 when they are completely perpendicular to the substrate surface. Intriguingly, as shown in Figure 8c, in the simulated doped film, cyclic MC-C3T3 exhibited a strong tendency to adopt a horizontal molecular orientation (S ¼ À0.34), which is reasonably consistent with the foregoing angle-dependent p-polarized PL results (S ¼ À0.46) (According to Equation (3), S for the doped films of MC-C3T3 and CTC were calculated to be À0.46 and 0.26, respectively, from the measured Θ values of 89% and 58%; A short movie of MD simulation demonstrating the co-deposition of MC-C3T3 and CBP can be found in the Supporting Information***). The slight underestimation of S in the MD simulation may originate from the short annealing time, which is much shorter than the timescale of the actual vacuum-deposition process. In contrast, acyclic CTC exhibited a relatively random molecular orientation (S ¼ À0.14, Figure 8d), presumably because of its flexible and distorted geometric structure. The large variation in the S values among the 100 CTC molecules may reflect the intrinsic characteristics of CTC, with no preferred orientation. Finally, we highlight that all-atom MD simulations can be effectively used to mimic the codeposition of EML, [29] thereby facilitating understanding of the PL and EL properties to a considerable extent.

Conclusions
A new TADF π-conjugated macrocycle, MC-C3T3, was successfully synthesized by combining three electron-donating 5-(N-carbazolyl)-phenylen-1,3-diyl units and three electronaccepting 6-phenyl-1,3,5-triazin-2,4-diyl units. The key electronic structures and photophysical properties of MC-C3T3 were determined by comparison with the acyclic homolog CTC from both computational and experimental perspectives. It was found that cyclic MC-C3T3 far outperformed acyclic CTC in OLEDs, where the former afforded %4-times higher EL efficiency, due to its distinct TADF ability. Another unique feature of MC-C3T3 is the preferential horizontal molecular orientation in thin films, which enhances the light outcoupling efficiency of OLEDs. This set of attractive features is expected to make the π-conjugated macrocycles new useful candidates for a wide range of optoelectronic applications. We envision that the modular design approach for synthesizing D-A-structured π-conjugated macrocycles will encourage further exploration of this class of unprecedented functional materials as not only different shapes and sizes become accessible, but also other functional building units can be incorporated.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. www.advancedsciencenews.com www.adpr-journal.com