A bipolar host based high triplet energy electroplex for an over 10 000 h lifetime in pure blue phosphorescent organic light-emitting diodes

Abstract

Ultimate device performances of blue phosphorescent organic light-emitting diodes, an external quantum efficiency of 27.6%, a device lifetime over 10 000 h at 100 cd m⁻², and pure blue color coordinates of (0.12, 0.13), were simultaneously achieved using a device architecture employing an electroplex host. A high triplet energy electron transport type host derived from a triazine core, a carbazole substituent, and a triphenylsilyl blocking unit was newly synthesized for the purpose of developing the high emission energy electroplex host. The electroplex host functioned as a carrier balancing host for high external quantum efficiency and a lifetime extending host of pure blue phosphorescent organic light-emitting diodes. This work demonstrates ultimate device performances, high external quantum efficiency and long lifetime, in pure blue phosphorescent organic light-emitting diodes. In particular, the lifetime limit of pure blue phosphorescent organic light-emitting diodes was overcome by demonstrating an over 10 000 h lifetime.

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The device performances of phosphorescent organic light-emitting diodes (PhOLEDs) have been advancing for the last twenty years^1–10 to meet the technical specification of commercial products in red and green organic light-emitting diodes. However, the performance level of blue PhOLEDs is still far from a practical level owing to a device lifetime insufficient for commercial use. Therefore, lifetime extension of blue PhOLEDs is the most urgent task to be resolved in OLED development.

In order to extend the lifetime, understanding of the degradation processes and proposing solutions to avoid the degradation pathways are essential. Several key degradation mechanisms have been proven to be critical to the device lifetime of blue PhOLEDs^11–13 and one of the most serious mechanisms is triplet exciton triggered degradation by triplet–triplet annihilation (TTA)^14–16 and triplet-polaron annihilation (TPA).^17–21 Therefore, a device development strategy to circumvent the TTA and TPA processes has been attempted for extended device lifetime.^22–24 One of the most powerful examples of TTA and TPA managing tactics in PhOLEDs is to employ an exciplex host which is a mixture of a hole transport type host and an electron transport type host.^25,26 The exciplex host can handle TTA with a broad emission zone and TPA by minimizing carrier trapping in the hosts and phosphors through exciplex formation in the host followed by energy transfer to the phosphors.^27,28 Therefore, exciplex hosts have been an effective strategy to elongate the device lifetime of PhOLEDs. However, the challenging issue of exciplex hosts is to increase the triplet energy because the singlet and triplet energies of the exciplex are diminished from those of the hosts constituting the exciplex. Although several exciplex hosts for blue phosphors have been reported in the literature, only a few exciplex hosts could harvest triplet excitons of pure blue phosphors. Furthermore, no exciplex host for pure blue phosphors can ensure the device lifetime. Only exciplex hosts for sky blue phosphors could show an extended lifetime.

An alternative to an exciplex host to suppress TTA and TPA is an electroplex host. An electroplex host performs like an exciplex host in that excitons are created in the host through complex formation and energy transfer is the main emission mechanism.²⁹ The difference is the driving force for the exciton formation because the electroplex is not generated without an electric field. Although exciplex and electroplex hosts behave similarly, electroplex hosts are advantageous for pure blue PhOLED applications because high emission energy can be obtained by weak binding of hole transport and electron transport type hosts. However, electroplex hosts for pure blue phosphors have not been reported and development of them is necessary to realize high efficiency and long lifetime pure blue PhOLEDs.

In this work, we developed a high emission energy electroplex host for application in high efficiency and long lifetime pure blue PhOLEDs. An electron transport type host derived from a carbazole substituted diphenyltriazine core, 9-(4-phenyl-6-(3-(triphenylsilyl)phenyl)-1,3,5-triazin-2-yl)-9H-carbazole (SiCzTrz), was synthesized to generate a high emission energy electroplex host with a common 3,3-di(9H-carbazol-9-yl)biphenyl (mCBP) host. The mCBP:SiCzTrz electroplex host was evaluated as the host of a pure blue-emitting fac-tris(5-(tert-butyl)-1,3-diphenyl-2,3-dihydro-1H-imidazo[4,5-b]pyrazine)iridium (Ir(cb)₃) phosphorescent emitter and provided a high external quantum efficiency (EQE) of 27.6% and an extrapolated half luminance lifetime (LT₅₀) over 10 000 h at an initial luminance of 100 cd m⁻² along with pure blue color coordinates of (0.12, 0.13). This work is the first demonstration of a high EQE over 25% and a more than 10 000 h lifetime in pure blue PhOLEDs with a y color coordinate below 0.15. The electroplex host enabled the simultaneous achievement of the high EQE, pure blue color coordinates, and long lifetime in pure blue PhOLEDs.

The SiCzTrz host used to develop the elecroplex host had a carbazole modified diphenyltriazine core structure surrounded by triphenylsilyl protecting groups. The carbazole unit was substituted onto the diphenyltriazine core structure to weaken the acceptor strength of the host for high emission energy in the electroplex host. The triphenylsilyl group was introduced into the core structure to increase the emission energy by suppressing intermolecular interaction between mCBP and SiCzTrz by spatially separating the triazine core structure from the mCBP host. They also can prevent excimer formation or strong intermolecular packing originating from the planar structure of the triphenyltriazine unit. The excimer or intermolecular packing reduces the emission energy of the host in the solid state and induces exciton quenching of the emitters. Therefore, the introduction of the triphenylsilyl group can increase the emission energy of the electroplex host in the solid state. Moreover, the triphenylsilyl group can improve the electron transport properties of the host by the hyperconjugation effect. The combination of the triphenylsilyl blocking group and the carbazole substituent on the triazine core structure was the strategy to develop an electroplex host for high EQE and long lifetime in pure blue PhOLEDs.

SiCzTrz was prepared by a three-step synthetic procedure as shown in Scheme 1. A brominated tetraphenylsilyl intermediate was prepared from triphenylsilylchloride and was attached to the triazine core structure by a Grignard's reaction procedure. The triphenylsilyl group modified intermediate was substituted with a carbazole unit for target molecule preparation. The final product was thoroughly purified by consecutive purification processes of column chromatography, recrystallization and vacuum sublimation to remove impurities harmful to the device operation lifetime. The purity of the final product was over 99.9% from high performance liquid chromatography analysis. Confirmation of the final chemical structure was carried out using chemical analytical methods such as ¹H and ¹³C nuclear magnetic resonance spectrometry, mass spectrometry and elemental analysis.

Scheme 1 Synthesis scheme of SiCzTrz (a) and chemical structures of mCBP and Ir(cb)₃ (b).

The basic photophysical properties of SiCzTrz were analyzed to justify the potential as a high triplet energy n-type host for pure blue phosphors. Ultraviolet-visible (UV-vis), fluorescence and low temperature (77 K) phosphorescence spectra were collected to understand the photophysical properties of the host (Fig. 1(a)). The UV-vis absorption spectra of the SiCzTrz host reflected strong π–π* absorption of the backbone structure with an absorption onset energy of 3.31 eV. The fluorescence spectrum of SiCzTrz provided a singlet energy of 3.23 eV and the phosphorescence spectrum in frozen tetrahydrofuran solution at 77 K delivered a triplet energy of 3.10 eV from the onset of the spectrum. The singlet and triplet energies of SiCzTrz were over 3.10 eV, which were high enough for energy transfer to a pure blue-emitting phosphor with a triplet energy of ∼2.80 eV. The solid state triplet energy of the host was also measured using the neat film at 77 K and it was 2.91 eV (Fig. 1(b)). The triplet energy was over 2.90 eV in the solid state due to the intermolecular packing inhibiting function of the triphenylsilyl protecting group. The rather broad emission spectrum of SiCzTrz in THF is caused by the charge transfer character stabilized by the polar THF solvent.

Fig. 1 (a) The UV-vis, fluorescence and phosphorescence spectra of SiCzTrz. The UV-vis and fluorescence spectra were measured in tetrahydrofuran solution. The phosphorescence spectrum was measured in frozen tetrahydrofuran solution at 77 K. (b) Low temperature (77 K) phosphorescence spectrum of the SiCzTrz neat film.

The highest occupied molecular orbital (HOMO) of SiCzTrz was estimated from the electrochemically determined oxidation potential (Fig. S1 in the ESI†).

The lowest unoccupied molecular orbital (LUMO) was estimated from the optical band gap of the UV-vis absorption spectrum and electrochemically determined HOMO. The HOMO/LUMO level of the SiCzTrz host was −6.06/−2.75 eV from the electrochemically determined HOMO and HOMO–LUMO gap from the UV-vis absorption spectrum. The HOMO level of SiCzTrz was relatively deep compared to that of mCBP due to the electron deficient triazine unit, and the LUMO level was also deep due to the electron poor triphenyltriazine core structure. This can be confirmed from the HOMO and LUMO simulation results in Fig. S2 (ESI†) calculated using the B3LYP 6-31G* level of theory with Gaussian 16 simulation software. The HOMO was dominated by the carbazole unit and the LUMO was governed by the diphenyltriazine unit. In particular, the LUMO level was much deeper than that of the common hole transport type host and was appropriate for electroplex formation with the carbazole based host.

The electron transport character of SiCzTrz was confirmed by current density data of single carrier devices. Fig. S3 (ESI†) shows the current density comparison of the hole only and electron only devices of SiCzTrz. The single carrier device data of SiCzTrz clearly proves that SiCzTrz is an electron transport type host with a higher electron current density than hole current density. Although carbazole was substituted onto the triazine core structure, the carrier transport properties of SiCzTrz were governed by the electron poor triazine core structure.

Basic material properties such as the glass transition temperature and thermal decomposition temperature are described in Fig. S4 in the ESI.† The glass transition temperature and the thermal decomposition temperature of the SiCzTrz host were 98.5 and 449 °C, respectively, demonstrating the thermal stability of SiCzTrz in the device operation and device fabrication processes. The rigid planar structure induced by hydrogen bonding between the triazine core structure and phenyl units thermally stabilized the SiCzTrz host, supporting the appropriateness of SiCzTrz as the host of OLEDs.

After confirming the basic material characteristics of SiCzTrz as the host of pure blue phosphors, SiCzTrz was mixed with a p-type mCBP host to develop a mixed host. Firstly, the electroplex formation between mCBP and SiCzTrz developed in this work was investigated. Prior to confirmation of the electroplex formation, exciplex formation between mCBP and SiCzTrz was examined by mixing the two hosts at a ratio of 50 : 50. The photoluminescence (PL) spectra of each host and the mixed host are shown in Fig. 2(a). Compared with the PL spectra of each host, the PL spectrum of the mixed host was relatively shifted to short wavelength relative to that of the SiCzTrz neat film, confirming that an exciplex is not formed in the mixed host. The short wavelength shift of the emission in the mixed host is due to the charge transfer (CT) nature of SiCzTrz. As mCBP is a non-polar host, the mixing of SiCzTrz with mCBP weakened the intermolecular interaction of the CT type SiCzTrz host, shifting the emission wavelength to short wavelength. Therefore, the mixed host of mCBP:SiCzTrz can be considered as an exciplex free mixed host.

Fig. 2 (a) PL spectra of mCBP, SiCzTrz and mCBP:SiCztrz solid films and (b) EL spectra of the device with the mCBP:SiCzTrz mixed host as an emitting layer.

The electroplex formation in the mixed host was identified by the red-shifted emission spectra in the electroluminescence (EL) device with the mixed host as the emitting layer (Fig. 2(b)). The device was fabricated as the following structure for EL measurements: ITO (50 nm)/BPBPA:HATCN (40 nm, HATCN 30%)/BPBPA (10 nm)/mCBP (10 nm)/mCBP:SiCzTrz (25 nm, 50 : 50%)/DBFTrz (5 nm)/ZADN (20 nm)/LiF (1.5 nm)/Al (200 nm). The peak wavelength of the mCBP:SiCzTrz electroplex was 460 nm relative to the 429.5 nm (PL peak wavelength) of the mCBP:SiCzTrz mixed film. The EL spectrum of mCBP:SiCzTrz appeared at a longer wavelength than that of the PL spectrum, confirming the complex formation in the EL device. The emission energy of the mCBP:SiCzTrz electroplex was 3.06 eV from the onset energy of the EL spectrum. The small HOMO level offset prevented exciplex formation between mCBP and SiCzTrz, but the large LUMO level offset drove the electroplex formation in the mixed host. The short wavelength shoulder in the EL spectra is due to extra hole transport layer emission by electron leakage.

As the electroplex emission energy was over 3.00 eV for exothermic energy transfer, the electroplex host was doped with a pure blue-emitting Ir(cb)₃ emitter to develop high efficiency and long lifetime pure blue PhOLEDs. The composition of the mCBP : SiCzTrz host was 50 : 50 in all electroplex hosts. The doping concentration of the Ir(cb)₃ emitter was 20%. The current density–voltage–luminance, EQE–luminance and electroluminescence (EL) spectra of the pure blue PhOLEDs are presented in Fig. 3(a)–(c). Three devices were fabricated to examine the device performances in different device architectures. A common device platform of ITO/HAT-CN/BPBPA/EBL/EML/DBFTrz/ZADN/LiF/Al was shared in the devices. The electron blocking layer (EBL) was PCZAC in device I and mCBP in device II. Device I and device II were bottom-emitting type devices. Additionally, device III with an mCBP blocking layer and top-emitting device structure instead of a bottom-emitting device structure was also fabricated. The detailed device structures are in the ESI.†

Fig. 3 The device performances of the 20% Ir(cb)₃ doped mCBP:SiCzTrz device. (a) Current density–voltage–luminance, (b) external quantum efficiency–luminance, and (c) EL spectra of pure blue PhOLEDs.

The current density–voltage data of the blue PhOLEDs show that device II injects holes more efficiently than device I due to the small energy barrier for hole injection between the exciton blocking layer and emitting layer. This can be confirmed in the energy level diagram in Fig. S5 of the ESI.† The turn-on voltages of device I and II defined as the voltage at 1 cd m⁻² were 2.7 and 2.4 V. However, the driving voltages at 100/1000 cd m⁻² were 3.8/4.8 and 4.1/5.2 V in device I and II, respectively, because of the good hole transport properties of PCZAC in spite of the energy barrier for hole injection. The driving voltages at 100 and 1000 cd m⁻² were further reduced in device III and were 3.4 and 4.7 V, respectively, due to the high EQE as shown in Fig. 3(b). The high EQE of device III due to the microcavity effect lowered the driving voltage. The turn-on voltage calculated from the onset point of the current density increase was similar in device II and III. Considering the turn-on voltage of 3.0 V and the driving voltage at 1000 cd m⁻² of 5.4 V in the mCBP reference device, the electroplex host based blue PhOLEDs exhibited significantly improved driving conditions of the devices due to facilitated electron injection through the SiCzTrz electron transport type host. This suggests that the SiCzTrz host is effective for electron injection and transport as an electron transport type host.

The EQE of device II was also much higher than that of device I. The maximum EQEs of device I and device II were 15.6 and 20.3%, respectively. As the PCZAC and mCBP EBLs have high triplet energy for triplet exciton blocking, the high EQE in device II might be due to optimized carrier balance. The lower current density of device II than that of device I suggests reduced hole density due to the large hole injection barrier as shown in the energy level diagram. The reduced hole density balanced injected carriers and enhanced the EQE of device II. This indicates that the electroplex host of mCBP:SiCzTrz fully harvested the triplet emission of the Ir(cb)₃ phosphor. The EQE of device II was much higher than that of the reference devices with the mCBP host in place of the mCBP:SiCzTrz electroplex host. The high EQE is due to the good electron transport properties for carrier balance and high triplet energy of the SiCzTrz host in the solid film due to the triphenylsilyl blocking group for efficient energy transfer. The high triplet energy of the SiCzTrz host in the solid film effectively harvested the triplet excitons of Ir(cb)₃ with little quenching by the host and the good electron transport properties due to the triphenylsilyl group additionally balanced the carriers. The EQE was even further enhanced in device III with the top-emitting device structure and the maximum EQE was 27.6% in device III. The microcavity effect in the top-emitting devices intensified the pure blue emission of Ir(cb)₃ and increased the EQE. The EQEs at 100 and 1000 cd m⁻² were also 27.6 and 25.6%, respectively, suggesting that the electroplex host is effective in managing the efficiency roll-off of pure blue PhOLEDs. The balanced carrier injection through the mCBP hole transport type host and SiCzTrz electron transport type host allowed high EQEs over a wide luminance range from 0 to 1000 cd m⁻². The maximum EQE, EQE at 100 cd m⁻² and EQE at 1000 cd m⁻² of device II were 20.3, 20.2, and 18.6%, respectively.

The EL spectra of the blue PhOLEDs are similar to each other in that the peak position of the EL spectra is 471 nm. However, the full width half maximum (FWHM) values of device I, II and III were 61, 60, and 36 nm, respectively. The top-emitting device structure of device III sharpened the EL spectrum by the microcavity effect. Therefore, the color coordinates of device I, II and III were (0.14, 0.19), (0.14, 0.19) and (0.12, 0.13), respectively. A pure blue emission color with a y color coordinate below 0.20 was obtained in the bottom-emitting device and the blue emission color was further deepened in the top-emitting device. The color coordinates of the devices were quite stable with the driving voltage although a little change of the color coordinates was observed possibly due to recombination zone change at different driving voltages (Table S2, ESI†).

The device lifetime of device I, II and III was investigated by recording the luminance change in constant current operation mode (Fig. 4). The luminance change of the devices against the device operation time at 3000 cd m⁻² shows that the device lifetimes of device I, II, and III at 50% of the initial luminance (LT₅₀) are 12.9, 18.6, and 23.5 h in contrast to 5.4 h of the reference devices. This supports the lifetime extending role of the mCBP:SiCzTrz electroplex host. As reported in our previous work, the lifetime extension of blue PhOLEDs by an electroplex host can be explained by reduced TTA and TPA mechanisms through the energy transfer process and carrier stability by separated hole and electron channels compared to the single host.²⁹ The light emission by energy transfer rather than charge trapping in the electroplex host would suppress the TPA degradation mechanism as reported previously,²⁹ which assisted the lifetime improvement. The good electron injection and transport character of the SiCzTrz host balances carriers and widens the recombination zone in the emitting layer in combination with the mCBP host, which manages the TTA degradation mechanisms. Moreover, the small singlet emission energy of the electroplex and SiCzTrz host compared with that of the mCBP host relieves the stress of the emitting layer, which can stabilize the emitting material in the degradation process. Additionally, the stability of the SiCzTrz host under positive polarons and negative polarons compared with the common electron transport type host also contributed to the long lifetime. The polaron stability test result of the SiCzTrz host using hole only and electron only devices of SiCzTrz (Fig. S6 in ESI†) relative to the triazine based 2,8-bis(4,6-diphenyl-1,3,5-triazin-2-yl)dibenzo[b,d]furan (DBFTrz) host reported previously shows that the SiCzTrz host is comparable to DBFTrz in terms of negative polaron stability, and is better than DBFTrz in terms of positive polaron stability.²⁹ Stability improvement under positive polarons was apparent in SiCzTrz due to the carbazone group in the molecular structure while keeping the same negative polaron stability as the triazine based electron transport type host. Although the electron transport type host mostly carries electrons, holes can be leaked to the n-type host, which degrades the device lifetime of mixed host based PhOLEDs. Therefore, the improved carrier stability under positive polarons can partially explain the extended lifetime of the SiCzTrz based electroplex host. In fact, this is the first meaningful lifetime value reported in pure blue PhOLEDs with a y color coordinate below 0.15. In other work, the y color coordinate was over 0.15 and even the lifetime was very short. Simultaneous improvement of the EQE, lifetime and pure blue color coordinates was limited in previous work. The lifetime data of pure blue PhOLEDs with a y color coordinate below 0.20 are summarized in Table 1. The 23.5 h lifetime of device III can be transformed into an over 10 000 h lifetime at 100 cd m⁻² using an acceleration factor of 1.8. This is the first demonstration of an over 10 000 h lifetime at 100 cd m⁻² in any high EQE pure blue OLEDs. A high EQE of 27.6% and LT₅₀ device operational lifetime over 10 000 h at 100 cd m⁻² were simultaneously reached in pure blue PhOLEDs with color coordinates of (0.12, 0.13) using an electroplex host platform. The new electroplex host created in this work contributed the ultimate device performances of the pure blue PhOLEDs and confirmed the potential of pure blue PhOLEDs as a substitute for pure blue fluorescent OLEDs.

Fig. 4 The device lifetime of device I, II, III, and reference devices at 3000 and 100 cd m⁻².

Table 1 Reported lifetime data of pure blue PhOLEDs with a y color coordinate <0.20

Name	CIE (x, y)	EQEmax (%)	Lifetime (h)	Ref.
Device I	(0.14, 0.19)	15.6	LT50/5900 h@100 cd m^−2	This work
Device II	(0.14, 0.19)	20.3	LT50/8460 h@100 cd m^−2	This work
Device III	(0.12, 0.13)	27.6	LT50/10 700 h@100 cd m^−2	This work
Device 3 (PtON1)	(0.14, 0.18)	3.2	LT70/801 h@100 cd m^−2	30
H:Ir3 (10 wt%)	(0.139, 0.165)	14.3	LT70/12.27 h	31
H:Ir3 (20 wt%)	(0.140, 0.176)	16.0	LT70/19.70 h	31
m-CBPPO (15%)	(0.13, 0.16)	24.8	—	32
TSPO1 (10%)	(0.14, 0.19)	17.1	—	32
A3	(0.15, 0.05)	10.27	—	33
3% 28	(0.16, 0.16)	—	—	34
6% PtON1 (10)	(0.15, 0.13)	25.2	—	35

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In conclusion, an electroplex host developed using the SiCzTrz host to increase the triplet energy of the host enabled a high EQE of 27.6%, a LT₅₀ over 10 000 h, and pure blue color coordinates of (0.12, 0.13) at the same time in pure blue PhOLEDs. Considering that this result was obtained from academia rather than industry, the EQE and lifetime of the pure blue PhOLEDs can be further enhanced to the commercialization level of high EQE pure blue PhOLEDs. This work demonstrated that an electroplex host can be the best host for high EQE and long lifetime in pure blue PhOLEDs.

Experimental

Detailed synthesis of SiCzTrz is explained in the ESI.†

Device fabrication and measurement

Bottom-emitting type and top-emitting type devices were produced in this work. The device stack of the bottom-emitting device was indium tin oxide (ITO, 150 nm)/BPBPA:HAT-CN (10 nm)/BPBPA (80 nm)/PCZAC or mCBP (10 nm)/emitting layer (30 nm)/DBFTrz (5 nm)/ZADN:LiQ (30 nm)/LiQ (1 nm)/Al (200 nm). The top-emitting device (device III) used a stack structure of Ag (100 nm)/ITO (8 nm)/BPBPA:HAT-CN (10 nm)/BPBPA (127 nm)/mCBP (10 nm)/emitting layer (30 nm)/DBFTrz (5 nm)/ZADN:LiQ (30 nm)/LiQ (1 nm)/Ag:Mg (15 nm). In the device structure, BPBPA is N,N,N′,N′-tetra[(1,10-biphenyl)-4-yl]-(1,10-biphenyl)-4,4′-diamine, HATCN is 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile, PCZAC is 9,9-dimethyl-10-(9-phenyl-9H-carbazol-3-yl)-9,10-dihydroacridine, ZADN is 2-[4-(9,10-di-naphthalen-2-yl-anthracene-2-yl)-phenyl]-1-phenyl-1H-benzimidazole, and LiQ is 8-hydroxyquinolinolato lithium. The emitting layer was mCBP:SiCzTrz:Ir(cb)₃ with a host composition of 50 : 50 and an Ir(cb)₃ doping concentration of 20%. The reference device had an emitting layer of mCBP:Ir(cb)₃ with a phosphor doping concentration of 20%. The device characterization was carried out using a Keithley 2400 source meter for the voltage scan and current density collection, and a CS2000 for the optical output measurement. The lifetime of the devices was monitored using a lifetime system equipped with an electrical source unit and Si photodiode.

Author contributions

Jun Yeob Lee and Mina Jung designed, synthesized and analysed the SiCzTrz host used in this study. Kyung Hyung Lee fabricated the devices. The manuscript was prepared by Mina Jung, Taekyung Kim and Jun Yeob Lee. Finally, all authors reviewed and approved the final manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by Nano Materials Research Program (2016M3A7B4909243) through the National Research Foundation of Korea funded by Ministry of Science, ICT and Future Planning.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9mh01268k

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