# Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence Alternating Donor–Acceptor Copolymers for Nondoped Solution-Processable OLEDs

George K Belousov, Aliaksei A Vaitusionak, Irina V Vasilenko, Melika Ghasemi, Viktorija Andruleviciene, Aliaksei Ivanchanka, Dmytro Volyniuk, Hern Kim, Juozas Vidas Grazulevicius, Sergei V Kostjuk

► To cite this version:

George K Belousov, Aliaksei A Vaitusionak, Irina V Vasilenko, Melika Ghasemi, Viktorija Andruleviciene, et al. Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence Alternating Donor–Acceptor Copolymers for Nondoped Solution-Processable OLEDs. Macromolecules, 2023, 56 (7), pp. 2686–2699. 10.1021/acs.macromol.2c02582. http://dx.doi.org/10.1021/acs.macromol.2c02582

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# Through-Space Charge-Transfer Thermally Activated Delayed Fluorescence Alternating Donor–Acceptor Copolymers for Nondoped Solution-Processable OLEDs

George K. Belousov, Aliaksei A. Vaitusionak, Irina V. Vasilenko, Melika Ghasemi, Viktorija Andruleviciene, Aliaksei Ivanchanka, Dmytro Volyniuk, Hern Kim, Juozas Vidas Grazulevicius,* and Sergei V. Kostjuk*

<figure><img src="image_2.png" /></figure>

**ABSTRACT:** A series of donor-acceptor alternating copolymers with $M_n \le 12,000$ g mol$^{-1}$ and moderate polydispersity ($D = 1.5-2.5$) were synthesized via RAFT copolymerization of styrene-based donor monomers and maleimide-type acceptor monomers in the presence of S-dodecyl-S'-($\alpha,\alpha'$-dimethyl-$\alpha''$-acetic acid)trithiocarbonate as a chain transfer agent. It was shown that synthesized alternating copolymers were characterized by high thermal stability ($T_{ds} = 375-465$ °C), and most of them exhibited exciplex emission via the through-space charge-transfer (TSCT) mechanism and displayed aggregation-induced emission. The color of the emission of synthesized copolymers largely depended on the nature of the acceptor monomer and could be changed from blue to orange depending on the copolymer structure. These copolymers in solid state demonstrated delayed fluorescence in the range between 0.35 and 6.2 $\mu$s and moderate photoluminescence quantum yield (2.12-6.32%) in air. Multicolor host-free solution-processable TADF OLEDs using synthesized alternating copolymers as emitters were fabricated, which showed good stability of electroluminescence spectra at different voltages. The highest external quantum efficiency of 7.84% was reached for the green emitting device.

## INTRODUCTION

Organic light-emitting diodes (OLEDs) have formed a tremendous area of research in chemistry and physics$^{1-3}$ because they are widely used for the fabrication of flat panel displays and different lighting devices.$^{4-6}$ OLED is a solid-state semiconductor device that is 100 to 500 nm thick and consists of several organic layers such as the hole injection layer, hole- and electron transport layers, and most importantly the emissive layer, which are sandwiched all together between two electrodes and deposited on a substrate.$^7$ Under external voltage, holes and electrons are injected from the anode and cathode and recombine within the emissive layer to produce photons. Therefore, emitters largely determine the performance of devices, and their selection is critical for the fabrication of highly efficient OLEDs.$^{3,8}$

Thermally activated delayed fluorescent (TADF) emitters, following prompt fluorescent and phosphorescent ones, represent the third generation of electroluminescent materials for OLEDs. Fluorescent emitters can only take advantages of singlet excitons for luminescence with a maximal internal quantum efficiency (IQE) of roughly 25%, whereas phosphorescent OLEDs have the capability to reach IQE of 100% due to the presence of heavy metals that allow the harvesting of both
<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Synthesis of (a) Donor and Acceptor Monomers and (b) Their RAFT-Mediated Alternating Copolymerization<sup>a</sup></figcaption></figure>

<sup>a</sup>The 3,6-dimethoxycarbazole fragment is located in the meta-position of styrene.

singlet and triplet excitons for phosphorescence through significant spin-orbit coupling.<sup>9-11</sup> TADF materials that contain no heavy metals are capable of harvesting both triplet and singlet excitons. The usage of heavy metal complexes leads to the increase in cost and decrease in the long-term stability of devices.

The application of organic TADF materials in OLEDs has led to the great progress since the discovery of a series of carbazolyl dicyanobenzene derivatives as promising emitters.<sup>12,13</sup> Since then, many TADF organic emitters covering the deep-blue to red emission spectra have been synthesized and used for the fabrication of efficient OLEDs.<sup>11,12</sup> However, their fabrication relies on the expensive vacuum deposition technique. In this respect, TADF polymers, which are suitable for simple, low-cost, and easily scalable solution (wet) processing methods such as spin-coating, ink-jetting, etc., have attracted significant interest.<sup>14-16</sup> In addition, polymeric emitters are characterized by much higher morphological and thermal stability than organic TADF materials that allows devices based on them to be exploited longer.

Among TADF polymers, three main categories are reported including dendrimers,<sup>14,17-19</sup> linear polymers with TADF units in a main chain,<sup>20-24</sup> and polymers with TADF units in the side chain.<sup>25-29</sup> Dendrimers similarly to small molecules are characterized by a well-defined structure, on the one hand, and due to the high molecular weight can be solution-processed into OLED devices displaying an external quantum efficiency (EQE) up to 24%, on the other hand. One serious limitation for the application of dendrimers in the construction of OLEDs lies in the high complicity of their synthesis.<sup>14-16</sup> Another strategy to construct TADF polymers is to connect the TADF moiety into the main chain, resulting in alternating

donor-acceptor (D-A type) or donor-spacer-acceptor (interval B-T-B type) conjugated as well as nonconjugated polymers. OLEDs fabricated using these TADF polymers for the preparation of emitting layers via solution processing methods showed promising EQE values of 23.5%. However, the synthesis of high-molecular-weight main-chain TADF polymers is difficult because of the steric hindrance. In addition, these polymers are characterized by limited solubility.<sup>14-16,20-24</sup> From this point of view, the design and synthesis of the side-chain TADF polymers, in which TADF units are incorporated into the polymer as pendant groups, are the easiest. One of the approaches toward such TADF polymers is the preparation of TADF monomers from the known TADF low-molecular-weight compounds followed by their copolymerization with donor monomers. Although OLEDs with deep-blue to yellow emission with good EQE (from 3 to 20%) were successfully fabricated by solution methods, these approaches suffer from a complicated multistep synthesis of TADF monomers.<sup>14,16,17,25,30-33</sup> Therefore, much attention during recent years has been focused on side-chain TADF polymers with through-space charge-transfer (TSCT), which consist of a nonconjugated polymer chain with spatial donor-acceptor architecture. The emission colors of OLEDs based on TSCT polymers cover the whole visible range reaching EQE values as high as 19%.<sup>34-38</sup> Despite the promising results, the simplification of the synthetic methods to TSCT polymers is strongly required for cost-efficient fabrication of OLEDs. On the other hand, with rare exceptions,<sup>38</sup> these polymers were synthesized using a conventional radical polymerization technique that results in high polydispersity and significant structural heterogeneity from one polymer chain to another or by ring-opening
metathesis polymerization leading to the contamination of polymers by heavy metal.<sup>39-41</sup> Aiming at overcoming the above-mentioned limitations of TADF polymers, we proposed a new synthetic strategy toward TSCT polymers consisting of the preparation of alternating donor-acceptor copolymers with a nonconjugated structure, where donor (D) and acceptor (A) side groups will be located close enough to enable through-space π-π interaction between them. It is known that perfectly alternating copolymers could be easily obtained through the copolymerization of styrenes as donor monomers with maleic anhydride or N-substituted maleimides as acceptor monomers as a result of the specific interaction between these monomers leading to the formation of a charge-transfer complexes.<sup>42-46</sup> Alternating copolymerization of styrene-maleimide pairs could be performed in a living fashion by exploiting the reversible addition-fragmentation chain-transfer (RAFT) mechanism, giving access to well-defined copolymers without any contamination by traces of metals.<sup>44-48</sup>

In this work, we synthesized four donor-containing styrene-
type monomers (D1-D4) and four new acceptor maleimide-
type monomers (A1-A4) in a cost-efficient approach using
scalable one- or two-step procedures (Scheme 1a). These
monomers were copolymerized by means of RAFT polymer-
ization, affording 16 new alternating copolymers (Scheme 1b),
which are characterized by high thermal stability. Most of these
alternating copolymers exhibited exciplex emission via the
through-space charge-transfer (TSCT) mechanism as well as
displayed aggregation-induced emission enhancement. The
color of the emission of the synthesized copolymers largely
depended on the nature of acceptor monomer and could be
changed from blue to orange depending on the copolymer
structure. Furthermore, the alternating copolymers in solid
state demonstrated delayed fluorescence in the range between
0.35 and 6.2 µs, making them promising materials for emitting
layers of multicolor solution-processable electroluminescent
devices. An EQE of 7.84% was reached for the green host-free
device based on the D4A2 copolymer.

## EXPERIMENTAL SECTION

**Materials.** Cyclohexanone (Sigma-Aldrich, ≥99%) was dried overnight with CaH₂ and then distilled from CaH₂ under reduced pressure. THF (Sigma-Aldrich, ≥99%) was treated with KOH and distilled twice from Na under an inert atmosphere. Toluene (Sigma-Aldrich, ≥99%) was refluxed with Na and distilled twice from Na under an inert atmosphere. Acetic anhydride was refluxed with AcONa (Sigma-Aldrich, ≥99%) and then distilled from AcONa under reduced pressure. S-Dodecyl-S'-(*α*,*α*'-dimethyl-*α*''-acetic acid)-trithiocarbonate (DDMAT) was synthesized according to the published procedure.<sup>49</sup> 2,2'-Azobisisobutyronitrile (AIBN) (Sigma-Aldrich, ≥98%) was recrystallized from ethanol. CDCl₃ (Euriso-top), CHCl₃ (Sigma-Aldrich, 99.5%), methanol (Sigma-Aldrich, 99.9%), and ethanol (Sigma-Aldrich, 96%) were used as received.

**Instrumentation.** ¹H (400 MHz) NMR spectra of the synthesized monomers were recorded in CDCl₃ at 25 °C on a Bruker Avance III. Mass spectra were obtained by electron impact mass spectrometry (EI-MS) on a GSMS-QP2010 Plus. Size exclusion chromatography (SEC) was performed on an Ultimate 3000 Thermo Scientific apparatus with an Agilent PLgel 5 µm MIXED-C (300 × 7.5 mm) column and one precolumn (PLgel 5 µm guard 50 × 7.5 mm) thermostated at 30 °C. The detection was achieved by a differential refractometer (thermostated at 35 °C). Tetrahydrofuran (THF) was eluted at a flow rate of 1.0 mL min⁻¹. The calculation of molecular weights and polydispersity was carried out using polystyrene standards (Polymer Labs, Germany). Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements

were carried out using a Netzsch STA (Simultaneous thermal
analysis) 449 F3 device at a heating rate of 20 °C min⁻¹ under
nitrogen flow. UV-vis and photoluminescence spectra of dilute
solutions were recorded with a PerkinElmer Lambda 25 UV-vis
spectrometer and an Edinburgh Instruments FLS980 spectrometer,
respectively. For these measurements, the dilute solutions of the
investigated compounds were prepared by dissolving them in a
spectral grade THF at 10⁻⁴ M concentration. An Edinburgh
Instruments FLS980 spectrometer and a PicoQuant LDH-DC-375
laser (wavelength 374 nm) as the excitation source were used for
recording photoluminescence (PL) decay curves. Photoluminescence
quantum yields (PLQYs) of THF solutions were recorded by using an
integrated sphere (inner diameter of 120 mm). Cyclic voltammetry
(CV) measurements were performed using a micro-Autolab III
(Metrohm Autolab) potentiostat-galvanostat. A three-electrode cell
equipped with a glassy carbon working electrode, an Ag/Ag⁺ reference
electrode, and a Pt wire counter electrode was employed. The
measurements were performed for anhydrous dichloromethane with
0.1 M of tetrabutylammonium hexafluorophosphate (Bu₄N(PF₆)) as
the supporting electrolyte under a nitrogen atmosphere at a scan rate
of 0.1 V s⁻¹. The measurements were calibrated using the ferrocene/
ferrocenium (FC) system as an internal standard.

**Device Fabrication.** Electroluminescent devices were fabricated by wet and vacuum techniques. Functional materials poly(2,3-dihydrothieno-1,4-dioxin)-poly(styrenesulfonate) (PEDOT:PSS), diphenyl-4-triphenylsilyl-phenylphosphineoxide (TSP01), and 2,2',2"-(1,3,5-benzinetriyl)-tris(1-phenyl-1-*H*-benzimi-dazole) (TPBi) were purchased from Sigma Aldrich and used as received. Tris(4-formylphenyl)amine tri-*N*-4-vinylbenzyl-*N*-phenylhydrazone (VM35) was synthesized according to a previously published procedure.⁵⁰ Devices were fabricated using glass substrates with prepatterned indium-tin-oxide (ITO) anodes (with a sheet resistance of 15 Ω/sq) from Ossila (with a possibility to deposit four OLEDs on a substrate). The substrates were cleaned in different solvents and in a UV ozone cleaner from Ossila. PEDOT:PSS layers were spin-coated in the air at 3000 rpm for 3 min and dried at 120° for 40 min. Layers of VM35 were spin-coated at 3000 rpm in a glove box from chlorobenzene solution (8 mg/mL) in an inert atmosphere. The VM35 layers were dried at 100 °C for 30 min followed by annealing at 200 °C for 45 min to achieve cross-linking.⁵⁰ As doping-free light-emitting layers, D1A3, D3A2, and D4A2 were spin-coated at 3000 rpm from tetrahydrofuran (4 mg/mL) and dried at 70 °C for 30 min. The device structures were finalized by thermal deposition of TPBi/LiF/Al at a vacuum ca. 2 × 10⁻⁶ mbar. The certificated photodiode PH100-Si-HA-D0 in combination with the PC-Based Power and Energy Monitor 11S-LINK (from STANDA), Keithley 2400C source meter, and Avantes AvaSpec-2048XL spectrometer was used to record electroluminescent characteristics.

**Synthesis of Monomers.** 3,6-Dimethoxycarbazole-containing monomers D1 and D2 were synthesized according to the procedure described earlier.<sup>47</sup> The donor styrene-type monomers D3 and D4 were synthesized using the modified method applied earlier for the synthesis of carbazole-substituted styrene and perfluorostyrene.<sup>51</sup> 10*H*-Phenoxazine (4.51 g, 24.6 mmol) (or 9,9-dimethylacridan (5.15 g, 24.6 mmol)), tris-(*dibenzyldieneacetone*) dipalladium (0) (*Pd*(*dba*)<sub>3</sub>, 0.6 g, 0.66 mmol), 1 M (*t*-Bu)<sub>3</sub>P in toluene (0.66 mL, 0.66 mmol), 4-bromostyrene (3 g, 16.38 mmol), and sodium *tert*-butoxide (9.42 g, 96.3 mmol) were dissolved under argon in 135 mL of dry toluene and stirred for 10 min at room temperature (preformation of the catalyst). The reaction mixture was heated at 90 °C for 4 h. After cooling, the reaction mixture was diluted with dichloromethane, and the organic phase was washed with water and brine. After being dried over Na<sub>2</sub>SO<sub>4</sub> and filtered, the solvent was removed, and the residue was purified by column chromatography using the mixture of hexane and dichloromethane in a volume ratio of 6:1 as the eluent. The target products were recrystallized from hexane. **10-(*-4*-Vinylphenyl)-10*H*-phenoxazine (D3).** The yield of white crystals was 80%; m.p. = 162 °C. MS (EI, 70 V), *m/z* (%): 285 (M⁺, 100).
<figure><img src="image_4.png" /><figcaption>**Figure 1.** (a) First-order plots and (b) $M_n$(SEC) and *D* vs conversion dependencies for D1 and A1 copolymerization and D1 homopolymerization using the AIBN/DDMAT initiating system at 70 °C in cyclohexanone at [M]/[I] = 40. The straight line corresponds to the theoretical $M_n$. Polymerization conditions: [AIBN] = 5 mM, [DDMAT] = 15 mM; for D1A1: [M] = [D1] + [A1] = 1 M ([D1] = [A1] = 0.5 M), for poly(D1): [M] = [D1] = 1 M.</figcaption></figure>

¹H NMR (400 MHz, CDCl₃, *δ*, ppm): 5.36 (d, 1H, CH = CH2), 5.83 (d, 1H, CH = CH2), 5.95 (d, 2H, Ar), 6.55–6.66 (m, 4H, Ar), 6.69 (d, 2H, Ar), 6.79 (dd, 1H, CH = CH2), 7.30 (d, 2H, Ar), 7.62 (d, 2H, Ar) (Figure S1a).

9,9-Dimethyl-10-(4-vinylphenyl)-9,10-dihydroacridan (D4). The yield of white crystals was 73%; m.p. = 130 °C. MS (EI, 70 V), *m/z* (%): 296 ([M−CH3]⁺, 100).

¹H NMR (400 MHz, CDCl₃, *δ*, ppm): 1.69 (s, 6H, CH3), 5.37 (d, 1H, CH = CH2), 5.86 (d, 1H, CH = CH2), 6.30 (d, 2H, Ar), 6.84 (dd, 1H, CH=CH2), 6.89–7.00 (m, 4H, Ar), 6.30 (d, 2H, Ar), 7.45 (d, 2H, Ar), 7.66 (d, 2H, Ar) (Figure S1b).

The acceptor maleimide-type monomers (A1-4) were synthesized in two steps by acylation of the corresponding substituted aniline by maleic anhydride with the formation of the corresponding maleeanilic acid followed by its cyclization. Maleic anhydride (1 g, 10.2 mmol) was dissolved in 10 mL of dry diethyl ester. A solution of 10.2 mmol of 4-aminobenzophenone (2.01 g) (or 2 aminobenzophenone (2.01 g) or 4-aminobenzonitrile (1.20 g) or 3 aminobenzonitrile (1.20 g)) in 50 mL of diethyl ether was added dropwise under argon to the maleic anhydride solution; after that, it was heated to boiling and stirred for 8 h, with the amount of product precipitated from the solution visually monitored. The obtained intermediate product was then filtered off, washed with pure diethyl ether, and dried. Then, 40 mL of acetic anhydride and 0.36 g of sodium acetate were added under argon to the dried intermediate. The resulting solution was stirred at 80 °C for 5 h. The cooled solution was precipitated into distilled water, centrifuged, dried from water, and then recrystallized from isopropyl alcohol. As a result, the final products were obtained.

1-(4-Benzoylphenyl)pyrrole-2,5-dione (A1). 2.1 g of yellow crystals (74% yield); m.p. = 156 °C. MS (EI, 70 V), *m/z* (%): 277 (M⁺, 100).

¹H NMR (400 MHz, CDCl3, *δ*, ppm): 6.91 (s, 2H, CH = CH), 7.50 (t, 2H, Ar), 7.55 (d, 2H, Ar), 7.61 (t, 1H, Ar), 7.81 (d, 2H, Ar), 7.92 (t, 2H, Ar) (Figure S1c).

1-(2-Benzoylphenyl)pyrrole-2,5-dione (A2). 2.0 g of yellow crystals (71% yield); m.p. = 172 °C. MS (EI, 70 V), *m/z* (%): 277 (M⁺, 100).

¹H NMR (400 MHz, CDCl3, *δ*, ppm): 6.65 (s, 2H, CH = CH), 7.38 (d, 1H, Ar), 7.42 (t, 2H, Ar), 7.49–7.57 (m, 2H, Ar), 7.64 (d, 1H, Ar), 7.68 (t, 2H, Ar), 7.73 (d, 2H, Ar) (Figure S1d).

1-(4-Cyanophenyl)pyrrole-2,5-dione (A3). 1.4 g of lemon-colored crystals (69% yield); m.p. = 140 °C. MS (EI, 70 V), *m/z* (%): 198 (M⁺, 100).

¹H NMR (400 MHz, CDCl3, *δ*, ppm): 6.91 (s, 2H, CH = CH), 7.60 (dt, 2H, Ar), 7.76 (dt, 2H, Ar) (Figure S1e).

1-(3-Cyanophenyl)pyrrole-2,5-dione (A4). 1.4 g of lemon-colored crystals (69% yield); m.p. = 125 °C. MS (EI, 70 V), *m/z* (%): 198 (M⁺, 100).

¹H NMR (400 MHz, CDCl3, *δ*, ppm): 6.91 (s, 2H, CH = CH), 7.59 (t, 1H, Ar), 7.65 (d, 1H, Ar), 7.69 (d, 1H, Ar), 7.75 (t, 1H, Ar) (Figure S1f).

**Copolymerization Procedures.** Copolymerization was carried out under a dry argon atmosphere in a Schlenk tube. Liquid reagents were transferred to the reactor via dry syringes against a continuous argon flow. In a typical polymerization experiment, 115 µL of the solution of DDMAT in cyclohexanone (30 mM) and 115 µL of the solution of AIBN in cyclohexanone (10 mM) were sequentially added to a crystalline mixture of D (115 mmol) and A (115 mmol). After three freeze-pump-thaw cycles, the reaction was started by immersing the Schlenk tube into an oil bath preheated to 70 °C. After a predetermined time, the whole solution was withdrawn and poured into an excess of ethanol. The precipitated polymers were separated from the solution by centrifugation and then were dried in a vacuum at 50 °C. Before analyses, copolymers were reprecipitated twice from CHCl₃ into methanol. Monomer conversions were determined gravimetrically. To investigate the kinetic and $M_n$(SEC)-conversion dependencies of the copolymerization process, an additional experiment was carried out with the withdrawing of aliquots of the reaction solution at a predetermined time.

## RESULTS AND DISCUSSIONS

**Synthesis of Monomers and Copolymers.** In our previous work, we showed that well-defined polymers could be successfully prepared from styrene bearing metoxy-substituted carbazoles (D1 and D2 in Scheme 1) via RAFT-mediated radical polymerization using S-dodecyl-S'-(*a*,*a'*-dimethyl-*a'*-acetic acid)trithiocarbonate (DDMAT) as a chain-transfer agent. These polymers were then successfully used as hole transporting materials in TADF-based OLEDs and hosts for phosphorescent solution-processed OLEDs.⁴⁷ Aiming at synthesizing polymeric TADF emitters, we focused on the preparation of the alternating copolymers of four styrene-based donor monomers (D1–D4, Scheme 1) with four acceptor maleimide-based monomers (A1–A4, Scheme 1). The selection of corresponding donor (3,6-dimethoxycarbazole, 9,9-dimethylacridane, phenoxazine) and acceptor moieties (benzophenone, benzonitrile) was based on the high efficiency of organic TADF emitters bearing these fragments.⁵²⁻⁵⁵ Donor monomers were synthesized in one stage via palladium catalyzed condensation of 1-bromo-4-vinyl-
<figure><img src="image_5.png" /><figcaption>Figure 2. ¹H NMR spectra of (a) D1A1 synthesized with equimolar ([D1]/[A1] = 50/50) and (b) nonequimolar ([D1]/[A1] = 60/40) comonomer ratio using the DDMAT/AIBN initiating system at 70 °C in cyclohexanone. The molar ratio of donor and acceptor units in copolymer was calculated as follows: (−D1−)/(−A1−) = 3·I(MeO)/(2·I(Ar) − (5I(MeO)/3)) = 1/1, where I(MeO) is the integral intensity of protons of the methoxy group and I(Ar) is the integral intensity of aromatic protons.</figcaption></figure>

benzene (D1, D3, D4) or 1-bromo-3-vinylbenzene (D2) with
corresponding donor molecules (Scheme 1). Acceptor
maleimide-type monomers (A1–A4) were obtained in two
steps by acylation of the corresponding substituted aniline by
maleic anhydride with the formation of the corresponding
maleanic acid followed by its cyclization (Scheme 1). All
acceptor maleimide-type (A1–A4) as well as donor styrene-
based D3 monomers were synthesized for the first time in
relatively high yields. It should be noted that the approach for
the synthesis of acceptor-type monomers developed in the
present work is a simple and atom efficient synthetic method,⁵⁶
which is suitable for fast access to a great variety of acceptor
monomers.

RAFT copolymerization using DDMAT as a chain-transfer agent and AIBN as initiator was chosen as a method to obtain donor–acceptor alternating copolymers due to the high efficiency of this system in controlling the radical copolymerization of a wide range of monomers.⁴⁸,⁵⁷ In addition, this technique allowed us to prepare well-defined homopolymers from D1 and D2 with controlled molecular weight (up to M<sub>n</sub> = 20,000 g mol<sup>-1</sup>) and relatively low polydispersity (D < 1.5).⁴⁷ The copolymerization of synthesized donor and acceptor monomers proceeded at a much faster rate than the homopolymerization of the corresponding donor monomer, which is consistent with the concept of charge-transfer-complex (CTC) formation.³⁰,⁴⁴,⁵⁸,⁵⁹ In fact, the rate of copolymerization of D1 and A1 is about 12 times higher than that of homopolymerization of D1: k<sub>p, app</sub> = 6.16 and 0.51 h<sup>-1</sup> for copolymerization and homopolymerization, respectively (Figure 1a).

As shown in Figure 1a, the first-order plot is linear, indicating that the concentration of active species remains constant during copolymerization similarly to homopolymerization of D1. The number-average molecular weights of the obtained copolymers increased in direct proportion to monomer conversion, whereas experimental values of the molecular weight slightly deviated from the theoretical line (Figure 1b). This underestimation of M<sub>n</sub> by SEC calibrated against polystyrene standards for structurally similar carbazole-substituted styrenes was validated by performing SEC with a triple-detection system. This phenomenon is explained by the difference in the hydrodynamic volumes of the analyzed polymer and polystyrene with the same molecular weight.⁵¹

These data indicate that RAFT copolymerization of D1 with A1, similarly to homopolymerization of D1, proceeds in a controlled fashion, providing alternating copolymers with controlled molecular weight in a range between M<sub>n</sub> = 3000 and 12,000 g·mol<sup>-1</sup> and moderate polydispersity (D = 1.39–2.17). It should be mentioned here that the polydispersity of alternating copolymer D1A1 is higher than that for poly(D1), pointing out that some side reactions can operated in the former case. This observation could be connected with the much higher stability of radicals generated upon the addition of CTC to growing macroradicals. An increase in the thermodynamic stability of the growing radical leads to less efficient interaction with the RAFT agent (DDMAT) that, in turn, increases the possibility of participation of the radical in side reactions such as, for example, bimolecular termination.

Aiming at confirming the formation of alternating copolymer
during the copolymerization of D1 and A1, we investigated the
obtained copolymer (D1A1) by ¹H NMR spectroscopy. The
spectrum of the copolymer with a conversion close to 100%
showed that the numbers of donor and acceptor units in the
polymer chain are equal (Figure 2a). This is already an indirect
confirmation of the alternating structure because, as is known
from the literature,⁶⁰,⁶¹ N-substituted maleimides have a low
tendency for homopolymerization by the radical mechanism.
However, to further confirm the alternating structure of the
copolymers, an additional experiment was carried out with the
copolymerization of D1 and A1 at a molar ratio of 60/40
according to the methodology reported elsewhere.⁴⁶ The
process was brought to an incomplete conversion (60%), at
which the maleimide monomer would not be completely
consumed in the reaction system. The spectrum of the
resulting polymer also showed the presence in its structure of
both monomer units in equimolar ratio (Figure 2b). This
observation proves that copolymerization of D1 with A1 is
much faster than homopolymerization of D1 as well as the
alternating structure of the resulting copolymer.

Following the developed methodology, 16 alternating copolymers from four donor and four acceptor monomers were then synthesized (Table 1).

As can be seen from Table 1, the copolymers were
synthesized in quantitative or close to quantitative yields.
The number-average molecular weight of most of obtained
alternating copolymers is close to the theoretical value, and
<table><caption>**Table 1.** RAFT Copolymerization of D1-D4 with A1-A4 Using the AIBN/DDMAT Initiating System<sup>a</sup></caption><thead><tr><th>copolymer</th><th>time (h)</th><th>conv. (%)</th><th>M<sub>n</sub>(theor)<sup>b</sup>, (g·mol<sup>-1</sup>)</th><th>M<sub>n</sub>(SEC), (g·mol<sup>-1</sup>)</th><th>D</th><th>T<sub>d</sub> (°C)</th></tr></thead><tbody><tr><td>D1A1</td><td>4</td><td>100</td><td>12,316</td><td>9577</td><td>1.75</td><td>409</td></tr><tr><td>D1A2</td><td>4</td><td>100</td><td>12,316</td><td>9885</td><td>1.73</td><td>402</td></tr><tr><td>D1A3</td><td>4</td><td>100</td><td>10,734</td><td>3195</td><td>2.99</td><td>394</td></tr><tr><td>D1A4</td><td>4</td><td>100</td><td>10,734</td><td>8330</td><td>1.78</td><td>375</td></tr><tr><td>D2A1</td><td>19.5</td><td>89</td><td>10,961</td><td>8275</td><td>1.95</td><td>421</td></tr><tr><td>D2A2</td><td>19.5</td><td>87</td><td>10,715</td><td>7481</td><td>1.91</td><td>417</td></tr><tr><td>D2A3</td><td>19.5</td><td>77</td><td>8265</td><td>3704</td><td>2.19</td><td>382</td></tr><tr><td>D2A4</td><td>19.5</td><td>89</td><td>9553</td><td>5197</td><td>2.33</td><td>392</td></tr><tr><td>D3A1</td><td>15</td><td>100</td><td>11,435</td><td>4266</td><td>2.75</td><td>405</td></tr><tr><td>D3A2</td><td>15</td><td>100</td><td>11,435</td><td>8945</td><td>1.65</td><td>404</td></tr><tr><td>D3A3</td><td>15</td><td>100</td><td>9853</td><td>887</td><td>2.29</td><td>399</td></tr><tr><td>D3A4</td><td>15</td><td>100</td><td>9853</td><td>4473</td><td>1.57</td><td>463</td></tr><tr><td>D4A1</td><td>3.5</td><td>85</td><td>10,163</td><td>6891</td><td>2.05</td><td>418</td></tr><tr><td>D4A2</td><td>3.5</td><td>100</td><td>11,957</td><td>7324</td><td>1.85</td><td><sup>d</sup></td></tr><tr><td>D4A3</td><td>3.5</td><td>100</td><td>10,375</td><td>1532</td><td>3.16</td><td>408</td></tr><tr><td>D4A4</td><td>3.5</td><td>100</td><td>10,375</td><td>5399</td><td>1.86</td><td>397</td></tr></tbody></table>

<sup>a</sup>Polymerization conditions: T = 70 °C; solvent: cyclohexanone; [M] = [D] + [A] = 1 M ([D] = [A] = 0.5 M), [AIBN] = 5 mM, [DDMAT] = 15 mM. <sup>b</sup>M<sub>n</sub>(theor) was calculated according to the following equation: M<sub>n</sub>(theor) = [D] + [A] / [AIBN] + [DDMAT] × (M<sub>D</sub> + M<sub>A</sub>) / 2 × conv. <sup>c</sup>5% Weight loss was determined by TGA in a nitrogen atmosphere at a heating rate of 20 °C/min. <sup>d</sup>Not determined.

they are characterized by moderate polydispersity (D = 1.5-2.3). In addition, SEC traces for most of synthesized alternating copolymers are symmetrical, confirming that copolymerizations proceeded in a controlled fashion (Figure S2). However, DDMAT-mediated copolymerization of all four donor monomers with the A3 acceptor monomer displayed poorer control over the reaction: the experimental values of M<sub>n</sub> are much lower in comparison with theoretical ones, and polydispersity is rather high (D = 2.2-3.2) (copolymers D1A3, D2A3, D3A3, and D4A3 in Table 1). Additionally, SEC traces of these four copolymers displayed tailing in the low-molecular-weight region indicating the operation of chain transfer reaction (Figure S2). This phenomenon could be tentatively attributed to the capability of additional delocalization of spin density of the growing radical terminated by maleimide unit A3 as a result of the presence of a cyano group in the para-position making this radical more stable (Figure S3a). As we mentioned above, the higher stability of the radical can lead to its less efficient interaction with DDMAT and, therefore, to increase the possibility of occurrence of side reactions. This assumption is confirmed by good control in the copolymerization of donor monomers with the A4 acceptor monomer, in which the CN group is located in the meta-position that makes its participation in delocalization impossible. Considering A1 and A2 acceptor monomers, in both cases, the benzoyl group is capable to stabilize the radical as a result of the delocalization of the spin density. However, steric factors make the benzoyl group not coplanar to the p-phenylenemaleimide fragment in the A1 structure as well as to the o-phenylenemaleimide fragment in the A2 structure (Figure S3b), which significantly reduces the π-π overlap, and therefore, the growing radical stabilization effect is minimized.<sup>62</sup>

The thermal properties of monomers (D1-D4 and A1-A4) and the synthesized alternating copolymers were briefly studied by DSC and TGA. The donor monomers showed endothermic melting signals in the first heating scan at 76 °C (D1), 62 °C (D2), 162 °C (D3), and 130 °C (D4), respectively, whereas the acceptor monomers showed these at 159 °C (A1), 172 °C (A2), 140 °C (A3), and 125 °C (A4), respectively (Table S1 and Figure S4). However, in the second heating scan, only glass transition temperatures were observed for the donor monomers, indicating that they were subjected to thermal self-polymerization during the first heating scan. Indeed, as we showed in our previous work,<sup>47</sup> SEC analysis of D1 and D2 after DSC measurement revealed the formation of a polymer with M<sub>n</sub> ≤ 3500 g·mol<sup>-1</sup> and high polydispersity (D ~6.0). This phenomenon was not detected for acceptor monomers A1-A4 because of the inability of radical homopolymerization of maleimide-type monomers<sup>45</sup> (Table S1 and Figure S4). The inability of formation of homopolymers from A1 to A4 is also confirmed by the much lower decomposition temperature for these monomers as compared with the donor counterparts (Table S1 and Figure S5a). The thermal stability of all the synthesized alternating copolymers was then estimated by TGA. According to data presented in Table 1 and Figure S5b, all copolymers showed high thermal stability: the values of temperature at which 5% loss of weight is observed (T<sub>d5</sub>) were in the range of 375-463 °C.

**Photophysical and Electrochemical Properties of the Solutions.** UV-vis absorption spectra of THF solutions of the monomers and alternating copolymers exhibit several bands in the region between 200 and 360 nm, which can be attributed to the lowest π-π* transition of the aromatic rings (Table 2 and Figure S6). Interestingly, the absorption spectrum of the mixture of donor-acceptor monomers roughly resembles the absorption spectra of the corresponding single monomers (Figure S7a). However, the UV-vis absorption spectra of the solutions of the alternating copolymers showed a visible hypochromic shift (12-17 nm) as compared to the spectra of the corresponding donor/acceptor monomer mixtures due to the decrease of the π-conjugation length<sup>39</sup> owing to the copolymer formation (Scheme 1b and Figure S7). The photoluminescence (PL) spectra of THF solutions of alternating copolymers in addition to the emission of donor and acceptor moieties exhibited exciplex emission peaks at the longer wavelengths (Figure 3a,b and Figures S8 and S9). This exciplex emission was not observed for THF solutions of equimolar mixtures of the corresponding donor and acceptor monomers (Figure 3a,b and Figure S9), indicating the necessity of the formation of a rigid copolymer structure for conformations to arise, at which donor and acceptor units are close enough for the through-space charge-transfer (TSCT).

Several interesting features were observed after the analysis of PL spectra of alternating copolymers. Most of the spectra contain several peaks, which often did not coincide with the emission of the corresponding donor and acceptor monomers (Figure 3b and Figure S8e,f). This phenomenon could be explained by the possibility of the simultaneous manifestation of several conformations in the solution, resulting in the different orientations of donor and acceptor monomer units that lead to the different mechanisms of photoluminescence. For example, when donor and acceptor monomer units are spatially distant from each other, the local excitation (LE) is more probable. LE from donor and acceptor monomer units in a polymer chain may be different from that of the
<table><caption>**Table 2.** Photophysical and Electrochemical Properties of the Solutions of the Synthesized Copolymers<sup>a</sup></caption><thead><tr><th>copolymers</th><th>λ<sup>a,b</sup><br>(nm)</th><th>λ<sup>a,c</sup><br>(nm)</th><th>τ/τ<sup>d</sup><br>(ns)</th><th>PLQY<sup>e</sup><br>(%)</th><th>E (abs)<sup>f</sup><br>(eV)</th><th>E<sub>e</sub><sup>g</sup><br>(eV)</th></tr></thead><tbody><tr><td>D1A1</td><td>238, 295,<br>354</td><td>396,<br>543</td><td>6/20</td><td>0.22</td><td>3.03</td><td>2.93</td></tr><tr><td>D1A2</td><td>234, 295,<br>355</td><td>399,<br>548</td><td>7/25</td><td>1.00</td><td>3.23</td><td>3.00</td></tr><tr><td>D1A3</td><td>232, 295,<br>354</td><td>388,<br>461</td><td>7/40</td><td>14.95</td><td>3.24</td><td>3.07</td></tr><tr><td>D1A4</td><td>224, 295,<br>354</td><td>388,<br>461</td><td>7/34</td><td>17.34</td><td>3.71</td><td>3.23</td></tr><tr><td>D2A1</td><td>241, 295,<br>355</td><td>387,<br>558</td><td>5/20</td><td>0.18</td><td>3.23</td><td>2.96</td></tr><tr><td>D2A2</td><td>239, 295,<br>354</td><td>390,<br>557</td><td>17/53</td><td>0.30</td><td>3.21</td><td>3.03</td></tr><tr><td>D2A3</td><td>232, 295,<br>354</td><td>390,<br>470</td><td>28/56</td><td>5.98</td><td>3.24</td><td>3.10</td></tr><tr><td>D2A4</td><td>235, 294,<br>354</td><td>389</td><td>6/22</td><td>7.62</td><td>3.66</td><td>3.26</td></tr><tr><td>D3A1</td><td>230, 241,<br>306</td><td>399,<br>615</td><td>5/34</td><td>0.08</td><td>2.67</td><td>2.70</td></tr><tr><td>D3A2</td><td>227, 240,<br>307</td><td>397,<br>625</td><td>5/13</td><td>0.10</td><td>2.59</td><td>2.77</td></tr><tr><td>D3A3</td><td>227, 306</td><td>398,<br>521</td><td>24/50</td><td>1.80</td><td>3.05</td><td>2.84</td></tr><tr><td>D3A4</td><td>227, 306</td><td>399,<br>523</td><td>15/32</td><td>1.89</td><td>3.05</td><td>3.00</td></tr><tr><td>D4A1</td><td>250</td><td>397,<br>561</td><td>9/74</td><td>0.50</td><td>3.77</td><td>2.90</td></tr><tr><td>D4A2</td><td>242, 272</td><td>397,<br>561</td><td>16/61</td><td>0.86</td><td>3.71</td><td>2.97</td></tr><tr><td>D4A3</td><td>228, 267</td><td>400,<br>467</td><td>5/38</td><td>3.15</td><td>3.62</td><td>3.04</td></tr><tr><td>D4A4</td><td>223, 272</td><td>400,<br>466</td><td>6/37</td><td>4.19</td><td>3.67</td><td>3.20</td></tr></tbody></table>

<sup>a</sup>Absorption and photoluminescence (λ<sub>exc</sub> = 325 nm) spectra were recorded for dilute THF solutions (10<sup>-4</sup> M) of the compounds at 298 K. <sup>b</sup>Wavelengths of absorption maxima. <sup>c</sup>Wavelengths of photoluminescence maxima measured under air at 298 K. <sup>d</sup>Lifetime of prompt (τ<sub>p</sub>) and delayed (τ<sub>d</sub>) fluorescence measured under air at 298 K. <sup>e</sup>Photoluminescence quantum yield (PLQY) measured by integrating sphere under nitrogen at 298 K. <sup>f</sup>Band gap determined from the absorption spectra of the films of the copolymers. <sup>g</sup>Band gap calculated as a difference between the HOMO of donor units (−D−) and LUMO of acceptor units (−A−) energies using the B3LYP/6-311++G(2df,2p)//B3LYP/6-31G(d,p) theory level.

corresponding monomers because of the decrease of the π-conjugation lengths of the monomer units in a polymer chain as compared to those of the monomer (Scheme 1b). In the case of a conformation in which D and A are close enough, through-space charge-transfer becomes possible. It is characterized by a lower energy emission in comparison with LE. Another important observation from PL spectra is that the maximum TSCT emission (the longest wavelength peak in the spectra) largely depends on the nature of the acceptor monomer unit. Indeed, λ<sub>PL</sub> is roughly similar for most of the synthesized alternating copolymers (with the exception of the D3 series) from the same acceptor monomer (Table 2, Figure S8, and Figure S10). A more precise tuning of the emission color could be achieved further by the change of the nature of the donor monomer (Figure S10). This finding opens the possibilities for the simple prediction of the emission color and, in turn, the design the corresponding alternating copolymers with the target color emission through the attachment of the designed acceptor fragment attached to maleimide function.

With the aim of confirming the charge-transfer character of the emission with the longest wavelength, the photoluminescence spectra of the solutions in the solvents of the different polarity were recorded (Figure 3c and Figures S11–S14). Although some spectra are rather complicated as a result of the simultaneous presence of different types of emissions, the general trend of the red shift of the emission peak of the longest wavelength with increasing polarity of the solvent is clearly observed (Figure 3c and Figures S11–S14). This red shift of PL peak is indicative of the emission through the TSCT process.<sup>38,47,63</sup> It should be noted that a limited number of solvents were tested here because of the insolubility of the synthesized alternating copolymers in the solvents of low polarity.

Most of the known luminescent materials typically display aggregation-caused quenching, whereas some TSCT materials have a special ability to exhibit aggregation-induced emission enhancement (AIEE).<sup>64,65</sup> Therefore, the possibility of displaying AIEE by the synthesized alternating copolymers was briefly investigated. PL spectra of the dispersions of the copolymers in THF-water mixtures with the different volume ratios of water (φ<sub>H2O</sub>) were recorded taking the same copolymer concentration (Figure S15). It should be noted that upon the addition of the certain amount of water into THF solutions of copolymers, the formation of disperse systems due to some precipitation of copolymers in the analyzed solutions caused a certain problem. In fact, scattering of the exciting radiation by the dispersed phase during the recording of the emission spectra led to two undesirable effects. First, the increase in the amount of scattered radiation led to the decrease in the radiation absorbed by the copolymer that resulted in the decrease of the intensity of the emission peak. Second, the presence of opalescence in the analyzed system increased the probability of the appearance of the second-order Rayleigh scattering peak, known as an artifact.<sup>66</sup> The wavelength of the artifact (λ<sub>ex</sub>) is twice the wavelength of the exciting radiation (λ<sub>ex</sub>) of 250 nm was used, which made λ<sub>ex</sub> equal to 500 nm and spoiled the visual image of the photoluminescence spectra of some copolymers. Despite the difficulties described above, the recorded PL spectra can be used for selection of copolymers exhibiting AIEE. However, for majority of the studied copolymers, the peak corresponding to TSCT emission for the solutions with φ<sub>H2O</sub> of 20% was less intense than that for the solution in THF. The further increase in φ<sub>H2O</sub> resulted in the gradual increase of intensity of the TSCT peak. These observations could be explained by two competing effects: AIEE and loss of intensity due to light scattering. Aiming at minimizing this negative effect, we then studied AIEE using a more polar solvent in which the synthesized copolymers showed better solubility. Indeed, the artifact peak was less pronounced or even absent when the measurements were performed using dimethylformamide (DMF): the intensity of emission gradually increased with the increase of the content of water in its mixture with DMF (Figure 3d and Figure S16). The data presented in Figure 3d and Figures S15 and S16 show that most of the synthesized alternating copolymers displayed AIEE.

Cyclic voltammetry (CV) was used for the estimation of HOMO and LUMO energies of the synthesized monomers as well as of the alternating copolymers. A number of the previously reported works show that HOMO and LUMO energy levels of TSCT copolymers correspond to the levels of
<figure><img src="image_6.png" /><figcaption>**Figure 3.** Photoluminescence spectra of the solutions of (a, b) donor (D1, D3) and acceptor (A3) monomers, their mixtures (D1/A2, D3/A3), and corresponding alternating copolymers D1A2 and D3A3. (c) Representative photoluminescence spectra of the D1A3 copolymer in the solvents of the different polarities. (d) Representative photoluminescence spectra of the dispersions of the D2A3 copolymer in the mixtures of DMF and water with increasing water amount ($\phi_{H2O}$).</figcaption></figure>

the monomers they were synthesized from.³⁶,³⁸,⁵¹ All donor and acceptor monomers demonstrated quasi-reversible oxidation (Figure S17a–h), whereas voltammograms of alternating copolymers showed a much more complex behavior and were characterized by poor reproducibility (Figure S17i–x). Therefore, in line with our previous work,⁴⁷ we used HOMO and LUMO energies of the monomers determined by CV (Table S2) to predict frontier orbital energies of the corresponding alternating copolymers. However, large differences in the values of energy gaps estimated from the absorption spectra and the HOMO and LUMO energies of the donor and acceptor monomers were observed (Table S2).

To understand the reasons of the observed discrepancy in the values of band gaps determined by the different approaches, quantum chemical calculations for monomers and alternating copolymers were performed using the density functional theory (DFT) employing the B3LYP/6-311++G(2df,2p)// B3LYP/6-31G(d,p) theory level. First of all, a good correlation between HOMO/LUMO energies determined by CV and obtained from DFT calculations was observed for donor and acceptor monomers (Table S2). However, the analysis of frontier orbitals showed that for acceptor monomers, LUMOs are localized exclusively on the maleimide moiety as a result of its conjugation with the acceptor unit. In strong contrast, in the structures imitating the acceptor monomer units in a copolymer (–A1– to –A4–, without a maleimide double bond), LUMOs are located mainly on the acceptor moieties (Figure 4). It should be noted that LUMO + 1 of acceptor monomers is similar to LUMO of

the same monomer unit in a copolymer (Figure 4 and Table S2).

<figure><img src="image_7.png" /><figcaption>**Figure 4.** (a–d) LUMO and (e–h) LUMO + 1 orbitals of acceptor monomers and (i–l) LUMO orbitals of the corresponding acceptor monomer units in a copolymer chain.</figcaption></figure>

Therefore, the calculation of the energy gaps of the copolymers as the difference between LUMOs of acceptor monomer and HOMOs of donor monomers is incorrect (Table S2). When an acceptor monomer unit is incorporated into a polymer chain, its frontier orbitals change dramatically in shape and energy (Figure 4). However, energy gaps for copolymers calculated using LUMO energies of –A1– to –A4– units or LUMO + 1 energies of acceptor monomers in conjunction with HOMO energies of donor monomers showed excellent correlation with $E_g$ obtained from absorption spectra (Table 2 and Table S3). Importantly, the shape and energy of HOMO orbital are similar for the donor monomer
before and after its incorporation in a polymer chain (Figure S18).

To estimate TADF properties of the solutions of the synthesized alternating copolymers in THF, photoluminescence decay curves were recorded under air at two different wavelengths of analysis (Figure S19), corresponding to the maxima of the photoluminescence spectra (Table 2; Figure 3a,b; and Figures S8 and S9). The emission at higher wavelength showed a longer lifetime than that analyzed at lower wavelength (Table 2), confirming the TSCT character of the former emission. However, the lifetime of delayed emission ($\tau_d$) did not exceed 100 ns probably because of the conformational flexibility of the polymer chain in the solution. Therefore, the films of the synthesized alternating copolymers were prepared and investigated.

**Photophysical Properties of the Solid Samples.** The UV-absorption spectra of the films of the alternating copolymers were much broader than those of the solutions (cf. Figures S6 and S20). PL spectra of the films of the synthesized copolymers showed predominantly only one broad peak corresponding to TSCT emission (Figure S21) in strong contrast to PL spectra of the solutions (Figure S8). In addition, the peaks in PL spectra of the films of the copolymers of all donor monomers with A1 and A2 acceptor monomers displayed a significant blue shift of ca. 24–80 nm in comparison to PL spectra of the solutions of same copolymers in THF. A smaller hypochromic shift between PL spectra of the films and solutions was detected for alternating copolymers of the donor monomers with acceptor monomers A3 and A4 (0–50 nm) (Tables 2 and 3). The largest difference between emission maxima of the films and of the solutions was observed for the copolymers of D3 with all acceptor monomers (30–80 nm).

Interestingly, the equimolar mixtures of donor and acceptor monomers give the exciplex emission in solid state (Figure

<table><caption>**Table 3.** Photophysical and Electrochemical Properties of the Films of the Synthesized Copolymers<sup>a</sup></caption><thead><tr><th>copolymers</th><th>λ<sub>abs</sub></th><th>λ<sup>a,b</sup> (nm)</th><th>λ<sup>a,c</sup> (nm)</th><th>τ<sup>d</sup> (ns)</th><th>τ<sup>d</sup> (μs)</th><th>PLQY<sup>e</sup> (%)</th></tr><tr><th></th><th></th><th></th><th>PL</th><th>p</th><th>d</th><th></th></tr></thead><tbody><tr><td>D1A1</td><td>355</td><td>519</td><td>103</td><td>0.79</td><td></td><td>2.12</td></tr><tr><td>D1A2</td><td>355</td><td>518</td><td>172</td><td>1.40</td><td></td><td>2.31</td></tr><tr><td>D1A3</td><td>355</td><td>473</td><td>49</td><td>0.35</td><td></td><td>3.30</td></tr><tr><td>D1A4</td><td>366</td><td>473</td><td>290</td><td>6.17</td><td></td><td>4.14</td></tr><tr><td>D2A1</td><td>355</td><td>515</td><td>158</td><td>1.38</td><td></td><td>1.80</td></tr><tr><td>D2A2</td><td>356</td><td>520</td><td>193</td><td>1.37</td><td></td><td>2.06</td></tr><tr><td>D2A3</td><td>355</td><td>467</td><td>196</td><td>2.28</td><td></td><td>3.07</td></tr><tr><td>D2A4</td><td>356</td><td>473</td><td>195</td><td>2.63</td><td></td><td>2.75</td></tr><tr><td>D3A1</td><td>306</td><td>551</td><td>304</td><td>2.21</td><td></td><td>2.03</td></tr><tr><td>D3A2</td><td>306</td><td>547</td><td>225</td><td>2.13</td><td></td><td>2.84</td></tr><tr><td>D3A3</td><td>306</td><td>491</td><td>72</td><td>0.65</td><td></td><td>3.66</td></tr><tr><td>D3A4</td><td>310</td><td>473</td><td>55</td><td>0.80</td><td></td><td>3.92</td></tr><tr><td>D4A1</td><td>264</td><td>521</td><td>388</td><td>2.34</td><td></td><td>4.27</td></tr><tr><td>D4A2</td><td>270</td><td>513</td><td>501</td><td>2.81</td><td></td><td>3.62</td></tr><tr><td>D4A3</td><td>268</td><td>467</td><td>240</td><td>2.71</td><td></td><td>3.42</td></tr><tr><td>D4A4</td><td>269</td><td>454</td><td>79</td><td>1.20</td><td></td><td>6.32</td></tr></tbody></table>

S21) in contrast to solution (Figure 3a,b and Figure S9). However, the color of the emission of alternating copolymers showed a significant red shift as compared to physical mixtures of corresponding donor and acceptor monomers (Figure S21). The photoluminescence quantum yields (PLQYs) of the dilute solutions and of the films of the synthesized copolymers were estimated in air. PLQY values higher than those listed in Table 3 are expected for the copolymers in the absence of oxygen. This expectation is supported by the data presented in Figure 5a,b and Figure S22. PLQYs of the films of most of the studied copolymers were found to be higher than those of the corresponding solutions as a result of the AIEE effect (see Tables 2 and 3). For example, the solution and film of D3A2 displayed PLQYs of 0.1 and 2.84%, whereas those of D4A2 showed values of 0.86 and 3.62%, respectively. The significant drop of PLQY of the films as compared to that of the solutions typically shows copolymers demonstrating aggregation-caused quenching. PLQYs of 17.34 to 4.14% were observed for the solution and film of D1A3, respectively. In general, PLQYs of the films of all alternating copolymers synthesized in this work range between 2.12 and 6.32% (Table 3).

The emission of the films of alternating copolymers was further investigated by PL decay measurements. The prompt fluorescence was observed in the range of 50–500 ns for the films of copolymers (Figure S22), which is ca. 2 orders of magnitude higher than $\tau_p$ observed for the solutions of the corresponding copolymers (Tables 2 and 3). The films of most of the copolymers demonstrated delayed fluorescence in the range between 0.35 and 6.2 µs (Table 3). However, the films prepared from the equimolar mixtures of donor and acceptor monomers did not show TADF (Figure S23), confirming the importance of copolymer composition.

To reveal in more detail the origin of delayed fluorescence of the developed copolymers, PL spectra and PL decay curves were recorded for films in the presence of oxygen and after deoxygenation, i.e., in air and in a vacuum (Figure 5a,b and Figure S22). Emission intensity enhancement was observed practically for all the copolymers after deoxygenation (Table S4). Overcoming triplet quenching by oxygen after the evacuation of air, triplets are harvested for emissive relaxations via reverse intersystem crossing (RISC), resulting in delayed fluorescence. The film of D4A2 exhibiting pronounced delayed fluorescence was selected for additional investigations of photophysical properties at different temperatures. Temperature dependencies of PL spectra and PL decay curves of D4A2 are shown in Figure 5c–e. TADF of the film of D4A2 was evidenced by the higher emission intensity recorded at 160 K in comparison to that observed at 78 K. It should be noted that the emission intensity of the film of D4A2 decreased with the further increase of the temperature from 160 to 300 K. This last observation is quite surprising since, because of TADF, the PL intensity would increase with the increase of the temperature. However, the quenching processes of exciplex emission (TADF exciplex) are intensified with the increasing temperature. The intensification of exciplex emission quenching is mainly related to the intensified molecular movements (rotations/vibrations). In other words, because the distances between the exciplex-forming donor and acceptor moieties are not fixed (e.g., by chemical bonds like in the conventional TADF compounds), quenching processes of exciplex emission are intensified with the increasing temperature. Another possible reason could be related to trapped charges as demonstrated in a recently published work.<sup>67</sup>

<footnote><sup>a</sup>Absorption and photoluminescence ($\lambda_{exc}$ = 325 nm) spectra of films from alternating copolymers were recorded at 298 K. <sup>b</sup>Wavelengths of absorption maxima. <sup>c</sup>Wavelengths of photoluminescence maxima recorded in air at 298 K. <sup>d</sup>Lifetimes of prompt ($\tau_p$) and delayed ($\tau_d$) fluorescence measured in air at 298 K. <sup>e</sup>PLQY measured by integrating sphere in air at 298 K. Films were obtained by spin-coating.</footnote>
<figure><img src="image_8.png" /><figcaption>**Figure 5.** (a) PL spectra and (b) PL decay curves recorded in air and in a vacuum and (c) PL spectra and (d, e) PL decay curves recorded at the different temperatures in an inert atmosphere of the film D4A2. PL decay curves were recorded using laser excitation (374 nm, 6 ns) except for the PL decay curve (e) that was recorded at 120 K using the microsecond lamp and corresponds mainly to the phosphorescence of D4A2. The inset shows zooming that, at the selected time range, demonstrates higher emission intensity at 160 K in comparison to that observed at 78 and 300 K (marked by arrows).</figcaption></figure>

The tails of PL decay curves at low temperatures are related to phosphorescence (e.g., at 120 K, phosphorescence with a lifetime of 142.9 µs was detected using the microsecond lamp (Figure 5e and Table S5)). Because of that, zooming has to be used to recognize the emission intensity enhancement in PL decays of the film of D4A2 with increasing temperature (Figure 5c,d, inset). Lower values of emission lifetimes of the film of D4A2 were observed at 300 K by multiexponential fitting relative to those recorded at 77 K (Figure 5e and Table S5). A similar observation was previously reported for TADF exciplexes and explained by nonemissive relaxation of excited triplet states at high temperatures.⁶⁸ Apparently, the non-emissive relaxation of triplets before RISC partly caused the

relatively low PLQY values of the synthesized alternating copolymers. The next step in the molecular design of the emitting polymers should be related to the restriction of such losses of triplets probably via creation of short donor-acceptor alternating sequences in a polymer chain, i.e., via preparation of multiblock type copolymers.

**Electroluminescent Properties.** To demonstrate applications of the developed alternating copolymers as light-emitting materials for multicolor solution-processable TADF OLEDs, D1A3 was selected as the blue emitter, D4A2 was used as the green emitter, and orange D3A2 was taken as the orange emitter. Their electroluminescent (EL) properties were investigated using the device structure ITO/PEDOT:PSS/
<figure><img src="image_9.png" /><figcaption>Figure 6. EL spectra of blue, green, and orange OLEDs based on D1A3, D4A2, and D3A2, respectively, recorded at the different voltages.</figcaption></figure>

VM-35(40 nm)/host-free light-emitting layer (20 nm)/TSPO1(10 nm)/TPBi (40 nm)/LiF(0.3 nm)/Al(120 nm). The hole-injecting (PEDOT:PSS), hole-transporting (VM-35), and light-emitting (D1A3, D4A2, or D3A2) layers were step-by-step spin-coated. Hole-blocking (TSPO1), electron-transporting (TPBi), and electron-injecting (LiF) layers and cathode (Al) were deposited by vacuum technology. The hole-transporting layer of VM35 was selected because of its high hole-mobility reaching $3.5 \times 10^{-3}$ cm$^2$/(V·s) at an electric field of $6.4 \times 10^5$ V/cm and ability of self-polymerization with a maximum polymerization rate at 209 °C.⁵⁰ The device structure was well designed. The stability of EL spectra recorded at the different voltages (Figure 6) confirms this statement. EL spectra of the devices are in very good agreement with the PL spectra of corresponding emitters (Figure S22). Thus, hole-electron recombination and exciton relaxation occur within the light-emitting layers. Practically the same EL spectra were obtained at different applied voltages.

The EL parameters such as turn-on voltage ($V_{on}$), maximum brightness, maximum current ($CE_{max}$), maximum power ($PE_{max}$), and external quantum efficiency ($EQE_{max}$) of the devices are collected in Table S6 and Figure S24. The highest $EQE_{max}$ of 7.84% was reached for the green device based on D4A2. This observation can be explained by the outstanding TADF properties of D4A2 (Figure 5). The appropriate optimization should certainly be considered for reaching low turn-on voltages or high external efficiencies. Nevertheless, at this stage, alternating copolymers show promising light-emitting, charge-injecting, and charge-transporting properties useful for the fabrication of multicolor solution-processable electroluminescent devices.

## CONCLUSIONS

In this work, we developed a new simple and cost-efficient strategy for the synthesis of polymeric through-space charge-transfer TADF emitters with easily tunable color of the emission from blue to orange. The strategy is based on RAFT-mediated alternating copolymerization of styrene-based donor monomers with acceptor maleimide-type monomers, which were obtained in one- or two-step scalable synthesis. The alternating copolymers are characterized by high thermal stability. Most of them exhibit exciplex emission via the through-space charge-transfer mechanism and display aggregation-induced emission. The alternating copolymers in solid state demonstrated delayed fluorescence with lifetimes between 0.35 and $6.2 \mu s$, making them promising materials

for emitting layers of multicolor solution-processable electroluminescent devices. The photoluminescence quantum yields of the solid films of the synthesized alternating copolymers are relatively low (2.12–6.32%) most probably because of nonemissive relaxation of triplets before reverse intersystem crossing. Multicolor hosts-free solution-processable TADF OLEDs were fabricated using the synthesized copolymers as blue, green, and orange emitters. The devices showed good stability of electroluminescence spectra at different voltages. The highest $EQE_{max}$ of 7.84% was reached for the green device based on the alternating copolymer of 9,9-dimethyl-10-(4-vinylphenyl)-9,10-dihydroacridan and 1-(3-cyanophenyl)-pyrrole-2,5-dione (D4A2) showing outstanding TADF properties. Although the synthesized alternating copolymers show promising light-emitting, charge-injecting, and charge-transporting properties that are useful for multicolor solution-processable electroluminescent devices, further optimizations in the synthetic approach are required for reaching lower turn-on voltages and/or higher external quantum efficiencies. The macromolecular design should be directed to the decrease of the nonemissive relaxation of triplets via a decrease of the length of donor-acceptor alternating sequences in a polymer chain. This design will include the preparation of a multiblock type copolymer where the short alternating sequences of donor-acceptor monomers will be placed along the polymer chain consisting of donor monomer units. The use of different acceptor monomers in the same polymer chain would allow reaching white emission from a single polymer emitter.

## ASSOCIATED CONTENT

### Supporting Information

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.macromol.2c02582.

¹H NMR spectra; SEC curves; additional thermal, photophysical, and electrochemical data; computational results; and EL device characteristics (PDF)

## AUTHOR INFORMATION

### Corresponding Authors

Juozas Vidas Grazulevicius – Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas 51423, Lithuania; ● orcid.org/0000-0002-4408-9727; Email: juozas.grazulevicius@ktu.lt

Sergei V. Kostjuk – Research Institute for Physical Chemical Problems of the Belarusian State University, 220006 Minsk, Belarus; Department of Chemistry, Belarusian State
## Authors

George K. Belousov – Research Institute for Physical Chemical Problems of the Belarusian State University, 220006 Minsk, Belarus; Department of Chemistry, Belarusian State University, 220006 Minsk, Belarus

Aliaksei A. Vaitusionak – Research Institute for Physical Chemical Problems of the Belarusian State University, 220006 Minsk, Belarus

Irina V. Vasilenko – Research Institute for Physical Chemical Problems of the Belarusian State University, 220006 Minsk, Belarus; Department of Chemistry, Belarusian State University, 220006 Minsk, Belarus

Melika Ghasemi – Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas 51423, Lithuania; orcid.org/0000-0003-3342-7310

Viktorija Andruleviciene – Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas 51423, Lithuania

Aliaksei Ivanchanka – Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas 51423, Lithuania

Dmytro Volyniuk – Department of Polymer Chemistry and Technology, Kaunas University of Technology, Kaunas 51423, Lithuania; orcid.org/0000-0003-3526-2679

Hern Kim – Environmental Waste Recycle Institute/ Department of Energy Science and Technology, Myongji University, Yongin, Gyeonggi-do 17058, Republic of Korea; orcid.org/0000-0002-6137-3201

Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.macromol.2c02582

## Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

## Notes

The authors declare no competing financial interest.

## ACKNOWLEDGMENTS

This work was supported by the State Committee on Science and Technology of the Republic of Belarus in the framework of the Korea–Belarus joint project X21KOPТ-004, National Research Foundation (NRF) grant funded by the Ministry of Science and ICT (MSIT) (2020K1A3A1A30103645), Republic of Korea and by European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie grant agreement no. 823720. This project has also received funding from the Research Council of Lithuania (LMTLT), agreement S-MIP-22-78.

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