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White-Light Emission from Cucurbituril-Based Host–Guest Interaction in the Solid State: New Function of Macrocyclic Host Yu Xia, Shiyan Chen, and Xin-Long Ni ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b02573 • Publication Date (Web): 28 Mar 2018 Downloaded from http://pubs.acs.org on March 28, 2018

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ACS Applied Materials & Interfaces

White-Light Emission from Cucurbituril-Based Host–Guest Interaction in the Solid State: New Function of Macrocyclic Host Yu Xia, Shiyan Chen, and Xin-Long Ni* Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Guizhou University, Guiyang, Guizhou 550025, China Keywords: cucurbituril, energy transfer, host-guest interaction, aggregation, solid white-light-emitting.

ABSTRACT: Energy transfer and interchange are central to fabricating white-light-emitting organic materials. However, increasing the efficiency of light-energy transfer remains a considerable challenge due to the occurrence of “crosstalk”. In this work, by exploiting the unique photophysical properties of cucurbiturils triggered host-guest interactions, the two complementary luminescent colors blue and yellow for white-light emission were independently obtained from a single fluorophore dye rather than energy transfer. Further study suggested that the rigid cavity of cucurbiturils efficiently prevented aggregation of the dye and improved its thermal stability in the solid state by providing a regular nano-sized fence for each encapsulated dye molecule. As a result, a novel macrocycle-assisted supramolecular approach for obtaining solid, white-light emitting organic materials with low cost, high efficiency, and easy scale-up was successfully demonstrated.

Introduction White-light-emitting organic diodes (WOLEDs), potentially allowing the fabrication of large-area, flexible, and transparent displays, have been widely investigated in recent years.1-4 In particular, the possibility of obtaining pure white light from a supramolecular assembly has attracted increasing interest because it would serve as an alternative approach to the conventional method for generating white light by color blending of multiple dye molecules.5-7 Compared to traditional organic synthesis, supramolecular assembly offers a convenient and low-cost strategy for white-light emission because the photophysical properties can be smartly tuned by simply varying the noncovalent interactions,8 such as hydrogen bonding,9-12 coordinative bonds,13-15 host–guest interactions,16-21 aggregation,22-24 and electrostatic interactions.25,26 The management of energy transfer, such as Forrier resonance energy-transfer (FRET) or through-bond energy-transfer (TBET) processes, within or between the donor–acceptor fluorophore dye components, is the crucial factor for such white-light emission. However, a key challenge that needs to be overcome in this approach is self-quenching [a fluorophore is often self-quenching at high concentrations, which is termed aggregation-caused quenching (ACQ)],27,28 which can seriously hinder practical application in the solid state. Retaining the high quantum efficiency and pure white-light emission upon transferring

the supramolecular assemblies from solution to solid substrates is another critical issue for device fabrication.7 Macrocyclic host receptors, as main components of the supramolecular family, have recently emerged as an elegant solution for the self-quenching of luminophores by host–guest encapsulation.29-33 In particular, Guo et al.,34 Tian and co-workers,35 and ourselves36 discovered that macrocyclic hosts, such as calixarenes, γ-cyclodextrins, and cucurbiuril[8] (Q[8] or CB[8]), have potential applications in fabricating smart luminescent materials, including those that are white-light emitting in aqueous solution. However, there are few examples was conducted for white-light emission in the solid state through the macrocycle-based host-guest interactions. In this respect, we report herein the study of white-light emission from a cucurbituril-based host–guest interaction in the solid state. Notably, the cooperation of Q[7] and Q[8] hosts with oligo(p-phenylenevinylene) (OPV)-based cationic dye (G1) gave a solid fluorescent material with improved thermal stability and high-quality white-light emission (Figure 1). More importantly, the present work indicates that white-light emission from the two complementary luminescent colors blue and yellow could be obtained independently from a single dye, rather than through energy transfer.

Results and discussion

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As mentioned above, white-light emission by supramolecular assembly is mainly based on energy transfer between different fluorophore dyes. According to Kasha’s rule, fluorophores always tend to attain the lowest possible vibrational states, generally resulting in monochromatic emission. Thus, the energy-transfer approach typically requires quenching or partial quenching of one or more of the higher energy emission pathways, thereby restricting the transitions that define the output spectrum (Figure S1).37,38 In particular, increasing the efficiency of light-energy transfer in a typical fluorescence energytransfer (FET) is a considerable challenge work, since the remains a rapid release of energy from a photoactive donor (excited-state lifetime: 10‒9 to 10‒8 s) results in an instantaneous high energy density, which cannot be effectively harvested by the acceptor molecules.39 Therefore, the design of white-light emitters requires readily tailorable different fluorophores and fine-tuning of the energytransfer processes between them, such as matched energy levels, concentrations, suitable orientation and distance, and so on.37, 38,40

gy region (shorter wavelength). According to Kasha’s rule, this is not beneficial and forbidden for energy transfer between the donor and acceptor moieties, whereas a white-light emission was occurred in that host-guest system, indicating the two complementary blue and yellow fluorescence emissions for white-light emission could be independent obtained rather than energy transfer. However, when we transferred the host–guest interaction of Q[8] with G1 that gave white-light emission in solution to the solid state, the white-light emission was no longer seen, and instead a yellow-green emission was observed. As shown in Figure S4, the fluorescence spectrum of the solid sample clearly revealed that the emission was mainly located in the yellow wavelength range. In order to clarify the situation, fluorescence spectra of G1 and the J-dimer complex of Q[8]/G1 in the solid state were determined. We found that the observed phenomenon could be attributed to aggregation of G1 in the solid state.41 In that case, yellow emission with maximum intensity at around 554 nm was observed for a solid sample of G1 (Figure S5). However, the yellow emission with maximum intensity at around 580 nm still can be noted in the solid state of Q[8]/G1 complex (Figure S6).

Figure 2. Schematic representations of (a) aggregationcaused quenching of a dye, and (b) cucurbit[n]uril-based host–guest interaction-induced emission of the dye, at different concentrations.

Figure 1. Schematic representation of Q[7], Q[8] (a) and G1 (b); (c) fluorescence spectra of Q[7]/G1 in solution and solid; (d) fluorescence spectra of Q[8]/G1 in solution and solid; (e) fluorescence spectra of the solid mixture of Q[7]/G1 and Q[8]/G1 complex, λex = 398 nm; (f) glass slides after painting with the Q[7]/G1-and Q[8]/G1 complex, and shows whitelight emission when exposed to the UV-light; (g) photograph of the 365-nm ultraviolet LED coated with the Q[7]/G1 and Q[8]/G1 complex when the LED is turn off; (h) the coated LED is turned on.

In our previous study,36 by virtue of the unique photophysical properties of the Q[8]/G1 complex (Figures S2S3), we found that the J-dimer Q[8]/G1 triggered absorption as the acceptor group was located in the higher ener-

As shown in Figure 2, from a structural viewpoint, the rigid nano-cavity-based cucurbituril units can be expected to be advantageous and highly efficient in preventing dye aggregation and aggregation-caused quenching in high concentrations and even in the solid state by providing a regular nano-sized fence for each of the encapsulated fluorophore dye molecules, compared to other approaches such as the use of nanoparticles42 or dye-loaded polymers.43,44 Most importantly, numerous host-guest studies on cucurbiturils have indicated that the Q[n]-based encapsulation of dye molecules generally results in profound changes in the absorption/emission wavelengths and quantum yields.29,45,46 For instance, the Q[7]-based complexation of a dye guest is generally characterized by an enhancement and/or blue shift of its fluorescence emission,47− 49 whereas in the case of a Q[8] system, a red-


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shift of the fluorescence emission is generally observed upon encapsulation of the dye guest, because of the ability of the larger host to bind two hetero- or homo-guests in its cavity.36,50−52

Figure 3. (a) UV/Vis spectra of G1 at increasing concentrations of Q[7] in aqueous solution (pH 7.2); (b) fluorescence spectra of G1 at increasing concentrations of Q[7] in aqueous solution (pH 7.2); (c) normalized absorption and fluorescence spectra of Q[7]/G1 and Q[8]/G1; (d) emission spectra of the solid mixture of Q[7]/G1 and Q[8]/G1 with excitation wavelengths varied from 368 to 418 nm.

Consequently, as proposed in Figure 1, we anticipated that the above-described novel photophysical properties of the Q[n]-based host–guest interaction might be exploited for the fabrication of a solid, white-light emitting material. Based on the previous study,36 we confirmed that the yellow-emitting Q[8]-based J-dimer complex of G1 could be stably formed in aqueous solution at high pH values and in the solid state (Figures S6-S7). In order to obtain optimal conditions for the realization of the blue emission of G1, we proceeded to ascertain whether motion can be blocked in the solid state by the Q[7] host. The absorption and emission properties of G1 with Q[7] were first evaluated in aqueous solution. From Figure 3a, it can be seen that the addition of increasing concentrations of the Q[7] host to a solution of G1 led to a significant bathochromic shift in the absorption maximum of this guest from 390 to 403 nm, which is similar as Q[8]/G1 system (Figure S8). However, compared to the large Stokes shift of Q[8]/G1 complex in the fluorescence emission, no significant emission shift was observed, but a remarkable emission enhancement was evident in Q[7]/G1 complex (Figure 3b). 1H NMR titration experiment (Figure S9) revealed that the aromatic moiety, including the ethylene unit, of G1 was partly encapsulated in the cavity of the Q[7] host in a 1:1 molar ratio, with an association constant (Ka) of 9.50 × 105 M−1 (Figure S10). The absolute fluorescence quantum yield (Φf(abs)) of G1 increased from 1.30% to 8.60%. The favorable photophysical

property of the Q[7]/G1 system can be attributed to: i) formation of the host–guest complex restricting the rotational and vibrational motions of the encapsulated fluorophore; ii) the host cavity shielding the guest G1 from quenchers (e.g., oxygen or polar water molecules). In particular, the small red-shift in the emission of Q[7]/G1 in the solid state (487 nm) compared to that in solution (478 nm) (Figure 1c and Figure S11) indicates that the Q[7] host inhibits the aggregation of G1 in the solid state and maintains its single-molecule emission. Therefore, a pure white-light emission with CIE coordinates of (0.33, 0.36) in the solid was successfully obtained in the cooperation of Q[7]/G1 and Q[8]/G1 host-guest interactions (Figure 1eh, and Figure S12). Additionally, as shown in Figure 3c, there was almost complete overlap between the normalized absorption spectra of Q[7]/G1 and Q[8]/G1 dimer complexes, and such that white light was continuously emitted on changing the excitation wavelength from 368 to 418 nm (Figure 3d). This provides an excellent example of the great advantage of the Q[n]-based host–guest interaction in facilitating multiple independent fluorescence emissions in a single dye system, as opposed to energy transfer from different fluorophore dyes53. Most interestingly, because of the similar binding abilities of G1 with the Q[7] and Q[8] hosts (Figure S13), it seems that the Q[7]-cavity-anchored G1 complex can help to control and improve the formation and quality of the white-light emission (Figures S14-15). For example, as shown in Figure S15a, upon addition of about 0.5 equivalents of the Q[7] host to a solution of G1, the blue fluorescence emission of this guest was obviously enhanced. On further adding Q[8] to the solution at up to 0.25 molar equivalents, white-light emission appeared. Notably, this white-light emission persisted on increasing the concentration of Q[8] up to 0.5 equivalents, as opposed to the yellow emission seen for the J-dimer complex of Q[8]/G1 in the absence of the Q[7] host (Figure S15b). Considering the excellent color lock ability of the host–guest interactions of Q[7] and Q[8] with G1 both in solution and in the solid state, this white-light emission derived from a simple synthetic protocol could be scaled-up to bulk quantities (video S1 in the Supporting Information). Furthermore, the application potential of macrocycleencapsulated fluorescent materials for practical whitelight emission has been demonstrated. A rough WLED was fabricated by simply coating a poly(methyl methacrylate) (PMMA)-fixed solid mixture of Q[7]/G1 and Q[8]/G1 dimer (2:1 molar ratio) on the surface of a commercially available ultraviolet LED chip. As illustrated in Figure 1g and h, the resultant WLED device emitted bright white light when a potential of 3.8 V was applied. To date, although a number of organic fluorescent materials have been proposed for the fabrication of white OLEDs, their thermal stabilities do not yet meet the commercial standards.2 Here, the thermal stability of G1 before and after encapsulation by Q[7] and Q[8] was ex-



amined by thermal analysis, namely by differential scanning calorimetry (DSC) and thermogravimetry (TG). As shown in Figure 4, the TGA curves clearly showed that the unbound G1 decomposed at 294.7 °C. However, after complexation with Q[7] and Q[8], its thermal stability increased dramatically, not decomposing until 453.6 °C and 456.4 °C, respectively. This high thermal stability may be ascribed to the stability of the Q[n] hosts, which protect the guest G1 in their rigid cavities. In addition, the fluorescence spectra suggested that the solid white-light emission properties of such host-guest interactions based is less influenced after being heated to high temperature (Figure S16).

Yu Xia and Shiyan Chen contributed equally

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the Natural Sciences Foundation of China (21302026), and the Science and Technology Talent Fund of Guizhou Province (20165656)..

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Figure 4. (a) Differential scanning calorimetry and (b) thermal gravimetric curves of pure G1, Q[7]/G1 and Q[8]/G1 measured in N2

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Conclusion In summary, we have demonstrated a novel macrocycle-based host–guest interaction approach for the design of white-emitting organic materials with good color purity and quality in the solid state. In particular, anchoring of the dye in the cavity of the host facilitates independent blue and yellow emissions from a single fluorophore. Distinct from energy-transfer, this represents a new strategy for attaining pure white light-emitting materials. Furthermore, the host-guest encapsulation improved the thermal stability of the fluorophore dye in the solid state, which is favorable for large-area display fabrication of OLEDs and their commercial applications. We expect that the present design strategy and the remarkable photophysical properties emanating from this host-guest interaction will help to extend applications of Q[n]s in functional materials.

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ASSOCIATED CONTENT Supporting Information. Full experimental details, UV−vis, fluorescence spectrum, NMR, ITC experiment, and video is available free of charge on the http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author [email protected] (X.-L. Ni) Author Contributions

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