Lighting Up AIEgen Emission in Solution by Grafting onto Colloidal

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Spectroscopy and Photochemistry; General Theory

Lighting Up AIEgen Emission in Solution by Grafting onto Colloidal Nanocrystal Surfaces Xiao Luo, Xue Liu, Tao Ding, Zongwei Chen, Lifeng Wang, and Kaifeng Wu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b02832 • Publication Date (Web): 18 Oct 2018 Downloaded from http://pubs.acs.org on October 20, 2018

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The Journal of Physical Chemistry Letters

Lighting Up AIEgen Emission in Solution by Grafting onto Colloidal Nanocrystal Surfaces Xiao Luo, Xue Liu, Tao Ding, Zongwei Chen, Lifeng Wang and Kaifeng Wu* State Key Laboratory of Molecular Reaction Dynamics, Dynamics Research Center for Energy and Environmental Materials, and Collaborative Innovation Center of Chemistry for Energy Materials, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning 116023, China

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ABSTRACT: Aggregation-induced emission luminogens (termed AIEgens) have emerged as an important class of light-emitting materials with many potential applications. As hinted by their names, AIEgens typically exhibit strong emission in the aggregated form for which the intramolecular motions that dissipate excited state energy are inhibited. Here we demonstrate that AIEgens, using carboxylated tetraphenylethylene (TPE-CA) as an example, can exhibit strong emission in the solution after being grafted onto colloidal nanocrystal (NC) surfaces. Using ultrafast transient absorption spectroscopy, we found that the intramolecular motions of TPE-CAs were strongly suppressed, with rates retarded by two orders of magnitudes, when they were grafted onto ZnS NCs. As a result, the emission quantum yield (QY) of TPE-CA-NC hybrids in solution was also enhanced by two orders of magnitudes compared to free TPE-CA molecules in solution. This opens a new avenue for constructing multi-functional hybrid materials capitalizing on the emission of AIEgens. In addition, this methodology can be extended to suppress aggregation-caused quenching (ACQ) of ACQphores in the solid-state form.

TOC GRAPHIC AIEgen emission

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Chromophores typically exhibit two types of emission changes in their aggregated forms: one is aggregation-caused quenching (ACQ) and the other is aggregation-induced emission (AIE).1 The former has long history2 and the reason for quenching has generally been attributed to π-π stacking of aromatic rings,3 whereas the latter is a relatively new phenomenon with its term not coined until 2001.4 As many light-emitting materials are targeted for solid state devices,5 AIE luminogens (AIEgens) are believed to be superior to ACQ luminophores (ACQphores) in these applications.1 In light of its technological significance, over the years, the AIE mechanism has been extensively studied both experimentally and theoretically.6-15 These studies have revealed that intramolecular motions, including both ration and vibration, dissipate the excited state energy of AIEgens nonradiatively (Scheme 1a; using tetraphenylethylene as an example) and restriction of intramolecular motion (RIM) in the aggregates is responsible for the observed AIE phenomenon (Scheme 1b).1 Recent years, hybrid materials comprising inorganic nanocrystals (NCs) and organic molecules have emerged as an important platform for various optoelectronic applications.16-25 These applications are often enabled by fundamental processes such as charge and energy transfer across the inorganic/organic interface.17,

20-21, 24

There is yet another type of important interaction mechanism between NCs and

molecules that could activate new functionalities, that is, host-guest like interactions.26-27 Molecular guests are grafted onto the surfaces of inorganic NC hosts and hence their mechanical motions and packing behaviors are modulated by the hosts. One of such examples is the so-called spherical nucleic acids (SNAs) consisting of densely packed nucleic acids (NAs) covalently attached to the surfaces of Au nanoparticles,28 which exhibit properties markedly different from linear NAs and are very suited for many biomedical applications.

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free intramolecular motion in solution

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c

restricted intramolecular motion in ligand-shell

Scheme 1.Free intramolecular motion of tetraphenylethylene (TPE) molecules in the solution quenches their emission (a); in aggregates, restricted intramolecular motion (RIM) due to molecular stacking leads to strong emission (b); when grafted onto nanocrystal surfaces, RIM by the ligand-shell also leads to strong emission (c). Here we utilize the host-guest type of interaction to construct AIEgen-NC hybrid materials which exhibit strong light emission in the solution form (Scheme 1c). The inorganic NC surfaces are densely packed with native long-chain ligands. Insertion of AIEgens to the ligand-shell leads to RIM due to steric hindrance effect and thus strong light emission. Specifically, our study shows an enhancement of emission yield by two orders of magnitude when tetraphenylethylene (TPE) molecules,9 a prototype AIEgen, are grafted onto ZnS NCs; transient absorption measurements reveal that retardation of intramolecular ration rate by also two orders of magnitude is responsible for the emission enhancement. This grafting-induced emission mechanism is leveraged to demonstrate hybrid materials with multi-fold functionalities. We also extend the host-guest interaction methodology to suppress emission quenching of ACQphores in the solid-state form. TPE molecules were chosen for this study because they are regarded as one of the most common AIEgens1 whose excited state energy dissipation dynamics have been extensively examined both experimentally29-30 and theoretically31. Moreover, TPE functioned with carboxyl groups (TPE-CA) is 4


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commercially available. These should greatly expedite our study on TPE-NC model systems. Figure 1a shows the absorption spectrum of TPE-CA molecules dissolved in ethanol, which has an absorption onset at ~380 nm. The absorption spectrum is slightly red-shifted when the solution is drop-casted onto a glass substrate to form a solid-state film. As expected for AIEgens, the emission of TPE-CA is negligible in ethanol solution but is strongly enhanced (by ~140 fold) in the solid-state film (Figure 1b). The photoluminescence (PL) quantum yield of the film excited at 330 nm is ~35 %.

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Figure 1.Absorption (a) and photoluminescence (PL; b) spectra of TPE-CA in ethanol (yellow), TPECA in solid film (orange), and TPE-CA grafted onto ZnS NC surfaces and dispersed in hexane (red). Note that three samples in (b) have the same absorbance at the excitation wavelength of 330 nm. Inset in 5



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(a) are the absorption and PL spectra of ZnS NCs. Inset in (b) is an enlarged view of the PL spectrum of TPE-CA in ethanol. The ZnS NCs used as the host were synthesized using a well-established hot-injection method;32 see Supporting Information (SI) for details. ZnS NCs were chosen due to their wide band gap, allowing for selective excitation of TPE molecules without resulting in charge and/or energy transfer between NCs and TPE-CA which are an interesting topic but are beyond the scope of this work. The ZnS NCs used here has an average diameter of 2.1 nm (See Figure S1 for their TEM image). Figure 1a inset shows the absorption and PL spectra of ZnS NCs dispersed in hexane. The absorption onset is at ~300 nm, which is indeed well separated from that of TPE-CA. The PL spectrum of NCs contains a band edge emission peak and a broad-band emission peak resulting from trap states. TPE-CA molecules were grafted onto ZnS NC surfaces via a simple sonication procedure; see SI for details. The carboxyl group on TPE-CA should covalently bind to the Zn2+ on NC surfaces. Since the surfaces of ZnS NCs are covered by stearic acid (SA) ligands, the grafted TPE-CA molecules are locally confined in the ligand shell. According to the absorption spectrum in Figure 1a and the extinction coefficients of ZnS NCs and TPE-CA (see SI for details), on average 5.2 TPE-CA molecules are bound to each NC. As shown in Figure 1b, the emission of TPE-CA-NC hybrids dispersed in hexane is strongly enhanced compared to that of TPE-CA in ethanol (by ~100-fold) and the emission yield reaches 25%.

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a

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Figure 2.(a) 2-D pseudo color plot of the transient absorption (TA) spectra of TPE-CA in ethanol excited by a 340 nm pulse. The excitation density was 140 μJ/cm2. Two excited state absorption features, A1 and A2, are labeled. (b) TA kinetics plotted at the A1 (blue squares) and A2 (red circles) of TPE-CA in ethanol. The black solid lines are multi-exponential fits to the kinetics. (c) 2-D pseudo color plot of the TA spectra of TPE-CA grafted onto NC surfaces in hexane excited by a 340 nm pulse. The excitation density was also 140 μJ/cm2. (d) TA kinetics plotted at the A2 of TPE-CA in ethanol (light red circles) and TPE-CA grafted onto NC surfaces in hexane (dark red triangles). The black solid lines are multi-exponential fits to the kinetics. In order to uncover the mechanisms for strongly enhanced emission of TPE-CA grafted onto NC surfaces, we performed transient absorption (TA) measurements. In these experiments, a 340 nm pump pulse, which selectively excites TPE-CA, was used to excite the samples and the pump-induced 7



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absorption changes were recorded by a white light probe pulse (see SI for details). Figure 2a shows the 2-D pseudo color plot of the TA spectra of TPE-CA in ethanol, which are dominated by two excited state absorption (ESA) features, labeled as A1 (~450 nm) and A2 (~600 nm) in the visible range. The kinetic evolutions of these two features are plotted in Figure 2b. Within ~10 ps, A2 shows an ultrafast decay, which leads to a complementary growth of A1. The time constant for this process is fitted as 6.3±0.2 ps. After that, both A1 and A2 decay to zero within ~100 ps, the time constant of which is 16.3±0.4 ps. According to previous TA and computational studies on TPE molecules,29-31 the 6.3 ps component can be assigned to twisting around and charge separation across the central C=C bond (denoted as intramolecular motion 1, IM-1). Specifically, this twisting and charge separation leads to the decay of initially generated excited state, which has strong absorption at A2, and the formation of a twisted, charge separated state which has strong absorptions at both A1 and A2. The following 16.3 ps component then reflects the nonradiative recombination of the latter state via torsion of the phenyl groups (denoted as IM-2). The TA spectra of TPE-CA in the film and TPE-CA grafted NCs in hexane, measured under the same conditions as those for Figure 2a, are displayed in Figure S2 and Figure 2c, respectively. These spectra also show A1 and A2 features. Since these two features give consistent kinetics, in Figure 2d we compare kinetics probed at A2 for TPE-CA in ethanol and TPE-CA grafted onto NCs in hexane. Using bi-exponential fitting, we find that the decay time constants of both IM-1 and IM-2 processes for TPECA grafted onto NCs are significantly prolonged as compared to free TPE-CA molecules (36±2 ps vs 6.3±0.2 ps for IM-1 and 1480±70 ps vs 16.3±0.4 ps for IM-2). Thus, by simply grafting TPE-CA onto NC surfaces, the intramolecular motion processes that nonradiatively dissipate excited state energy are suppressed by up to ~100 fold, which explains the strongly enhanced emission of TPE-CA on NC surfaces. This RIM effect presumably arises from the closely-packed SA ligands that confine the freespace motion of TPE molecules. Previous studies on Rhodamine molecules also reported similar 8


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confinement-induced prolonging of emission lifetime.25, 33 We performed TA measurements on TPECA grafted onto ZnS NCs with various diameters from 1.8 nm to 3.3 nm and found that RIM effect was similarly strong for these samples (Figure S3), suggesting that RIM is very robust despite that the order of surface-packed ligands depends on the curvature and thus the diameter of NCs.34

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Figure 3. (a) PL spectra of Mn-doped ZnS NCs (yellow solid line), TPE-CA grafted onto ZnS NCs (orange dashed line), and TPE-CA grafted Mn-doped ZnS NCs (wine dash-dotted line).All samples are 9



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dispersed in hexane. (b) Emission of TPE-CA grafted Mn-doped ZnS NCs plotted on CEI chromaticity coordinates. Inset is a photograph of the white-emitting sample. The strong emission of AIEgens grafted onto NC surfaces offers an opportunity to exploit the combination of functionalities from both AIEgens and NCs for multi-functional hybrid materials as the NCs can host many physical properties. For example, Mn2+ can be incorporated into NCs for both sensitized emission and magnetism: excitation energy transfer from the NC host to Mn2+ leads to a strong yellowish emission,35-38 while on the other hand Mn2+ also renders the NCs paramagnetic.39 As a demonstration, we prepared Mn2+-doped ZnS NCs (see SI for details) and grafted TPE-CA onto these NCs. The absorption spectra of Mn2+-doped NCs with and without TPE grafted are shown in Figure S4. As the absorption features of NCs and TPE are separated, the functionality of TPE-CA grafted Mn2+doped NCs (TPE-CA-Mn:NCs) depends on the excitation wavelength. Specifically, if using a >350 nm excitation light, TPE molecules are selectively excited; in this case, the TPE-CA-Mn:NCs are dualfunctional magneto-fluorescent hybrids that may find important biomedical applications such as multimodal imaging.40 Alternatively, if using a ≤350 nm excitation light, both TPE and NCs are excited; in this case, the emission spectrum of the hybrid covers almost the whole visible range. As shown in Figure 3a, the emission spectrum of Mn:NCs has two broad-band peaks centered around ~420 nm and 600 nm, which can be assigned to trap-states emission of NCs and Mn2+ emission, respectively. The emission of surface-grafted TPE molecules almost exactly compensates the color window at ~500 nm. As a result, the emission color of the hybrids approaches an ideal white light (Figure 3b inset). Indeed, when plotted on CEI chromaticity coordinates, the value for the hybrid (0.3242, 0.3708) is very close to that of the standard white light (0.33, 0.33). The PL quantum yield of this white-light-emitter is ~7%, which is not high but can in principle be improved in the future by optimizing Mn2+-doped NCs and using more emissive AIEgens.

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Figure 4.(a) 2-D pseudo color plot of the TA spectra of PCA in the film excited by a 340 nm pulse. The excitation density was 196 μJ/cm2. The dominant ESA feature is labeled. (b) TA kinetics plotted at the ESA of PCA in ethanol (yellow squares) and in solid film (orange triangles), and PCA grafted onto ZnS NC surfaces in hexane (red circles) and in solid film (wine diamonds). The black solid lines are multiexponential fits to the kinetics. The idea of utilizing host-guest interaction between NCs and organic molecules can also be extended to ACQphores. ACQphores exhibit very weak emission in the solid-state form primarily due to the π-π stacking of the phenyl rings. By grafting ACQphores onto NC surfaces, the π-π stacking behavior can be inhibited. To illustrate this idea, we studied carboxylated pyrene (PCA), a typical ACQ molecule (see Figure S5 for absorption spectra), using TA spectroscopy. Figure 4a shows the TA spectra of PCA in the film, which are dominated by a broad-band excited state absorption (ESA) feature. The kinetics of this ESA feature measured for PCA in ethanol and in the film, and PCA grafted onto ZnS NCs in hexane and in the film are plotted in Figure 4b. As expected, the ESA lifetime of PCA in the film (averaged time constant ~41±2 ps) is ~200-fold shorter than that of PCA in ethanol (~8140±120 ps). When grafted onto NC surfaces, the ESA lifetime of PCA (in hexane) is slightly shortened (~6800±240 ps), likely due to stacking of PCA with SA ligands on NC surfaces. Most importantly, the ACQ

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behavior of PCA-NC is much weaker than that of PCA. The lifetime of PCA-NC in the film is ~3280±270 ps, only ~2-fold shorter than that of PCA-NC in hexane. In summary, we have studied the light emission and ultrafast excited state dynamics of AIEgens grafted onto inorganic NC surfaces. The local confinement environment provided by the host material, ligandcapped ZnS NCs, enhanced the emission quantum yield of TPE molecular guests in the solution by ~100-fold. Transient absorption measurements revealed that both types of intramolecular motions, twisting around/charge separation across the central C=C bond and torsion of the phenyl groups, were strongly suppressed when TPE molecules were grafted onto NC surfaces. The host-guest interaction strongly enhancing the emission of AIEgens was deployed to demonstrate multi-functional, white-lightemitting hybrids comprising TPE grafted onto Mn2+ doped ZnS NCs. In addition, we briefly showed this type of interaction could also be extended to suppress emission quenching of ACQ molecules in the solid state form. AUTHOR INFORMATION Corresponding Author * [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial supports from the Ministry of Science and Technology of China (2018YFA028703) and the National Natural Science Foundation of China (21773239), Dalian Institute of Chemical Physics, and Collaborative Innovation Center of Chemistry for Energy Materials (iChEM-2011). ASSOCIATED CONTENT 12


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Supporting Information. Figures S1-S5, sample preparations, TA experiment set-ups, estimation of the number of attached TPE-CA molecules per NC.

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