Phenanthroimidazole Derivative as an Easily Accessible Emitter for

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Phenanthroimidazole Derivative as an Easily Accessible Emitter for Non-Doped Light-Emitting Electrochemical Cells Madayanad Suresh Subeesh,† Kanagaraj Shanmugasundaram,† Chozhidakath Damodharan Sunesh,† Thao P. Nguyen,‡ and Youngson Choe*,† †

Department of Polymer Science and Chemical Engineering, Pusan National University, Busan 609-735, South Korea Department of Chemistry, Pohang University of Science and Technology, Pohang 37673, South Korea

‡

S Supporting Information *

ABSTRACT: We report a versatile approach to harvest electroluminescence from a nondoped light-emitting electrochemical cell (LEC) using an easily accessible phenanthroimidazole derivative. The authors investigated two different types, (i) ionic and (ii) neutral phenanthroimidazole derivatives by modifying our previously reported LEC emitter. Sky-blue electroluminescence was achieved by applying these modified emitter in LEC devices. In comparison to the parent molecule, a highly contrasting performance was exhibited by all the modified emitters except the neutral butyl derivative (nbpypn). By employing an ionic molecule (ihpypn) in a fully solution-processed typical LEC device structure, a peak brightness of 711 cd/m2 was observed at a current efficiency of 0.18 cd/A. Our champion device (ihpypn-LEC) presented a 5-fold increase in maximum brightness at a ten times higher current density than its parent molecule. These peak brightness values are among the best comparing to those reported for LECs with the corresponding emission colors. Even though the neutral molecules did not show any high electroluminescence, their current efficiency at maximum brightness has improved 20 times when compared to its parent molecule utilized device. The study reveals that substituents on imidazole nitrogen has a critical impact on its performance in the LEC devices. This result is even more encouraging, considering that our molecular design can be applied to the majority of the imidazole derivatives and may open-up a plausible way of enriching the library of emitters for LECs with efficient and easily obtainable small organic molecules.

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scientific community.7,8 Among the important achievements is the report on color tunable fiber-shaped polymer light-emitting electrochemical cells.9 It has thus been proven that LECs have an enormous amount of potential to replace OLEDs in various applications and can function in its role as an advanced lighting technique. Even though LECs have been actively pursued with many impressive advancements, limitations do exist. One of the main issues for the development of LECs is the lack of proper emitters in the LEC library. To date, iTMCs and polymer materials mostly encompass the materials used for LEC devices.7 Cationic biscyclometalated complexes of IrIII are by far the most utilized phosphorescent materials for LECs and have passed several milestones10,11 when compared to other reported emitters for LECs. However, most of the high performances reported for this class of compounds are based on complexes emitting around the greenish region of the spectrum, which obviously indicates that the pure blue emission remains devilishly difficult for ionic iridium complexes.12,13 In

INTRODUCTION In the past decade, purely organic compound based electronics rendered phenomenal advances to the next generation of lighting technologies. Organic light-emitting diodes (OLEDs) are the ultramodern lighting devices which have reached its edge of complete commercialization without paying attention to its high cost.1 Even though OLEDs reign over the reported lighting sources and continue to impress with their versatile applications, they may fail to bring down the cost of embedded devices. In this respect, researchers all over the world anticipate light-emitting electrochemical cells (LECs) to be the best choice to replace expensive OLEDs in the very near future.2 LECs show unique characteristics that offer tremendous potential for successful implementation in solid state lighting.3 Amenability toward various solution processing techniques along with simple device architecture feature LECs as a low cost lighting source.4 Due to the initial electrochemical doping mechanism for the formation of p−n junctions,5 in the current state, LECs may find difficulty in reaching application in high end displays, but they can surely fit into the cutting edge of general lighting applications,6 which will significantly bring down the cost of mass production of those devices. Recent advances in LECs have garnered a broad interest in the © 2015 American Chemical Society

Received: August 12, 2015 Revised: September 13, 2015 Published: September 22, 2015 23676

DOI: 10.1021/acs.jpcc.5b07871 J. Phys. Chem. C 2015, 119, 23676−23684

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The Journal of Physical Chemistry C

phenanthroimidazole-based fluorophores, which is so far scarcely addressed. One of the most important properties of phenanthroimidazole derivatives is that they readily form donor−acceptor systems when tailored with various aromatic groups with minimal structural modifications, a property that empowers them to play an important role as electroluminescent materials.17 Among others, phenanthroimidazole derivatives possess various attractive features in terms of conductivity, luminescence, and color tunability.26,27 Moreover, the different bonding natures of two nitrogen atoms endow this class of functional chromophores with a wide range of structural tunability by taking advantage of the known synthetic procedures.28−30 We believe that advanced low-cost lighting technology, viz., LECs with potential organic materials like phenanthroimidazole derivatives, will be a key tool for materials science in the coming years. In this article, we report the synthesis and characterization of an easily accessible phenanthroimidazole derivative with different side chains. We then applied the synthesized compounds in the LEC device configuration to study their performances. With the exception of the components in the active layers, the device structures are the same for neutral and ionic molecules. Two different strategies were used to process the active layers for neutral and ionic molecules. A tricomponent blend was used to fabricate neutral molecule based LECs, which consist of our organic molecule with an ion transporting polymer (poly(ethylene oxide)) and an inorganic salt (lithium trifluoromethylsulfonate).19 However, a single component active layer was used to fabricate LECs based on ionic emitters. A simple spin-coating approach was used to process all the layers, except the electrodes for the devices.

addition, utilization of rare and expensive metals like iridium may raise questions about its future availability and costs.14 However, for the polymer materials, laborious synthesis and purification steps remain as well-known drawbacks.5 All these facts lead to the conclusion that new emitters are inevitable for further advances in LECs. The compatibility to use easily obtainable small organic molecules scaled OLEDs up for achievements like high brightness, color purity, color tunability, long device lifetime, and so on,15−18 whereas LECs still find difficulty in utilizing easily obtainable small organic molecules in their active layers. The limiting factors for the small molecules to find application in LECs are their solubility issues in common organic solvents from which LECs are usually fabricating. Thus, it is highly desirable to develop a strategy that can design and discover soluble small organic molecules for the enrichment of LEC library. The concept of neutral small organic molecule light-emitting electrochemical cells was first introduced by Hill et al. with a perylene-based organic molecule as the active material.19 Since ionic species are inevitable for electrochemical doping and the formation of p−n junctions in LEC devices, their device structure was similar to a conventional polymer light-emitting electrochemical cell (pLEC) which uses additional ions in its active later.20 In this respect, Tang et al. utilized a green and red emitting organic molecule to construct a better LEC device and achieved reasonable performances as well.5 These reports demonstrated that easily purified small molecules can effectively function in their role as an emissive material in LECs. Nonetheless, the materials used for their studies were synthesized via complex synthetic steps, which may significantly increase the cost and effort for developing improved LEC devices. In a different approach in terms of active material as well as LEC device structure, Kervyn et al. used a UV emitting borozine derivative for the fabrication of LEC devices, but unfortunately the device performance was poor even with the use of air sensitive cathode layers.21 In addition to the neutral organic small molecules, ionic counterparts have also found application in LECs and presented a method to eliminate the need for a tricomponent blend in the LEC device fabrication. Most of the ionic organic molecules reported so far for LECs are endorsed with fluorine derivatives, which means that this area of research was not well explored.22,23 However, after all these reports, an easily obtainable organic emitter for LEC devices was farfetched until our recent report on a simple phenanthroimidazole derivative based light-emitting electrochemical cell, although the outcome was counterproductive.24 From a series of analytical studies, it was identified that unsubstituted nitrogen acts as a poison to the LEC device performance by interacting with the ions in the active layer. With these considerations in mind, we continued to put our endeavors on the same emitter to generate a better LEC device out of the same chromophoric core.24 Hence herein, the same core was used to tailor different alkyl spacers to reduce intraand intermolecular interactions in the solid states.25 We considered that a comparative study of neutral with ionic penanthroimidazole derivatives should be a highly significant move for the development of efficient organic emitters for the future generation of LEC devices. To this end, the authors followed up by investigating two different types of (i) ionic and (ii) neutral phenanthroimidazole derivatives. However, the ultimate aim of this study was meant for designing a strategy which can enrich the LEC library with easily accessible

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EXPERIMENTAL SECTION Synthesis of nhpypn. A 250 mL RB flask was charged with PYPN (1g, 2.389 mmol, and 1 equiv), tetrabutyl ammonium bromide (0.32 g, 1 mmol), 10 mL 50% KOH solution in distilled water, and 20 mL butanone. The reaction mixture was stirred under an Ar atmosphere for 30 min in a conventional oil bath at 50 °C, sequentially, and 1-bromohexane (0.67 mL, 4.778 mmol, and 2eq) was added to the reaction mixture. Then the stirring was continued overnight at the same temperature to complete the reaction. The resulting mixture was cooled down and extracted several times with dichloromethane, and the organic layer was washed with water and dried over anhydrous Na2SO4. After removing the solvent, the crude product was purified by column chromatography on silica gel using ethyl acetate:hexane (1:3), and the resulting greenish yellow oil was dissolved in a minimum quantity of dichloromethane and vaporized under vacuum to obtain a yellowish orange solid, which is then dried in vacuum at 60 °C. Yield: 0.96 g (80%). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.57 (t, J = 7 Hz, 3 H), 0.85−1.05 (m, 6 H),1.85−1.78 (m, 2 H),4.43 (t, J = 8.0 Hz, 2 H), 7.74−7.73(m, 5 H), 7.95 (d, J = 9.2 Hz, 1 H), 8.04−8.09 (m, 2 H), 8.16−8.22 (m, 3 H), 8.27(d, J = 8.4 Hz, 2 H), 8.3− 8.34 (m, 2 H), 8.76 (d, J = 8.4 Hz,1 H), 8.9 (dd, J = 8.2, 1.2 Hz, 1 H). 13C NMR (125 MHz, [D6]DMSO) δ (ppm): 151.0, 137.4, 131.6, 130.7, 130.2, 130.1, 128.6, 128.5, 128.3, 127.6, 127.2, 127.0, 126.9, 126.6, 126.0, 125.7, 125.5, 125.4, 125.1, 124.5, 124.4, 124.3, 123.8, 123.5, 122.9, 121.9, 121.2, 46.0, 30.0, 29.2, 24.9, 21.3, 13.2. MS (FAB, m/z): [M + H]+ Calcd for C37H30N2, 503.24; Found, 503.25. Calcd for C37H30N2 (%): C, 88.41; H, 6.02; N, 5.57. Found: C, 74.09; H, 4.71; N, 4.40. 23677

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g (93%). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.864−0.943 (m, 3 H), 1.16−1.29 (m, H), 1.724−1.794 (m, 4 H), 3.7 (s, 3 H), 4.48 (t, J = 8.0 Hz, 2 H),7.6−7.74 (m, 5 H), 8.02−8.08 (m, 3 H), 8.16−8.29 (m, 7 H), 8.3 (d, J = 8 Hz, 1 H), 8.7 (d, J = 8.0, 2 H),8.8−8.85 (m, 2 H). 13C NMR (125 MHz, [D6]DMSO) δ (ppm): 152.5, 139.1, 136.4, 133.2, 132.3, 131.8, 131.7, 129.9, 129.8, 129.7, 129.0, 128.6, 128.4, 128.4, 128.3, 127.8, 127.1, 127.0, 126.9, 126.8, 126.6, 126.2, 125.9, 125.7, 125.5, 125.1, 124.6, 124.5, 124.4, 123.2, 122.9, 122.5, 50.0, 47.4, 36.7, 30.5, 29.9, 26.0, 25.8. MS (FAB, m/z): [M − PF6]+ Calcd for C41H35N4+, 583.29; Found, 583.29. Calcd for C41H35F6N4P (%): C, 67.58; H, 4.84; N, 7.69. Found: C, 68.09; H, 5.22; N, 7.69. Synthesis of ibpypn. The product was prepared by refluxing bbpypn (0.5 g, 0.903 mmol, 1 equiv) in 5 mL 1methyl imidazole for 15 h. After cooled down to room temperature, the unreacted starting material was removed by precipitating the intermediate by adding excess ethyl acetate to the reaction mixture, the obtained off white precipitate was dissolved in dichloromethane and excess KPF6 (0.83 g, 4.515 mmol, 5 equiv) in methanol was added to the stirring solution under air. Subsequently, the reaction mixture was washed several times with water and the organic phase was dried over anhydrous Na2SO4, followed by rotary evaporation to get a yellow sticky solid. Yield: 0.35 g (55%). 1H NMR (400 MHz, CDCl3) δ (ppm): 1.3−1.45 (m, 2 H), 1.6−1.7 (m, 2 H), 3.6 (s, 3 H), 3.8 (t, J = 7 Hz, 2 H), 4.55 (t, J = 8.0 Hz, 2 H), 7.37 (dt, J = 17.6, 1.2 Hz, 2 H), 7.5 (s, 1 H), 7.64−7.79 (m, 4 H), 7.9 (d, J = 4.8 Hz, 1 H), 8.13−8.19 (m, 2 H), 8.28−8.42 (m, 5 H), 8.5 (d, J = 8 Hz, 1 H), 8.6 (d, J = 8.0, 1 H), 8.7 (d, J = 7.9 Hz, 1 H), 8.9 (d, J = 8 Hz, 1 H), 9 (dd, J = 8.1, 1.2 Hz, 1 H). 13C NMR (125 MHz, [D6]DMSO) δ (ppm): 152.5, 139.1, 136.4, 133.4, 132.3, 131.8, 131.8, 129.9, 129.8, 129.7, 129.7, 129.1, 128.5, 128.4, 128.3, 127.8, 127.2, 126.9, 126.8, 126.7, 126.4, 126.2, 125.7, 125.5, 125.2, 124.5, 124.4, 124.3, 123.2, 122.8, 122.5, 49.6, 47.1, 36.7, 27.5, 27.3. MS (FAB, m/z): [M − PF6]+ Calcd for C39H31N4+, 555.25; Found, 555.26. Calcd for C39H31F6N4P (%): C, 66.85; H, 4.46; N, 8.00. Found: C, 63.76; H, 4.65; N, 6.90.

Synthesis of nbpypn. Using a similar synthetic procedures for nhpypn, the target compound was obtained as yellowish orange solid. Yield: 0.85 g (75%). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.59 (t, J = 7 Hz, 3 H), 0.83−1.8 (m, 4 H), 4.44 (t, J = 8.0 Hz, 2 H), 7.73−7.75(m, 4 H), 7.96−8.34 (m, 11 H), 8.75−8.9 (m, 2 H). 13C NMR (125 MHz, [D6]DMSO) δ (ppm): 151.0, 137.5, 131.6, 130.7, 130.2, 130.0, 128.7, 128.5, 128.3, 127.6, 127.2, 126.9, 126.6, 126.0, 125.7, 125.5, 125.1, 125.0, 124.6, 124.5, 124.3, 123.7, 123.5, 122.8, 121.9, 121.3, 46.0, 31.5, 18.6, 12.9. MS (FAB, m/z): [M + H]+ Calcd for C35H26N2, 475.21; Found, 475.22. Calcd for C35H26N2 (%): C, 88.58; H, 5.52; N, 5.90. Found: C, 73.01; H, 4.70; N, 4.45. Synthesis of bhpypn. A mixture of PYPN (1 g, 2.389 mmol, 1 equiv) and potassium tert-butoxide (0.54 g, 4.778 mmol, 2 equiv) in 25 mL anhydrous THF was stirred for 30 mints under an Ar atmosphere at 50 °C. Then 1,6dibromohexane (2.94 mL, 19.112 mmol, 8 equiv) was added to the yellowish orange reaction mixture and stirred overnight at 50 °C in a conventional oil bath. After being brought back to room temperature, the mixture was extracted several times with dichloromethane and then organic phase was washed with water to remove excess base and sequentially dried over anhydrous Na2SO4. Further purification was done by column chromatography on silica gel using ethyl acetate/hexane (1:3) as eluent. The obtained oil was dissolved in a small amount of dichloromethane and vaporized under vacuum to get yellowish orange solid, which is then dried in vacuum at 60 °C. Yield: 1.11 g (80%). 1H NMR (400 MHz, CDCl3) δ (ppm): 0.99− 1.02 (m, 4 H), 1.43−1.46 (m, 2 H), 1.18−1.84 (m, 2 H), 2.9 (t, J = 6.6 Hz, 2 H), 4.46 (t, J = 8.0 Hz, 2 H), 7.6−7.7 (m, 4 H), 7.9 (d, J = 9.2 Hz), 8.04−8.12 (m, 2 H), 8.16−8.23 (m, 3 H), 8.25−8.3 (m, 3 H), 8.32−8.35 (m, 2 H), 8.7 (d, J = 8 Hz,1 H), 8.9 (dd, J = 8.2, 1.2 Hz,1 H). 13C NMR (125 MHz, [D6]DMSO) δ (ppm): 151.0, 137.8, 132.0, 131.1, 130.8, 130.5, 129.0, 128.7, 128.8, 127.9, 127.3, 127.1, 126.9, 126.6, 126.0, 125.8, 125.5, 125.4, 125.1, 124.6, 124.5, 124.4, 123.1, 123.0, 122.8, 121.9, 121.1, 16.5, 35.0, 31.8, 24.9, 21.8, 13.9. MS (FAB, m/z): [M + H]+ Calcd for C37H29BrN2, 581.15; Found, 581.16. Calcd for C37H29BrN2 (%): C, 76.42; H, 5.03; N, 4.82. Found: C, 71.99; H, 4.66; N, 4.20. Synthesis of bbpypn. Using a similar synthetic procedures for bhpypn, the product was obtained as yellowish orange solid. Yield: 0.79 g (60%). 1H NMR (400 MHz, CDCl3) δ (ppm): 1.96−2.03 (m, 4 H), 2.9 (t, J = 6.6 Hz, 2 H), 4.49 (t, J = 8.0 Hz, 2 H), 7.64−7.74 (m, 4 H), 7.96 (d, 9.2 Hz, 1 H), 8.04−8.09 (m, 2 H), 8.16−8.23 (m, 3 H), 8.26−8.35 (m, 4 H), 8.76 (d, J = 8 Hz, 1 H), 8.85−8.89 (m, 2 H). 13C NMR (125 MHz, [D6]DMSO) δ (ppm): 151.1, 137.6, 131.6, 130.7, 130.3, 130.1, 128.8, 128.5, 128.3, 127.7, 121.2, 126.9, 126.7, 126.1, 125.8, 125.6, 125.1, 125.0, 124.6, 124.5, 124.3, 123.8, 123.7, 122.8, 121.9, 121.4, 46.1, 33.0, 19.0, 13.5. MS (FAB, m/z): [M + H]+ Calcd for C35H25BrN2, 553.12; Found 553.13 Calcd for C35H25BrN2 (%): C, 75.95; H, 4.55; N, 5.06. Found: C, 74.09; H, 4.21; N, 4.30. Synthesis of ihpypn. A 100 mL RB flash was charged with bhpypn (0.5 g, 1.194 mmol, 1 equiv) and 5 mL 1-methyl imidazole, and the resulting mixture was stirred under an Ar cover at 100 °C for 12 h. After cooled down to room temperature, the final product was obtained by stirring the reaction mixture with excess KPF6 (1.1 g, 5.97 mmol, 5 equiv) for 5 h under air and precipitated by adding water. The white solid obtained was filtered and washed several times with water and hexane, then dried at 55 °C for 24 h in vacuum. Yield: 0.81

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RESULTS AND DISCUSSION The fluorescent core molecule (PYPN) was synthesized according to our previous reports, 24 which was then precipitated in dichloromethane and washed with water, hexane, and dried in vacuum. The obtained solid was used for the next step of reaction without further purification. Scheme 1 outlines the synthesis of neutral phenanthroimidazole derivatives, 1-hexyl-2-(pyren-1-yl)-1H-phenanthro[9,10-d]imidazole (nhpypn) and 1-butyl-2-(pyren-1-yl)-1Hphenanthro[9,10-d]imidazole (nbpypn) as well as the ionic derivatives 3-methyl-1-(6-(2-(pyren-1-yl)-1H-phenanthro[9,10d]imidazol-1-yl)hexyl)-1H-imidazol-3-ium hexafluorophosphate(V) (ihpypn) and 3-methyl-1-(4-(2(pyren-1-yl)-1H-phenanthro[9,10-d]imidazol-1-yl)butyl)-1Himidazol-3-ium hexafluorophosphate(V) (ibpypn). All the compounds have been synthesized in such a way as to keep the core untouched, and to eliminate the energetically active hydrogen from the imidazole ring. Both of the ionic molecules were obtained by alkylation of the parent molecule with corresponding dibromoalkane followed by coupling with 1methylimidazole. The final product was obtained by an ion exchange reaction with excess KPF6. For making this study more systematic, we tried to incorporate two more compounds 23678

DOI: 10.1021/acs.jpcc.5b07871 J. Phys. Chem. C 2015, 119, 23676−23684

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The Journal of Physical Chemistry C Scheme 1. Synthetic Routes for nhpypn, nbpypn, ihpypn, and ibpypn

Figure 1. UV−vis absorption (plane lines) and PL spectra (dotted lines). nhpypn (red), nbpypn (green), ihpypn (black), and ibpypn (blue) in dilute THF solutions.

in each series of emitters, i.e., one with an ethyl side chain and other having an ethane linker between core and imidazolium group. Unfortunately, neutral ethyl derivative showed poor solubility in common organic solvents, which forced us to eliminate it from this study. However, a different approach was carried out to obtain the ethyl bridged ionic emitter due to the extreme toxicity of dibromoethane. It involves alkylation of free nitrogen with bromoethanol, followed by conversion of end hydroxyl to bromide using PBr3 at 0 °C,31 sequentially, couple with imidazole to generate ionic side group, but poor yields of target compounds as well as the persistence of ionic side products in the hydroxyl to bromide conversion step held us from the next stage of reactions. Even though alkyl halides readily react with 1-methyl imidazoles, the product concentration of ibpypn was lower compared to its hexyl counterpart (ihpypn), which implies that the lengthier alkyl bridges are more sterically preferred than the shorter ones in this molecule, which obviously reflected in the synthesis of ionic ethyl derivatives. In order to make sure the existence of steric factor in this molecule, we thus proceed to synthesize an ionic propyl bridged derivative. The same aforementioned difficulty of poor yield was observed in the ionization step of propyl derivatives. Considering all these factors, we decided to limit our study with butyl and hexyl spacers in both series of emitters. In addition, all the compounds were fully characterized by NMR spectroscopy, mass spectroscopy, and elemental analysis (see details in the Experimental Section). The UV−vis absorption and photoluminescence (PL) spectra of all the synthesized compounds in dilute anhydrous tetrahydrofuran solutions are displayed in Figure 1. Being the key feature of solution processability, solubilities of the synthesized compounds were investigated with a range of solvents having different polarities. It was observed that the chain length increases the solubilities of compounds in common organic solvents, irrespective of the polarity of solvents used. The new neutral compounds show enhanced solubility over the parent compound in tetrahydrofuran and DMF.32 In contrast, ionic compounds exhibited much better solubility in common organic solvents, especially in acetonitrile, which makes it more attractive due to the fact that thin film processing from acetonitrile is effortless. These high solubilities may facilitate the production of free-standing films on solution processing. Since the core structure is same, all the compounds

exhibited identical absorption peaks at 242, 258, 277, 312, 328, 345, and 360 nm. The absorption spectra obviously exhibit a typical phenanthroimidazole derivative characteristic of an intense band from 312 to 360 nm, which can be attributed to the delocalized π−π* transition of the phenanthroimidazole.17 The absorption bands in the region from ∼250 to 300 nm might originate from a combination of π−π* transitions of the aromatic segments.33 As expected, both of the neutral compounds exhibited similar structures with less blue emission peaks centered on 445 nm, whereas, ionic compounds exhibited emission peaks at 453 and 450 nm for ihpypn and ibpypn, respectively. It is noteworthy that all molecules exhibited intense blue fluorescence in solution processed thin films, which is highly desirable for electroluminescent applications.17 The emission characteristics of these molecules in solutions indicate that different alkyl chains have negligible influence on the photo luminescence behavior of these emitters in solutions. Thin film emissions (Supporting Information Figure S1) of nhpypn, nbpypn, ihpypn, and ibpypn were observed around 459,462, 463, and 458 nm, respectively. In comparison to the solution emission, thin film fluorescence is slightly red-shifted, the deviation of the emission maximum may be due to closer packing of molecules in the amorphous thin film states. From the thin film emission, it is noted that the ionic series exhibited much lower spectral shifts from emission in solution than those of neutral derivatives, which clearly indicates that the ionic compounds have a lower aggregation tendency than that of neutral ones. Irrespective of their charge, all of the molecules exhibit similar photoluminescence quantum yields (PLQYs)34 in solution (Table 1), whereas in thin films, ionic derivatives give slightly better PLQYs, calculated to be 0.35 and 0.36, respectively, for ibpypn and ihpypn, which underline the observation that ionic side chains effectively reduce molecular aggregation in this type of molecules and thus reduces aggregation induced fluorescence quenching in the solid states. Additionally, the optical band-gaps calculated from the onset of absorption were 3.06, 3.1, 3.07, and 3.09 eV, respectively, for nhpypn, nbpypn, ihpypn, and ibpypn in THF solutions. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy distribution were determined using density functional theory (DFT) 23679

DOI: 10.1021/acs.jpcc.5b07871 J. Phys. Chem. C 2015, 119, 23676−23684

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The Journal of Physical Chemistry C Table 1. Key Physical Properties of the Compounds compounds

Tda (°C)

λPL,maxb (nm)

ΦFc

λPL,maxd (nm)

ΦFe

Egf

nhpypn nbpypn ihpypn ibpypn

339 376 386 366

445 445 453 450

0.65 0.64 0.64 0.65

459 462 463 458

0.31 0.30 0.36 0.35

3.06 3.10 3.07 3.09

a

Td is the thermal-decomposition temperature at a weight percentage of 95%. bSolution emission in dilute THF solutions. cSolution photoluminescence quantum yield (PLQY) in THF measures against DPA as standard. dEmission maximum in spin coated thin films. ePLQY in thin film. fOptical energy gap calculated from onset of absorbance spectra.

values can be applied to the ionic derivatives. In addition, dihedral angles between the phenanthroimidazole core and the C2-substituted pyrene were calculated to be 61.17, 61.33, 61.28, and 61.36° for nhpypn, nbpypn, ihpypn, and ibpypn, respectively. Even though DFT does not provide any reasonable evidence for the presence of steric repulsion in ionic molecules, it was observed that the dihedral angle increases slightly with the decrease in length of alkyl spacers employed, which may have resulted from the steric repulsion in shorter alkyl chain tailored structures. According to DFT, the stability is higher for ionic hexyl bridged molecule (ihpypn) and decreases with a decrease in the length of the alkyl chains, which is obviously reflected in the synthesis of shorter alkyl chain incorporated ionic molecules. Stability values for ionic compounds were tabulated in Table S1. Further understanding of the steric interaction was hindered by the absence of single crystal X-ray studies due to the strong oiling out tendency of these molecules.35 As expected, the influence of alkyl spacers directly on the emission properties are very small due to the lack of contribution to the formation of frontier molecular orbitals. However, it may influence the packing of molecules in the thin films36 and can thus influence properties like PLQY indirectly, vide supra. A DFT optimized structure of ionic derivatives was included in Figure S2. Thermal properties of synthesized compounds were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) under an inert atmosphere. As shown in Figure 3, the decomposition temperatures (defined as temperatures at which the material loss is 5% of its weight) were measured to be 339, 376, 386, and 366 °C for nhpypn, nbpypn, ihpypn, and ibpypn, respectively. The high thermal stabilities of these compounds indicate that all these

calculations (Figure 2). Due to the fact that prediction of HOMO energy distribution for ionic side group incorporated

Figure 2. Optimized molecular geometry and the electronic distribution of HOMO and LUMOs.

molecules using DFT calculation cannot be regarded as reliable and being that the core structure is the same for all compounds, neutral derivatives are used for calculating both HOMO and LUMO structures. As depicted in the Figure 2, the HOMO energy levels are distributed over the entire core, while LUMO confines on the pyrene moiety. The estimated values are −5.16 and −1.68 eV respectively, for HOMO and LUMO energy levels of neutral molecules, and as mentioned earlier, the same

Figure 3. (a) TGA curves for nbpypn (red), nhpypn (green), ibpypn (blue), and ihpypn (black) and (b) DSC curves for nbpypn. 23680

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Figure 4. Current density−voltage−luminescence (J−V−L) characteristics of the LECs based on the molecules (a) nhpypn, (b) nbpypn, (c) ihpypn, and (d) ibpypn.

nbpypn or nhpypn, poly(ethylene oxide), and lithium triflate in the weight ratio 1:0.1:0.185 (organic molecule: PEO:LiCF3SO3) was spun from a solution of THF and cyclohexanone (1:0.57) onto PEDOT:PSS coated ITO glass. Sequentially, aluminum (100 nm) was thermally evaporated for a cathode contact. In the case of ionic derivatives (ibpypn or ihpypn), a 2% solution of organic molecules in acetonitrile was used as a unique component for processing their active layers. Detailed device fabrication procedures are explained in the SI. All the devices revealed sky-blue emissions (Figure S3), but interestingly, ionic derivatives exhibited lesser red shift than neutral ones from their thin film PL spectra. The EL emissions of nhpypn, nbpypn, ihpypn, and ibpypn were observed around 491, 503, 487, and 484 nm, respectively. The red shift of EL from thin film PL was calculated to be 32 and 41 nm respectively, for nhpypn and nbpypn and 24 and 26 nm, respectively, for ihpypn and ibpypn molecules. Even though EL emissions of all the compounds exhibited considerable red shift, it is noteworthy that ionic compound embedded devices show the best blue CIE coordinates among the four LEC devices (Figure S4). In particular, ibpypn shows a CIE of (0.181, 0.270), which is closer toward the blue region among the constructed LECs. Since the emission wavelength can be tuned with ease for this class of compounds, our observation seems meaningless, but considering that the same chromophoric core is behaving differently in a similar environment (ionic molecules) needs attention. All these spectral shifts on bias

materials have the potential to be implemented in organic electroluminescent devices. The key thermal characteristics are summarized in Table 1. From DSC measurements, the nbpypn shows an endothermic melting transition at 183 °C in the first heating process and a glass transition at 102 °C on a second heating process. No obvious glass transition and melting temperatures were observed for nhpypn, ibpypn, and ihpypn on repeated scannings, which may be due to their smaller heat capacities and crystallinities. However, all the modified emitters show lower thermal stabilities compared to the parent molecule,24 which clearly indicates that incorporation of side chains can reduce thermal stabilities in these types of molecules. Nevertheless, the overall thermal characteristics suggest that all the molecules can be annealed at high temperatures in the device manufacturing steps.

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DEVICE CHARACTERIZATION In the current study, we fabricated four LEC devices consisting of neutral and ionic compounds, with the following structure ITO/PEDOT:PSS/Active layer/Al. Other than the components in the active layer, all the fabricated device structures remain the same. Neutral molecules are blended with ion transporting polymers and an inorganic salt for in situ electrochemical doping and formation of a p−n junction during operation,5 whereas ionic molecules are used without any additional components. In order to process the active layer in neutral molecule based devices, a tricomponent blend of 23681

DOI: 10.1021/acs.jpcc.5b07871 J. Phys. Chem. C 2015, 119, 23676−23684

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LECs, the maximum brightness and maximum efficiency are 49 cd/m2 (at 0.18 cd/A) and 0.29 cd/A (at B = 5.1 cd/m2), respectively, for the nbpypn LEC and 278 cd/m2 (at 0.31 cd/ A) and 0.38 cd/A (B = 62 cd/m2), respectively, for the nhpypn based device. Compared to the performance of ionic compound based LECs, neutral derivatives fail to impress in the context of maximum brightness, and notably the butyl derivative (nbpypn) exhibited poor EL performances among the processed devices. This implies that quenching of fluorescence may be more pronounced in its thin films due to the shorter alkyl spacer, which may not be sufficient for reducing intermolecular interactions in the emissive layer. In addition, the poor electroluminescence from neutral molecules can also be due to the improper formation of doping regions in their active layers. Since the concentration of components in the active layer has a profound influence on the device performance of neutral molecules,43 we tried other mass ratios as well, but phase separation tendency of neutral emitters held us from careful tuning of optimum mass concentrations. Furthermore, the overall device performance suggests that single straight alkyl chains are not enough for hindering intermolecular interactions in the solid state for neutral phenanthroimidazole derivatives, while the incorporation of ionic side chains in the phenanthroimidazole derivatives seems like a plausible way of generating improved solution processable emitters for LEC devices.

are probably due to the combined effect of subtle LEC operational mechanisms along with the intra- and intermolecular interaction of molecules in the active layers.37,38 However, the spectral shifts exhibited by neutral derivatives are not severe compared to the parent molecule, thus suggesting that our design principles work well to some extent in eliminating the unusual red shift of EL spectrum for neutral phenanthroimidazole derivatives. However, the shorter alkyl chain derivative (nbpypn) shows more similar EL characteristics toward its parent molecule (PYPN) and thus pinpoints the effect of the alkyl chain length of imidazole nitrogen on the performance of phenanthroimidazole derivatives in LECs. A constant voltage scanning was employed in this study for the investigation of maximum brightness that can be achievable with this prototype device, and accordingly, all other parameters were calculated. Typical LEC performance was exhibited by all the constructed devices (Figure 4). All the device characteristics are summarized in Table 2. The lowest turn-on voltage of 5.8 V Table 2. EL Performance of the Synthesized Compounds active material

ELmaxa

Von (V)b

nhpypn nbpypn ihpypn ibpypn

491 503 487 484

6.5 8.5 5.8 8.7

Lmax (cd m−2)c

Effmax (cd A−1)d

278 49 711 586

0.38 0.29 0.2 0.19

(0.31) (0.18) (0.18) (0.1)

(62) (5.1) (98) (174)

CIE (x,y)e 0.193, 0.211, 0.191, 0.181,

0.362 0.386 0.309 0.270

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CONCLUSIONS We designed and synthesized a series of easily accessible, highly soluble, strongly luminescent emitters from a single phenanthroimidazole core through simple synthetic procedures and characterized those emitters by various spectroscopic techniques. Additionally, LEC devices were fabricated using both neutral and ionic phenanthroimidazole derivatives. It is noteworthy that all of the constructed devices displayed emissions in the sky blue region of the spectrum, demonstrating that phenanthroimidazole derivatives can function as a better solution processable emitter when tailored with lengthier alkyl spacers, especially those having ionic end groups. Our best device performance was obtained from the ionic phenanthroimidazole derivative (ihpypn) under the LEC device structure, which yields a maximum brightness of 711 cd/m2 at a current efficiency of 0.18 cd/A. In comparison to its parent molecule (pypn), around a 5-fold increase in the maximum brightness

Maximum luminescence. bTurn-on voltages at 1 cd m−2. cMaximum luminescence (current efficiency in cd A−1 at maximum luminescence is given in parentheses). dMaximum efficiency (brightness in cd m−2 at maximum efficiency is in parentheses). eCommission International de I’Eclairage coordinates (CIE) measured at 50 mA. a

(at B > 1 cd/m2) was presented by the devices with ihpypn as the active material. The same device outplays other with a maximum brightness of about 711 cd/m2 at a current efficiency of 0.18 cd/A, whereas the device based on ibpypn exhibited a slightly lower performance than its analogue, with a maximum brightness of 586 cd/m2 at a current efficiency of 0.1 cd/A. These peak brightness values lie among the best reported for LECs in the same emission colors.11,39−42 The maximum efficiencies are 0.2 cd/A (at brightness (B) = 98 cd/m2) and 0.19 cd/A (at B = 174 cd/m2), respectively, for the ihpypn and ibpypn LECs (Figure 5). For the neutral molecules based

Figure 5. Current efficiency versus brightness of (a) neutral and (b) ionic compounds based LECs. 23682

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(6) Tang, S.; Pan, J.; Buchholz, H. A.; Edman, L. White light from a single-emitter light-emitting electrochemical cell. J. Am. Chem. Soc. 2013, 135, 3647−3652. (7) Meier, S. B.; Tordera, D.; Pertegás, A.; Roldán-Carmona, C.; Ortí, E.; Bolink, H. J. Light-emitting electrochemical cells: recent progress and future prospects. Mater. Today 2014, 17, 217−223. (8) Xiong, Y.; Li, L.; Liang, J.; Gao, H.; Chou, S.; Pei, Q. Efficient white polymer light-emitting electrochemical cells. Mater. Horiz. 2015, 2, 338−343. (9) Zhang, Z.; Guo, K.; Li, Y.; Li, X.; Guan, G.; Li, H.; Luo, Y.; Zhao, F.; Zhang, Q.; Wei, B.; Pei, Q.; Peng, H. A colour-tunable, weavable fibre-shaped polymer light-emitting electrochemical cell. Nat. Photonics 2015, 9, 233−238. (10) Tamayo, A. B.; Garon, S.; Sajoto, T.; Djurovich, P. I.; Tsyba, I. M.; Bau, R.; Thompson, M. E. Cationic Bis-cyclometalated iridium(III) diimine complexes and their use in efficient blue, green, and red electroluminescent devices. Inorg. Chem. 2005, 44, 8723−8732. (11) He, L.; Duan, L.; Qiao, J.; Dong, G.; Wang, L.; Qiu, Y. Highly efficient blue-green and white light-emitting electrochemical cells based on a cationic iridium complex with a bulky side group. Chem. Mater. 2010, 22, 3535−3542. (12) Hu, T.; He, L.; Duan, L.; Qiu, Y. Solid-state light-emitting electrochemical cells based on ionic iridium(iii) complexes. J. Mater. Chem. 2012, 22, 4206−4215. (13) Yang, C.-H.; Mauro, M.; Polo, F.; Watanabe, S.; Muenster, I.; Fröhlich, R.; De Cola, L. Deep-blue-emitting heteroleptic iridium(III) complexes suited for highly efficient phosphorescent OLEDs. Chem. Mater. 2012, 24, 3684−3695. (14) Costa, R. D.; Tordera, D.; Orti, E.; Bolink, H. J.; Schonle, J.; Graber, S.; Housecroft, C. E.; Constable, E. C.; Zampese, J. A. Copper(i) complexes for sustainable light-emitting electrochemical cells. J. Mater. Chem. 2011, 21, 16108−16118. (15) Huang, J.; Su, J.-H.; Li, X.; Lam, M.-K.; Fung, K.-M.; Fan, H.-H.; Cheah, K.-W.; Chen, C. H.; Tian, H. Bipolar anthracene derivatives containing hole- and electron-transporting moieties for highly efficient blue electroluminescence devices. J. Mater. Chem. 2011, 21, 2957− 2964. (16) Justin Thomas, K. R.; Velusamy, M.; Lin, J. T.; Chuen, C.-H.; Tao, Y.-T. Chromophore-labeled quinoxaline derivatives as efficient electroluminescent materials. Chem. Mater. 2005, 17, 1860−1866. (17) Yuan, Y.; Chen, J.-X.; Lu, F.; Tong, Q.-X.; Yang, Q.-D.; Mo, H.W.; Ng, T.-W.; Wong, F.-L.; Guo, Z.-Q.; Ye, J.; Chen, Z.; Zhang, X.H.; Lee, C.-S. Bipolar phenanthroimidazole derivatives containing bulky polyaromatic hydrocarbons for nondoped blue electroluminescence devices with high efficiency and low efficiency roll-off. Chem. Mater. 2013, 25, 4957−4965. (18) Liu, H.; Bai, Q.; Yao, L.; Zhang, H.; Xu, H.; Zhang, S.; Li, W.; Gao, Y.; Li, J.; Lu, P.; Wang, H.; Yang, B.; Ma, Y. Highly efficient near ultraviolet organic light-emitting diode based on a meta-linked donoracceptor molecule. Chem. Sci. 2015, 6, 3797−3804. (19) Hill, Z. B.; Rodovsky, D. B.; Leger, J. M.; Bartholomew, G. P. Synthesis and utilization of perylene-based n-type small molecules in light-emitting electrochemical cells. Chem. Commun. 2008, 6594− 6596. (20) Pei, Q.; Yu, G.; Zhang, C.; Yang, Y.; Heeger, A. J. Polymer lightemitting electrochemical cells. Science (Washington, DC, U. S.) 1995, 269, 1086−1088. (21) Kervyn, S.; Fenwick, O.; Di Stasio, F.; Shin, Y. S.; Wouters, J.; Accorsi, G.; Osella, S.; Beljonne, D.; Cacialli, F.; Bonifazi, D. Polymorphism, fluorescence, and optoelectronic properties of a borazine derivative. Chem. - Eur. J. 2013, 19, 7771−7779. (22) Chen, H.-F.; Liao, C.-T.; Chen, T.-C.; Su, H.-C.; Wong, K.-T.; Guo, T.-F. An ionic terfluorene derivative for saturated deep-blue solid state light-emitting electrochemical cells. J. Mater. Chem. 2011, 21, 4175−4181. (23) Shanmugasundaram, K.; Subeesh, M. S.; Sunesh, C. D.; Chitumalla, R. K.; Jang, J.; Choe, Y. Synthesis and photophysical characterization of an ionic fluorene derivative for blue light-emitting electrochemical cells. Org. Electron. 2015, 24, 297−302.

and over a 10-fold increase in the current efficiency at peak brightness was displayed by both of the ionic derivatives in the LEC devices. The maximum brightness values exhibited by ihpypn-LECs are among the best compared to LEC devices reported for corresponding emission colors. However, neutral molecules lag behind their ionic counterparts in terms of maximum brightness, but contrastingly, their current efficiency at peak brightness was improved over 20 times when compared to its core molecule based device. However, the neutral molecule (nhpypn) exhibited a 2-fold improvement in maximum brightness from its parent molecule in the LEC device. All these factors reveal that the length and nature of alkyl spacers have tremendous influence over their electroluminescent properties in LEC devices. Moreover, the present report indicates that phenanthroimidazole derivatives have a huge amount of potential to be used as active materials in LECs, even though their performance has to improve. Work is in progress toward improving LEC device performance via tuning the core structure of emitter for enhanced carrier transport balance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b07871. General procedure; thin film PL; CIE chromaticity diagram of ionic molecules; DFT optimized structures of ionic molecules; DFT calculated stability data of ionic molecules; and EL spectra of devices (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.C.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research has been supported by Basic Science Research program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (NRF-2013R1A1A4A03009795) and Brain Korea 21 Plus project.

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REFERENCES

(1) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. White organic light-emitting diodes with fluorescent tube efficiency. Nature 2009, 459, 234−238. (2) Pei, Q.; Yang, Y. G.; Zhang, C.; Heeger, A. J. Polymer lightemitting electrochemical cells: In situ formation of a light-emitting p− n junction. J. Am. Chem. Soc. 1996, 118, 3922−3929. (3) Matyba, P.; Yamaguchi, H.; Chhowalla, M.; Robinson, N. D.; Edman, L. Flexible and metal-free light-emitting electrochemical cells based on graphene and PEDOT-PSS as the electrode materials. ACS Nano 2011, 5, 574−580. (4) Pertegás, A.; Tordera, D.; Serrano-Pérez, J. J.; Ortí, E.; Bolink, H. J. Light-emitting electrochemical cells using cyanine dyes as the active components. J. Am. Chem. Soc. 2013, 135, 18008−18011. (5) Tang, S.; Tan, W.-Y.; Zhu, X.-H.; Edman, L. Small-molecule lightemitting electrochemical cells: evidence for in situ electrochemical doping and functional operation. Chem. Commun. 2013, 49, 4926− 4928. 23683

DOI: 10.1021/acs.jpcc.5b07871 J. Phys. Chem. C 2015, 119, 23676−23684

Article

The Journal of Physical Chemistry C (24) Subeesh, M. S.; Shanmugasundaram, K.; Sunesh, C. D.; Won, Y. S.; Choe, Y. Utilization of a phenanthroimidazole based fluorophore in light-emitting electrochemical cells. J. Mater. Chem. C 2015, 3, 4683− 4687. (25) Sunesh, C. D.; Mathai, G.; Choe, Y. Constructive effects of long alkyl chains on the electroluminescent properties of cationic iridium complex-based light-emitting electrochemical cells. ACS Appl. Mater. Interfaces 2014, 6, 17416−17425. (26) Skonieczny, K.; Ciuciu, A. I.; Nichols, E. M.; Hugues, V.; Blanchard-Desce, M.; Flamigni, L.; Gryko, D. T. Bright, emission tunable fluorescent dyes based on imidazole and π-expanded imidazole. J. Mater. Chem. 2012, 22, 20649−20664. (27) Zhang, Y.; Lai, S.-L.; Tong, Q.-X.; Chan, M.-Y.; Ng, T.-W.; Wen, Z.-C.; Zhang, G.-Q.; Lee, S.-T.; Kwong, H.-L.; Lee, C.-S. Synthesis and characterization of phenanthroimidazole derivatives for applications in organic electroluminescent devices. J. Mater. Chem. 2011, 21, 8206− 8214. (28) Zhuang, S.; Shangguan, R.; Huang, H.; Tu, G.; Wang, L.; Zhu, X. Synthesis, characterization, physical properties, and blue electroluminescent device applications of phenanthroimidazole derivatives containing anthracene or pyrene moiety. Dyes Pigm. 2014, 101, 93− 102. (29) Benelhadj, K.; Massue, J.; Retailleau, P.; Ulrich, G.; Ziessel, R. 2(2′-Hydroxyphenyl)benzimidazole and 9,10-phenanthroimidazole chelates and borate complexes: solution- and solid-state emitters. Org. Lett. 2013, 15, 2918−2921. (30) Park, S.; Kwon, J. E.; Kim, S. H.; Seo, J.; Chung, K.; Park, S.-Y.; Jang, D.-J.; Medina, B. M.; Gierschner, J.; Park, S. Y. A White-lightemitting molecule: frustrated energy transfer between constituent emitting centers. J. Am. Chem. Soc. 2009, 131, 14043−14049. (31) He, D.; Zhuang, H.; Liu, H.; Liu, H.; Li, H.; Lu, J. Adjustment of conformation change and charge trapping in ion-doped polymers to achieve ternary memory performance. J. Mater. Chem. C 2013, 1, 7883−7889. (32) Yan, Y.-N.; Lin, D.-Y.; Pan, W.-L.; Li, X.-L.; Wan, Y.-Q.; Mai, Y.L.; Song, H.-C. Synthesis and optical behaviors of 2-(9-phenanthrenyl)-, 2-(9-anthryl)-, and 2-(1-pyrenyl)-1-alkylimidazole homologues. Spectrochim. Acta, Part A 2009, 74, 233−242. (33) Huang, T.-H.; Lin, J. T.; Chen, L.-Y.; Lin, Y.-T.; Wu, C.-C. Dipolar dibenzothiophene S,S-dioxide derivatives containing diarylamine: materials for single-layer organic light-emitting devices. Adv. Mater. 2006, 18, 602−606. (34) Eaton, E. F. Reference materials for fluorescence measurement. Pure Appl. Chem. 1988, 60 (7), 1107−1114. (35) Kiesow, K.; Tumakaka, F.; Sadowski, G. Experimental investigation and prediction of oiling out during crystallization process. J. Cryst. Growth 2008, 310, 4163−4168. (36) Zhang, F.; Hu, Y.; Schuettfort, T.; Di, C.; Gao, X.; McNeill, C. R.; Thomsen, L.; Mannsfeld, S. C. B.; Yuan, W.; Sirringhaus, H.; Zhu, D. critical role of alkyl chain branching of organic semiconductors in enabling solution-processed n-channel organic thin-film transistors with mobility of up to 3.50 cm2 V−1 s−1. J. Am. Chem. Soc. 2013, 135, 2338−2349. (37) Kalinowski, J.; Fattori, V.; Cocchi, M.; Williams, J. A. G. Lightemitting devices based on organometallic platinum complexes as emitters. Coord. Chem. Rev. 2011, 255, 2401−2425. (38) Wang, Y.-M.; Teng, F.; Hou, Y.-B.; Xu, Z.; Wang, Y.-S.; Fu, W.F. Copper (I) complex employed in organic light-emitting electrochemical cells: Device and spectra shift. Appl. Phys. Lett. 2005, 87, 233512−233513. (39) Fernandez-Hernandez, J. M.; Ladouceur, S.; Shen, Y.; Iordache, A.; Wang, X.; Donato, L.; Gallagher-Duval, S.; de Anda Villa, M.; Slinker, J. D.; De Cola, L.; Zysman-Colman, E. Blue light emitting electrochemical cells incorporating triazole-based luminophores. J. Mater. Chem. C 2013, 1, 7440−7452. (40) Pertegas, A.; Shavaleev, N. M.; Tordera, D.; Orti, E.; Nazeeruddin, M. K.; Bolink, H. J. Host-guest blue light-emitting electrochemical cells. J. Mater. Chem. C 2014, 2, 1605−1611.

(41) Yang, C.-H.; Beltran, J.; Lemaur, V.; Cornil, J.; Hartmann, D.; Sarfert, W.; Fröhlich, R.; Bizzarri, C.; De Cola, L. Iridium metal complexes containing N-heterocyclic carbene ligands for blue-lightemitting electrochemical cells. Inorg. Chem. 2010, 49, 9891−9901. (42) He, L.; Duan, L.; Qiao, J.; Zhang, D.; Wang, L.; Qiu, Y. Enhanced stability of blue-green light-emitting electrochemical cells based on a cationic iridium complex with 2-(1-phenyl-1H-pyrazol-3yl)pyridine as the ancillary ligand. Chem. Commun. 2011, 47, 6467− 6469. (43) Fang, J.; Matyba, P.; Edman, L. The design and realization of flexible, long-lived light-emitting electrochemical cells. Adv. Funct. Mater. 2009, 19, 2671−2676.

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