Efficient emissive fluorene-based p–n conjugated porous materials for near-white electroluminescence: benefits of metal-free Friedel–Crafts green polymerization

Chuan-Xin Weia, Lu-Bing Baia, Xiang Ana, Ya-Min Hana, Yu-Qin Liua, Meng-Na Yub, Jin-Yi Lin*ac, Zong-Qiong Linc, Ling-Hai Xie*b, Man Xua, Qing-Huang Zhengd, Jian-Feng Zhaoa, Jian-Pu Wanga and Wei Huang*ac
aKey Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing, China. E-mail: iamjylin@njtech.edu.cn; wei-huang@njtech.edu.cn
bCentre for Molecular Systems and Organic Devices (CMSOD), Key Laboratory for Organic Electronics and Information Displays & Jiangsu Key Laboratory for Biosensors, Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing University of Posts & Telecommunications, 9 Wenyuan Road, Nanjing, China. E-mail: iamlhxie@njupt.edu.cn
cShaanxi Institute of Flexible Electronics (SIFE), Northwestern Polytechnical University (NPU), 127 West Youyi Road, Xi'an, Shaanxi 710072, China
dFujian Senry Green Wallpaper CO. LTD, Zhangzhou, Fujian, China

Received 17th June 2018 , Accepted 31st August 2018

First published on 31st August 2018


Efficient emissive fluorene-based p–n conjugated porous materials (CPMs) have been constructed via an effective Friedel–Crafts green polymerization. For metal-free and room-temperature processing, CPMs exhibit high fluorescence quantum efficiency of 32%, allowing the fabrication of near-white polymer light-emitting diodes with CIE of (0.30, 0.38).


Conjugated organic frameworks with various unique tailored mechanical, electrical and optical properties show wide applications in chemical sensors and bio- and organic electronics. Particularly, conjugated porous materials (CPMs) have attracted tremendous attention and intensive efforts in gas storage1–5 and separation,6 catalysis,7,8 capacitors,9,10 sensing11,12 and detection13–15 due to their large specific surface area and accurate molecular recognition. Through novel molecular designs and reasonable chemical tailoring methods, various porous materials have been synthesized such as polymers of intrinsic microsporocytes (PIMs), covalent organic frameworks (COFs),1 conjugated micro-porous polymers (CMPs)16 and porous aromatic frameworks (PAFs) obtained by the Suzuki reaction17 or the Yamamoto coupling reaction.18 However, most of the synthetic methods for these porous materials are environmentally unfriendly metal-catalysed polymerizations, which require harsh reaction conditions, high cost and cumbersome post-treatment. The residual metal ion in the porous materials is the main factor that significantly affects the emission property and exciton behavior in the solid state. Undoubtedly, these drawbacks limit the exploration of new material systems and their applications. In this regard, it is urgent to exploit a low-cost and convenient method to construct CPMs and extend their practical applications.

Over the past decades, inexpensive and efficient Friedel–Crafts (F–C) reactions have been explored to fabricate CPMs to solve the problems mentioned above; these problems include decreasing material performance due to residual aluminium as well as the requirement for high temperature and strong acid.1,11,12,15,19–21 In our previous studies, we designed and synthesized a series of diarylfluorene derivatives through metal-free and mild F–C reactions, and we also constructed polymers of PVK through F–C click post-functionalization at room temperature.22,23 We were also the first to develop and synthesize a kind of diarylfluorene-based π-conjugation-interrupted polymer by F–C C–H bond polymerization.24,25 Therefore, herein, we have rapidly and readily synthesized a type of CPMs by adopting 9,9,9-triphenyl-9H,9H,9H-[2,2:7,2-terfluorene]-9,9,9-triol (TPhOH) as the p-type building block and 2,4,6-trichloro-1,3,5-triazine (TCTA) as the n-type linkage to construct porous networks by catalysing with boron trifluoride ether at room temperature and dissolving in dichloromethane (Scheme 1). Remarkably, the metal-free and mild reaction conditions resulted in a high photoluminescence quantum yield (PLQY). In addition, we also preliminarily fabricated a near white polymer light-emitting diode (PLED) based on this porous material as the emissive layer. To the best of our knowledge, this is the first reported PLED based on CPMs prepared via a green method.


image file: c8tc02975j-s1.tif
Scheme 1 Diagram of the preparation of TPhOH–TCTA by Friedel–Crafts reactions.

The chemical structure of our CPMs was examined by Fourier transform infrared (FT-IR) spectroscopy, solid-state 13C cross-polarization magic-angle spinning (CP/MAS) NMR (ss 13C NMR), 1H NMR (400 MHz, chloroform-d), X-ray photoelectron spectroscopy (XPS) and elemental analysis (EA). Fig. 1a shows the FT-IR spectra of TPhOH–TCTA networks, which are in agreement with previously reported networks.6,12,20,21 The absence of the C–Cl stretching vibration band at 851 cm−1 in TPhOH–TCTA indicated complete activation of the C–Cl bond. This band together with the absorption band of the triazine ring shifted from 1352 cm−1 (C–N) and 1480–1506 cm−1 (C[double bond, length as m-dash]N) for cyanuric chloride to 1384 cm−1 and 1641 cm−1 in TPhOH–TCTA, respectively; this implied a complete reaction between the two synthesis monomers. In addition, the strong stretching band appearing at around 1075 cm−1 in TPhOH–TCTA indicated the formation of an ether linkage (C–O–CH3), which was suspected to be formed during washing TPhOH–TCTA with methanol and the unreacted chlorines in the triazine rings being replaced by methoxy groups. The structure of TPhOH–TCTA was further confirmed by 13C CP-MAS NMR spectra (Fig. S1, ESI). The signal at 126 ppm was ascribed to unsubstituted phenyl carbons in the section of TPhOH, whereas the signal at 139 ppm belonged to substituted phenyl carbons attached to other aromatic rings. As shown by spectroscopy of TPhOH–TCTA, the peak at 147 ppm was assigned to substituted phenyl carbons linked to the triazine rings. In addition, the signals of quaternary carbon atoms of TPhOH-based units in this polymer were also found at 64 ppm. Elemental analysis data showed that the elemental contents of carbon, hydrogen and nitrogen were 86.5%, 4.92% and 0.18%, respectively. The low nitrogen content indicated that there were few triazine ring units in the polymer, which resulted in a very weak peak at 170 ppm for the carbon on the triazine ring. Therefore, the signal at 170 ppm was very weak, which caused its peak height to be significantly lower than those of the other signal peaks. Furthermore, the XPS data of TPhOH–TCTA were also used to analyse the information about polymer composition (Fig. S2, ESI). The C 1s spectrum of TPhOH–TCTA could be assigned to sp2 carbon atoms of the TPhOH moiety and the triazine node (284–285 eV). The N 1s spectrum showed a signal observed around 398.7 eV, which could be assigned to nitrogen atoms within the triazine units (C–N[double bond, length as m-dash]C). In fact, the self-F–C polymerization of TPhOH also induced the formation of CPMs, as we previously reported.25 We also characterized the residual groups in the prepared TPhOH–TCTA polymer by 1H NMR (Fig. S4, ESI); there was no residual hydroxyl unit in our CPMs. To further confirm the structure, the hyper-cross-linked structure of TPhOH–TCTA was also demonstrated by energy dispersive spectrometry (EDS), which also illustrated that the main building block TPhOH and the linker unit TCTA were fully connected. Through the above structural characterizations, we can conclude that the crosslinking products of TCTA and TPhOH have been obtained successfully. Furthermore, elemental carbon and nitrogen were observed with an efficient, dense and sparse distribution in the analyzed samples (Fig. 1b and c). Hence, we can also effectively conclude that TPhOH and TCTA-based CPMs are obtained via metal-free green F–C polymerization.


image file: c8tc02975j-f1.tif
Fig. 1 (a) FT-IR spectra of TPhOH–TCTA, TPhOH and TCTA. (b) Elemental carbon and (c) nitrogen analysed by EDS.

The porosity parameters for TPhOH–TCTA were quantitatively analyzed from the sorption and desorption isotherms of nitrogen at 77 K (Fig. 2a), which showed a moderate characteristic of adsorption and desorption. The BET surface area and Langmuir surface area for TPhOH–TCTA were moderately 179 m2 g−1 and 258 m2 g−1, respectively (Table S1, ESI). The pore size and distribution in TPhOH–TCTA were derived from the sorption isotherms by the nonlocal density functional theory (NLDFT). TPhOH–TCTA showed three pores sizes of 2.4, 3.4 and 6.6 nm. For a clear illustration, we observed that the reef-like net morphology of the organic polymers observed by field-emission scanning electron microscopy (SEM) reflected the typical characteristic of CPMs (Fig. 2b). We could also clearly see alternately dark and bright spot structures through high-resolution transmission electron microscopy (Fig. 2c). The SEM and TEM images strongly demonstrated the porous nature of this conjugated porous material. Meanwhile, thermogravimetric analysis was employed to confirm the good thermal stability of the obtained porous materials. Notably, the thermal analysis showed that TPhOH–TCTA retained 95% weight at 337 °C under a nitrogen atmosphere and showed a char yield of 86 wt% when heated to 600 °C under a nitrogen atmosphere. It can be accepted that this material has good thermal stability (Fig. S5, ESI).


image file: c8tc02975j-f2.tif
Fig. 2 (a) Adsorption (filled) and desorption (empty) isotherms of N2 at 77 K for TPhOH–TCTA. Inset shows the pore size distributions by NLDFT model; (b) SEM and (c) TEM images of TPhOH–TCTA.

Interestingly, although our CPMs are insoluble, they can be efficiently dispersed in some common organic solvents such as toluene and CHCl3, which is useful for the fabrication of optoelectronic devices. Therefore, we also investigated the optical properties of our CPMs in various states (Fig. 3). As shown in Fig. 3a, the maximum absorption peak of TPhOH–TCTA dispersed solution was at about 352 nm, which was assigned to the main conjugated backbone of TPhOH (Fig. S6, ESI). Amazingly, our dispersed solution exhibited efficient deep-blue emission with two peaks of 404 and 420 nm. To check the film-forming ability of our CPMs, AFM measurements were obtained to investigate the film morphology. As shown in Fig. 3b, a continuous and smooth film was obtained via a spin-coating process with a roughness of about 5.2. Besides, cross-linked and interpenetrated surface morphology was also found in our 2D AFM image, which was consistent with our TEM results (Fig. S, ESI). Interestingly, this spin-coated film also showed a maximum absorbance peak at 352 nm similar to the dispersed solutions. However, unlike the result for the diluted solution, there was an additional emission peak at 530 nm in the PL spectra of our TPhOH–TCTA spin-coated films. In our previous study, all pure fluorene F–C cross-linked polymers with any n-type units exhibited deep-blue emission without any emission at longer wavelengths.25 Moreover, during the chemical reaction, the hydroxyl group was removed in the presence of a boron trifluoride diethyl ether complex; thus, this could not be formed by anthrone. To explain this phenomenon, the lifetimes of three fluorescence emission peaks were measured (Fig. 3d). After curve fitting, the fluorescence lifetimes at 415, 428 and 520 nm were 0.13, 0.21 and 8.53 ns, respectively. Therefore, the introduction of TCTA in our p–n CPMs that induces charge transfer is a reasonable explanation for this phenomenon. It is worth noting that this effect is weaker in the solution state but is significantly enhanced in the solid state. Thus, we can obtain deep-blue light in the dispersed solution and near-white light in the spin-coated film. In addition, we investigated the effect of film thickness on the emission properties (Fig. S7, ESI). We observed that the fluorescence intensity of the emission peaks decreased with reducing film thickness, which is associated with a lower density of fluorophore centers in our films. As expected, our TPhOH–TCTA dispersed solution and spin-coated films showed relatively higher PLQYs of 69% and 32%. As a reference, well-studied PFO diluted solution and amorphous film have a PLQY of 95% and 51% at the same experimental condition, respectively. In fact, residual metal ions in the conventional CPMs network also resulted in lower PLQYs because of the significant photoluminescence quenching effect. Therefore, the unique property of our fluorene-based CPMs with higher PLQY is benefitted from metal-free and room-temperature Friedel–Crafts green polymerization. At the same time, we conducted the scanning of excitation–PL emission mapping of CPMs film (Fig. 3c). All the optical properties of our CPMs in various states showed good luminous performance.


image file: c8tc02975j-f3.tif
Fig. 3 (a) Absorption and PL spectra of our CPMs dispersed solution and spin-coated films. (b) AFM image of our CPMs spin-coated film. (c) Excitation–PL emission mapping of CPMs film. (d) Lifetime decay profiles of emission bands of CPMs film.

To further confirm the higher PLQY and weaker photoluminescence quenching effect in our CPMs, we also prepared preliminary PLED devices. We created five-layer devices with a configuration of ITO/PEDOT:PSS (40 nm)/emissive layer (50 nm)/TPBi (20 nm)/LiF (0.8 nm)/Al (100 nm). [ITO, indium tin oxide; PEDOT:PSS, poly(3,4-ethylenedioxythiophene)polystyrene sulfonate; TPBi, 2,2′,2′′(1,3,5-benzinetriyl)tris(1-phenyl-1-H-benzimidazole).] After applying an external voltage, the prepared device was lit to emit near-white light (Fig. 4b). Similar to the PL spectrum, EL spectra of our CPMs consisted of two emission peaks at 403 and 540 nm (Fig. 4b). According to the Chromaticity diagram, the Commission Internationale de L’Eclairage (CIE) coordinates were at (0.30, 0.38), which indicated that this material can be used as a near white light material. In some sense, the lower fraction of n-type TCTA in our CPMs is the fundamental factor for inducing this significant near-white light emission. This demonstrated the potential applications of the material in the field of luminescence. The device characteristics of TPhOH–TCTA exhibited a turn-on voltage of 5.1 V and a maximum brightness of 54 cd m−2 from the voltage–luminance and voltage–current density graphs. To the best of our knowledge, this is the first example of using high emissive CPMs for fabricating near-white light PLEDs.


image file: c8tc02975j-f4.tif
Fig. 4 Device configuration and EL spectrum of our spin-coated CPMs film.

In summary, we described effective, convenient and green BF3·Et2O-mediated Friedel–Crafts polymerizations to prepare conjugated porous materials for constructing PLEDs, which avoided the existence of metal ions in the catalyst residue and the need for harsh reaction conditions. Friedel–Crafts alkylation using easily available TCTA as a triazine node and electron-rich aromatic compounds (TPhOH) yielded conjugated porous polymers with a surface area of up to 258 m2 g−1. We used the prepared conjugated porous materials to fabricate electroluminescent devices. To the best of our knowledge, this is the first report about the application of porous materials in the field of organic light-emitting for near white electroluminescence. Although the electroluminescence performance is average, the study of the electroluminescence of conjugated porous materials, especially for emissive host–guest systems, can open up new ideas for researchers.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The work was supported by the National Natural Science Foundation of China (61874053, 21504041, 21502091, 21502092, 21774061), National Key Basic Research Program of China (973) (2015CB932200), Natural Science Funds of the Education Committee of Jiangsu Province (18KJA430009), “High-Level Talents in Six Industries” of Jiangsu Province (XYDXX-019), Natural Science Foundation of Jiangsu Province (BK20171470), the open research fund from Key Laboratory for Organic Electronics and Information Displays and State Key Laboratory of Supramolecular Structure and Materials at Jilin University (sklssm201808), Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX18-1121).

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c8tc02975j
Chuanxin Wei and Lubin Bai are equally contributed to this work.

This journal is © The Royal Society of Chemistry 2018