Highly e
fficient exciplex emission in solid-state
light-emitting electrochemical cells based on
mixed ionic hole-transport triarylamine and ionic
electron-transport 1,3,5-triazine derivatives
Hsiao-Fan Chen,aChih-Teng Liao,bHai-Ching Su,*bYun-Shiuan Yeha and Ken-Tsung Wong*a
Highly efficient exciplex emission in solid-state light-emitting electrochemical cells (LECs) was achieved by
mixing two ionicfluorescent emitters with opposite bipolarity. A triarylamine derivative as a hole-transport
material (HTM) and a 1,3,5-triazine derivative as an electron-transport material (ETM) were selected due to their superior carrier mobilities. The ionic character of HTM and ETM was achieved by chemically attaching three pendant methylimidazolium groups as peripheries, rendering them amenable to LEC applications. Strong exciplex green emission can be observed either from photoluminescence in solid states or electroluminescence in LEC devices when HTM and ETM are mixed. By optimizing the mixing ratio, the
external quantum efficiency and power efficiency can be achieved up to 3.04% and 10.29 lm W1,
respectively. These results are among the highest reported forfluorescent LECs.
Introduction
Organic light-emitting devices (OLEDs) have attracted much attention due to their promising potential in display and solid-state lighting applications.1,2To optimize the carrier injection
and transport characteristics, hole-transport materials (HTM) and electron-transport materials (ETM) are oen utilized simultaneously in either a single-layered blend device or a discrete multilayered device. When an HTM and an ETM are in close contact, either in a blend or at the interface between the thin lms of the two materials, an exciplex can be formed through charge transfer in the excited states between the lowest unoccupied molecular orbital (LUMO) level of the acceptor and the highest occupied molecular orbital (HOMO) level of the donor.3Since exciplex emission is different from either the donor
or the acceptor monomer emission, it offers a feasible way to tune the emission wavelength of OLEDs. Several single-color4–11
and white12–17 OLEDs based on exciplex emission have been
reported. Although exciplex emission is generally weak and inefficient, several studies about efficient OLEDs based on exci-plex emission were reported.7,9–11,14,16–19Good mobility balance
and large offsets in energy levels of HTM and ETM have been pointed out to be important requirements for optimizing device efficiencies of exciplex-based OLEDs.10Similar carrier mobilities
would increase the number of electron–hole pairs formed for exciplex emission at similar current densities. Large offsets in energy levels between HTM and ETM facilitate carrier accumu-lation at the HTM–ETM interface, enhancing the possibility for exciplex formation. With a judiciously chosen HTM–ETM pair, high external quantum efficiencies (EQE) >3% photons/elec-trons have been reported for exciplex-based OLEDs.10
In this work, we demonstrate efficient exciplex emission in light-emitting electrochemical cells (LECs). Compared with conventional OLEDs, solid-state LECs20,21 possess several
promising advantages. LECs generally require only a single emissive layer, which can be easily processed from solutions, and can conveniently use air-stable electrodes. The emissive layers of LECs contain mobile ions, which can dri toward electrodes under an applied bias. The spatially separated ions induce electrochemical doping (oxidation and reduction) of the emissive materials near the electrodes, i.e. p-type doping near the anode and n-type doping near the cathode.20 The doped
regions induce ohmic contacts with the electrodes and conse-quently facilitate the injection of both holes and electrons, which recombine at the junction between p- and n-type regions. As a result, a single-layered LEC device can be operated at very low voltages (close to Eg/e, where Eg is the energy gap of the
emissive material and e is the elementary charge) with balanced carrier injection, giving high power efficiencies. Exciplex-based LECs employing a blend of p- and n-type luminescent conju-gated polymers have been reported.22However, the EQE
ach-ieved in these devices is rather low (0.02%). To enhance the device efficiency of exciplex-based LECs, two novel ionic HTM
aDepartment of Chemistry, National Taiwan University, Taipei 10617, Taiwan. E-mail: [email protected]; Fax: +886-2-33661667; Tel: +886-2-33661665
bInstitute of Lighting and Energy Photonics, National Chiao Tung University, Tainan 71150, Taiwan. E-mail: [email protected]; Fax: +886-6-3032535; Tel: +886-6-3032121-57792
Cite this:J. Mater. Chem. C, 2013, 1, 4647 Received 20th March 2013 Accepted 24th May 2013 DOI: 10.1039/c3tc30518j www.rsc.org/MaterialsC
Materials Chemistry C
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and ETM are proposed to provide large offsets in energy levels for efficient exciplex formation. The design of HTM and ETM which are amenable for LEC application mainly requires introduction of movable ions. We simply attached three 1-methylimidazolium hexauorophosphates in the terminal of alkyl peripheries onto the hole-transport tris(4-biphenyl)amine and electron-transport 2,4,6-tris(3-biphenyl)triazine to afford HTM and ETM, respectively. Both tris(4-biphenyl)amine and 2,4,6-tris(3-biphenyl)triazine have been proven to possess decent hole and electron mobilities of ca. 1.5 104and 1.2 104 cm2 V1 s1, respectively.23,24 In the mixed HTM–ETM
emissive layer, balance of carrier mobility could be conveniently tailored by tuning the mixing ratio of the two materials. Hence, with balanced carrier transport and large offsets in energy levels at the HTM–ETM interface, high EQEs and power efficiencies up to 3% and 10 lm W1, respectively, can be obtained in single-layereduorescent LECs based on exciplex emission.
Results and discussion
Synthesis
Scheme 1 outlines the syntheses of HTM and ETM. HTM-Br and ETM-Br were synthesized via modied one-pot Suzuki
coupling of tris(4-bromophenyl)amine25 and
2,4,6-tris(3-bro-mophenyl)triazine24 with 1-bromo-4-(6-bromohexyl)benzene26
in 39 and 41% yields, respectively. Aer treating HTM-Br and ETM-Br with an excess amount of 1-methylimidazole followed by ion-exchange with potassium hexauoro-phosphate, we isolated HTM and ETM in 65 and 84% yields, respectively.
Electrochemical properties
Fig. 1 displays the electrochemical characteristics of HTM and ETM, as probed using cyclic voltammetry in acetonitrile. A reversible oxidation [at +0.43 V vs. (Fc/Fc+)] was observed for HTM while ETM exhibited a reversible reduction [at 2.00 V vs. (Fc/Fc+)]. Both oxidation and reduction occur at the core of the molecules. The supreme oxidation or reduction reversibility is attributed to the stable core structures of HTM (triarylamine) and ETM (1,3,5-triazine), making them good candidates for the formation of a stable charge-transfer complex in the excited state (exciplex). By adopting the reversible redox potentials, the HOMO of HTM and the LUMO of ETM can be calculated as 5.23 and 2.80 eV, respectively.
Scheme 1 Syntheses of HTM and ETM.
Photoluminescence studies
Fig. 2a presents the UV-visible absorption and photo-luminescence (PL) spectra of HTM and ETM in acetonitrile solutions (105M). HTM shows two electronic transition bands at 270 and 340 nm. The shorter wavelength is assigned to be the electronic transition of biphenyl moieties and is red-shied by 25 nm as compared to that of tris(biphenyl-4-yl)-amine due to the introduction of alkyl peripheries, while the longer wave-length component is the electronic transition involving the entire amine moiety.23On the other hand, there is only one
electronic transition band with lmaxat 264 nm for ETL which is
consistent with the absorption regime of 2,4,6-triphenyltriazine derivatives.24 The lack of the electronic transition at longer
wavelengths indicates a limited conjugation between biphenyl and 1,3,5-triazine due to meta-conjugation. For solution PL, HTM and ETM exhibited similar structurelessuorescence at 423 and 428 nm with PL quantum yields of 0.49 and 0.06, respectively. Both absorption and PL spectra of HTM : ETM with 1 : 1 weight ratio are approximately the superposition of individual spectra, showing no indication of intermolecular interaction between HTM and ETM presumably due to insuffi-cient concentration. Inlms (Fig. 2b), the absorption spectrum of HTM exhibited comparable peak intensities at 265 and 350 nm as compared to those in the acetonitrile solution. The change in relative peak intensities might be due to the overall effect of different degrees of molecular exibility in solutions and inlms and as well as the dielectric environment, causing different oscillator strengths in their electronic transitions.27,28
Notably, ETM showed a red-shied absorption peak at 280 nm with a signicantly broadened tail from 370 nm to 600 nm, indicating a great extent of intermolecular interaction due to a better planarity of the 2,4,6-triphenyltriazine core structure. The absorption spectrum of the mixed sample with 1 : 1 weight ratio in thin lm showed no sign of intermolecular interaction between HTM and ETM in the ground state.
PL spectra of thinlms containing various mixing weight ratios of HTM–ETM on quartz substrates are shown in Fig. 2c. The PL emission peaks of pure ETM and HTMlms are 400 and 425 nm, respectively. It is noted that some excimer emission around 550 nm was observed in the PL spectrum of pure ETM lms. The appearance of excimer emission from ETM lms is
evidence for the substantial intermolecular interaction, in line with the observation of the signicantly broadened tail aer 370 nm in the solid-state absorption spectrum. When the two materials are mixed, the PL emission of the pristine lms is totally quenched and a new emission band centered at 545 nm is observed. To clarify this phenomenon, the HOMO and LUMO levels of both materials estimated by CV measurements as well as the energy gaps estimated from the absorption onset of the solid lms are depicted in Fig. 3. Deep-blue emission of the pristine lms is attributed to deactivation of the monomer excited states of ETM and HTM. The new green emission would be inferred to the emission of exciplex formed between ETM and HTM. The energy gap of exciplex emission is determined by the energy difference between the LUMO level of the acceptor molecule and the HOMO level of the donor molecule. As shown in Fig. 3, the photon energy of exciplex emission can be esti-mated to be 2.43 eV, which matches well with the measured value (2.28 eV, Fig. 1). Slightly smaller energy gaps in thinlms as compared to those in solutions are due to the environmental polarization. To further examine the new emission band, the intermolecular distance between HTM and ETM was adjusted
Fig. 1 Cyclic voltammograms of HTM and ETM. All potentials were recordedvs. Ag/AgCl (saturated) as a reference electrode and calibrated with the ferrocene/ ferrocenium redox couple. A glassy carbon electrode was used as the working electrode; scan rate 100 mV s1.
Fig. 2 Absorption and/or PL spectra of HTM and ETM with various mixing weight ratios in acetonitrile solution (105M) (a) and thin films on quartz substrates [(b) and (c)].
by dispersing the HTM–ETM mixture (HTM : ETM ¼ 1 : 3, weight ratio) in poly(methylmethacrylate) (PMMA) lms. As shown in Fig. 4, the relative amount of new green emission with respect to the monomer emission reduces as the PMMA concentration increases. It reveals that steric hindrance provided by an inert polymer matrix effectively suppresses the new green emission. Such results further conrm that the green emission arises from HTM–ETM exciplex emission.
Electroluminescence studies
To study the electroluminescence (EL) properties of exciplex-based LECs, EL characteristics of LEC devices exciplex-based on emissive layers containing various mixing ratios of HTM and ETM were measured and are summarized in Table 1. The LECs have the structure of indium tin oxide (ITO) (120 nm)/poly(3,4-ethyl-enedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (30 nm)/emissive layer (200 nm)/Ag (100 nm). Emissive layers con-taining ETM concentrations of 100, 75, 50, 25 and 0 wt% are
denoted as Devices E100, E75, E50, E25 and E0, respectively. The EL spectra of LEC devices based on emissive layers con-taining various mixing weight ratios of HTM and ETM are depicted in Fig. 5. The EL spectrum of the pure HTM device (E0) resembles the PL spectrum of pure HTM lms (Fig. 2c). However, ETM shows signicantly different PL and EL spectra. The monomer emission band in the blue region is suppressed and the excimer emission band in the green region is signi-cantly enhanced (cf. Fig. 2c and 5). This phenomenon may be related to self-heating of the EL devices during operation.29It
would result in a spatial rearrangement of molecules and consequently promote the formation of intermolecular excimer species. When HTM and ETM are mixed, the EL spectra are insensitive to the mixing ratios and are in good agreement with the PL spectra of mixinglms (cf. Fig. 2c and 5). It is noted that
Fig. 3 Energy levels of HTM and ETM.
Fig. 4 PL spectra of thinfilms of a HTM–ETM mixture (HTM : ETM ¼ 1 : 3, weight ratio) dispersed in PMMA on quartz substrates.
Table 1 Summary of the LEC device characteristics
Device (ETM concentration) Bias (V) tmaxa (min) Lmaxb (cd m2) hext,maxc (%) hp,maxd (lm W1) Lifetimee (min) E100 (100 wt%) 3.8 V 13.8 0.01 0.13 0.33 9.3 4.2 V 12.7 0.02 0.15 0.35 5.7 4.6 V 8.8 0.03 0.20 0.42 3.6 E75 (75 wt%) 3.0 V 12.1 0.29 3.04 10.29 10.0 3.4 V 5.9 0.54 2.98 8.89 19.6 3.8 V 5.8 0.75 2.56 6.85 17.1 4.2 V 4.6 1.17 2.61 6.30 9.6 4.6 V 3.5 1.61 2.67 5.89 6.8 E50 (50 wt%) 3.8 V 3.5 9.69 0.90 2.45 1.2 4.2 V 3.5 10.15 0.79 1.97 1.1 4.6 V 2.0 12.87 0.78 1.76 0.8 E25 (25 wt%) 3.8 V 2.3 2.35 0.08 0.20 12.0 4.2 V 1.4 3.31 0.09 0.21 10.4 4.6 V 1.2 4.13 0.08 0.18 9.3 E0 (0 wt%) 3.8 V 18.3 0.09 0.0074 0.0034 21.2 4.2 V 14.1 0.21 0.0047 0.0020 14.9 4.6 V 11.3 0.27 0.0058 0.0022 11.4
aTime required to reach the maximal brightness.bMaximal brightness
achieved at a constant bias voltage. cMaximal external quantum
efficiency achieved at a constant bias voltage. dMaximal power
efficiency achieved at a constant bias voltage. eThe time for the
brightness of the device to decay from the maximum to half of the maximum under a constant bias voltage.
Fig. 5 EL spectra of LEC devices based on emissive layers containing various mixing weight ratios of HTM and ETM. Devices with ETM concentrations of 100, 75, 50, 25 and 0 wt% are denoted as E100, E75, E50, E25 and E0, respectively.
the EL spectrum of devices with a mixed HTM–ETM emissive layer is red-shied as compared to that of pure ETM devices (Fig. 5). Hence, it reveals different mechanisms responsible for EL emissions of mixed HTM–ETM and pure ETM devices. The EL emission of mixed HTM–ETM devices arises from HTM– ETM exciplex emission while excimer emission dominates the EL spectrum of pure ETM devices.
All LECs based on various mixing ratios of HTM and ETM showed similar dependent EL characteristics. The time-dependent brightness and current density of Device E75 under a constant bias of 3.0 V are shown in Fig. 6a. Aer the bias was applied, the device current monotonically rose and then grad-ually decreased with time. On the other hand, the brightness rst increased with the device current, reaching a local maximum value before undergoing a gradual decrease to a local minimum value at ca. 7.5 min. Then the brightness rose again and reached the maximum value. Finally, the brightness monotonically decreased with the device current. The reason for the second rise in brightness will be explained later. The time required for the brightness to reach its maximum value decreased as the bias voltage increased (Table 1) due to a higher accumulation rate of mobile ions to facilitate the formation of electrochemically doped regions under a higher electriceld. It is noted that the device response of mixed HTM–ETM devices (E75, E50 and E25) was faster than that of pure HTM (E0) or ETM (E100) devices (Table 1). A mixed HTM–ETM emissive layer would facilitate carrier injection as compared to purelms of HTM and ETM (Fig. 3). Fewer accumulated ions near electrodes are consequently required to signicantly enhance carrier injection, accelerating device turn-on.30 Aer reaching the
maximum brightness, the brightness then dropped with time
with a rate depending on the bias voltage. Under a constant bias, the lifetime of each device, dened as the time required for the brightness of the device to decay from the maximum value to half of the maximum value, decreased upon increasing the bias voltage (Table 1). It results from the fact that a higher current density induced by a higher bias voltage leads to a higher rate of irreversible multiple oxidation and subsequent decomposition of the emissive material, thereby accelerating the degradation of the LEC devices.31
The EQE and power efficiency of Device E75 under a constant bias of 3.0 V are shown in Fig. 6b. Immediately aer a forward bias was applied, the EQE was rather low because of unbalanced carrier injection. During the formation of the doped regions near the electrodes, the balance of the carrier injection improved and, accordingly, the EQE of the device increased rapidly. The peak EQE and peak power efficiency for Device E75 under 3.0 V were 3.04% and 10.29 lm W1, respectively. Aer reaching the peak device efficiency, the device current was still increasing while the brightness was decreasing. Thus, the device efficiency deteriorated rapidly. Before the doped regions were well established, i.e., the device current was still increasing, the thickness of the intrinsic layer between the p-and n-type doped layers gradually reduced p-and thus the electric eld in the intrinsic layer increased with time.32The
recombi-nation zone in the intrinsic layer may keep moving due to eld-dependent carrier mobilities and gradually formed doped layers, which enhance carrier injection efficiency with time. The brightness and device efficiency would reduce as the recombi-nation zone approaches electrodes due to exciton quenching near the doped regions33–35 while the brightness and device
efficiency would recover when the recombination zone moves away from electrodes. However, the recovered device efficiency would not be as high as the maximum value due to the degra-dation of the emissive material during the LEC operation.36
Similar dual-peak characteristics in brightness and device effi-ciency were also observed in LECs with thicker emissive layers (>200 nm).37,38 Compared with a thinner emissive layer, a
thicker emissive layer is benecial in keeping the recombina-tion zone away from the doped regions. Since exciton quench-ing reduces when the recombination zone is movquench-ing toward the center of the emissive layer, recovering of brightness and device efficiency can be measured in such devices.
Maximum brightness as a function of maximum current density for all LEC devices is shown in Fig. 7. The device current under similar bias voltages (3.8–4.6 V) increases signicantly as the percentage of the HTM concentration increases. It reveals that the hole mobility of HTM would be higher than the electron mobility of ETM. The brightnessrst increases as the propor-tion of HTM increases and then decreases when the percentage of HTM concentration exceeds 50%. Increase in brightness as the proportion of HTM increases results from improved carrier balance in the mixed ETM–HTM layer. However, further increase in HTM concentration deteriorates carrier balance and thus device efficiency signicantly reduces (Table 1), rendering lowered brightness.
Although the EL spectra of the mixed HTM–ETM LECs were insensitive to the mixing ratios, the device efficiency is strongly
Fig. 6 (a) Brightness (solid symbols) and current density (open symbols) and (b) EQE (solid symbols) and power efficiency (open symbols) plotted with respect to time under a constant bias voltage of 3.0 V for Device E75.
dependent on the mixing ratios (Table 1). For pure HTM and ETM devices, the device current would be predominantly uni-polar and thus the recombination zone would be located near doped regions, resulting in severe exciton quenching and rather low device efficiencies. In devices based on emissive layers containing mixed hole- and electron-transport materials, carrier balance would be better and thus the recombination zone would be pushed away from doped regions. As a result, exciton quenching near the doped regions would be reduced and the device efficiency can be improved. When 25 wt% ETM was mixed in pure HTM LECs, an order of magnitude in enhance-ment of EQE can be obtained (cf. Devices E25 and E0, Table 1). Further order-of-magnitude enhancement of EQE was measured by increasing the concentration of ETM to 50 wt% (cf. Devices E50 and E25, Table 1). Finally, the EQE achieved in the device based on 75 wt% ETM was up to ca. 2.6%, which is over 350 increment in device efficiency as compared to that of pure HTM devices under similar bias voltages (3.8–4.6 V) (cf. Devices E75 and E0, Table 1). With this optimized mixing ratio, even higher EQE and power efficiency up to ca. 3.0% and 10.3 lm W1
were measured by lowering the bias voltage to 3.0 V. These results are among the highest reported for uorescent LECs. Therefore, it conrms that efficient LECs based on exciplex emission can be realized by simultaneously adopting two strategies: (i) a suitable HTM–ETM pair should be judiciously chosen to provide a large energy level offset for efficient exciplex formation; (ii) to optimize device efficiency, balance of carrier mobility could then be tailored by tuning the mixing ratio of the two materials.
Recently, enhanced singlet exciplex emission from OLEDs has been reported.39,40It was achieved by production of singlets
via triplet–triplet annihilation, i.e., triplet fusion39or thermally
assisted delayeduorescence.40The latter takes place in
emit-ters with a small singlet–triplet gap (small electron exchange energy), which facilitates triplets to undergo reverse intersystem crossing back into the singlet manifold. Efficient exciplex emission in LECs based on the HTM–ETM pair reported in this work may also result partially from the two mechanisms shown above. However, triplet fusion would more likely be responsible for enhanced singlet exciplex emission since the singlet–triplet gaps for ETM and HTM (0.16 and 0.38 eV, respectively) are not small enough to efficiently activate reverse intersystem crossing.
Conclusion
In summary, we have designed and synthesized ionic HTM and ETM by chemically attaching three pendant methylimidazolium groups as peripheries of hole-transport tris(4-biphenyl)amine and electron-transport 2,4,6-tris(3-biphenyl)triazine, respec-tively. The ionic character of HTM and ETM renders them amenable to LEC application. The core structures of HTM and ETM possess excellent electrochemical reversibility for oxida-tion and reducoxida-tion, as well as high hole and electron mobilities, respectively, potential to generate efficient exciplex emission when HTM and ETM are mixed in the solid state. We observed a new green emission band centered at 545 nm in the mixedlm with weight ratio of HTM and ETM ranging from 3 : 1 to 1 : 3. The green emission can also be suppressed by the addition of the inert polymer matrix PMMA, indicating that the green emission arises from HTM–ETM exciplex. The EL properties of exciplex-based LECs have been examined by incorporating the HTL emissive layers containing ETM concentrations of 100, 75, 50, 25 and 0 wt%. The EL spectra of the mixed HTM–ETM LECs are insensitive to the mixing ratios and are in good agreement with the PL spectra of mixed lms. However, the device effi-ciency is strongly dependent on the mixing ratios. In devices based on emissive layers containing mixed hole- and electron-transport materials, carrier balance would be better and thus the recombination zone would be pushed away from doped regions. By optimizing the mixing ratio, the EQE and power efficiency can be achieved up to 3.04% and 10.29 lm W1,
respectively. These results are among the highest reported for uorescent LECs. Therefore, we believe that efficient LECs based on exciplex emission can be obtained by two simple strategies– judicious selection of the HTM–ETM pair to provide large energy level offset and tuning the mixing ratio of the two materials to balance the carrier mobility.
Experimental section
General experiments
1H and13C NMR spectra of compounds were collected on a 400
MHz spectrometer at room temperature. Redox potentials of all complexes were determined by cyclic voltammetry (CV) at a scan rate of 100 mV s1in acetonitrile solution (1.0 mM). A glassy carbon electrode and a platinum wire were used as the working electrode and the counter electrode, respectively. All potentials were recorded versus the Ag/AgCl (saturated) reference electrode and calibrated with the ferrocene/ferrocenium redox couple. Oxidation CV was performed using 0.1 M tetra-n-butylammo-nium hexauorophosphate (TBAPF6) in acetonitrile as the
supporting electrolyte. For reduction CV, 0.1 M tetra-n-buty-lammonium perchlorate (TBAP) in acetonitrile was used as the supporting electrolyte. Photophysical characteristics of HTM and ETM in solutions were measured at room temperature by using acetonitrile solutions (105 M) of all compounds. The thickness of spin-coatedlms measured by prolometry is 200 nm. UV-visible absorption spectra were recorded on a spectro-photometer (HITACHI U2800A). PL spectra were measured with auorescence spectrophotometer (HITACHI F9500).
Fig. 7 Maximum brightness as a function of maximum current density for all LEC devices.
Synthesis of tris(40-(6-bromohexyl)biphenyl-4-yl)amine (HTM-Br). A mixture of 1-bromo-4-(6-bromohexyl)benzene (1.94 g, 6.06 mmol), bis(pinacolato)diborane (1.54 g, 6.06 mmol, donated by Frontier Scientic), bis(dibenzylideneacetone)-palladium(0) (27 mg, 0.03 mmol), biphenyl-2-yl-dicyclohexyl-phosphane (42 mg, 0.12 mmol), potassium acetate (1.19 g, 12.1 mmol), and dioxane (20 mL) was reuxed for 15 min. A suspension of tris(4-bromophenyl)amine (964 mg, 2.00 mmol) and potassium phosphate (7.72 g, 36.3 mmol) in dioxane (10 mL) and deionized water (15 mL) was added to the solution and reuxed for a further 1.5 h. Aer dioxane was removed by rotary evaporation, the reaction mixture was extracted with CH2Cl2and dried over MgSO4. The crude product was puried
by column chromatography on silica gel (CH2Cl2/hexane¼ 1/4)
to afford a pure product (748 mg, 39%) as a sticky colorless liquid.1H NMR (CD 2Cl2, 400 MHz) d 7.55–7.52 (m, 12H), 7.26 (d, J¼ 8.0 Hz, 6H), 7.22 (d, J ¼ 8.0 Hz, 6H), 3.45 (t, J ¼ 6.8 Hz, 6H), 2.67 (t, J¼ 8.0 Hz, 6H), 1.89 (quin., J ¼ 6.8 Hz, 6H), 1.68 (quin., J¼ 7.2 Hz, 6H), 1.54–1.39 (m, 12H);13C NMR (CD 2Cl2, 100 MHz) d 146.6, 141.3, 137.9, 135.4, 128.7, 127.6, 126.5, 124.3, 35.3, 33.9, 32.7, 31.1, 28.3, 28.0; HRMS (m/z, FAB+) calcd for C54H6079Br3N
959.2276, found: 959.2285; calcd for C54H6079Br281BrN
961.2255, found: 961.2277; calcd for C54H6079Br81Br2N
963.2235, found: 963.2212; calcd for C54H6081Br3N 965.2214,
found: 965.2202.
Synthesis of tris(40 -(6-(3-methylimidazolium)hexyl)biphenyl-4-yl)amine tris(hexauorophosphate) (HTM). A mixture of tris(40-(6-bromohexyl)biphenyl)-4-ylamine (636 mg, 0.66 mmol) and 1-methylimidazole (0.17 mL, 2.18 mmol) was dissolved in acetonitrile (3 mL) and reuxed for 12 h. The reaction mixture was concentrated and extracted with CH2Cl2 and aqueous
solution of potassium hexauorophosphate (30 mL, 0.13 M). The crude product was puried by column chromatography on alumina (ACN/CH2Cl2¼ 1/9 to 9/1) to afford a pure product (600
mg, 65%) as a white solid.1H NMR (DMSO-d6, 400 MHz) d 9.06
(s, 3H), 7.73 (s, 3H), 7.66 (s, 3H), 7.60 (d, J¼ 8.0 Hz, 6H), 7.54 (d, J¼ 8.0 Hz, 6H), 7.24 (d, J ¼ 8.0 Hz, 6H), 7.12 (d, J ¼ 8.0 Hz, 6H), 4.13 (t, J¼ 6.8 Hz, 6H), 3.83 (s, 9H), 2.59 (t, J ¼ 6.8 Hz, 6H), 1.80 (quin., J¼ 6.8 Hz, 6H), 1.59 (quin., J ¼ 6.8 Hz, 6H), 1.31–1.28 (m, 12H);13C NMR (DMSO-d6, 100 MHz) d 146.1, 141.1, 137.0, 136.5, 134.7, 128.9, 127.5, 126.1, 124.1, 123.6, 122.3, 48.8, 35.7, 34.6, 30.6, 29.3, 28.0, 25.4; HRMS (m/z, ESI+) calcd for C66H78F18N7P3
1403.5228, found: 1403.5210.
Synthesis of 2,4,6-tris(40 -(6-bromohexyl)biphenyl-3-yl)-triazine (ETM-Br). A mixture of 1-bromo-4-(6-bromohexyl)-benzene (1.78 g, 5.55 mmol), bis(pinacolato)diborane (1.41 g, 5.55 mmol, donated by Frontier Scientic), bis(dibenzylide-neacetone)palladium(0) (25 mg, 0.03 mmol), biphenyl-2-yl-dicyclohexylphosphane (39 mg, 0.11 mmol), potassium acetate (1.09 g, 11.1 mmol), and dioxane (20 mL) was reuxed for 30 min. A suspension of 2,4,6-tris(3-bromophenyl)triazine (1 g, 1.83 mmol) and potassium phosphate (7.07 g, 33.3 mmol) in dioxane (10 mL) and deionized water (15 mL) was added to the solution and reuxed for further 1 h. Aer dioxane was removed by rotary evaporation, the reaction mixture was extracted with CH2Cl2and dried over MgSO4. The crude product was puried
by column chromatography on silica gel (CHCl3/hexane¼ 1/2)
to afford a pure product (700 mg, 41%) as a sticky colorless liquid.1H NMR (CDCl3, 400 MHz) d 9.00 (s, 3H), 8.74 (d, J ¼ 8.0 Hz, 3H), 7.84 (d, J¼ 8.0 Hz, 3H), 7.68–7.63 (m, 9H), 7.33 (d, J ¼ 8.0 Hz, 6H), 3.43 (t, J¼ 6.8 Hz, 6H), 2.71 (t, J ¼ 8.0 Hz, 6H), 1.90 (quin., J¼ 6.8 Hz, 6H), 1.73 (quin., J ¼ 7.2 Hz, 6H), 1.58–1.41 (m, 12H);13C NMR (CDCl 3, 100 MHz) d 171.8, 142.1, 141.6, 138.3, 136.8, 131.1, 129.1, 129.0, 127.7, 127.5, 127.2, 35.5, 33.9, 32.7, 31.3, 28.5, 28.0; HRMS (m/z, FAB+) calcd for C57H6079Br3N3
1023.2337, found: 1023.2327; calcd for C57H6079Br281BrN3
1025.2317, found: 1025.2291; calcd for C57H6079Br81Br2N3
1027.2296, found: 1027.2267; calcd for C57H6081Br3N3
1029.2276, found: 1029.2268.
Synthesis of 2,4,6-tris(40 -(6-(3-methylimidazolium)hexyl)-biphenyl-3-yl)triazine tris(hexauorophosphate) (ETM). A mixture of 2,4,6-tris(40-(6-bromohexyl)biphenyl-4-yl)triazine (650 mg, 0.62 mmol) and 1-methylimidazole (0.17 mL, 2.55 mmol) was dissolved in acetonitrile (3 mL) and reuxed for 12 h. The reaction mixture was concentrated and a solution of potassium hexauorophosphate (700 mg, 3.80 mmol) in deionized water (30 mL) was added and stirred for 10 min. The appeared solid was washed with deionized water and collected to afford a pure compound (780 mg, 84%) as a white solid.1H
NMR (DMSO-d6, 400 MHz) d 9.07 (s, 3H), 8.93 (s, 3H), 8.73 (d, J ¼ 8.0 Hz, 3H), 7.99 (d, J¼ 8.0 Hz, 3H), 7.76–7.72 (m, 12H), 7.68 (s, 3H), 7.37 (d, J¼ 8.0 Hz, 6H), 4.14 (t, J ¼ 7.2 Hz, 6H), 3.83 (s, 9H), 2.65 (t, J¼ 8.0 Hz, 6H), 1.79 (quin., J ¼ 7.2 Hz, 6H), 1.63 (quin., J¼ 7.2 Hz, 6H), 1.37–1.30 (m, 12H);13C NMR (DMSO-d6, 100 MHz) d 171.6, 142.5, 141.3, 137.5, 136.9, 136.5, 131.6, 130.2, 129.5, 128.0, 127.2, 126.9, 124.0, 122.7, 49.2, 36.1, 35.1, 31.0, 29.7, 28.4, 25.8; HRMS (m/z, ESI+) calcd for C69H78N93+
1032.6380, found: 1032.6381.
Fabrication and characterization of LEC devices
ITO-coated glass substrates were cleaned and treated with UV/ozone prior to use. A poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) layer (30 nm) was spin-coated onto the ITO substrate in air and then the structure was baked at 150C for 30 min. The emissive layers were then spin-coated from an acetonitrile solution containing various mixing weight ratios of HTM and ETM (HTM : ETM¼ 0 : 100, 25 : 75, 50 : 50, 75 : 25 and 100 : 0 for Devices E100, E75, E50, E25 and E0, respectively) with a solute concentration of 150 mg mL1. The thickness of the emissive layer is ca. 200 nm. Preparation and spin-coating processes of all solutions were carried out under ambient conditions. Aer spin-coating the emissive layer, the samples were baked at 70C for 10 h in a nitrogen glove box (oxygen and moisture levels below 1 ppm) and then subjected to thermal evaporation under a 100 nm thick Ag top contact in a vacuum chamber (ca. 106 torr). The electrical and emission characteristics of the LEC devices were measured using a source-measurement unit and a Si photodiode calibrated with a Photo Research PR-650 spectroradiometer. All device measurements were performed under a constant bias voltage in a nitrogen glove box. EL spectra of LEC devices were recorded using a calibrated CCD spectrograph.
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