Fluorescent Supramolecular Polymers Based on Pillar[5]arene for

Letter pubs.acs.org/macroletters

Fluorescent Supramolecular Polymers Based on Pillar[5]arene for OLED Device Fabrication Xiye Yang,†,‡ Wanqing Cai,†,‡ Sheng Dong,† Kai Zhang,† Jie Zhang,*,† Feihe Huang,*,§ Fei Huang,*,† and Yong Cao† †

State Key Laboratory of Luminescent Materials and Devices, Institute of Polymer Optoelectronic Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China § State Key Laboratory of Chemical Engineering, Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *

ABSTRACT: A series of AA/BB-type supramolecular polymers (SP1−3) based on pillar[5]arene host−guest interactions was developed and their photoelectric properties were further evaluated. The formation of SP1 was confirmed by multiple measurements via nuclear magnetic resonance and specific viscosity studies. The electroluminescence properties of SP1−3 were also investigated. As a result of the efficient energy transfer caused by the exciton trapping on narrow band gap guest G2, by applying a doping strategy, the light-emitting color of the resulting polymers could be easily turned from blue to green. Meanwhile, photoluminescent efficiencies up to 81.6% were obtained. All the supramolecular polymers prepared in this work were utilized as the emissive layers (EMLs) in light-emitting devices and a maximum luminance efficiency (LE) of nearly 5 cd A−1 was achieved.

ince the concept of supramolecular polymers was first introduced into the scientific research field by J.-M. Lehn decades ago, they have drawn a lot of attention and significant efforts for unique chemical/physical properties.1 Linear supramolecular polymers are formed by alternative noncovalent interactions including multiple hydrogen bonding, metal coordination, host−guest interactions, and aromatic stacking between certain functionalized monomers.2−5 Supramolecular polymers not only show traditional polymeric characteristics, but also possess inborn advantages, such as good solution processability and facile formation of sophisticated copolymers compared to conventional polymers.6−8 Therefore, novel supramolecular polymers have been designed and applied to various fields, such as drug delivery,6 bioimaging7 and multiple stimuli-responsive applications.8 It is noteworthy that progress has also been made in the fast-developing organic semiconductors area.9−11 For example, Meijer et al. initially reported the pioneering work of using hydrogen-bonded supramolecular copolymers for organic light-emitting diodes (OLEDs),11 despite the low device performance, which was less than 0.1 cd A−1, this work broadened the potential materials choices for OLED fabrication. Recently, supramolecular polymers based on crown ether/ammonium salt host−guest interactions applied into OLEDs were also reported.12−14 These supramolecular light-emitting polymers had device performances comparable to analogous traditional conjugated polymers.12 However, the brightnesses of these devices were lower than the analogous traditional conjugated polymers. We speculate that the ammonium salt units may be a possible reason for this

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© XXXX American Chemical Society

problem. Thus, it is a necessity to develop new approaches addressing this issue. As a new generation of supramolecular macrocyclic hosts after crown ethers,15−17 cyclodextrins,5 calixarenes18 and cucurbiturils,19 pillar[n]arenes, specifically pillar[5]arenes discussed in this paper, which was first reported by Ogoish et al. in 2008,20 have been developed vigorously due to their simple preparation procedure21 and unique chemical structures.22−24 Functional supramolecular polymer systems based on pillar[n]arene derivatives emerged since then. Remarkable progress has been made on obtaining supramolecular polymers in solution and in the solid state.25−28 One of the astonishing merits that pillar[n]arenes possess is they form host−guest complexes with both cationic molecules and neutral molecules bearing electron-withdrawing groups. Thus, we projected that a well-designed pillar[n]arene-based supramolecular polymer system utilizing neutral guests that avoid the presence of mobile ions may be an optimal choice for OLED fabrication. However, to our best knowledge, pillar[n]arene-based supramolecular polymers for organic optoelectronic devices fabrication has never been reported. Herein, we report a series of fluorescent supramolecular polymers (SP1, SP2, and SP3) driven by host−guest interactions between pillar[5]arene and the imidazole group bearing a short alkyl chain29,30 (Scheme 1). Received: April 25, 2017 Accepted: June 8, 2017

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DOI: 10.1021/acsmacrolett.7b00309 ACS Macro Lett. 2017, 6, 647−651

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ACS Macro Letters

Scheme 1. Chemical Structures of H1, G1, and G2, the Construction of Supramolecular Polymers with Different Emitting Colors, and Model Device Structure Fabricated in This Work

Monomers H1 and G1 were designed based on fluorene derivatives that lead to a blue emission. Another guest molecule G2 was synthesized based on fluorene-co-benzothiadiazole derivatives with a green-emitting character as the dopant unit in the SPs. In order to obtain a linear polymer structure, the assembly behavior between H1 and G1 was first investigated. The formation of the linear polymer was confirmed by multiple measurement methods including 1H NMR spectroscopy, twodimensional diffusion-ordered 1H NMR spectroscopy (2D DOSY) and specific viscosity studies. Furthermore, electroluminescence properties and device performances of SP1−SP3 were investigated. It turned out that green light-emitting supramolecular polymers were effectively obtained by doping G2 into the system and device performance was greatly improved and the emitting brightness doubled that of the previous report.12 The details of the synthesis of the monomers are demonstrated in the Supporting Information. Both host and guest monomers have excellent solubility in common organic solvents such as CHCl3, toluene, o-xylene, and so on. All these SPs could be obtained by simply mixing the monomers in certain solvents, such unprecedented easy approach makes pillar[5]arene-based supramolecular polymers even more suitable for solution processed device applications. The emission colors of the SPs could be well tuned from blue to green by modifying the constituent of the main chain. The doping ratios of G2 were 0, 10, and 30%, corresponding to SP1, SP2, and SP3, respectively. According to previous studies, the host−guest complexes based on pillar[5]arene showed enhanced binding constants in nonpolar solvents or when the solvent is too bulky to be included in the cavity.31 Thus, the solvent choice was diversified for polymerization procedures. The low-toxic solvent o-xylene was used for device fabrication in this study, for example. The host−guest interaction behavior between H1 and G1 was first investigated via 1H NMR. Figure 1 shows the proton

Figure 1. 1H NMR (500 MHz, CDCl3, 298 K) spectra of (a) G1, (b) H1⊃G1 with molar ratio H1/G1 = 1:1 at 20 mM, and (c) H1.

NMR spectra of H1, G1, and H1⊃G1 (1:1 molar ratio at 20 mM) in CDCl3. Obviously, the proton NMR spectra of H1⊃G1 are concentration-dependent and the evidence for host−guest complexation is also revealed as the concentration increased (Figure S10). The key proton signals from the pillar[5]arene dimer H1 including the phenyl protons (Ha, Hb, and Hc), methylene protons (Hd), and methoxyl protons (He and H f ) exhibited downfield shifts. Concurrently, the conjugated oligomer proton peaks shifted downfield because of the weakening of π−π stacking. On the other hand, the proton signals of imidazole groups (Ha′ , Hb′ ) from G1 underwent a remarkable upfield shift as well as that observed from the proton signals of the alkyl chain (H′d, H′e , H′f , H′g, H′h, H′i ). This is due to the shielding effect of the electron-rich pillar[5]arene cavity. Meanwhile, this result clearly suggests that both the alkyl chains and the imidazole moieties of G1 are fully threaded through the pillar[5]arene rings of H1. In addition, the formation of high molecular weight aggregates was confirmed by the broadening of the proton signals at high concentrations, which is about 10 mM in this case.32 It is noteworthy that no apparent transformation occurred on the shape of the proton signals of the alkyl chains from the lightemitting units, indicating that the formation of a well-ordered linear supramolecular polymer would not be influenced by the existence of the alkyl chains. 648

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ACS Macro Letters 2D diffusion-ordered 1H NMR spectroscopy experiments were carried out with H1⊃G1 (1:1 molar ratio) in CDCl3 at different concentrations to investigate the formation of supramolecular polymers. Figure S11 illustrate the diffusion coefficient against concentration ranging from 1 mM to 40 mM; as the concentration increased from 1 to 40 mM, the measured weight-averaged diffusion coefficient of H1⊃G1 decreased from 2.27 × 10−10 to 0.67 × 10−10 m2 s−1, revealing that linear supramolecular polymers were formed based on previous studies.33 The viscosity study was performed in chloroform; Figure S12 is the double logarithmic plot of a specific viscosity versus the initial concentrations of equimolar solutions of monomers H1 and G1. At low concentrations, the curve has a slope of 0.263, exhibiting a linear relationship between the specific viscosity and the initial concentration, which is characteristic for noninteracting assemblies of constant size.34 While the concentration increased over the critical polymerization concentration (CPC), a sharp rise with the slope up to 1.923 was observed, suggesting that a linear supramolecular polymer was formed. It turns out that the CPC value is about 8.28 mM by calculation, which matches perfectly with the speculation proposed from the 1H NMR analysis. Meanwhile, it is a rather low concentration compared with previous reported supramolecular polymers based on crown-ether.12−14 This result facilitates the approach to film thickness control with spin coating procedure and would be further beneficial for the fabrication of solution processed organic electronic devices. In order to develop the application of the SPs for solution processable OLED devices, the photophysical properties including absorption and photoluminescent (PL) spectra of both the monomers and the supramolecular polymers in solid film were initially investigated. As shown in Figure 2, both H1 and G1 exhibited similar UV−visible absorption and PL spectra to those of typical fluorene oligomers, while G2 displayed two absorption peaks and characteristically green emission with a peak at around 550 nm as expected.35 The absorption spectrum of SP1 was practically identical to these of H1 and G1, while the supramolecular polymers SP2 and SP3 exhibited a broader main absorption peak at around 360 nm and a minor absorption peak at around 440 nm attributed to the doping of G2. The minor absorption became stronger with the increased doping ratio of G2, indicating that the green-emission groups were incorporated into the supramolecular polymer main chain.16 The PL spectra (Figure 2b) show that SP1 displayed similar emission main peaks to those of H1 and G1. Both SP2 and SP3 exhibited a green emission at around 550 nm, the emission peak at 420 nm for SP1 was completely quenched due to efficient excitation energy transfer from H1 and G1 to G2. The PL peak was red-shifted slightly as the doping ratio increased from 10 to 30%, which was similar to those analogous traditional conjugated copolymers.36,37 The PL quantum yields of the SPs were obtained under excitation by a HeCd laser. The efficiencies were remarkably increased from 58.6% for SP1 to 81.6% for SP2 and 73.7% for SP3, respectively. Interestingly, the quantum yield diminished as the doping ratio increased; such a fact may be due to the trapping mechanism for such a wide-narrow band gap copolymer system, which has been widely investigated in conventional conjugated copolymer research.37 As the formation of supramolecular polymers was confirmed, we investigated the electroluminescence (EL) properties of the supramolecular polymers SPs. All the supramolecular polymers

Figure 2. (a) UV−visible absorption and (b) PL spectra of H1, G1, G2, SP1, SP2, and SP3 in thin film.

SPs prepared in this work were utilized as the emissive layers (EMLs) in light-emitting devices. The device configuration was designed and fabricated as ITO/PEDOT:PSS/poly(9-vinylcarbazole) (PVK)/EMLs/PFN/Ba/Al, where poly[9,9-bis(3′(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN) worked as the electron transporting layer, which could facilitate electron injection from the cathode.38 PVK was introduced as hole transporting layer, which could effectively decrease the leakage current and led to better device performance.39 The current density (J) and brightness (L) versus voltage (V) curves are shown in Figure 3a; as expected, devices fabricated based on the supramolecular polymers SP2 and SP3 showed higher brightness and efficiencies compared with SP1. A maximum luminance efficiency (LE) nearly 5 cd A−1 was achieved as shown in Table 1. Moreover, the blueemitting device based on SP1 also exhibited comparable maximum LE at 0.62 cd A−1 to those based on analogous traditional conjugated polymers, polyfluorene homopolymers, for example.40,41 It is noteworthy that the brightness of the devices achieved in this work doubled that of our previous report, which is probably due to getting rid of mobile ions in the system by utilizing neutral guests for supramolecular polymer formation. The relatively high turn-on-voltages were attributed to the application of PVK, which is a low conductivity hole transporting material.39 Figure 3b shows the EL spectra of the devices. The EL emission derived from SP1 was red-shifted significantly compared with its PL emission due to the existence of emissive on-chain keto defects as typical polyfluorene homopolymers.42,43 Nevertheless, with the doping 649

DOI: 10.1021/acsmacrolett.7b00309 ACS Macro Lett. 2017, 6, 647−651

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ACS Macro Letters

by the exciton trapping on narrow band gap guest G2 led to an improvement on the PL efficiencies. Furthermore, we demonstrated the first OLED device utilizing pillar[5]arene based supramolecular polymers as the emissive layer. Maximum LEs for SP1, SP2, and SP3 were achieved at 0.62, 3.88, and 4.88 cd A−1, respectively, which are comparable efficiencies to those of analogous conventional conjugated polymers, indicating that pillar[n]arenes could be an ideal choice for supramolecular polymer-based optoelectronic applications. This study inspired us to take a deeper insight on pillar[n]arene-based polymers and further illuminate their applications in organic semiconductor area.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00309. Synthesis of the monomers, device fabrication, and characterization (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xiye Yang: 0000-0001-8273-9535 Jie Zhang: 0000-0001-8536-5436 Feihe Huang: 0000-0003-3177-6744 Fei Huang: 0000-0001-9665-6642

Figure 3. (a) J−L−V and (b) EL spectra in the device with configuration of ITO/PEDOT:PSS/PVK/EML/PFN/Ba/Al.

Table 1. EL Performance of the SPs in the Devices with Configuration of ITO/PEDOT:PSS/PVK/EML/PFN/Ba/Al EMLs

Vona (V)

Bb (cd/m2)

LEc (cd/A)

Bmaxd (cd/m2)

LEmaxe (cd/A)

SP1 SP2 SP3

6.5 13.4 10.2

64 334 415

0.59 3.29 3.90

166 692 913

0.62 3.88 4.88

a

Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was financially supported by the Natural Science Foundation of China (Nos. 21520102006, 51403070, and 51521002), and the Science and Technology Project of Guangdong Province (No. 2014B090914002), and Foundation of Guangzhou Science and T echnology Project (201707020019).

−2 b

At the luminance of 1 cd m . Brightness at the current of 10 mA/ cm2. cLuminous efficiency at current density around 10 mA/cm2. d Maximum brightness. eMaximum luminance efficiency.

of G2, the resulting supramolecular polymer-based devices exhibited an exclusively green emission. Since G2 has a narrow band gap, as mentioned before, the units act as charge traps in the copolymers, so the energy transfer efficiency among the polymers is enhanced as well as the hole−electron recombination ratios by doping. This result greatly inspired us to take continuous investigation on pillar[n]arene-based supramolecular for optoelectronic applications. Needless to say, the easy approach to obtain versatile supramolecular polymers as mentioned before also launched a bunch of possibilities in future research work. In conclusion, new fluorescent pillar[5]arene supramolecular polymers based on host monomer H1 and two neutral guest monomers G1 and G2 were prepared. Linear supramolecular polymers with fluorescent character for OLED fabrication were successfully prepared and confirmed by multiple methods. As the doping strategy was successfully applied in these SPs, SP1 and SP2−3 exhibited blue-emitting and green-emitting, respectively, as expected. The efficient energy transfer caused

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