A diphenylamino-substituted quinacridone derivative: red fluorescence based on intramolecular charge-transfer transition

Abstract

Quinacridone (QA) derivatives are long-established emitting materials for use in organic light-emitting diodes (OLEDs). However, their role in OLEDs has generally been limited to green/yellow-green emitters. Herein, we report a QA-based red emissive OLED material, NPh₂-QA, constructed through the introduction of two strongly electron-donating diphenylamino groups onto the QA core of green emissive N,N′-dioctyl-substituted QA. The green-to-red change is due to an alteration of the electronic transition responsible for the population of the S₁ state, i.e. from a QA-centered π → π* transition to an intramolecular charge-transfer transition. NPh₂-QA can be easily synthesized and it exhibits efficient red emission, with a fluorescence quantum yield of 0.56 in toluene, as well as good thermal stability, with a decomposition temperature of 444 °C. By adopting NPh₂-QA as a dopant emitter, for the first time, an efficient and bright QA-based red OLED has been realized.

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Introduction

Quinacridone (QA, Chart 1) and its derivatives are widely used organic pigments with exceptional colour and weather fastness.¹ Moreover, they exhibit interesting supramolecular assembly² and polymorphism,³ are useful for field-effect transistors,⁴ and have photoluminescent,⁵ electroluminescent (EL)⁶ and photovoltaic⁷ properties. Among these properties, the EL behaviour, which has been realized in organic light-emitting diodes (OLEDs), is especially attractive as it can be utilized in flat-panel displays and lighting.^8–10 As QAs usually exhibit intense green/yellow-green emission originating from the π → π* transition of the QA backbone in the dispersed state,¹¹ they are usually used as green/yellow-green dopant emitters in OLEDs.⁶ Although a few QAs with slightly redder emission colours are known,^5c QAs emitting red light, an important light component for displays and lighting applications, have been seldom reported, and efficient red OLEDs based on QA derivatives have not been produced. Recently, we reported a red emissive QA derivative constructed through the expansion of π-conjugation by fusing four alkyl-substituted thiophene rings to the QA backbone.¹² However, difficulties encountered during vacuum deposition caused by the reasonably large molecular weight of this compound restricted its application in OLEDs to a certain extent. In addition, although some green-emissive QA derivatives show red emission in the solid state due to strong intermolecular interactions,^2c,5d these are also not suitable for use in red OLEDs because aggregation strongly quenches their emission intensity.

Chart 1 Molecular structures and transition types for NPh₂-QA and C₈-QA.

In our exploration of QA derivatives suitable for red OLEDs, we have attempted to construct donor–acceptor (D–A) systems by introducing strongly electron-donating substituents onto the QA backbone, because D–A systems featuring intramolecular charge-transfer (ICT) transitions can lower their emission energy.¹³ In line with this strategy, a QA derivative NPh₂-QA has been synthesized with two diphenylamino donor groups added onto the skeleton of the known parent molecule C₈-QA (Chart 1). This compound exhibits bright red emission in dilute toluene solution, and efficient, bright red OLEDs have been produced by employing it as a red dopant in the emitting layer. Herein, detailed photophysical, electrochemical, theoretical, thermal, and EL studies relating to NPh₂-QA are reported. In order to elucidate the effect of structural modification, the properties of NPh₂-QA and C₈-QA are compared.

Results and discussion

Synthesis of NPh₂-QA

The synthetic procedure of NPh₂-QA is illustrated in Scheme 1. Firstly, two octyl chains were introduced onto the QA core to provide a soluble derivative, C₈-QA, and then iodination was employed to afford the key precursor I₂-QA.^7h Finally, a Buchwald–Hartwig coupling of I₂-QA and diphenylamine provided the target compound NPh₂-QA. The compound shows excellent solubility in common organic solvents, such as toluene (>50 mg mL⁻¹). Starting from a cheap industrial QA dye, NPh₂-QA was readily and economically synthesized in three steps with an overall yield of 54%. This facile and low-cost synthesis is an advantage of NPh₂-QA in comparison with other red fluorescent materials, such as well known DCM-type dyes.¹⁴

Scheme 1 Procedure for the synthesis of NPh₂-QA.

Photophysical properties

The photophysical properties of NPh₂-QA and C₈-QA were preliminarily studied and compared in toluene. As shown in Fig. 1, C₈-QA exhibits absorption (λ_abs) and emission (λ_em) maxima at 514 and 525 nm, respectively, with a small Stokes shift of 408 cm⁻¹. In contrast, NPh₂-QA exhibits much longer λ_abs and λ_em values of 566 and 602 nm, respectively, with a moderate Stokes shift of 1057 cm⁻¹. The bathochromic shifts in λ_abs and λ_em are as large as 52 and 77 nm, respectively, reflecting the effect of the electron-donating diphenylamino groups. The larger Stokes shift of NPh₂-QA implies there is a larger degree of structural relaxation in the excited state.¹⁵ A comparison of photographs of the toluene solutions under daylight and UV light (365 nm) are much more impressive (Fig. 1). The yellow solution of C₈-QA exhibits intense green fluorescence with a high fluorescence quantum yield (Φ_F) of 0.96. In sharp contrast, the pink solution of NPh₂-QA shows strong red emission with a good Φ_F of 0.56.

Fig. 1 Absorption (solid line) and fluorescence (dashed line) spectra of C₈-QA and NPh₂-QA measured in toluene (ca. 10⁻⁵ M). Inset: photographs of the toluene solutions under daylight and UV light (365 nm).

Next, the transition types of the compounds from their ground to excited state were determined by studying their emission solvatochromism. As shown in Fig. 2, the fluorescence spectrum of C₈-QA is almost unchanged when increasing the solvent polarity in terms of λ_em and its spectral profile. This is consistent with a π → π* transition without ICT character. In contrast, the fluorescence spectrum of NPh₂-QA becomes red-shifted and broadens dramatically when changing from low-polarity toluene (λ_em = 602 nm) to highly polar CH₃CN (λ_em = 674 nm), indicating the existence of a highly polarized excited state.^13c,d,16 A large dipole-moment increase of 13.4 D from the ground to excited state, derived from the linear fit of a Lippert–Mataga plot (Fig. S1 and S2 and Table S1†), suggests an ICT transition for NPh₂-QA.¹⁷ Upon increasing solvent polarity, the Φ_F value for C₈-QA is almost unchanged, while that of NPh₂-QA exhibits a dramatic decrease from 0.56 in toluene to 0.02 in CH₃CN (Table S2†). This further confirms the proposed transition types of these compounds.

Fig. 2 Fluorescence spectra of C₈-QA (solid line) and NPh₂-QA (dashed line) in various solvents.

The emission properties of the compounds in solid thin films (thickness: ca. 100 nm) prepared by spin-coating of CHCl₃ solutions were also studied. NPh₂-QA shows a deep red emission (λ_em = 666 nm, Φ_F = 0.03) in the film state (Fig. S3†), which is much redder than the fluorescence of the C₈-QA film (λ_em = 557 nm, Φ_F = 0.06). A fluorescence microscopy image of the solid sample of NPh₂-QA is shown in Fig. S4.†

Electrochemical properties

The electrochemical properties of NPh₂-QA were characterized by cyclic voltammetry (CV) and compared with those of C₈-QA (Fig. 3). NPh₂-QA exhibits one reduction and two oxidation processes in its cyclic voltammogram, while only one reduction and one oxidation event can be observed for C₈-QA. The onset reduction potentials of NPh₂-QA (−1.86 V vs. Fc/Fc⁺) and C₈-QA (−1.84 V vs. Fc/Fc⁺) are very close. However, the onset oxidation potential of NPh₂-QA (0.10 V vs. Fc/Fc⁺) is significantly lower than that of C₈-QA (0.60 V vs. Fc/Fc⁺), which suggests that oxidation is easier after the introduction of the electron-rich diphenylamimo groups. From the CV data, the HOMO/LUMO energy levels were calculated to be −4.90/−2.94 and −5.40/−2.96 eV for NPh₂-QA and C₈-QA, respectively, by assuming that the HOMO level of ferrocene lies 4.8 eV (ref. 18) below the vacuum level. In comparison with C₈-QA, NPh₂-QA possesses a comparable LUMO level, but a much higher HOMO level. Correspondingly, NPh₂-QA possesses a smaller HOMO–LUMO gap (1.96 eV) compared to C₈-QA (2.44 eV), which may be related to the red-shifted absorption spectrum of NPh₂-QA.

Fig. 3 Cyclic voltammograms of NPh₂-QA and C₈-QA. The reduction (left) and oxidation (right) were measured in THF and CH₂Cl₂, respectively. The onset potentials are given against a ferrocene/ferrocenium (Fc/Fc⁺) redox couple.

Theoretical calculations

In order to gain deeper insight into the electronic structures and transition characters of the compounds, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were carried out at the B3LYP/6-31G* level. As shown in Fig. 4, the HOMO and LUMO of C₈-QA are both located on the π-skeleton of the QA core. In contrast, the orbitals of NPh₂-QA are distributed differently: the LUMO is located on the QA core, but the HOMO is dispersed over the whole π-system including the diphenylamino groups. The calculated LUMO levels of the two compounds are very close, but the calculated HOMO level of NPh₂-QA is much higher than that of C₈-QA (Fig. 4), which is consistent with the trend deduced from the CV data. The TDDFT results indicate that the S₀ → S₁ transitions of both compounds are dominated by HOMO → LUMO transitions. Therefore, population of the S₁ state in C₈-QA can be attributed to a π → π* transition localized on the QA core, while the S₀ → S₁ transition of NPh₂-QA features ICT from the diphenylamino groups to the QA core. The calculated excitation energy for the S₁ state of NPh₂-QA (2.24 eV) is significantly smaller than the corresponding value for C₈-QA (2.74 eV). This explains the trend observed in the absorption spectra and confirms that a conversion in the transitions from π → π* to ICT is responsible for the spectral red shift of NPh₂-QA.

Fig. 4 Calculated frontier molecular orbitals, energy levels, excitation energies and oscillator strengths for the two compounds. The octyl groups were replaced by methyl groups in order to simplify the calculations.

Thermal properties

The thermal properties of NPh₂-QA were studied using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). NPh₂-QA has a high decomposition temperature (T_d5, which corresponds to a 5% weight loss) of 444 °C and a high melting point (T_m) of 203 °C (Fig. S5 and S6†). These values are higher than the corresponding values for C₈-QA (T_d5 = 370 °C, T_m = 179 °C).^6h The improved thermal stability of NPh₂-QA is related to its larger molecular weight. In addition, NPh₂-QA shows a weaker crystallization tendency than C₈-QA as evidenced by the formation of a glass (glass transition temperature: 69 °C) for NPh₂-QA and the recrystallization (at 158 °C) of C₈-QA upon cooling melted samples of these compounds (Fig. S6 and S7†). The weaker crystallization tendency of NPh₂-QA could be explained by it having a more twisted π-skeleton caused by the introduction of the non-planar diphenylamino groups. The good thermal properties of NPh₂-QA are beneficial for enhancing device performance when used in OLEDs.

Electroluminescent properties

The electroluminescent properties of NPh₂-QA were studied using an OLED with a configuration of [ITO/NPB (40 nm)/Alq₃: 3 wt% NPh₂-QA (30 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al]. In the device, NPh₂-QA-doped Alq₃ (tris(8-hydroxyquinoline)aluminium) was used as the emitting layer; pure NPB (N,N′-di-(naphthalen-1-yl)-N,N′-diphenylbenzidine) and Alq₃ served as the hole-transporting and electron-transporting layers, respectively; and ITO (indium tin oxide) and LiF/Al were utilized as the anode and cathode, respectively. The device exhibited saturated red emission with an emission maximum at 628 nm and Commission Internationale de l’Éclairage (CIE) coordinates of (0.62, 0.36) (Fig. 5). The EL spectrum was stable over a variety of driving voltages (Fig. S8†). The red EL emission of NPh₂-QA is impressive considering that C₄-QA (a N,N′-dibutyl-substituted analogue of C₈-QA) exhibits green EL emission with an emission maximum at 540 nm and CIE coordinates of (0.32, 0.65) (Fig. 5b).^6f The NPh₂-QA-based device exhibited a turn-on voltage of 4.0 V, a maximum current efficiency of 1.08 cd A⁻¹, a maximum power efficiency of 0.85 lm W⁻¹, and a maximum external quantum efficiency of 1.05% (Fig. S9–S11†). The device was bright with a maximum luminance of 6239 cd m⁻² at 12.4 V (Fig. S9†). To the best of our knowledge, this is the first efficient and bright red OLED based on a QA derivative.

Fig. 5 (a) The EL spectrum of NPh₂-QA; (b) CIE coordinates for the EL devices based on NPh₂-QA and C₄-QA (a N,N′-dibutyl-substituted analogue of C₈-QA).

Conclusions

In summary, a red emissive QA derivative, NPh₂-QA, was constructed through the introduction of two strongly electron-donating diphenylamino groups onto the QA core of green emissive C₈-QA. The emission-colour change from green for C₈-QA to red for NPh₂-QA can be attributed to a conversion of the transitions from π → π* to ICT, as confirmed by both emission solvatochromism and theoretical studies. The facile syntheses, intense fluorescence and good thermal stability of NPh₂-QA enable it to act as a red emitter in OLEDs. By employing this compound as a dopant in the emitting layer, an efficient and bright QA-based red EL device has been produced for the first time. This study discloses an effective strategy for the design of red emissive QA derivatives and expands the utility of QA derivatives to red OLEDs. Further development of QA-based red emitters with improved EL performance is ongoing in our laboratory.

Experimental

General information

Anhydrous xylene used in the synthesis was distilled from sodium/benzophenone under a nitrogen atmosphere. Other solvents and reagents for organic synthesis were obtained from commercial sources and used without further purification. C₈-QA and I₂-QA were synthesized according to our previous work.^7h All reactions were performed under a N₂ atmosphere. ¹H and ¹³C{¹H} NMR spectra were recorded using a Bruker Avance 400 MHz spectrometer (400 MHz for ¹H and 100 MHz for ¹³C) in CD₂Cl₂. The chemical shifts in the ¹H NMR spectra are reported in ppm using the residual protons in the solvent as an internal standard, and those in the ¹³C{¹H} NMR spectra are also reported using the solvent signal as an internal standard. High-resolution mass spectra were measured with a Bruker micrOTOF Focus spectrometry system with the APCI ionization method . UV-vis absorption spectra were measured with a Shimadzu UV-2550 spectrometer. Emission spectra were measured with a PerkinElmer LS45 spectrometer. The Φ_F values given in the paper are absolute fluorescence quantum yields determined using an Edinburgh FLS920 spectrometer combined with a calibrated integrating sphere. The dilute solutions (ca. 10⁻⁵ M) were prepared in spectroscopic grade solvents for photophysical measurements. Cyclic voltammetry measurements were obtained using a BAS 100W instrument at a scan rate of 100 mV s⁻¹. A three-electrode configuration was used for the measurements with a platinum disk as the working electrode, a platinum wire as the counter electrode, and an Ag/Ag⁺ electrode as the reference electrode. A 0.1 M solution of tetrabutylammonium hexafluorophosphate (TBAPF) in dry CH₂Cl₂ (for oxidation) or THF (for reduction) was used as the supporting electrolyte. The calculations were performed using the Gaussian 09 program¹⁹ at the B3LYP/6-31G* level of theory. The long octyl groups were replaced by methyl groups in order to simplify the calculations. DSC measurements were performed on a NETZSCH DSC204 instrument at a heating rate of 10 °C min⁻¹ under nitrogen. Two heating–cooling cycles were measured. TGA measurements were performed on a TAQ500 thermogravimeter at a heating rate of 10 °C min⁻¹ under nitrogen.

Fabrication and characterization of the OLED devices

Electronic grade NPB, Alq₃ and LiF were obtained from commercial sources. Commercial ITO-coated glass (10 Ω per square) was used as the starting substrate. Before device fabrication, the ITO glass was carefully pre-cleaned and then treated with oxygen plasma for 2 min. NPB, NPh₂-QA-doped Alq₃, Alq₃ and a cathode composed of LiF and aluminium were sequentially deposited onto the ITO substrate by vacuum deposition at a pressure of 10⁻⁶ Torr. The electrical characteristics of the devices were measured with a Keithley 2400 sourcemeter. The EL spectra and luminance of the devices were obtained using a PR650 spectrometer. All measurements were carried out on the devices without encapsulation at room temperature under ambient conditions.

Synthesis of N,N′-Di(n-octyl)-2,9-bis(diphenylamino)quinacridone (NPh₂-QA)

To a mixture of I₂-QA (2.00 g, 2.54 mmol), diphenylamine (1.03 g, 6.09 mmol), t-BuOK (1.37 g, 12.2 mmol) and Pd(OAc)₂ (50 mg, 0.22 mmol) in anhydrous xylene (50 mL) was added t-Bu₃P (10% pentane solution, 0.50 mL, 0.21 mmol). The resulting mixture was then heated at reflux for 2 h. After cooling to room temperature, H₂O (50 mL) was added to quench the reaction. The mixture was extracted two times with EtOAc (100 mL). The combined organic phase was washed with H₂O (20 mL) and brine (20 mL), then dried over anhydrous Na₂SO₄ and filtered. After concentration of the filtrate under reduced pressure, the resulting solid was purified by silica gel column chromatography (hexane/CH₂Cl₂ = 2/5, R_f = 0.50) and subsequent recrystallization from CH₂Cl₂/hexane to afford 1.62 g (1.86 mmol, 73%) of NPh₂-QA as a purple powder. ¹H NMR (400 MHz, CD₂Cl₂): δ 8.66 (s, 2H), 8.17 (d, J = 2.8 Hz, 2H), 7.57 (dd, J = 9.2, 2.8 Hz, 2H), 7.48 (d, J = 9.2 Hz, 2H), 7.29 (t, J = 8.0 Hz, 8H), 7.13 (d, J = 8.8 Hz, 8H), 7.05 (t, J = 7.6 Hz, 4H), 4.46 (t, J = 6.8 Hz, 4H), 2.02–1.94 (m, 4H), 1.62–1.54 (m, 4H), 1.50–1.28 (m, 16H), 0.88 (t, J = 6.8 Hz, 6H); ¹³C{¹H} NMR (100 MHz, CD₂Cl₂): δ 177.5, 148.2, 141.8, 139.1, 135.9, 132.6, 129.8, 126.3, 124.2, 123.3, 122.4, 121.9, 116.7, 113.7, 46.7, 32.2, 29.7, 29.7, 27.5, 27.3, 23.0, 14.2; HRMS (APCI) m/z: 871.4932 [M + H]⁺ (calcd 871.4946).

Acknowledgements

This work was supported by the National Basic Research Program of China (2015CB655000), the National Natural Science Foundation of China (91333201) and Program for Chang Jiang Scholars and Innovative Research Team in University (No. IRT101713018).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: NMR spectra and additional thermal, photophysical and electroluminescence data and spectra. See DOI: 10.1039/c6ra01094f

‡ C. Wang and S. Wang contributed equally to this work.

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