AIE-active mechanochromic materials based N-phenylcarbazol-substituted tetraarylethene for OLED applications

Abstract

Multifunctional optoelectronic materials are important in both theoretical and practical aspects. In this study, three luminogens based on N-phenylcarbazol-substituted tetraarylethene, namely, NPCE, MeNPCE and MeONPCE, were designed and synthesised via Friedel–Crafts acylation and the McMurry coupling reaction. All of the luminogens show typical aggregation-induced emission (AIE) characteristics with high solid-state efficiencies of up to 83%. The dyes also exhibit excellent thermal stability (T_d of up to 434 °C) and prominent morphological stability. In addition, only MeONPCE reveals obvious mechanochromism (emission wavelength change of up to 64 nm). This proved that by introducing a methoxy group into one of the phenyl rings at the para position, mechanochromic materials can be easily obtained. The organic light-emitting diodes (OLEDs) that use these dyes as the nondoped emission layer emit cyan light with current efficiency and external quantum efficiency of 7.87 cd A⁻¹ and 3.87%, respectively. In comparison, the multilayer organic light-emitting diodes adopting MeONPCE as the nondoped emission layer reveal that MeONPCE is an eximious p-type light emitter.

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Introduction

Organic fluorescent dyes have drawn considerable attention because of their unique photochemical properties. Efficient organic fluorescent materials have been designed and synthesised for application in various fields, such as chemosensors and organic light-emitting diodes (OLEDs).^1–7 However, most traditional dyes suffer from the thorny aggregation-caused quenching (ACQ) problem due to molecular aggregation. Therefore, most practical applications of organic fluorescent materials were restricted, especially for optoelectronics and biophotonics.^8–11 Fortunately, Tang's group discovered an intriguing aggregation-induced emission (AIE) phenomenon in contrast with the ACQ effect; AIE dyes more efficiently emit in the aggregated state than in the dissolved form. The AIE phenomenon is caused by the restriction of intramolecular rotations (RIR) in the aggregated solid, which offers the possibility of obtaining high solid-state efficiency and of an active area of research for their potential applications.^12–14

Recently, a great deal of attention is being paid to the AIE-active mechanochromic materials and electroluminescent materials. On the one hand, solid emitters that show changes in fluorescence colour upon mechanical stimuli can be applied in memory chips, sensors, camouflaging security inks and other optoelectronic devices because of their fundamental importance and potential applications.^7,15–19 AIE molecules with a propeller-like twisted conformation can bring about loose molecular packings, which are easily destroyed under external mechanical stimuli, resulting in changes in fluorescence colour.¹⁶ Hence, AIE-active compounds have been considered as a well of mechanochromic materials. On the other hand, AIE molecules exhibit high solid fluorescence quantum efficiency owing to restrictions in intramolecular motion, which are helpful in enhancing the maximum external quantum efficiency (EQE_max) of the light emitting layer in OLEDs. Normally, AIE-active luminogens, even nondoped OLEDs, exhibit great performances owing to their high solid-state efficiencies.^20–22

Although AIE-active mechanochromic or electroluminescent luminogens are no longer novel materials after years of widespread investigation, multifunctional optoelectronic materials with AIE-active, mechanochromic and electroluminescence properties are rarely found. To achieve a multifunctional optoelectronic material, herein, a singly N-phenylcarbazol-substituted tetraarylethene luminogen, namely, MeONPCE (Chart 1), was designed and synthesised. Carbazole units in MeONPCE facilitate electroluminescence and hole transportation.^23,24 More importantly, methoxy groups can influence the packing pattern of MeONPCE in the solid state, thereby facilitating the procurement of mechanochromic materials by conversion of morphology or creating polymorphs, and enhancing solid state emission by restricting intramolecular rotation.^25,26 For comparison, NPCE and MeNPCE were also designed and synthesised (Chart 1).

Chart 1 Chemical structures and photophysical properties of NPCE, MeNPCE and MeONPCE; λ_aps and λ_gs stand for the maximum emission wavelength of as-prepared solid and ground solid, Φ_F,s and Φ_F,aps stand for the quantum efficiencies of the dye molecules in dilute solution and in as-prepared solid, respectively, α_AIE stands for the corresponding AIE factor.

The three compounds exhibit typical AIE-active properties, excellent thermal stability and high solid-state photoluminescence quantum yields (PLQY) of up to 83%. The organic light-emitting diodes utilizing these dyes as light emitter and hole transporter simultaneously emit cyan light with current efficiency and EQE of 7.87 cd A⁻¹ and 3.87%, respectively. As expected, MeONPCE also shows distinct mechanochromism (emission wavelength change of up to 64 nm) and hole-transporting properties as good as those of 4,4′-bis [N-(1-naphthyl)-N-phenyl-amino] biphenyl (NPB).

Results and discussion

Synthesis

The molecular structures of the three new AIE luminogens were designed. Scheme S1† shows the synthetic route of the carbazole-containing AIE luminogens NPCE, MeNPCE and MeONPCE. First, the Friedel–Crafts acylation of benzoyl chloride derivatives and 9-phenyl-carbazole synthesised 1a–1c,²⁷ which were converted into the three target compounds by a Zn/TiCl₄-catalysed McMurry coupling reaction afterwards.^28,29 All of the intermediates and final products were carefully purified and fully characterized by NMR and mass spectroscopy, from which satisfactory data corresponding to the expected molecular structures were obtained. Details of the synthetic procedures and characterization data are presented in the ESI.† McMurry coupling of NPCE, MeNPCE and MeONPCE, each resulted in two different isomers with a ratio of about 1 : 1, calculated from the ¹H NMR spectra. All of the luminogens are soluble in common organic solvents, such as tetrahydrofuran, chloroform, toluene and dichloromethane, but are insoluble in water.

Optical properties

When the target compounds were dissolved in pure THF, no visible light was observed under UV light, whereas their aggregated powders emitted bright yellow-green light and blue light. The emission spectra of the luminogens were further determined in THF and THF/water mixtures. Water was used because it is a typically poor solvent for the target compounds in which they are aggregated. The target compounds are almost non-fluorescent in pure THF solutions (10 μM). As depicted in Fig. 1A, at water fractions (f_w) ≤ 60%, the fluorescence intensity of MeONPCE shows extremely weak signals, which is attributed to the dissolved luminogens with active intramolecular rotations. When the water fraction is increased to 70%, the emission intensity of MeONPCE increases because the solvating power of the mixture is worsened, thereby leading to molecular aggregation. The molecular aggregation can result in planarized conformations, which generate increased effective conjugation lengths, thus producing much redder emissions³⁰ (Fig. 1A). Moreover, the mixture polarity also has a little impact on the spectral shift for D–π–D conjugated molecules. At a f_w of 90%, the emission is significantly strengthened by <144-fold compared with that in pure THF (Fig. 1B). Moreover, the contrast images given in the inset of Fig. 1B show the AIE nature of MeONPCE. Similar phenomena are also observed in NPCE and MeNPCE, with an emission enhancement of >111- and 102-fold, respectively, with 90% water content compared with those in pure THF (Fig. S9 and S10†). Clearly, the emission of the luminogens is induced by aggregate formation. In a dilute solution, the rotation of multiple phenyl rings effectively consumes exciton energies and the molecules are nonluminescent in solution. In the aggregated state, the restriction of RIR allows the dye molecules to emit intensely. However, the aqueous mixtures show no precipitation in the macroscopic view, indicating that the aggregates are nanodimensional. To prove this point, transmission electron microscopy (TEM) images were obtained (Fig. S11†) and the absorption of the luminogens in THF and THF/water mixtures were tested. As shown in Fig. S12,† their absorption spectra show obvious leveling-off tails in the long-wavelength region with high water content, which is due to the scattering effect of the luminogen nanoparticles.^31,32

Fig. 1 (A) Emission spectra of MeONPCE in THF and THF/water mixtures with varying water fractions (f_w). (B) Emission intensity increase of MeONPCE in different aqueous mixtures. Concentration: 10 μM; excitation wavelength: 360 nm. The images in (B) are MeONPCE in THF and in 10/90 THF-water mixture taken under 365 nm UV light illumination.

To evaluate the emissions quantitatively, the quantum efficiencies of the dye molecules in dilute solution (Φ_F,s) and in as-prepared solid (Φ_F,aps) states were further investigated. The Φ_F,s values of NPCE, MeNPCE and MeONPCE are as low as 0.1%, 0.1% and 0.1%, respectively, which were estimated in THF using quinine sulfate (Φ_F,s = 54% in 0.1 N H₂SO₄) as standard. However, the Φ_F,aps of NPCE, MeNPCE and MeONPCE, which were measured using a calibrated integrating sphere, are as high as 82%, 66% and 83%, respectively, with the corresponding AIE factors (α_AIE = Φ_F,aps/Φ_F,s) of 820, 660 and 830 (Chart 1). These results further validate their AIE activity. In addition, MeNPCE shows a lower efficiency of 66% than NPCE and MeONPCE, which might be attributed to the vibration and rotation of the methyl group, thereby leading to exciton energy consumption even in the solid state.⁸

Mechanochromism

The AIE characteristic and high solid-state efficiency render the dyes promising mechanochromic materials.¹⁶ Therefore, their solid emission properties were examined by grinding the as-prepared solids of the luminogens. Interestingly, aside from the high quantum efficiencies of the solid states, another remarkable difference was that the luminogen MeONPCE showed mechanochromic fluorescent behaviour. As shown in Fig. 2, the as-prepared solid powder of MeONPCE emits a sky-blue light at 441 nm wavelength. When the as-prepared solid powder was ground with a mortar, the sky-blue powder turned into cyan-emissive solids with a maximum emission at 505 nm. Normally, luminogens with mechanochromic character can be easily restored by annealing or fuming treatment. However, when MeONPCE was heated at 60, 80, 100 and 120 °C for more than 5 h (even overnight), the cyan emission colours could not be restored. The result proves its stable conformations due to the high glass-transition temperature (T_g = 134.2 °C) of the ground powders (Fig. 3A). Such steady conformation and emission are beneficial for optoelectronic applications.³³ Nevertheless, upon further fuming with dichloromethane or ethyl acetate vapour for 3 min, the original blue emission is recovered. The reversibility of mechanochromic conversion was checked by the grinding-vapour exposure processes. As depicted in Fig. S13,† switching between blue and green emission colours can be repeated with many cycles without fatigue because of the non-destructive nature of the mechanical stimuli. These surveys further demonstrate the remarkable morphological stability of the amorphous solids. The mechanochromic fluorescent behaviour of luminogens normally transforms from crystals to amorphous solids,^34–36 and the amorphous solids are always applied in OLEDs. Thus, the research suggests that MeONPCE has potential applications for EL materials because of its excellent thermal stability and efficient solid-state emission.

Fig. 2 (A) Emission spectra of the as-prepared, ground and fumed MeONPCE solids, (B) their XRD patterns and (C and D) their images taken under UV illumination.

Fig. 3 (A) DSC of the as-prepared and ground MeONPCE solids. (B) TGA thermograms of MeONPCE recorded under nitrogen atmosphere at 10 °C min⁻¹ scan rates.

To obtain information on the mechanism, powder X-ray diffraction (XRD) analysis was conducted. The as-prepared powder exhibits a very intense and sharp diffraction peak, as shown in Fig. 2B, which is indicative of the regular crystalline structure. No diffraction peak is observed with mechanical grinding, which reflects a disordered molecular packing. When fumed with solvent, sharp diffraction peaks emerge again, which indicated the restoration of an ordered crystalline lattice. The results further prove that mechanochromism was extremely associated with the arrangement of the molecules, which highly influenced the photophysical properties.³⁷ However, similar phenomena are not observed in NPCE and MeNPCE. Mechanochromism usually depends on the mode of molecular packing.¹⁶ This further proved that by introducing a methoxy group in one of the phenyl rings at the para position, mechanochromic materials can be easily obtained.

Thermal and electrochemical properties

In terms of device fabrication and operation, high thermal and morphological stability is vitally important. Thus, the thermal properties of the luminogens were evaluated. Thermogravimetric analysis (TGA) (Fig. 3 and S14†) results indicated that all the luminogens are thermally stable, with T_d (defined as the temperature at which a sample loses its 5% weight) values from 278 to 434 °C. The curves obtained by differential scanning calorimetry (DSC) (Fig. 3A), which was used to study the behaviour of as-prepared MeONPCE under heating, showed two glass-transition temperatures (T_g1 = 35.9 °C and T_g2 = 93.9 °C) caused by the coexistence of cis- and trans-isomers of the MeONPCE molecule. The T_g value of NPCE and MeNPCE are as high as 106.6 and 138.6 °C, respectively. These performances are sufficient for their potential applications in OLED preparation.

Carbazole derivatives are known to possess high energy levels of the highest occupied molecular orbital (HOMO) and excellent charge transport properties.³⁸ Thus, the electrochemical properties of the luminogens were investigated by cyclic voltammetry at a 50 mV s⁻¹ scan rate. As shown in Fig. 4, MeONPCE displayed a reversible oxidation process in dry DCM solution. The oxidation peak was found at 0.80 V. Therefore, a HOMO energy level was obtained from the onset oxidation potential corresponding to −5.2 eV. The E_g value of 3.34 eV was obtained from solution absorption spectra according to the published method.^39,40 The LUMO of MeONPCE was −1.86 eV, obtained by subtracting its E_g value from its HOMO value. Similar results were obtained in NPCE and MeNPCE (Fig. S15†). The band gap energies and HOMO and LUMO levels of NPCE, MeNPCE and MeONPCE are summarized in Table S1.† The electrochemical properties that characterize the dye molecules can be applied in electroluminescent devices.⁴¹

Fig. 4 Representative cyclic voltammogram of MeONPCE measured in dry dichloromethane solution with 0.1 M TBAPF₆ at 25 °C.

Electroluminescence properties

The high photoluminescence (PL) efficiency and good thermal stability of the luminogens encouraged us to evaluate their electroluminescence (EL) properties. Three simple OLEDs were fabricated with the non-doped dyes as the emissive layer. NPB doped with 6% 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) was used as the hole-injection layer (HIL), and 1,3,5-tris(N-phenylbenzimidazole-2-yl)benzene (TPBI) was used as the electron-transporting layer, followed by 1 nm LiF as the electron injection layer. Hence, the basic device structure is ITO/NPB:F4-TCNQ (20 nm, 6%)/X (30 nm)/TPBI (40 nm)/LiF (1 nm)/Al, where X is NPCE (device I), MeNPCE (device II) or MeONPCE (device III). All devices emit cyan light with maximum EL (λ_EL) of 498, 487 and 492 nm, respectively, which are similar to the PL emissions of the amorphous solids (502, 498 and 499 nm, respectively). The results reveal that the EL originated from the amorphous film (Fig. S16†). Moreover, the device with MeNPCE as emissive layer (EML) exhibits a maximum EQE of 3.87%, which is quite high for a non-doped EML OLED (Table 1). All three devices have turn-on voltages below 3.5 V, which strongly suggest that NPCE, MeNPCE and MeONPCE possess good hole-transporting properties. The three AIE materials possess a slightly higher HOMO level than NPB (5.4 eV); hence, the hole-injection barrier between HIL and EML can be neglected. Therefore, the order of the hole-transporting capability can be roughly estimated from their current density–voltage (J–V) characteristics (Fig. 5A): MeONPCE > MeNPCE > NPCE. This order agrees well with the donor power of the three compounds.

Table 1 EL performances of NPCE, MeNPCE and MeONPCE^a

Device	λmax (nm)	Von (V)	CEma (cd A^−1)	EQE (%)
a Device structures: without HTL, ITO/NPB:F4-TCNQ (20 nm, 6%)/X (30 nm)/TPBI (40 nm)/LiF (1 nm)/Al; X = NPCE (I), MeNPCE (II), MeONPCE (III); with HTL, ITO/NPB:F4-TCNQ (20 nm, 6%)/NPB(10 nm)MeONPCE (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (IV). Abbreviations: λEL = EL maximum, Von = turn-on voltage at 1 cd m^−2, CEmax = maximum current efficiency, EQEmax = maximum external quantum efficiency.
I	498	4.4	6.45	2.77
II	485	4.4	7.87	3.87
III	490	3.2	6.44	2.90
IV	494	3.2	6.82	2.75

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Fig. 5 (A) Current density and (B) luminance vs. voltage characteristics of the four devices.

For comparison, a device with a 10 nm NPB instead of 10 nm dye layer in device III was constructed, and its configuration is ITO/NPB:F4-TCNQ (20 nm, 6%)/NPB (10 nm)/MeONPCE (20 nm)/TPBI (40 nm)/LiF (1 nm)/Al (device IV). Devices IV and III had almost identical J–V characteristics (Fig. 5A), which was a strong hint that MeONPCE has similar hole-transporting capability to NPB. Significantly, the CIE coordinates were almost unchanged from (0.23, 0.37) to (0.23, 0.39), as shown in Fig. S17.† These results imply the inherent hole-transporting capability of carbazole-containing luminogens. Fig. 5B shows that all the devices had better luminance when the current was set to compliance. Fig. 6 describes the current efficiency (CE) and EQE vs. voltage characteristics of the four devices. All EL data are summarized in Table 1. Although these devices are not yet further optimized, the results clearly prove that carbazole-containing AIE luminogens as solid light emitters or hole-transporting materials are promising for the construction of advanced OLEDs.

Fig. 6 (A) Current efficiency and (B) external quantum efficiency (%) vs. voltage characteristics of the four devices.

Conclusions

The rational design of luminescent molecules is of importance to achieving efficient multifunctional optoelectronic materials with AIE-active, mechanochromic and electroluminescence properties. In this study, three dyes based on N-phenylcarbazol-substituted tetraarylethene, namely, NPCE, MeNPCE and MeONPCE, were designed and synthesized. The impacts of substituent groups in tetraarylethene on AIE-active, mechanochromic and electroluminescence properties of the luminogens were investigated in detail. With the increase of the electron-donating ability of the substituents, it was found that the hole-transporting properties were enhanced in the OLEDs. The three luminogens, all show typical AIE characteristics with high solid-state efficiency of up to 83%, excellent thermal stability (T_d up to 434 °C) and high morphological stabilities. Among them, only MeONPCE exhibits remarkable mechanochromic conversion ( up to 64 nm). Moreover, the mechanochromism can be repeatedly switched many times without fatigue by simple grinding–fuming processes, which indicated that the introduction of hydrophilic groups in hydrophobic tetraarylethene can easily change the molecule packing pattern on the solid state to obtain mechanochromic materials. The OLEDs that were fabricated using the luminogens as both hole-transporting and light-emitting layer materials showed high CE_max and EQE of up to 7.87 cd A⁻¹ and 3.87%, respectively. To compare MeONPCE with a traditional hole-transporting material, a device with HTL was constructed, and the results indicate that the hole-transporting ability of NPB and MeONPCE are almost the same. The results show that carbazole-containing AIE luminogens are promising solid light emitters or hole-transporting materials for the construction of advanced OLEDs.

Acknowledgements

This work was financially supported by the Key Programs for Science and Technology Development of Shihezi University (Rczx201017, GXJS2013-ZDGG02) and the Postgraduate Technology Innovation Program of Xinjiang (XJGRI2014052).

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, characterization data, absorption and emission spectra, and other materials of synthetic procedures. See DOI: 10.1039/c4ra16069j

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