To design luminescent polymers with through-space charge transfer, selection of polymer backbone, donor and acceptor plays the key role in determining the photophysical and electroluminescent properties of the resulting polymers. In this work polystyrene is selected as backbone because it provides the non-conjugated connection between donor and acceptor, while acridan is chosen as donor because of its good electron-donating ability as well as the rigid bridged structure. Moreover, triarylboron units are used as acceptors because they exhibit weak electron-accepting capability owing to the empty p_z orbital of boron that is capable to participate in π-conjugation with aryl groups. To tune the emissive color, three kinds of substituents hydrogen (H), fluorine (F), and trifluoromethyl (CF₃) are introduced to the polymers, denoted as PH-05–PH-20, PF-05–PF-20, and PTF-05–PTF-20, respectively (Figure 1). The three substituents are selected because their electron affinity are gradually increased in order of H, F, and CF₃, which can enhance the CT strength between the acridan donor and the triarylboron acceptor, allowing the modulation of TSCT emission of the resulting polymers. In addition, the content of acceptors are tuned at 5, 10, and 20 mol% to explore the influence of relative ratio between donor and acceptor on photophysical properties of the polymers.

Synthetic routes of the polymers are outlined in Scheme 1. The monomer Mon-H was prepared by a two-step procedure where the commercially available 4-bromovinylbenzene was first lithiated with n-BuLi, and then reacted with dimesityfluoroborane (Mes₂BF) to afford the product in yield of 56%. For Mon-F and Mon-TF, 1-bromo-3,5-difluorobenzene (1) and 1-bromo-3,5-bis(trifluoromethyl)benzene (3) were first lithiated by lithium diisopropylamide (LDA) and then treated with Mes₂BF to afford the bromide intermediates 2 and 4. Consequently 2 and 4 were cross-coupled with vinyltrifluoroborate or tributylvinylstannane under palladium-catalyzed conditions to afford the desired monomers. With the monomers in hand, TSCT polymers were synthesized by free radical polymerization of the corresponding vinyl-functionalized acridan and triarylboron monomers using 2-azoisobutyronitrile as initiator and tetrahydrofuran (THF) as solvent. The content of triarylboron acceptors of the polymers are controlled through feed ratio (5–20 mol% for PX-5–PX-20). For comparison, three triarylboron model compounds bearing no vinyl groups and two control polymers with only acridan donors and triarylboron acceptors are also synthesized (Scheme S1). Number-average molecular weights (M_ns) of the polymers measured by gel permeation chromatography using polystyrene as standard exhibit typical values of 9–45 KDa with polydispersity index (PDI) of 1.44–1.73 (Table 1). The decomposition temperatures (T_d) of the polymers with 5% weight loss under nitrogen are higher than 300°C, while glass transition temperatures (T_g) for the polymers are observed at 170–210°C (Figure S1). There are no exothermic peaks produced by crystallization within the scanning range, indicating the amorphous nature of the polymers. The TSCT polymers are readily soluble in common organic solvents, such as toluene, chloroform, tetrahydrofuran and chlorobenzene, ensuring the formation of high-quality films through solution process.

The UV–vis absorption and fluorescence spectra of the polymers in toluene at 298K are shown in Figure 2, with the data summarized in Table 1. The polymers show similar absorption peaks at 290 nm which are mainly attributed to π-π^* transition of acridan and triarylboron units under diluted solutions. PL spectra of the polymers exhibit weak emission bands at 374 nm coming from the acridan unit, together with a strong, broad, and featureless emission bands at longer wavelength. These featureless bands are red-shifted compared with those of acridan and triarylboron units as well as the acridan- and triarylboron-containing homopolymers (Figure 2A and Figure S2). Moreover, these emissions show strong positive solvation effect as the polarity of the solvent increases. For instance, the emission maxima (λ_{em, max}) of PH-20 shifts from 417 nm in cyclohexane to 476 nm in THF (Figure S3), confirming that the emissions are originating from CT transition between the donors and acceptors. By increasing electron-accepting ability of the triarylboron acceptors, the CT emission can be red-shifted from 438 nm (PH-05) to 465 nm (PF-05) and 494 nm (PTF-05). The content of the acceptor also have influence on the emission wavelength of the polymers. As the content of the acceptor increases from 5 to 20 mol%, the emission wavelength is red-shifted by 2–7 nm, consistent with the observations for triazine-based TSCT polymers (Shao et al., 2017). Different from those in solution, PL spectra of the polymers in film state show only CT emission from 429 to 483 nm (Figure 3), indicating that excited state energy of acridan donor has been completely transferred to the CT emissive species. The PLQYs of the polymers determined by integrating sphere is 26–27% for PH-05–PH-20, 27–38% for PF-05–PF-20 and 34–53% for PTF-05–PTF-20 (Table 1). As the content of acceptor increases, PLQYs of the polymers do not decrease, implying the weak concentration quenching effect of the TSCT emissive species.

To investigate the TADF properties of the polymers, PL decay characteristics are measured in nitrogen and air. As shown in Figure 4 and Table 1, under nitrogen, all polymers in toluene displays distinctive delayed emissions with lifetimes (τ_d) in microsecond scale, together with prompt emissions with lifetimes (τ_p) in nanosecond scale. For example, PH-5, PF-5, and PTF-5 show τ_ds of 0.19, 0.49, and 0.86 μs, respectively. The content of triarylboron units has slightly influence on τ_d, with values of 0.21, 0.46, and 0.98 μs detected for PH-20, PF-20, and PTF-20, respectively. Under air, the delayed components are not detectable for all the polymers, indicating that the delayed emission is arising from triplets which can be quenched by oxygen, consistent with typical TADF behaviors. To further explore the TADF character, ΔE_STs of the polymers were determined from the onset of fluorescence spectra at room temperature and phosphorescence spectra at 77 K (Figure S4), which are smaller than 0.1 eV (Table 1). Such small ΔE_STs are consistent with the TADF effect since the rapid RISC process can be favored by small ΔE_ST to convert non-forbidden triplet excitons to radiative singlet excitons.

To get insight into the electronic structures of the triarylboron-based TSCT polymers, frontier orbital distributions were investigated by density functional theory (DFT) calculations. It is found that for all the polymers with different substitution patterns, the HOMOs are predominantly located on the acridan units, whereas the LUMOs are distributed over the triarylboron acceptors, suggesting the CT character of the polymers (Figure 5). Moreover, the LUMO level of the polymers decreases from −1.61 to −1.75 eV and −1.84 eV as the substituent changes from H to F and CF₃, indicating that the electron-accepting ability becomes stronger. Since the HOMO and LUMO are well-separated, the polymers show close singlet state (S₁) and triplet state (T₁) energy levels with the ΔE_ST values estimated to be ~zero according to time-dependent density functional theory (TD-DFT) calculations, which are consistent with the experimental ΔE_ST values.

It is noteworthy that the triarylboron-based TSCT polymers exhibit aggregation-induced emission (AIE) effect through measuring their PL spectra in THF/water mixed solvents (Luo et al., 2001; Hong et al., 2011; Mei et al., 2015). As shown in Figure 6, PTF-20 in pure THF solution shows a weak emission band at ~520 nm. As water is added, a slight increase in PL intensity was observed, which is accompanied with blue-shift of emission band. When the weight water (f_w) is higher than 60%, the PL intensity increases drastically. Especially, at f_w of 99%, the polymer shows a dramatic increased intensity that is ~33 times higher than the initial THF solution (f_w = 0). Similar AIE effect is also observed for PH-20 and PF-20 with emission enhanced by ~6 and ~18 folds, respectively, as f_w increases from 0 to 0.99, which can be attributed to aggregation-induced dipole-dipole interaction between donors and acceptors.

To investigate the electroluminescent properties of the triarylboron TSCT polymers, solution-processed OLEDs were fabricated with device configuration of ITO/PEDOT:PSS (40 nm)/polymer (40 nm)/TSPO1 (8 nm)/TmPyPB(42 nm)/LiF (1 nm)/Al (100 nm) (Figure 7). Here PEDOT:PSS stands for poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) which serves as the hole-injection layer. TSPO1 (diphenyl(4-(triphenylsilyl)phenyl)phosphine oxide) (Mamada et al., 2011) and TmPyPB (1,3,5-tri(m-pyrid-3-yl-phenyl) benzene) (Su et al., 2008) act as the exciton blocking layer and the electron-transporting layer, respectively. As shown in Figures 7D–F, the polymers containing H substituents (PH-05, PH-10, and PH-20) show multiple emission bands at ~420 and ~480 nm. The former emission bands are similar to those observed in film-state PL spectra and therefore can be assigned to the CT emission. However, the origin of the latter emission bands is not clear yet. Similar behavior is also observed for PF-05–PF-20, showing CT emission at ~460 nm and unattributable emission bands at ~570 nm. Despite of this, the polymer bearing CF₃ groups (PTF-05–PTF-20) show mainly CT emission regardless of the triarylboron content, with CIE coordinates in range of (0.25, 0.39)–(0.29, 0.47).

EQE–luminance and current density–voltage–luminance characteristics of the devices are shown in Figures 7G–I and Figure S5. The device performance is summarized in Table 2. All the devices show low driving voltages at 3.0–3.4 V, implying the good carrier injection and transport from the electrodes. The device efficiency of the TSCT polymers is dependant on both the substituent and the content of triarylboron units. For example, from PH-05 to PF-05 and PTF-05, the maximum LE increases from 5.5 to 16.3 cd A⁻¹, and the maximum EQE increases from 2.9 to 5.7%. This observation is consistent with the enhanced PLQYs of the polymer films. Meanwhile, from PTF-05 to PTF-20 with increasing triarylboron content, the maximum LE increases slightly from 16.3 to 20.1 cd A⁻¹, implying the negligible concentration quenching effect in the polymers. We note that the maximum EQEs of PTF-05–PTF-20 with values of 5.7–7.0% are much higher than the upper limit of conventional fluorescent materials (EQE = 5%), confirming the contributions of the triplets for EL emission, and indicating the promising potential of triarylboron TSCT polymers to serve as luminescent materials.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.