Highly twisted bipolar molecules for efficient near-ultraviolet organic light-emitting diodes via a hybridized local and charge-transfer mechanism

Abstract

Near ultraviolet (NUV) electroluminescence (EL) is very important but seldom reported for being short of short-wavelength emitters. Herein, we report two highly twisted terphenyl-based donor–acceptor molecules exhibiting excellent thermal and morphological stability and nearly quantitative NUV luminescence. Via a hybridized local and charge-transfer (HLCT) process, excellent NUV EL is achieved with a maximum external quantum efficiency of 6.97% and CIE_x,y of (0.16, 0.06) in a nondoped OLED. The device shows excellent colour purity with a small full width at half maximum of 48 nm. Our research demonstrates that highly twisted structures can efficiently tune the luminescence wavelength of bipolar molecules to the NUV region and achieve high-performance NUV EL.

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Introduction

Near ultraviolet (NUV) organic light-emitting diodes (OLEDs) have attracted extensive attention for their potential applications.^1–9 However, NUV OLEDs with good performances are very limited due to the scarcity of luminescent emitters with emission peaks around 400 nm and Commission Internationale de L’Eclairage (CIE) coordinate y values of less than 0.06.^10–22 Usually, NUV emissions are achieved by emitters with short conjugation and low molecular weight. Unfortunately, short-conjugation molecules often show weak luminescence and low carrier transport ability, while molecules with low molecular weights exhibit inferior thermal and morphological stabilities.^23,24 These factors are detrimental to preparing high-performance OLED devices. As a result, design of emitters for efficient NUV OLEDs is a considerable challenge.^25–33 On the other hand, the exciton utility of luminescent molecules is a significant factor for electroluminescence efficiency. For the reason of achieving theoretically the full use of excitons, thermally activated delayed fluorescence (TADF) emitters are widely used.^34–41 TADF materials often exhibit a severe intramolecular charge transfer (ICT) effect and seldom emit NUV light. Furthermore, efficiency roll-off in TADF emitter-based OLEDs is also a common problem due to their long-delayed lifetime.³⁷ Fluorescent emitters with hybridized local and charge-transfer (HLCT) excited states, also coined as the “hot exciton” mechanism, can make full use of the excitons theoretically via reverse intersystem crossing (RISC) from high-lying triplet states (T_n, n > 1) to singlet states and have aroused extensive interest.^38–42 From the view of electronic structure, HLCT emitters should meet the requirements of a large ΔE(T_n ↔ T₁) and a small ΔE(T_n ↔ S_m), where n ≥ 2 and m ≥ 1.⁴³ Considering the PL spectra, the CT energy level of HLCT molecules decreases with an increase in the solvent polarity due to the solvatochromic effect, by which the CT and LE levels can be evaluated.⁴⁴ HLCT molecules radiate via a locally excited (LE)-emissive state, preventing long-wavelength shift, and exhibit small efficiency roll-off by rapid RISC.^45–47 By modulating the LE and CT states in molecules, several NUV OLEDs have been fabricated based on HLCT molecules.^6,48–55 Nevertheless, highly efficient and stable NUV OLEDs are still being urgently developed.

HLCT-featured molecules usually exhibit low to moderate luminescence with longer wavelengths due to the CT state, leading to the lack of NUV luminogens.⁵⁵ Increasing the twisting angles in the donor/acceptor (D/A) group by incorporating steric groups is an appealing method for weakening the electronic and intermolecular interaction.^55–59 In this work, large hindrance groups dimethylphenylsilyl- and methyldiphenylsilyl- were used to reduce the electronic D/A conjugation and π–π stacking in the phenanthroimidazole (PI) derivatives. As expected, a highly twisted structure is conducive to achieving NUV emission with nearly quantitative fluorescence quantum yields and high thermal/morphological stability. The nondoped OLEDs radiate NUV light (Commission Internationale de L’Eclairage (CIE) coordinates of (0.161, 0.063) for C2MPI and (0.162, 0.058) for C2PPI as emitters) with emission maxima of 404 and 406 nm, respectively. The maximum external quantum efficiencies (EQE_max) are as high as 6.97% and 5.55% for devices using C2MPI and C2PPI as emitters, respectively.

Results and discussion

Synthesis

Target molecules and their preparation methods are outlined in Scheme 1. Compounds 3 and 4 were obtained by lithiation and substitution with the corresponding chlorosilane followed by palladium-catalyzed amination. Intermediate 6 was synthesized by boration of compound 5 which was obtained by Debus–Radziszewski imidazole synthesis from phenanthrene-9,10-dione. Via palladium-catalyzed Suzuki-Miyaura coupling with compound 6 and compound 3 or 4, C2MPI and C2PPI were prepared and characterized by ¹H NMR, ¹³C NMR, and HRMS.

Scheme 1 Synthetic routes and molecule structures of C2MPI and C2PPI.

Thermal and photophysical properties

High decomposition temperatures (T_d) of 463 and 481 °C were achieved for C2MPI and C2PPI, respectively. The transition temperature (T_g) for C2MPI was 110 °C, while there was no T_g detected for C2PPI (Fig. S1, inset, ESI†). These results indicate that the thermal/morphological stabilities of the molecules are good for solid emission.

The photophysical properties were investigated and are shown in Table 1, Tables S1 and S2 (ESI†). The absorption and emission spectra in different solvents are shown in Fig. 1a and b, Fig. S2 and S3 (ESI†). The bands around 330 to 341 nm are ascribed to S₀ → S₁ transitions of molecules. The weak peaks around 364 nm should be associated with the CT transition.^49,60 The absorption spectra are affected little by solvent polarity, which indicates that the chromophores possess small dipole moments in the ground states (Fig. S2 and Table S1, ESI†). Comparatively small solvatochromism of emission with 19 and 21 nm was found from nonpolar to polar solvents for C2MPI and C2PPI, respectively, which indicates small dipole moments for the two dyes in the excited states (Fig. S3, ESI†). The dyes exhibit very high fluorescence quantum efficiencies as high as 0.99 and there is no obvious concentration quenching with the increase of emitters in films with sharper emission spectra as compared to those in solutions (Fig. 1a, b and Fig. S4, ESI†). Short lifetimes at the nano-level were detected, exhibiting single-component characteristics. (Fig. 1c and d). These results infer that these materials are potential candidates for device application.

Table 1 Photophysical, thermal and morphological measurements of C2MPI and C2PPI

	λ abs (nm)	ε abs (M cm^−1)	λ em (nm)	Δνem-abs^d (cm^−1)	Φ F	τ F (ns)	T g/Td^g (°C)
a Absorption maxima in toluene. b Molar extinction coefficient. c Emission maxima in toluene and in neat films. d Stokes shift. e Absolute fluorescence quantum yields in toluene/neat films. f τ F is PL lifetime in toluene/doped films. g Glass transition and decomposition temperatures. nd = not detected.
C2MPI	328	48 400	402/425	5612	0.86/0.99	1.63/1.69	110/463
C2PPI	330	69 000	410/425	5913	0.90/0.99	1.44/1.02	nd/481

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Fig. 1 (a) Absorption and emission spectra in toluene. (b) Emission in neat films. (c) Transient PL spectra of C2MPI and C2PPI in toluene and (d) Transient PL spectra of C2MPI and C2PPI in neat films.

Electrochemical nature and calculation study

The electrochemical properties of C2MPI and C2PPI were investigated by cyclic voltammetry (CV) performed in 0.1 M Bu₄NPF₆ dichloromethane. C2MPI and C2PPI exhibit irreversible one-electron oxidation/reduction waves with an E_onset of 0.80/−2.61 and 0.79/−2.58 eV versus Fc/Fc⁺ (0.13 V), respectively. By E_HOMO/LUMO = −(4.8 eV + E_onset), the HOMOs/LUMOs are calculated to be −5.60/−2.19 and −5.59/−2.22 eV for C2MPI and C2PPI, respectively. From the absorption spectra, the optical energy gaps (E_gs) were estimated as 3.25 and 3.22 eV for C2MPI and C2PPI, respectively (Fig. 2 and Table 2). As a result, the LUMOs for C2MPI and C2PPI, determined from E_g = E_HOMO − E_LUMO, are −2.35 and −2.33 eV, respectively, and they are very close to those calculated from reductive potentials in CV. Apparently, the HOMOs, LUMOs and E_gs of C2MPI are very close to those of C2PPI.

Fig. 2 Cyclic voltammograms of C2MPI and C2PPI, measured in N,N-dimethylformamide (negative) and in dichloromethane (positive) containing tetra-n-butylammonium hexafluorophosphate (0.1 M) at a scan rate of 50 mV s⁻¹.

Table 2 Electrochemical properties and energy gaps of C2MPI and C2PPI

	E ^oxdonset (V)^a	E ^redonset (V)^a	HOMO^b (eV)	LUMO (eV)	E g (eV)	θ 1 (°)	θ 2 (°)	HOMO^d (eV)	LUMO^d (eV)	E g (eV)
a E onset vs Fc/Fc^+. b Estimated by CV measurements calculated with EHOMO/LUMO = −(4.8 + Eonset) eV and Eg = ELUMO − EHOMO. c Optical energy gap determined by the absorption cutoff and LUMO calculated by E^optg = ELUMO − EHOMO. d Calculated by the optimized structures in DFT calculations, and θ1 and θ2 mean the twist angles between the benzyl rings in the terphenyl linker. e E ^optg = ELUMO − EHOMO.
C2MPI	0.80	−2.61	−5.60	−2.19^b/−2.35^c	3.41^b/3.25^c	61.65	68.40	−5.19	−1.18	4.01
C2PPI	0.79	−2.58	−5.59	−2.22^b/−2.37^c	3.37^b/3.22^c	61.62	59.23	−5.15	−1.22	3.83

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Information on molecular orbitals (MO) was obtained by DFT with Gaussian16 package by B3LYP/6-31G(d,p).⁶¹Fig. 3a shows the molecular configurations and the frontier MO as well as their energy levels. C2MPI and C2PPI exhibit localized HOMOs on the imidazole ring and the conjugated aryls. However, the tertiary-butyl substituted phenyl ring almost makes no contribution to MO due to its poor conjugation with the imidazole centre. The dyes show delocalized LUMOs distributed in the conjugation bridge backbone and the imidazole ring. Obviously, the silyl groups on the terphenyl disrupt the conjugation between carbazolyl and PI and affect the electron distribution. The electron distributions reveal that the lowest transitions of these molecules are mainly associated with the HOMO to LUMO transitions, which was manifested by the TD-DFT calculation (Table S3, ESI†). The charge distribution with a large HOMO–LUMO overlap reveals that C2MPI and C2PPI exhibit LE-dominated HLCT character, which matches the solvent effects well. Twisting angles of 59–68° were found in the terphenyls, indicating that these molecules have a more twisted linker compared to the molecules unsubstituted or with less steric substituents (Fig. S5, ESI†).^62,63 Such torsions can influence the electronic structures in the molecules and keep the molecules from close stacking in the solid states. The large energy gaps (E_gs), calculated from HOMOs–LUMOs or by CV and absorption cutoffs, are favourable for NUV emission (Table 2).

Fig. 3 (a) Configuration and frontier molecular orbital diagrams for C2MPI and C2PPI by B3LYP/6-31G(d,p). (b) Singlet/triplet energy levels and SOC matrix elements of C2MPI and C2PPI in the gas phase, calculated by the TD-DFT method from optimized S₁ geometries. (Section 2.3).

The pathway of exciton utility was studied by TD-DFT calculations by B3LYP/6-31G(d,p) basic sets and their excited states were investigated (Fig. 3b). The splitting energy between the singlet and triplet energy levels (ΔE_S1–T1) is 0.90 and 0.83 eV for C2MPI and C2PPI, respectively. Generally, a small ΔE_S1–T1 (<0.1 eV) is required to achieve an efficient TADF process. Therefore, the TADF process is unfavourable for both materials. Considerable spin–orbit coupling (SOC) was found between S₁ and T_7–9. Therefore, the high performances for the non-doped devices can be ascribed to hRISC of excitons.^55,64,65 The natural transition orbital (NTO) calculations also indicate that the S₀ → S₁ transition displays LE-dominated HLCT characteristics with a large overlap of “hole” and “particle” for C2MPI and C2PPI. The “hole” is dispersed on the molecular backbone and a “particle” is located on the linker and imidazole ring. Balanced CT/LE components were found in S₁ and T_7–9 and this is beneficial for achieving hRISC and radiative transition from S₁ to S₀ during electroluminescence in devices (Fig. S6, ESI†).^66,67

Electroluminescence performance

Non-doped OLEDs were fabricated using C2MPI and C2PPI as emitters with a configuration of ITO/HATCN (5 nm)/TAPC (X nm)/TCTA (5 nm)/Emitter (20 nm)/TPBi (40 nm)/LiF (1 nm)/Al (Device B1, Emitter = C2MPI, X = 60; Device B5, Emitter = C2PPI, X = 50). HATCN (1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile), TAPC (bis(di-4-tolylaminophenyl)cyclohexane), TCTA (tris(4-carbazoyl-9-ylphenyl)amine) and TPBi (1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene) are used as hole injection, hole-transporting, electron-blocking, and electron-transporting layers, respectively (Fig. 4).

η_ext = γ × η_PL × η_r × η_out (1)

Fig. 4 Nondoped device configuration, molecules used and their energy levels.

The EL profiles are shown in Fig. 5 and the device performances are listed in Table 3. Devices B1 and B5 emit NUV lights close to the blue index (0.15, 0.06) with CIE_x,y of (0.161, 0.063) and (0.162, 0.058) for C2MPI and C2PPI, respectively. Their devices show emission maxima at 404 nm (C2MPI) and 406 nm (C2PPI) with low turn-on voltages of 3.9 and 3.3 V for C2MPI and C2PPI, respectively. Impressively, narrow EL spectra with small FWHMs of 48 nm and 54 nm are observed at 3.9 V for device B1 and 3.3 V for device B5, respectively, showing that the devices exhibit excellent colour purity (Fig. 5). Moreover, the nondoped devices exhibit merely short-wavelength emission, which indicates that the excitons recombine completely within the emitting-layer and the radiation derives from the singlet excitons only. The device efficiencies are tabulated in Table 3. Devices B1 and B5 exhibit excellent EL performances and the EQEs are larger than 5%. Note that device B1 has a very high maximum EQE of 6.97% and a maximum exciton utility efficiency (EUE) of 35% calculated by eqn (1), in which γ is the balance factor of carriers (100%), η_PL is the neat-film fluorescence efficiency (99%) for C2MPI, and η_out stands for the light extraction rate (20%).⁶⁸ The EUE of device B1 significantly exceeds 25%, which indicates the utility of triplet excitons. To explore the mechanism of exciton utility, plots of luminance versus current density were made and good linearity is observed, excluding the triplet–triplet annihilation process. Meanwhile, large energy gaps between S₁ and T₁ of C2MPI (0.90 eV) and C2PPI (0.83 eV) are observed (Fig. 3b), indicating that the TADF process is not favoured (usually energy splitting is less than 100 mV between S₁ and T₁ for the TADF process).⁶⁹ So, devices B1 and B5 exhibit excellent performances via an HLCT process, which is also supported by the calculation of excited energy levels and SOC.

Fig. 5 Device configuration (a) Luminance–voltage–current density characteristics. (b) External quantum efficiency–luminance characteristics. (c) EL spectra. (d) Luminance versus current density (symbol) and the fitted curve plot (line) of devices.

Table 3 EL results of C2MPI and C2PPI in devices

Emitter	Device	λ EL (nm)	Fwhm^b (nm)	V (V)	L (cd m^−2)	η C (cd A^−1)	η P (lm W^−1)	η ext (%)	CIE^e (x, y)
						Maximum value/at 100 cd m^−2
a Emission maxima. b Full width at half maximum (nd, the spectra are not intact because light <380 nm cannot be collected.). c Turn-on voltage at 1 cd m^−2. d The luminescence (L), current efficiency (ηc), power efficiency (ηP) and external quantum efficiency (ηext) of the devices: maximum values/values at 100 cd m^−2. e CIE coordinates at 100 cd m^−2.
C2MPI	B1	404	48	3.9	3500	1.75/1.34	1.20/0.55	6.97/4.57	(0.161,0.063)
	B2	388	nd	4.9	681	0.60/0.59	0.24/0.20	1.96/1.93	(0.189,0.114)
	B3	396	nd	4.9	795	0.69/0.68	0.25/0.24	2.60/2.57	(0.184,0.099)
	B4	396	nd	4.5	1042	0.87/0.84	0.33/0.31	3.30/3.20	(0.179,0.086)
C2PPI	B5	406	54	3.3	1900	1.83/1.57	1.69/0.88	5.55/4.76	(0.162,0.058)
	B6	406	56	3.3	1877	1.67/1.51	1.53/0.84	4.20/3.79	(0.165,0.067)
	B7	406	54	3.3	2057	1.78/1.64	1.62/0.89	4.63/4.27	(0.166,0.065)
	B8	406	53	3.3	2274	1.90/1.76	1.73/0.95	4.98/4.61	(0.166,0.064)

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Doped devices were fabricated using 5–20% C2MPI as the emitter and 2,8-bis(diphenylphosphoryl)dibenzo[b,d]furan (PPF) as the host and device B1 exhibited superior efficiency and colour purity to devices B2–B4. (Fig. S7–S10, ESI,†Table 3). Besides, changing the electron-transporting layer or adding additional carrier blocking layers in the devices doesn’t help to improve the efficiencies using C2PPI as the emitter (devices B6–B8). These results indicated that functional layers with more appropriate HOMO/LUMO alignments are needed to achieve better device performances in future work (Fig. S7–S10, ESI,†Table 3).

Conclusions

In this study, two highly twisted D–A molecules, carbazolyl and phenanthroimidazole end-capped with terphenyls, were designed and synthesized. The chromophores show good thermal and morphological stabilities. The dyes emitted NUV light in nonpolar solvents and in the solid states. Nondoped OLEDs were prepared and good EL efficiencies were achieved due to the utility of the triplet excitons via the HLCT process. The non-doped OLED, fabricated with C2MPI as the emitting-layer, radiates NUV light peaking at 404 nm with CIE_x,y of (0.161, 0.063) and a small FWHM of 48 nm. The device also showed a very high maximum EQE of 6.97%. DFT and TD-DFT calculations combined with photophysical investigation revealed the HLCT characteristic of these dyes and the high EUE was derived from the conversion of high-lying triplet excitons in the excited states. This study manifests that twisted terphenyl conjugated D–A molecules can be used as emitters to achieve high performance NUV OLEDs.

Experimental section

Synthesis. Compound 1. To a solution of 1,4-dibromobenze (1.3 g, 5.5 mmol), and chlorodimethylphenylsilane (2.0 mL, 12 mmol) in THF (20 mL), lithium diisopropylamide (6.0 mL, 2 M) was added at −78 °C under nitrogen. The resultant mixture was stirred for 1 h at −78 °C and then allowed to warm to room temperature. Upon completion, 20 mL of water was added to the reactants and the mixture was extracted with ethyl acetate (20 mL × 2). The combined ethyl acetate was washed with brine (20 mL), dried over sodium sulphate and evaporated to give an oil. This residue was purified to give 2.7 g of compound 1 in 96% yield. ¹H NMR (400 MHz, CDCl₃) δ 7.53–7.51 (m, 4H), 7.43 (s, 2H), 7.41–7.34 (m, 6H), 0.65 (s, 12H). ¹³C NMR (100 MHz, CDCl₃) δ 143.27, 141.01, 136.43, 134.22, 129.37, 127.88, −2.06. HR-MS: Calcd. For C₂₄H₂₄Br₂Si₂ [M + H]⁺ 502.9783. Found 502.9799.

Compound 2. Using 1,4-dibromobenze (2.6 g, 11.0 mmol), chlorodiphenylmethylsilane (5.6 g, 22.0 mmol), lithium diisopropylamide (12.0 mL, 2.0 M), and THF (20 mL), 4.0 g of compound 2 was prepared in 59% yield by the same method reported for compound 1. ¹H NMR (400 MHz, CDCl₃) δ 7.51–7.49 (m, 8H), 7.42–7.37 (m, 10H), 7.36–7.35 (m, 2H), 7.28 (s, 2H), 0.97 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 142.61, 142.31, 135.17, 134.62, 129.96, 128.00, −3.15. HR-MS: Calcd. For C₃₄H₂₈Br₂Si₂ [M + H]⁺ 627.0096. Found 627.0105.

Compound 3. A solution of compound 1 (2.7 g, 5.4 mmol), 9H-carbazole, 9-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl) phenyl]- (1.0 g, 2.7 mmol), potassium carbonate (0.3 g, 2.2 mmol), and tetrakis(triphenylphosphine)palladium (0.2 g, 0.14 mmol) in toluene (40 mL) and methanol (10 mL) was heated to 85 °C and stirred overnight under nitrogen. The reaction mixture was cooled and washed with H₂O and brine. The organic phase was collected, dried, and evaporated to a residue, which was columned to give compound 3 (0.85 g, 54%) as a white solid. ¹H NMR (400 MHz, CDCl₃) δ 8.09 (d, J = 8.0 Hz, 2H), 7.76 (s, 1H), 7.36 (m, 2H), 7.31–7.28 (m, 11H), 7.25–7.21 (m, 4H), 7.11 (d, J = 8.4 Hz, 2H), 0.64 (s, 6H), 0.26 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ 148.02, 143.99, 141.05, 139.85, 138.75, 138.30, 137.59, 136.36, 134.36, 134.02, 132.52, 131.14, 130.97, 129.36, 128.98, 128.04, 127.90, 126.36, 126.29, 126.06, 123.16, 120.46, 119.22, 109.95, −0.90, −2.28. HR-MS: Calcd. For C₄₀H₃₆BrNSi₂ [M + H]⁺ 666.1570. Found 666.1580.

Compound 4. Using compound 2 (4.0 g, 5.4 mmol), 9H-carbazole, 9-[4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]- (1.0 g, 2.7 mmol), potassium carbonate (0.3 g, 2.2 mmol), tetrakis(triphenylphosphine)palladium (0.25 g, 0.22 mmol), toluene (40 mL) and methanol (10 mL), 1.0 g of compound 4 was prepared in 47% yield by the same method reported for compound 3. ¹H NMR (400 MHz, CDCl₃) δ 8.16 (d, J = 7.6 Hz, 2H), 7.65–7.59 (m, 5H), 7.47–7.26 (m, 26H), 7.14 (d, J = 8.4 Hz, 1H), 1.04 (s, 2H), 0.90 (s, 1H), 0.40 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 146.86, 140.84, 140.72, 139.65, 138.94, 137.77, 136.11, 135.85, 135.35, 135.15, 135.10, 131.08, 130.86, 129.54, 129.24, 128.05, 127.97, 127.90, 126.11, 125.92, 123.40, 120.35, 12.00, 119.93, −2.39, −2.84. HR-MS: Calcd. For C₅₀H₄₀BrNSi₂ [M + H]⁺ 790.1883. Found 790.1895.

Compound 5. A mixture of 9,10-phenanthrenedione (2.1 g, 10.0 mmol), 4-tertbutylaniline (4.50 g, 0.05 mol), p-bromobenzaldehye (1.90 g, 10 mmol), ammonium acetate (3.10 g, 40.0 mmol) and acetic acid (150 mL) was added into a round-bottomed flask and degassed with nitrogen. The reaction was stirred under reflux for 24 h. The mixture was cooled to rt and filtered to give a white solid. The solid was washed with H₂O (80 mL) and 25 mL of methanol to afford 4.0 g of compound 5 as white powder in 67% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.85 (d, J = 6.8 Hz, 1H), 8.76 (d, J = 8.4 Hz, 1H), 8.70 (d, J = 8.4 Hz, 1H), 7.75–7.42 (m, 1H), 7.67–7.65 (m, 1H), 7.61 (d, J = 8.4 Hz, 2H), 7.53–7.49 (m, 1H), 7.47–7.45 (m, 2H), 7.42–7.40 (m, 4H), 7.19 (dd, J = 8.4, 0.8 Hz, 1H), 1.46 (s, 9H).

Compound 6. A mixture of compound 5 (5.1 g, 10.0 mmol), bis(pinacolato)diboron (3.0 g, 12.0 mmol), potassium acetate (3.0 g, 30.0 mmol), Pd(dppf)Cl₂ (0.2 g, 3.0 mmol) and dry dioxane (50 mL) was refluxed under nitrogen for 24 h. The reaction mixture was cooled to room temperature and filtered to give a grey solid, which was purified to give 4.1 g of compound 6 as a white solid in 74% yield. ¹H NMR (400 MHz, CDCl₃) δ 8.79 (d, J = 8.0 Hz, 1H), 8.66 (d, J = 8.4 Hz, 1H), 8.61 (d, J = 8.4 Hz, 1H), 7.63 (m, 3H), 7.55–7.48 (m, 5H), 7.42–7.38 (m, 1H), 7.32(d, J = 8.0 Hz, 2H), 7.18–7.14 (m, 2H), 7.06 (d, J = 8.0 Hz, 1H), 1.36 (s, 9H), 1.25 (s, 12H).

C2MPI. A solution of compound 6 (1.8 g, 3.3 mmol), compound 3 (0.7 g, 1.1 mmol), Pd(PPh₃)₄ (0.1 g, 0.1 mmol), aq. K₂CO₃ (0.5 mL, 2 M), toluene (50 mL) and methanol (10 mL) were stirred at 85 °C overnight under nitrogen. Upon completion, a filtrate was obtained by filtration, which was evaporated to afford a grey residue. The residue was columned and recrystallized to afford 234 mg of C2MPI as a white solid in 21% yield. ¹H NMR (400 MHz, CD₂Cl₂) δ 8.61 (d, J = 8.4 Hz, 2H), 8.56 (d, J = 8.4 Hz, 1H), 7.97 (d, J = 7.6 Hz, 2H), 7.59–7.55 (m, 1H), 7.49–7.47 (m, 3H), 7.43 (s, 1H), 7.40 (s, 1H), 7.32–7.26 (m, 5H), 7.25–7.21 (m, 8H), 7.17–7.09 (m, 14H), 6.90 (d, J = 8.0 Hz, 2H), 1.26 (s, 9H), 0.15 (s, 6H), 0.00 (s, 6H). ¹³C NMR (100 MHz, CD₂Cl₂) δ 153.75, 150.73, 148.36, 146.87, 144.77, 143.50, 141.26, 139.96, 138.05, 138.00, 136.99, 136.88, 136.12, 134.30, 134.21, 131.84, 129.71, 129.45, 129.24, 128.99, 128.57, 128.11, 127.70, 127.47, 126.73, 126.55, 126.35 125.90, 125.30, 124.41, 123.68, 123.60, 122.88, 121.41, 120.62, 120.31, 111.38, 34.89, 29.53, −0.45. HR-MS: Calcd. For C₇₁H₆₂N₃Si₂ [M + H]⁺ 1012.4486. Found 1012.4477.

C2PPI. The same procedure for C2MPI was used using compound 6 (2.5 g, 2.0 mmol), compound 4 (1.2 g, 0.6 mmol), Pd(PPh₃)₄ (0.2 g, 0.06 mmol), aq. K₂CO₃ (0.3 mL, 2 M), toluene (50 mL) and methanol (10 mL) to obtain 198 mg C2PPI as a white solid in 29% yield. ¹H NMR (400 MHz, CD₂Cl₂) δ 8.81 (d, J = 8.3 Hz, 2H), 8.76 (d, J = 8.1 Hz, 1H), 8.16 (d, J = 7.7 Hz, 2H), 7.78 (m, 1H), 7.68 (d, J = 8.5 Hz, 3H), 7.54–7.28 (m, 40H), 7.21 (d, J = 8.3 Hz, 2H), 6.98 (d, J = 8.0 Hz, 2H), 1.48 (s, 9H), 0.45 (s, 3H), 0.15 (s, 3H). ¹³C NMR (100 MHz, CDCl₃) δ 153.74, 147.35, 147.14, 143.12, 141.13, 139.07, 138.96, 137.73, 137.56, 136.85 136.70, 136.54, 135.49, 135.42, 130.95, 129.66, 129.63, 129.58, 129.42, 129.08, 128.92, 128.54, 128.25, 128.23, 127.67, 127.44, 126.70, 126.44, 125.15, 124.39, 123.58, 122.83, 121.37, 120.56, 120.28, 110.29, 34.74, 26.51, −2.52, −3.16. HR-MS: Calcd. For C₈₁H₆₆N₃Si₂ [M + H]⁺ 1136.4786. Found 1136.4790.

Spectroscopic measurements, Device fabrication and DFT calculations are displayed in the ESI.†

Author contributions

Z. Z. and Z. L. contributed equally to this work. All the authors were involved in the analysis and interpretation of data. Z. Zhong did the synthesis work, characterized the photophysical properties, and analyzed part of the data. Z. Liu fabricated the devices, did the EL measurements and analyzed the data. S. Geng and H. Li did part of the photophysical and thermal measurements. X. J. Feng prepared the original draft and wrote the manuscript. Z. Zhao prepared the original draft and wrote the manuscript. H. Lu prepared the original draft and wrote the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the National Science Foundation of China (21871072), the Natural Science Foundation of Guangdong Province (2019B030301003) and the Zhejiang Provincial Natural Science Foundation of China (ZJNSF LY17E030004) for financial support for this work.

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, additional spectroscopic properties, NMR spectra and HRMS spectra. See DOI: https://doi.org/10.1039/d2tc04421h

‡ These authors contributed equally to this work.

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