Eicient and Tunable White-Light Emission Using a Dispersible Porous Polymer

studied for similar applications. This synthetic approach ofers the user much scope for modiication, which can be exploited when designing processable porous materials for a speciic end application. Already this approach has been exploited to design materials capable of che-mosensing, photocatalysis, and now white-light emission. In the near future, the group aims to report work on the fabrica-tion of ilms/membranes from these materials to complement and build on their current uses in solution.

The ORCID identiication number(s) for the author(s) of this article can be found under https://doi.org/10.1002/marc.202000176.

DOI: 10.1002/marc.202000176
dual emission. [25,26] Porous materials may ofer a unique solution to this problem given that they are capable of encapsulating multiple organic molecules, which could then be used to simultaneously emit light to generating white-light. For this to be realized, it is pivotal that the porous material is also solution processable so it is able to maximize encapsulation of the luorophores and allow for processing for the desired end application. [27,28] In recent years, there have been a handful of reports of other soluble porous polymers which use alkyl side chains to aid solubility. In 2012, Cheng et al. reported a soluble conjugated microporous polymer (SCMP-1) with a Brunauer-Emmett-Teller (BET) surface area of 505 m 2 g −1 using t-butyl pyrene monomers with a M n of 4340 g mol −1 similar to that of a irst generation polyhedral oligomeric silsesquioxane centered pyrene dendrimer. [29] Bandyopadhyay et al. reported another CMP type material this time using a dioctylcyclopentyl monomer, which could be fabricated as a powder, solution and as nanoparticles with a BET surface area of 405 m 2 g −1 . [30] Wood and co-workers synthesized the irst soluble hypercrosslinked polymers, synthesized by intramolecularly crosslinking individual poly(styrene) chains at high dilution with surface areas ranging from 187 to 724 m 2 g −1 . [31] Though these examples of soluble MOPs were found to be both porous and soluble, their synthesis either involved high costs, numerous synthetic steps, or large quantities of carcinogenic solvent. Furthermore, these examples are all very niche and do not provide a generic route to soluble MOPs and hence are unlikely to ever lead to commercial use.
In 2019, we reported the irst example of a versatile route to synthesize dispersible microporous polymers via the reversible addition-fragmentation chain-transfer (RAFT) mediated polymerization of divinylbenzene (DVB) and fumaronitrile (FN) using a hydrophilic poly(ethylene glycol) (PEG) macrochain transfer agent (macro-CTA). [32] This was performed in a water/ethanol solvent mixture using a polymerization-induced self-assembly (PISA) approach. The result of this synthesis was a material consisting of a porous core made solution dispersible by the presence of the long hydrophilic outer PEG polymer chains. These porous polymer particles demonstrated long-term stability in a wide variety of diferent solvents. Our approach has recently been exploited by Ferguson et al. to perform photocatalytic reactions by introducing the crosslinker 4,7-bis(4-vinylphenyl)benzo[c] [1,2,5]thiadiazole into the cores of the samples during polymerization. [33] This highlights the versatility of the synthetic approach as potentially any vinylic mono mer is capable of being polymerized and incorporated into the inal product. Microporous organic polymers (MOPs) are a class of micro porous material, which are both chemically and thermally stable (they can be boiled in acid/base without the loss of porosity) and possess high apparent surface areas, some as high as 5000 m 2 g −1 . [1][2][3] Furthermore, they can be tuned toward speciic applications such as gas separation [4][5][6] and storage, [7,8] photocatalysis, [9,10] chemosensing, [11,12] hydrogen evolution, [13][14][15] and more. [16,17] One disadvantage of MOPs however is their lack of solubility in common organic solvents due to their extended crosslinked network structure, which ensures their permanent porosity. This lack of solubility hinders the processing of the materials thereby limiting their potential applications despite their many interesting and advantageous properties. [18] There are many subclasses of MOP including conjugated microporous polymers (CMPs) [19,20] and hypercrosslinked polymers (HCPs) [21,22] but only one subclass combines permanent porosity with solubilitypolymers of intrinsic microporosity (PIMs). [23,24] The linear yet rigid and contorted structure of PIMs is advantageous to their solution processability, but they too are not without their own drawbacks which includes a limited number of building blocks and a relative lack of functionality compared to other MOPs.
White-light emission from one single material can be dificult to achieve because simultaneous emission of three primary colors to cover the entire visible region of light is needed. As such the vast majority of reports use metal complexes or the complex design of small organic molecules capable of this Building on this work, we now report the synthesis of dispersible microporous polymers to produce white light achieved by post-synthetic modiication (PSM) of a DVB and acrylic acid (AA) solution-stable porous dispersion capable of encapsulating the luorescent dye rhodamine B. Previously, FN was polymerized as a comonomer alongside DVB to produce dispersible microporous polymer particles. Though efective, fumaronitrile cannot be readily functionalized hence further applications or PSM of this material is quite limited. In order to introduce functionality into the core of the particles the use of AA was investigated due to its potential scope for further PSM.
Initially, the synthesis of an insoluble porous material was targeted via conventional free radical polymerization of DVB and AA in toluene (see sections S1 and S4 in the Supporting Information). This reaction resulted in an insoluble porous polymer in good yield (88%) with a BET surface area of 457 m 2 g −1 and set the precedence for the synthesis of a dispersible analogue.
As previously reported, the use of a RAFT-PISA approach using a PEG macro-CTA to polymerize DVB alongside a suitable comonomer yields a dispersible microporous polymer (Scheme 1). [33] Hence, the same methodology was applied to the synthesis of PEG 113 -b-DVB 800 -co-AA 200 . PSM of this material was carried out on the acrylic acid functionality embedded into the core of the material using 9-anthracenemethanol via esteriication to yield PEG 113 -b-DVB 800 -co-AA/An 200 .
The solid state 13 C cross-polarization magic angle spinning (CP/MAS) NMR spectra of both PEG 113 -b-DVB 800 -co-AA 200 and PEG 113 -b-DVB 800 -co-AA/An 200 (Figure 2a) show signals at 13 and 40 ppm due to the presence of -CH 2 -and -CH-backbone alkyl carbons indicating that the polymerization of the two monomers was successful. Further resonances in both spectra at 137 ppm are attributed to the quaternary aromatic carbons while those at 128 ppm are attributed to aromatic C-H Ar carbons. The signals at 145 ppm are assigned to the carboxylic acid carbons present due to the successful incorporation of the acrylic acid monomer into the inal material. The peak at 113 ppm is due to unreacted vinyl groups most likely present as a result of incomplete polymerization of divinylbenzene. There is a small peak at 70 ppm present in both samples, which highlights the successful inclusion of the PEG macro-CTA into the inal product. Unfortunately, due to the small amount of anthracene present in the post-synthetically modiied sample, it was not possible to resolve any peaks relating to the inclusion of anthracene aromatic groups in this material. However, it was possible to conirm the presence of anthracene in the modiied sample using a combination of UV-vis and luorescence emission spectroscopy (Figure 1). Under UV irradiation, both samples demonstrate luorescence emission in the UV region between 330 and 350 nm. However, in the case of PEG 113 -b-DVB 800 -co-AA/An 200, there are two new peaks present in the blue visible region of light (400-450 nm), which is owed to the incorporation of the anthracene units into the core of the material (Figure 1; Section S6, Supporting Information).
Nitrogen gas adsorption studies were carried out at 77 K, to study the porosity of both samples (Figure 2b). Initially, PEG 113b-DVB 800 -co-AA 200 was calculated to have a speciic BET surface area of 201 m 2 g −1 though after PSM this decreased slightly to 188 m 2 g −1 most likely due to the inclusion of some bulky aromatic anthracene units within the porous core of the material. Both materials were found to be lower in surface area than the insoluble analogue, which was also found to be the case for the previously reported divinylbenzene and fumaronitrile analogue. [32] In contrast to the DVB/FN analogue, PEG 113 -b-DVB 800co-AA 200 showed little uptake of gas in the low relative pressure region (<0.1 P/P 0 ) however incremental uptake was observed over a relative pressure range of 0.2-0.8 P/P 0 indicative of mesoporosity within the materials. This is demonstrated by the pore size distribution analysis (see inset of Figure 2b), which shows a pore size centered around 3 nm, slightly larger than that of a microporous material. Uptake in the high relative pressure range (>0.9 P/P 0 ) is due to the aggregated morphology observed in the transmission electron microscopy (TEM) (see Figure 3a,b) images and similar to that of the DVB/FN analogue. The fact that the surface area has decreased after PSM is indicative of the anthracene units being located within the pores of the material and not merely adsorbed onto the surface of the sample.
Both PEG 113 -b-DVB 800 -co-AA 200 and PEG 113 -b-DVB 800 -co-AA/ An 200 formed stable dispersions upon sonication in methanol, which allowed for the determination of the particle size via dynamic light scattering (DLS) (Figure 3c). It was expected that the size of the particles would be afected by the solvent as was found for the DVB/FN analogues where the core was shown to swell in good solvents while bad solvents were not able to swell the core. In PEG 113 -b-DVB 800 -co-AA 200 , the core is composed of both DVB and AA units whereas in the PEG 113 -b-DVB 800 -co-AA/An 200 sample a signiicant proportion of the AA units have been reacted with 9-anthracenemethanol, which alters the solubility of the material. DLS results showed that PEG 113 -b-DVB 800co-AA 200 was indeed larger than PEG 113 -b-DVB 800 -co-AA/An 200 in methanol with calculated hydrodynamic radii of 1124 and 870 nm, respectively. This is perhaps to be expected given that methanol is a poor solvent for anthracene, which now makes up part of the core of the material.
White light can be generated through the combination of a blue and yellow emitter, which covers most of the visible spectral range (400-800 nm). Being both porous and dispersible, PEG 113 -b-DVB 800 -co-AA/An 200 can be used to encapsulate organic compounds in solution. Therefore, the yellow emitting organic luorophore rhodamine B was mixed with a dispersion     of the blue emitting PEG 113 -b-DVB 800 -co-AA/An 200 in order to try and generate a white light emitting solution.
Through ine-tuning of the rhodamine B concentration (Section S7, Supporting Information), it was possible to generate a white-light emitting solution upon addition of 1.63 ppm of rhodamine B to a dispersion (1 mg mL −1 ) of PEG 113 -b-DVB 800 -co-AA/ An 200 (Figure 4). Under UV irradiation (λ max = 355 nm) a whitelight emitting solution was created with commission internationale de l'éclairage (CIE) coordinates of X = 0.33, Y = 0.32, close to that of an idealized white light source (X = 0.33, Y = 0.33). The correlated color temperature (CCT) of the solution was 5239 K, close to that of horizon daylight (5000 K). The quantum yield of this solution was calculated and found to be 38%, which is comparable to most other porous materials that range from 11-38% (see Section S7 in the Supporting Information).
In summary, a porous and solution processable material was synthesized via the controlled radical polymerization of DVB and AA mediated using a PEG-based macro-CTA. Postsynthetic modiication of this material using 9-anthracenemethanol resulted in a blue emitting dispersion in which the anthracene is present within the core of the sample. Encapsulation of a yellow emitting luorophore, rhodamine B, allows for the generation of a white-light emitting solution with CIE coordinates close to that of ideal white light (X = 0.32, Y = 0.32). The quantum yield of the solution (38%) is comparable to many other porous materials studied for similar applications. This synthetic approach ofers the user much scope for modiication, which can be exploited when designing processable porous materials for a speciic end application. Already this approach has been exploited to design materials capable of chemosensing, photocatalysis, and now white-light emission. In the near future, the group aims to report work on the fabrication of ilms/membranes from these materials to complement and build on their current uses in solution.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.