Highly Efficient Electroluminescent Materials with High Color Purity Based on Strong Acceptor Attachment onto B–N-Containing Multiple Resonance Frameworks

Open AccessCCS ChemistryRESEARCH ARTICLE6 Jun 2022Highly Efficient Electroluminescent Materials with High Color Purity Based on Strong Acceptor Attachment onto B–N-Containing Multiple Resonance Frameworks Yincai Xu, Chenglong Li, Zhiqiang Li, Jiaxuan Wang, Jianan Xue, Qingyang Wang, Xinliang Cai and Yue Wang Yincai Xu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Chenglong Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Zhiqiang Li Jihua Laboratory, Foshan 528200, Guangdong Province Google Scholar More articles by this author , Jiaxuan Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Jianan Xue State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Qingyang Wang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author , Xinliang Cai State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Google Scholar More articles by this author and Yue Wang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 Jihua Laboratory, Foshan 528200, Guangdong Province Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202101033 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The development and enrichment of organic materials with narrowband emission in longer wavelength regions beyond 515 nm still remains a great challenge. Herein, a synthetic methodology for narrowband emission materials has been proposed to functionalize multiple resonance (MR) skeletons and generate a universal building block, namely, the key intermediate DtCzB-Bpin, which can be utilized to construct multifarious thermally activated delayed fluorescence (TADF) materials with high color purity through a simple one-step Suzuki coupling reaction. Based on this unique synthetic strategy, a series of efficient narrowband green TADF emitters has been constructed by localized attachment of 1,3,5-triazine and pyrimidine derivatives-based acceptors onto B–N-containing MR frameworks with 1,3-bis(3,6-di-tert-butylcarbazol-9-yl)benzene (DtCz) as the ligand. The precise modulation of the acceptor is an intelligent approach to achieve bathochromic shift and narrowband emission simultaneously. The DtCzB-TPTRZ-based organic light-emitting diode (OLED) exhibits pure green emission with Commission Internationale de L’Eclairage (CIE) coordinates of (0.23, 0.68), a maximum external quantum efficiency (EQE) of 30.6%, and relatively low efficiency roll-off. Download figure Download PowerPoint Introduction Purely organic thermally activated delayed fluorescence (TADF) materials have been established as one of the most promising emitters for organic light-emitting diodes (OLEDs), which can capture electro-generated dark triplet excitons for light emission and achieve highly efficient conversion of electric energy to light via an endothermally assisted reverse intersystem crossing (RISC) process from the lowest triplet (T1) state to the lowest singlet (S1) state.1–4 According to Boltzmann statistics, a sufficiently small singlet–triplet energy splitting (ΔEST) between the S1 and T1 is indispensable in guaranteeing an effective RISC process, which can be achieved by diminishing the overlap of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).5–9 A commonly adopted strategy is to apply intramolecular charge transfer (ICT) configuration with the aid of a donor–acceptor (D–A) molecular skeleton, which inevitably induces a Stokes shift. Meanwhile, broad emission spectra are generated from vibronic coupling between the ground state and singlet excited state as well as structural relaxation of the S1 state.10 As for display applications, broadband emission lacks high color purity and cannot achieve a wide color gamut display, which are both important for accurate regeneration of authentic colors of the image content.11,12 Although excellent color purity can be obtained by cutting off the margin region of the original broadband electroluminescence (EL) with color filters or optical microcavities, the drawback is that these treatments sacrifice the actual efficiency value of OLEDs.13,14 Encouragingly, the most highlighted advantage of the D–A structure is the extreme flexibility of emission maximum regulation spanning, which is wide enough within the visible spectral region due to the ICT characteristics. Recently, multiple resonance (MR) type TADF materials based on B–N-containing conjugated molecules composed of rigid skeletons with alternate arrangements of HOMO and LUMO, have shown considerable potential in fabricating highly efficient OLEDs with extraordinary color purity (Scheme 1a).15–24 MR-type TADF molecules demonstrate unique excitonic features of narrow full-width at half-maximum (FWHM), giant oscillator strength (f), large extinction coefficient, and near-unity photoluminescence quantum yield (PLQY). Although it is relatively easy to synthesize MR-type TADF emitters with high efficiency and narrowband emission in blue and sky-blue regions, narrowband TADF emitters in longer wavelength regions with emission maximum over 515 nm remain scarce. To construct a wide color gamut full-color display, it requires not only narrowband deep blue emitters but also ultrapure green and red emitters that comply with the Commission Internationale de L’Eclairage (CIE) coordinates requirements defined by National Television System Committee (NTSC). Therefore, it is critical to continue to explore emitters with narrowband emission in longer wavelength regions for both academic research and commercial applications. Scheme 1 | (a) MR framework. (b) Diagram sketch of attaching acceptors onto the MR framework. Download figure Download PowerPoint Only a few MR-type TADF molecules emitting in longer wavelength regions have been reported until now.19–23 The MR molecules were synthesized via a tandem lithiation–borylation–annulation reaction or one-shot electrophilic C–H borylation reaction. Therefore, the adopted synthesis method for target molecules was only based on the borylation of aromatic amine ligands previously synthesized, which severely restricts the expansion of the MR molecular family. The reported synthetic method for MR molecules suffers several major disadvantages. First, the low yield of the borylation reaction involving large aromatic amine ligands with complex structures leads to a complex final reaction mixture, which contains the product molecule, amine ligands, borylation isomers, and other by-products with very similar polarities and similar or very near molecular weights, imposing difficulties in the separation and purification. Second, for the ligands with stronger electron acceptor moieties, such as 1,3,5-triazine, pyridazine, pyrimidine, and pyrazine groups, the borylation on the para-carbon position of the acceptor-substituted phenyl ring is not conducive. In other words, this class of ligands cannot directly coordinate with boron and form a MR framework ( Supporting Information Figure S1). These electron withdrawing groups (EWGs) can reduce the electron density of para-carbon atoms and thus, suppress the C–H borylation reaction with boron tribromide/triiodide.24 Even if alkyllithium is employed as a dehydrogenation/dehalogenation reagent for the ligands with the most EWGs, such as cyano or carbonyl groups, the side chemical reactions can take place at high temperature (ca. 60 °C) and transform cyano or carbonyl groups into undesired or unforeseen groups.22 In principle, the introduction of strong acceptors into the para-carbon position of B-substituted phenyl rings can remarkably depress the LUMO energy level (Scheme 1b), and slightly change that of HOMO, resulting in an obvious decrease of the band gap and red-shift emission.19,20 A precisely localized introduction of a strong acceptor should be an efficient approach to shift the emission to a longer wavelength region. Therefore, developing a modified methodology of MR framework with strong auxiliary acceptors is a significant challenge for achieving organic emitters with narrowband emission in longer wavelength regions. Herein, we present a straightforward synthetic methodology to functionalize the MR skeleton that can be attached onto strong acceptors. In this method, the parent molecule DtCzB was selected as the original skeleton (Figure 1), which could be converted into the key intermediate DtCzB-Bpin with high yield with catalytic amounts of di-mu-methoxobis(1,5-cyclooctadiene)diiridium(I) ([Ir(COD)(OCH3)]2) and 4,4′-di-tert-butyl-2,2′-bipyridine (dtbpy) (Figure 1b). Then, a wide variety of functional groups can be introduced by a one-step uncomplicated Pd-mediated Suzuki coupling reaction, resulting in diverse efficient emitters with high color purity. Moreover, it can be inferred that this synthetic method of functionalizing DtCzB can be extended to its carbazole-based analogues. Herein, four representative molecules are presented and employed to demonstrate the outstanding performance of this synthetic approach (Figure 1a). Importantly, the resultant 2,4,6-triphenyl-1,3,5-triazine 1,3-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzene boron (DtCzB-TPTRZ)-based OLED exhibits pure green emission with CIE coordinates of (0.23, 0.68), a remarkable maximum external quantum efficiency (EQE) of 30.6%, and relatively low efficiency roll-off. Figure 1 | (a) Molecular structures of the investigated compounds. (b) Synthetic procedures: (1) B2Pin, [Ir(COD)(OCH3)]2, dtbpy, THF, reflux. (2) Ar-X (X = Cl or Br), K2CO3, Pd(PPh3)4, THF, water, reflux. Download figure Download PowerPoint Experimental Methods Synthesis of materials All reagents were purchased from Energy Chemical Co. (Shanghai, China) and J&K Scientific Ltd. Co. (Beijing, China) and used immediately without further purification. Schlenk technology was performed under nitrogen in all reactions. Synthetic procedures are shown below in detail. Target compounds were initially purified by column chromatography and further purified by temperature-gradient sublimation under high vacuum. Synthesis of DtCzB The synthetic process can referred to previous literature.21 Synthesis of DtCzB-Bpin The catalytic agents [Ir(COD)(OCH3)]2 (43.1 mg, 0.065 mmol) and dtbpy (34.9 mg, 0.13 mmol)were added to the suspension of DtCzB (4.20 g, 6.5 mmol) and bis(pinacolato)diboron (B2Pin) (1.68 g, 6.6 mmol) in extra dry tetrahydrofuran (THF) (60 mL) at room temperature. Then the mixture was bubbled with nitrogen for another 5 min, heated to reflux and stirred for 24 h. After cooling to room temperature, the reaction mixture was directly concentrated under reduced pressure and purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (1∶2) to afford a yellow solid (4.50 g). Yield: 90%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9.14 (d, J = 1.9 Hz, 2H), 8.79 (s, 2H), 8.53 (d, J = 8.8 Hz, 2H), 8.48 (d, J = 1.8 Hz, 2H), 8.27 (d, J = 2.0 Hz, 2H), 7.74 (dd, J = 8.8, 2.1 Hz, 2H), 1.67 (s, 18H), 1.54 (s, 18H), 1.49 (s, 12H). 13C{1H} NMR (151 MHz, chloroform-d) δ/ppm: 145.17, 144.55, 143.68, 141.64, 138.48, 129.77, 126.93, 124.59, 123.75, 121.65, 120.75, 117.10, 114.44, 113.77, 84.34, 35.18, 34.82, 32.21, 31.87, 25.09. Electrospray ionization mass spectrometry (ESI-MS) (M) m/z: 765.93 [M]+ (calcd: 766.48). Anal. Calcd for C52H60B2N2O2: C, 81.46; H, 7.89; N, 3.65. Found: C, 81.48; H, 7.93; N, 3.68. Synthesis of DtCzB-DPTRZ 2-chloro-4,6-diphenyl-1,3,5-triazine (160.6 mg, 0.6 mmol), DtCzB-Bpin (383.3 mg, 0.5 mmol), and potassium carbonate (K2CO3) (138 mg, 1 mmol) were added with water (2 mL) and THF (16 mL). The mixture was bubbled with nitrogen for another 5 min, and tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)4) (28.9 mg, 0.025 mmol) was added under high flow nitrogen. Then the mixture was heated to reflux and stirred for 12 h. After cooling to room temperature, the reaction mixture was extracted with dichloromethane and water, the combined organic layer was condensed in vacuum, and then the crude product was purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (2∶1) to afford a yellow solid (239.8 mg). Yield: 55%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9.57 (s, 2H), 8.94 (s, 2H), 8.79 (d, J = 7.4 Hz, 4H), 8.56 (d, J = 8.7 Hz, 2H), 8.34 (d, J = 1.8 Hz, 2H), 8.14 (d, J = 2.0 Hz, 2H), 7.67 (t, J = 7.1 Hz, 2H), 7.61 (dd, J = 8.4, 6.2 Hz, 6H), 1.67 (s, 18H), 1.56 (s, 18H). The signal of 13C was not detected due to the poor solubility of the target compound. ESI-MS (M) m/z: 870.73 [M]+ (calcd: 871.48). Anal. Calcd for C61H58BN5: C, 84.02; H, 6.70; N, 8.03. Found: C, 84.08; H, 6.71; N, 8.09. Synthesis of DtCzB-TPTRZ, DtCzB-PPm and DtCzB-CNPm Each were synthesized in the same way as 2,4-diphenyl-1,3,5-triazine 1,3-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzene boron (DtCzB-DPTRZ), but 2-chloro-4,6-diphenyl-1,3,5-triazine was replaced with equivalent stoichiometric amounts of 2-(4-bromophenyl)-4,6-diphenyl-1,3,5-triazine, 5-bromo-2-phenylpyrimidine, and 5-bromo-2-cyanopyrimidine, respectively. DtCzB-TPTRZ Yellow solid (293.9 mg). Yield: 62%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9.10 (d, J = 19.0 Hz, 2H), 9.04–8.95 (m, 2H), 8.86–8.71 (m, 4H), 8.64–8.54 (m, 2H), 8.51–8.41 (m, 4H), 8.26 (d, J = 9.6 Hz, 2H), 8.18–8.07 (m, 2H), 7.71 (dt, J = 8.9, 1.9 Hz, 2H), 7.59 (dq, J = 15.8, 8.2, 7.7 Hz, 6H), 1.68 (s, 18H), 1.55 (s, 18H). 13C{1H} NMR (151 MHz, chloroform-d) δ/ppm: 171.65, 171.22, 145.60, 145.44, 145.02, 144.74, 141.76, 138.31, 136.24, 136.12, 132.53, 129.83, 129.01, 128.63, 128.00, 127.21, 124.52, 123.67, 122.53, 121.72, 120.72, 117.37, 114.17, 107.01, 35.19, 34.83, 32.21, 31.85. ESI-MS (M) m/z: 946.57 [M]+ (calcd: 947.51). Anal. Calcd for C67H62BN5: C, 84.88; H, 6.59; N, 7.39. Found: C, 84.89; H, 6.66; N, 7.38. DtCzB-PPm Yellow solid (238.5 mg). Yield: 60%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 9.27–9.15 (m, 2H), 8.96 (s, 2H), 8.65 (dd, J = 6.6, 3.7 Hz, 2H), 8.39 (dt, J = 5.3, 2.1 Hz, 2H), 8.31–8.15 (m, 6H), 7.62 (dt, J = 6.7, 2.6 Hz, 5H), 1.65 (s, 18H), 1.53 (s, 18H). 13C{1H} NMR (151 MHz, chloroform-d) δ/ppm: 155.53, 145.53, 144.81, 141.55, 138.22, 138.04, 136.87, 131.86, 131.21, 129.64, 128.81, 128.40, 127.16, 124.55, 123.66, 122.71, 121.54, 120.94, 117.42, 114.12, 105.58, 35.17, 34.80, 32.15, 31.80. ESI-MS (M) m/z: 793.80 [M]+ (calcd: 794.45). Anal. Calcd for C56H55BN4: C, 84.62; H, 6.97; N, 7.05. Found: C, 84.69; H, 6.99; N, 7.10. DtCzB-CNPm Orange solid (238.0 mg). Yield: 64%. 1H NMR (500 MHz, chloroform-d), δ/ppm: 8.16 (s, 4H), 7.96 (d, J = 9.0 Hz, 2H), 7.82–7.77 (m, 2H), 7.59 (s, 2H), 7.48 (d, J = 8.4 Hz, 2H), 7.21 (s, 2H), 1.54 (s, 18H), 1.49 (s, 18H). 13C{1H} NMR (151 MHz, chloroform-d) δ/ppm: 155.86, 146.04, 145.11, 144.78, 143.92, 141.38, 137.81, 135.88, 129.58, 127.17, 124.69, 123.64, 121.20, 117.57, 116.99, 115.84, 113.97, 105.60, 32.10, 31.77. ESI-MS (M) m/z: 742.84 [M]+ (calcd: 743.42). Anal. Calcd for C51H50BN5: C, 82.35; H, 6.78; N, 9.42. Found: C, 82.40; H, 6.82; N, 9.48. Synthesis of 9-(2-bromophenyl)-9H-3,9′-bicarbazole The synthesis procedure is presented in Scheme 2. 120 mL solution of anhydrous N,N-dimethylformamide (DMF) containing 9H-3,9′-bicarbazole (10.3 g, 31.0 mmol) was slowly added into a mixture of cesium carbonate (Cs2CO3) (14.7 g, 45.0 mmol) and 60 mL anhydrous DMF, and then 25 mL anhydrous DMF solution containing 2-bromofluorobenzene (5.2 g, 30.0 mmol) was injected. The mixture was heated and stirred at 140 °C for 12 h. After cooling to room temperature, the reaction mixture was poured into ice water (1000 g). The white powder solid was filtered out and dried in vacuum, and then further purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (2:5) to afford a white solid (13.1 g). Yield: 90 %. 1H NMR (500 MHz, CD2Cl2) δ/ppm: 8.31 (d, J = 1.9 Hz, 1H), 8.17 (d, J = 7.7 Hz, 2H), 8.13 (d, J = 7.8 Hz, 1H), 7.93 (d, J = 8.0 Hz, 1H), 7.60 (dd, J = 7.2, 1.7 Hz, 2H), 7.54 (dd, J = 8.5, 2.0 Hz, 1H), 7.50–7.38 (m, 6H), 7.32–7.25 (m, 4H), 7.13 (d, J = 8.2 Hz, 1H). ESI-MS (M) m/z: 487.85 [M]+ (calcd: 487.40). Anal. Calcd for C66H74BrN3: C, 73.93; H, 3.93; N, 5.75. Found: C, 74.13; H, 3.99; N, 5.81. Scheme 2 | Synthetic procedures of PhCzBCz. Download figure Download PowerPoint Synthesis of 9-(2-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-3,9′-bicarbazole 9-(2-bromophenyl)-9H-3,9′-bicarbazole (PhBCzBr) (13.0 g, 26.7 mmol), 9-phenyl-9H-carbazol-3-ylboronic acid (8.4 g, 29.4 mmol), and K2CO3 (7.4 g, 53.4 mmol) were added with water (50 mL) and THF (200 mL). The mixture was bubbled with nitrogen for another 5 min, and Pd(PPh3)4 (1.5 g, 1.3 mmol) was added under high flow nitrogen. Then the mixture was heated to reflux and stirred for 12 h. After cooling to room temperature, the reaction mixture was extracted with dichloromethane and water, the combined organic layer was condensed in vacuum, and the crude product was purified by column chromatography with a mixed eluent of dichloromethane/petroleum ether (4∶1) to afford a white solid (12.1 g). Yield: 70%. 1H NMR (500 MHz, CD2Cl2), δ/ppm: 8.12 (d, J = 8.2 Hz, 3H), 7.98 (d, J = 7.8 Hz, 1H), 7.85 (d, J = 7.6 Hz, 1H), 7.82 (d, J = 1.6 Hz, 1H), 7.74 (d, J = 7.8 Hz, 1H), 7.71–7.61 (m, 3H), 7.50 (t, J = 7.7 Hz, 2H), 7.41–7.29 (m, 9H), 7.28–7.14 (m, 7H), 7.11 (dd, J = 8.6, 1.7 Hz, 1H), 7.04 (d, J = 8.5 Hz, 2H). 13C{1H} NMR (151 MHz, CD2Cl2) δ/ppm: 142.69, 142.18, 141.42, 140.96, 140.42, 137.71, 134.97, 132.38, 130.81, 130.32, 130.17, 129.65, 128.83, 127.79, 127.18, 126.84, 126.42, 126.32, 126.19, 125.48, 124.34, 123.49, 123.31, 123.04, 120.72, 120.44, 120.32, 120.30, 119.99, 119.84, 119.50, 116.66, 110.72, 110.17, 109.71. ESI-MS (M) m/z: 650.42 [M]+ (calcd: 649.80). Anal. Calcd for C61H58BN5: C, 88.72; H, 4.81; N, 6.47. Found: C, 88.92; H, 4.89; N, 6.57. Theoretical calculation method A B3LYP method, including Grimme’s dispersion correction with a 6-31G (d, p) basis set, was used to fully optimize the geometries of the ground state in a gas state by Gaussian 09 software package. The properties of the excited state were calculated by time-dependent density functional theory (TDDFT) with the same theory level as DFT. The HOMO and LUMO were visualized with Gaussview 5.0. A detailed calculation of reorganization energies can be found in the Supporting Information. Results and Discussion Synthesis and characterization Starting from the parent molecule DtCzB, four target molecules were successfully prepared through two easy handling steps. The key step in obtaining the target compounds was the successful synthesis of the DtCzB-Bpin precursor. In the presence of catalytic amounts of [Ir(COD)(OCH3)]2 (1% molar stoichiometric ratio) and dtbpy (2% molar stoichiometric ratio), which are commercially available at low prices, the intermediate DtCzB-Bpin can be easily transformed via DtCzB and B2Pin with high yield,25 and then employed as a building block for the construction of versatile compounds through a one-step Suzuki coupling reaction. The preparation processes are robust enough to manufacture target compounds at a commercial scale. Detailed synthetic procedures are shown in the Experimental Methods, and the NMR spectra of all compounds are shown in Supporting Information Figures S2–S6. Computational simulations and photophysical properties To determine the implication and discrepancy of various peripheral EWG substituents with respect to geometric and electronic characteristics, ground (S0) state geometries were initially optimized by utilizing DFT, and the geometric configurations of S1 states were simulated using TDDFT. The HOMO/LUMO distributions, energy band gaps (Egap), oscillator strengths, and electrostatic potential (ESP) distributions are illustrated in Figure 2. The HOMOs of four compounds are approximately identical to that of the parent molecule, which mainly distribute on the nitrogen atoms and carbon atoms at ortho/para-positions in the DtCzB moiety. The LUMOs are predominantly localized on the boron atom and the carbon atoms at its ortho/para-positions in the vicinity of the DtCzB moiety and partially extended to the appended EWG moieties to various extents. Assisted by ESP analysis, the LUMO energy levels of the four compounds are significantly pulled down by the partial negative potential induced by EWGs, which can enhance the ICT strength, narrow the Egaps, and generate red-shift emission. The reduction in oscillator strengths of the investigated compounds, as compared to the parent molecule, is attributed to the enhanced ICT properties. However, the oscillator strength values still maintain at high levels, facilitating high PLQYs. Intriguingly, when triazine moiety bonds directly to the DtCzB core, the intramolecular hydrogen bonds (C–H···N) are induced, and the DPTRZ moiety and DtCzB core are interlocked together ( Supporting Information Figure S7).26,27 Consequently, the molecule DtCzB-DPTRZ displays a nearly planar structure based on the double intramolecular hydrogen bonds between the DPTRZ segment and DtCzB core. The other three molecules [DtCzB-TPTRZ, 2-phenylpyrimidine 1,3-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzene boron (DtCzB-PPm), and pyrimidine-2-carbonitrile 1,3-bis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzene boron (DtCzB-CNPm)] without intramolecular hydrogen bonds exhibit distorted molecular configurations and large dihedral angles (θ) of around 37° between EWG moieties and DtCzB core in the ground state. For the DtCzB-DPTRZ molecule, the optimized S0 and S1 have very similar configurations, and the dihedral angles are close to each other, suggesting minimum structural deformation upon switching between S0 and S1 states. For the other three molecules, S0 and S1 show obviously different configurations and dihedral angles, directing relatively significant structural deformation upon transformation between S0 and S1 states. Figure 2 | Calculated HOMO and LUMO distributions, Egaps, oscillator strengths, and molecular surface ESPs in S0 geometries of the investigated compounds (measuring scale ×10−2, red and blue indicate negative and positive ESP, respectively). Download figure Download PowerPoint To quantitatively understand geometrical deformation, reorganization energies (λ) were calculated according to the Marcus theory, which is closely associated to the excitation and emission processes and have a significant impact on the emission spectral profile. After the Franck–Condon vertical absorption transition, the internal conversion and vibrational relaxation lead to the geometrical deformation of the molecule and then relax to the emission state, which obeys Kasha’s rule: the photons must be emitted from the lowest-lying singlet or triplet excited states of the molecules with spin multiplicity and independent of excitation wavelength.28 In principle, the lesser reorganization energies are prerequisites in generating small Stokes shifts and narrowband emission. As depicted in Figures 3a–3d, the reorganization energies were calculated to be 0.16, 0.22, 0.17, and 0.33 eV for DtCzB-DPTRZ, DtCzB-TPTRZ, DtCzB-PPm, and DtCzB-CNPm, respectively, which are larger than that of DtCzB (0.13 eV) ( Supporting Information Figure S8), but still relatively small. The above theoretical calculations reveal that the EWG substituents have limited influence on the parent skeleton in terms of the frontier molecular orbitals distributions and reorganization energies. Therefore, it is reasonable to conjecture that the newly-constructed compounds should have narrowband emission. The four molecules attached with acceptors may inherit the original “fingerprint” features of DtCzB and possess the MR gene. According to the decomposition results of the reorganization energies from S1 to S0 ( Supporting Information Figure S9), the representative normal vibration modes with large contribution to reorganization energies are dominantly located in the low-frequency region (<500 cm−1) for DtCzB-TPTRZ, DtCzB-PPm, and DtCzB-CNPm.29 The large reorganization energies mainly stem from the distinct out-of-plane bending vibration of the molecular skeleton and the vibration of the EWG moiety ( Supporting Information Figure S10), and the internal rotation between EWG moiety and DtCzB core ( Supporting Information Figure S7). Generally, enhanced low-frequency and suppressed high-frequency vibronic coupling are conducive to narrow FWHM.30–32 For DtCzB-DPTRZ, the representative normal vibration modes with large contributions to reorganization energies are nearly distributed in all-frequency regions. However, the observed reorganization energies for the different normal vibration modes of DtCzB-DPTRZ have extremely small values (<20 cm−1). This is because the major discrepancy between S0 and S1 states of DtCzB-DPTRZ is only the slight stretching vibration between the DtCzB core and triazine ring with small average changes in bond length of 0.031 Å and negligible changes in dihedral angle. In this situation, the total reorganization energies of DtCzB-DPTRZ (0.16 eV) are not much higher than that of the parent molecule DtCzB (0.13 eV). However, the stretching vibration in the high-frequency mode (788 cm−1) may lead to the generation of a shoulder peak (vibronic emission band) in the emission spectrum owing to the involvement of vibrational progression separated by vibrational frequency.30 Based on the above analysis, it can be concluded that lowering the reorganization energies of the excitation and emission processes can be achieved by restricting the structural deformation between the ground and excited states, which may eventually impart small Stokes shifts and narrowband emissions to TADF molecules. Figure 3 | Optimized S0 and S1 structures, single point energies, and reorganization energies (λ) of DtCzB-DPTRZ (a), DtCzB-TPTRZ (b), DtCzB-PPm (c), and DtCzB-CNPm (d). Download figure Download PowerPoint The preliminary photophysical properties of four compounds consisting of UV–vis absorption and PL spectra were recorded in dilute toluene solution (1 × 10‒5 M). As depicted in Figures 4a–4d and Table 1, the electronic absorption spectra display an intense absorption band peaking at 507, 477, 474, and 481 nm for DtCzB-DPTRZ, DtCzB-TPTRZ, DtCzB-PPm, and DtCzB-CNPm, respectively, which correspond to ICT absorption transitions. They all exhibit perfect green fluorescence with emission peaks at 521, 501, 499, and 515 nm, small Stokes shifts of 14, 24, 25, and 34 nm, and narrow FWHMs of 24, 27, 25, and 36 nm, respectively. Additionally, they show positive solvatochromism and solvent-dependent spectral shape features, thereby confirming typical ICT characteristics ( Supporting Information Figure S11 and Table S1), which are consistent with the theoretically calculated results ( Supporting Information Table S2). For DtCzB-DPTRZ, the solubility is very poor, which can be reasonably understood by the strong intramolecular interactions caused by the large molecular conjugated plane, which demonstrates the existence of intramolecular hydrogen bonds between the DPTRZ segment and DtCzB core. The flattening of the molecular structure leads to enhanced π-conjugation and ICT states, rendering the emission substantially red-shifted in comparison with DtCzB. Simultaneously, the emission spectrum exhibits small FWHM, accompanied with an appreciable shoulder peak that has significant ramifications on color purity, which originates from the high-frequency vibronic coupling corresponding to the large molecular conjugated plane. To eliminate the shoulder peak, the phenyl bridge was introduced into DtCzB-TPTRZ to elude the direct connection between the