Bidirectional Photoswitching via Alternating NIR and UV Irradiation on

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Functional Inorganic Materials and Devices

Bidirectional Photoswitching via Alternating NIR and UV Irradiation on a Core-Shell UCNP-SCO Nanospheres Yang-Hui Luo, Jing-Wen Wang, Wen Wang, Xiao-Tong He, DanLi Hong, Chen Chen, Tao Xu, Qiyue Shao, and Bai-Wang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04455 • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 26, 2018

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ACS Applied Materials & Interfaces

Bidirectional Photoswitching via Alternating NIR and UV Irradiation on a Core-Shell UCNP-SCO Nanospheres

Yang-Hui Luo, * [a] Jing-Wen Wang, [a] Wen Wang, [b] Xiao-Tong He, [a] Dan-Li Hong, [a] Chen Chen, [a] Tao Xu, [b] Qiyue Shao [c] and Bai-Wang Sun* [a]

[a]

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, P.R. China. E-mail: [email protected] (LYH) [email protected] (SBW).

[b]

SEU-FEI Nano-Pico Center, Key Lab of MEMS of Ministry of Education, Southeast University, Nanjing 210096, P.R. China

[c]

School of Materials Science and Engineering, Jiangsu Key Laboratory for

Advanced Metallic Materials, Southeast University, Nanjing 211189, P.R. China

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Abstract: Bidirectional photoswitching of molecular materials under ambient condition is of significant importance. Herein, we present for the first time that a

core-shell

UCNPs-SCO

nanosphere

(UCNPs

=

up-conversion

nanophosphors, SCO = spin-crossover), which was composed of a UCNPs core (NaYF4: 20 mol%Yb3+, 1 mol%Er3+) and an SCO iron( Ⅱ ) shell ([Fe(H2Bpz)2(bipy-COOH)], bipy-COOH

=

H2Bpz

=

dihydrobis(1-pyrazolyl)borate,

4,4’-diacarboxy-2,2’-bipyridine),

can

be

reversibly

photoswitched between the high-spin and low-spin states at room temperature in the solid state, via alternating irradiation with NIR (near Infrared, λ = 980 nm) and ultraviolet (λ = 310 nm) light. What’s more, this reversible spin-state switching was accompanied by variation of fluorescent spectrum and dielectric constants. The strategies here integrate the SCO iron(II) complex into UCNPs-SCO nano-sphere for molecular photo-switching may open a new area in the development of photo-controlled molecular devices. Keywords: bidirectional photoswitching, spin-crossover, UCNPs, alternating irradiation, synergetic effect, NIR, UV.

2


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INTRODUCTION The design and preparation of spin-crossover (SCO) materials is one of the main activities in the field of molecular switches in recent years, with respect to not only the basic science, but also potential applications.1, 2 The spin state of SCO materials can be switched between low-spin (LS) and high-spin (HS) electronic states upon external triggers, such as temperature, pressure, guest molecules, or light. This SCO process usually accompanied by reversibly switching of magnetic, conductive, color, and other physicochemical properties.3-5 Such characteristics entrust the possibility of SCO materials as highly promising candidates for future applications in molecular electronics and spintronics, communication networks, ultra-high-density memory systems, displays, and sensors. 6-8 For the purpose of practical applications, the spin-crossover should be ideally operated under ambient conditions: at room temperature and atmospheric pressure. 9, 10 Much attentions have therefore been paid to the manipulation of SCO materials that are switchable at ambient conditions, and there indeed have a handful examples that featuring thermally induced spin-state-transition under ambient conditions been developed.11-15 However, compared with thermal-induction, light-triggered SCO is more attractive owing to the short response times, low power dissipation, and high selectivity. 16-18 Thus, SCO materials that are switchable by means of a light-irradiation at

3



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ambient conditions would of significant importance. Note that, this purpose has been realized in the light-induced excited spin state trapping (LIESST) effect at the single-molecule level.

19-21

This effect, however, can only be operated

below 50 K as the light-induced excited state rapidly relaxes to the ground state within nanoseconds at room temperature. Thus, control of the electronic spin state on a genuine molecular level under ambient conditions remains a huge challenge. More recently, Fink and Khusniyarov et al.

22

have

demonstrated the reversible photo-switching at room temperature in the solid state for a molecular spin-crossover iron(II) complex Fe(H2Bpz)2(o-btphen) (H2Bpz

=


dimethyl-3-thienyl)-1,10-phenan-throline),

by

o-btphen alternating

=

5,6-bis(2, irradiation

5with

ultraviolet (λ = 282 nm) and visible light (λ > 400 nm). Essentially, this approach is considered to be ligand-driven light-induced spin-state transition or light-driven coordination-induced spin-state switching which can be attributed to the photo-isomerizable of diarylethene ligand. These prominent results, on the one hand, demonstrated the sensitive responses of the electronic spin state for Fe(H2Bpz)2(phen) system to ligand field; On the other hand, it prompts us to explore the more attractive low-energy light (e.g., NIR light),

instead

of

high-energy

ultraviolet

or

visible

light,

to

drive

photo-isomerization reactions under ambient conditions. Meanwhile, Tao et al. 23

have demonstrated the synergetic effect between SCO and fluorescent

4


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emission

in

one-dimensional

hybrid

complexes

([Fe(4-NH2-1,2,4-triazole)3](ClO4)2 polymers grafting fluorophores). More recently, Liu et al. 24 have found the energy transfer from the fluorophore to the FeII ion, through photoinduced SCO. All these prominent results have suggested that we can turn the electronic spin state of SCO complexes by using fluorescence. Again, this has prompts us to explore the more attractive low-energy light, instead high-energy fluorescence, synergetic effects involving SCO properties. Based on the literature research Tao23 and Liu UCNPs

24

25-27

and the results of Khusniyarov

22

,

combining of the photo-responsive SCO complexes with

(up-conversion

nanophosphors)

into

UCNPs-SCO

composite

materials may represent the attractive target of low-energy light induced SCO under ambient conditions. UCNPs, which are particularly interesting systems consisting of certain lanthanide dopants (such as Tm3+ and Er3+) embedded in a nano-crystalline host lattice, can convert low-energy continuous-wave (CW) NIR (near Infrared) light in-situ into higher-energy light that emitted in the ultraviolet, visible or fluorescent regions, via the sequential absorption of two or more low-energy photons owning to the presence of multiple long-lived 4f electronic states which equally spaced in a ladder-like configuration.

25-27

UCNPs have been widely investigated in the field of biomedical (imaging and photodynamic therapy)

28

and photochemistry (photoisomerization and

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photocleave) 29, 30 as nano-scopic “light bulbs”, attributing to the less damaging and deeper penetrating character of NIR light. Several examples about the utilization of NIR-light-induced thermal effects as well as NIR-light-induced photochemical reactions have been reported.27-30 However, incorporating UCNPs into photo-responsive SCO materials to achieve NIR-light-induced molecular switching have not been reported.

Scheme 1. Design of the core-shell UCNPs-SCO composite nanospheres with alternating NIR and UV light irradiation (left) and illustration of the bidirectional photoswitching via alternating NIR and UV irradiation (right). Herein, we report the preparation and photoswitching properties of a novel core-shell UCNPs-SCO composite nanospheres, which was fabricated through accurate self-assembly process. Where, the SCO iron( Ⅱ ) shell ([Fe(H2Bpz)2(bipy-COOH)],

H2Bpz

=


bipy-COOH = 4,4’-diacarboxy-2,2’-bipyridine) was coordinated solidly to the surfaces of UCNPs core (NaYF4: 20 mol%Yb3+, 1 mol%Er3+), forming a core-shell nano-structure (Scheme 1). Note that, this coordination, on the one hand,

has

altered

the

ligand-field

strength

6


of

bipy-COOH

ligand

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(Ligand-driven); on the other hand, has strengthened the synergetic effects between the UCL (up-conversion luminescence) of UCNPs and SCO (Light-driven). More importantly, as we have expected, for the first time, the reversible spin-state switching between HS and LS state, via alternating NIR (λ = 980 nm) and UV (λ = 310 nm) light irradiation under ambient conditions in the solid state has been realized. In addition, the fluorescent properties and dielectric constants of the UCNPs-SCO composite nanospheres at different spin-states have been investigated. The strategies here, integrating the SCO iron(II) complex into UCNPs-SCO nanosphere for molecular photo-switching, may open a new area in the development of new bidirectional photo-switching materials for widespread applications.

RESULTS AND DISCUSSION The Manuscript is Structured as Follows: In the first section, we demonstrate the successfully assembly of UCNPs core module and the iron(Ⅱ) SCO shell module into core-shell nanospheres. For such core-shell nanostructure, NIR light (980 nm) penetrates the iron(Ⅱ) SCO shell direct to the UCNPs core, and the UCL that emitted by the UCNPs core can be absorbed by the iron(Ⅱ) SCO shell. As a result, spin-crossover of the iron(Ⅱ) SCO shell from HS to LS state was achieved. Interestingly, an opposite process was observed upon exposure the NIR light-irradiated samples to UV light (310 nm). The phenomenon indicated the succeed reversible

7



photo-switching

on

a

core-shell

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UCNPs-SCO

nanosphere,

without

photo-isomerizable ligands and/or redox-active centers, in the solid-state under ambient conditions. In the second section, the potential of molecular switching

applications

photo-luminescent

has

and

been

demonstrated

photo-dielectric

by

properties

investigating of

this

the

core-shell

nanospheres. In our design strategy, the active groups functionalized complex 1 [Fe(H2Bpz)2(bipy-COOH)] was chosen based on the following reasons: (i). The electronic state of this kind of complexes are sensitive to ligand-field and external stimuli (guest molecules, pH, pressure, heat, and light); 31-34 (ii). The active COOH groups can effectively coordinated to the lanthanide dopants on the surfaces of UCNPs to form a solidly core-shell nano-structure; (iii). This kind

of

core-shell

(ligand-field-induced

nano-structure and

may

realize

light-induced)

SCO.

the

“dual-induced”

Meanwhile,

the

hexagonal-phase UCNPs, which was composed of NaYF4: 20 mol%Yb3+, 1 mol%Er3+, was chosen as the UCL source because this kind of UCNPs showed the most superior up-conversion efficiency of green emission within 500-700 nm,

36

which matches the MLCT (metal-to-ligand charge transfer)

absorption band from the center Fe2+ ion dπ-orbitals into π*-orbitals of the bipy-COOH ligand in 1,36 making it possible for synergetic effects between the UCL and SCO.

8


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Synthesis and Characterization of Core-Shell UCNPs-SCO Nanosphere. Er3+-doping hexagonal-phase UCNPs were synthesized by following the previously reported procedures

37

(see Experimental Section of Supporting

Information for details) and characterized by transmission electron microscopy

Figure 1. (a) TEM and (b) HRTEM photographs of the as-prepared Er3+-doping hexagonal-phase UCNPs; (c) A photograph of green emission

9



from the UCNPs in a hexane solution excited under a 980 nm CW laser; (d) An optical image of the powder samples of UNNPs, complex 1 and nano-spheres UNNPs@1. (e) TEM and (f) HRTEM photographs of the constructed nano-spheres UNNPs@1. (TEM), scanning electron microscopy (SEM), HNMR spectrum and power X-ray diffraction (XRD) analysis (Figure S1). The TEM and SEM images of the as-prepared oleate-capped UCNPs (UCNPs@OA), as well as the un-capped UCNPs clearly shows hexagonal plates with high size uniformity, and an extremely narrow size distribution with an average diameter of about 32 nm (Figure 1 and Figure S2-3). Furthermore, high-resolution TEM (HRTEM) image (Figure 1b) revealed the lattice distances of 0.52 nm, corresponding to a hexagonal phase of NaYF4. The energy-dispersive X-ray (EDX) analysis indicates the presence of Na, Y, Yb, F, and Er in the un-capped UCNPs (Figure S4). All these aforementioned results demonstrated the successful preparation of the desired UCNPs. Under excitation with a 980 nm CW laser, the as-prepared UCNPs showed green emission (Figure 1c) with UCL emission peaks located at 520, 527, 539, 549, 652 and 660 nm (Figure 2a), corresponding to the 2H11/2 to 4I15/2, 4S3/2 to 4I15/2, and 4F9/2 to 4I15/2 transitions, respectively. Note that, all of the UCL emission peaks overlapping perfectly the broad MLCT absorption bands between 500 and 700 nm of 1 as expected (Figure 2a). 10


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Through the “ligand-exchange” self-assembly process (see Experimental Section for details), the purple-colored UCNPs-SCO composite nanospheres UCNPs@1 was formed and precipitated, from the white-colored UCNPs and dark-blue-colored complexes 1 (Figure 1d). It is worth noting that, the visual color change, on the one hand, indicated the successful fabrication of core-shell nanospheres; on the other hand, suggested the variation of ligand-field strength on bipy-COOH ligands after coordination. What’s more, the purple-colored UCNPs@1 can be well dispersed in chloroform, demonstrating that this kind of surfaces modification does no influence on the dispersion of UCNPs. The successful fabrication of UCNPs@1 was further confirmed by TEM and SEM, TGA, EDX analysis and the IR spectra. TEM and SEM photographs of UCNPs@1 (Figure 1e, f, and Figure S2, S5) clearly shown nanospheres with high size uniformity and an extremely narrow size distribution with an average diameter of about 38 nm. Especially, compared with the UCNPs@OA, the HRTEM photograph in Figure 1f have demonstrated the coordination of 1 around the whole surface of UCNPs, which was in accordance with the presence of elements C, B, N, O and Fe in UCNPs@1 (Figures S6). In addition, IR spectra (Figure S7) shows that the coordination of 1 to the UCNPs has resulted in dramatically decrease of the stretching vibration model of -COOH groups at around 2928 and 2851 cm-1, while increase of the bending

11



vibration model of -COO- groups at around 750 cm-1. TGA measurements (Figure 2b) revealed that the binding with UCNPs almost have no influence on the decomposition of 1 upon heating, the mass-loss before 500 °C can be attribute to the decomposition of SCO moieties. As a result, the binding ratio of 1 was calculated to be about 6% (weight ratio). The successful fabrication of UCNPs@1 was also evidenced by the UV-vis absorption spectrum, as shown in Figure 2a, a blue-shift (~50 nm) of both MLCT and d-d transition band of LS state iron(II) complex was observed,

Figure 2. Comparison between the (a) UV-vis absorption, the UCL emission and (b) TGA profiles of UCNPs, complex 1 and UCNPs@1, respectively; Variation of the (c) IR spectra and (d) UV-vis absorption spectral in 12


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cyclohexane solution of UCNPs@1 under 980 nm laser with different excitation times were presented. suggesting that the coordination of 1 with UCNPs has resulted in partial spin-state transition to give a mixture of HS and LS state species,31-34 which can be attributed to the increase in the π-accepting ability on bipy-COOH ligand (“ligand-driven” effect). Noted that, compared with UCNPs, the coordination of 1 has resulted in quenching of the emission bands at around 540 nm (Figure 2a), suggesting that the up-converted green light (2H11/2 to 4I15/2 transition) from the UCNPs was efficiently absorbed by the SCO complex 1. Thus, the CW NIR-light-induced ambient SCO would be expected in UCNPs@1. Bidirectional Photoswitching of UCNPs@1. To demonstrate the “light-driven” effect, the influences of NIR irradiation on the IR and UV-vis absorption spectrum, both in solution and in solid state, have been investigated. As shown in Figure 2c, the IR spectra of UCNPs@1 were measured after irradiated for different times with interval of 2 mins, at a laser density of 3.0 W/cm2. Upon irradiation, the vibrational peaks center at 3450 and 750 cm-1 were decreased, while the peaks center at 2928 and 1100 cm-1 increased. These spectral changes suggested that the UCL emission may have affected the coordination interactions and electronic state of complex 1. Note that, after irradiated about 8 mins, no further spectral change was 13



observed, that’s to say, 8 mins is the optimal irradiation time for UCNPs@1 to realize the “light-driven” effect in the solid state. While for the UV-vis absorption spectra of UCNPs@1 in a cyclohexane solution, both the MLCT and d-d transition bands were increased upon irradiation (Figure 2d), suggesting a typical spin-state transition from HS to LS state.31-34 While in the solid state, the UV-vis absorption also showed an increase in both the MLCT and d-d transition bands (Figure 3a and Figure S7), but more significantly than in solution, especially for the MLCT band (two times larger than the LS state d-d transition band, Figure 3b). These results suggested that the CW NIR-light-induced SCO within UCNPs@1 is more

Figure 3. (a) UV-vis absorption spectral change of UCNPs@1 in solid-state upon excitation with a 980 nm CW laser; and (b) Comparison between the variation of absorption intensity for LS state d-d transition and MLCT bands; (c) 14


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UV-vis absorption spectral change of the NIR-light-induced samples in solid-state upon excitation with a 310nm light; (d) Multiple photo-switching as a function of the intensity of the absorption band at 550 nm. Red circles: NIR light strengthening; black circles: UV light attenuating; (e) NIR-induced emission increase (λ = 980nm, total time:10 mins, from black to red), and (f) subsequent emission decrease with UV light (λ = 310 nm, total time:12 mins, from red to black). effective in solid state than in solution, which thus promote the practical applications in the solid state. However, the optimal irradiation time in the solid state was extend to about 12 mins, which can be attribute to the problems with the penetration depth of NIR light into the bulk samples of UCNPs@1. One points worth mentioning is that, the irradiation of UCNPs@1 in cyclohexane solution with 980nm CW laser has induced the decrease of absorption band at 262 nm (Figure 2d), which thus inspired us to explore a UV light to reversibly turning the spin-state of UCNPs@1 that were triggered by NIR light. 22 As we have expected, a 310nm UV light was found can realize this ambition. Upon exposure with 310nm UV light, the UV-vis absorption spectra of UCNPs@1, that was triggered by NIR-light, have undergo a decrease of the bands in near-UV and visible regions (Figure 3c). However, exposure to UV light did not lead to complete transition from HS to LS state. More importantly, subsequent irradiation with NIR light triggered the LS to HS transition again, 15



thus the whole reversible photo-induced SCO cycles can be repeated with infinite times (Figure 3d). Photo-induced Fluorescent and Dielectric Constants. Photo-induced reversibly SCO was expected to induce bidirectional switching of the other chemical-physical properties in UCNPs@1. To further demonstrate the potential of UCNPs@1, we followed multiple photo-switching upon in-situ irradiation with alternating NIR and UV light by fluorescent spectroscopy and dielectric constant measurements. As shown in Figure 3e, the fluorescence spectra were continuously recorded at regular time intervals of 2 mins, where, the emission peak in 440-480nm region, which refer to the iron(Ⅱ) SCO shell, increased considerably upon NIR irradiation (0-10 mins). While these bands decreased upon irradiation with UV light (3-12 mins) (Figure 3f) Thus, the photo-induced bidirectional switching cycles of luminescent properties for UCNPs@1 was demonstrated, which was in good agreement with the photo-induced SCO cycles determined by electronic absorption spectroscopy.

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Figure 4. Comparison between the temperature dependence for real part (a-c) and loss part (d-f) of dielectric constants for nanospheres UCNPs@1 measured before (a and d) and after irradiated with NIR (b and e) then followed by UV light (c and f). The dielectric loss quantifies inherent dissipation of electromagnetic energy. Note that, SCO not only leads to spin arrangement, but also induce electric polarization.

38-41

Hence, we further investigated the dielectric properties of

UCNPs@1 in the frequency and temperature ranges of 0.5-1000 kHz and 290-100 K. Before irradiation, UCNPs@1 exhibited dielectric anomalies in temperature range -50 - 20oC (Figure 4a and d), accompanied by significant frequency dependency: in the lower temperature range (-70 - -20oC), a positive correlation with frequency was observed, while in the upper temperature range (-20 - 20oC), the correlation is negative. Noticeably, the dielectric loss showed

17



an obvious dielectric relaxations process in the measured frequency range, with the dielectric anomalies temperature varied from -46oC at 500Hz to 20oC at 1000 kHz (Figure 4d). The origin of the dielectric anomalies may be associated with the “ligand-driven” effect. After irradiation with 980 nm CW laser, the dielectric responses were about 10 times larger than that before irradiation, demonstrating the significant influence of NIR light on the local polarizability of UCNPs@1. On the one hand, these two different kinds of frequency dependency were preserved, however, the turning points of the frequency dependency have coincidence to one point at around -48oC (Figure 4b), a point where the dielectric constant has shown no frequency dependency. On the other hand, the temperature range for dielectric relaxations process was decreased and narrowed to be -60 - -39 oC in the measured frequency range (Figure 4e). While upon exposure to UV light, the dielectric responses were changed in the opposite direction of the NIR irradiation, as we have expected: the turning point of the frequency dependency was increased to around -38oC (Figure 4c), and the temperature range for dielectric relaxations process was increased to be -50 - -28oC region (Figure 4f), demonstrating a bidirectional photo-switching of dielectric properties on UCNPs@1. CONCLUSIONS

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In summary, the herein presented UCNPs-SCO nanosphere system, which was composed of a UCNPs core and an iron(Ⅱ) SCO shell, represents the first example of a molecular photo-switching in core-shell nanostructure, which fulfils the general requirements for implementation in functional molecular devices: reversible switching, operating at ambient condition, in the solid state, and at the molecular level. The alternating irradiation with NIR and UV light, not only induced reversible SCO for iron(Ⅱ) SCO shell, but also triggered reversible fluorescent and dielectric responses. Except for the fluorescent and dielectric properties, switching of the other physical-chemical properties in this kind of core-shell nanostructure can be expected. EXPERIMENTAL SECTION Materials. All syntheses were performed under ambient conditions and all the chemicals were of analytical grade and used without further purification. The materials for UCNPs: NaOH, NH4F, ethanol, chloroform, toluene, cyclohexane, trifluroacetic acid (CF3COOH) and HCl were purchased from Sinopharm Chemical Reagent Co. Ltd (China). Oleic acid (OA) and 1-octadecence (OD) were purchased from Alfa Aesar. Rare earth oxides Y2O3, Yb2O3 and Er2O3 were purchased from Beijing HWRK Chem Co., LTD (China). The iron(Ⅱ) SCO complex 1 was prepared in our previously work.35 Characterization. For UCNPs: The size and morphologies of UCNPs were determined by using a transmission electron microscope (TEM; Tecnai-G2 20 19



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E-TWIN 200 KV) and Field emission scanning electron microscope (FESEM, HITACHI S-4800 20 kV). Energy-dispersive X-ray (EDX) analysis of UCNPs were performed during SEM measurements. The samples for TEM imaging were prepared by dispersing the nanoparticles in chloroform or cyclohexane at a concentration of 0.1% w/v, TEM copper grids (SPI Supplies/Structure Probe, Inc. US) were then immersed inside the nanoparticle dispersions for 10 s and dried in air. X-Ray powder diffractions (PXRD) were performed on Ultima IV diffractometer equipped with Cu Kα radiation (λ = 1.5418 Å) in the range 5-50°at room temperature. NMR spectra were recorded on a Varian FT-600 MHz instrument. Up-conversion luminescence spectra were measured with a fluorescence spectrometer (Edinburgh, LFS -920) using an external continuous-wave(CW) 980 nm laser (Xi’an Saipulin Laser Technology Institute, China) with tunable power of 0-3W as the excitation source. For the composite nano-spheres UCNPs@1: Infrared spectra were recorded on a SHIMADZU IR prestige-21 FTIR-8400S spectrometer in the spectral range 4000-500 cm-1, with the samples in the form of potassium bromide pellets. UV-vis absorption spectra (both in solution and in solid state) were recorded with a Shimadzu UV-3150 double-beam spectrophotometer. Thermogravietric

analysis

(TGA)

profiles

were

performed

using

a

Mettler-Toledo TGA/DSC STARe System at a heating rate of 10K min-1, under an atmosphere of dry N2 flowing at a rate of 20 cm3min-1 over a temperature

20


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range from 50 °C to 800 °C. The fluorescence spectra were obtained on a Horiba Fluoro Max 4 Spectro-fluorometer. The TEM, SEM, EDXA and up-conversion luminescence spectra of UCNPs@1 were measured by the same procedures as UCNPs. The irradiation of composite nano-spheres samples for the IR spectra, UV-vis absorption spectra, and fluorescence spectra were performed by alternating using a CW 980 nm NIR laser (SD980-5000G3, Xi’an Saipulin Laser Technology Institute, China) with Laser power density of 1.5 W cm-2 and a 230 W Hg arc lamp with a 310 ± 5 nm bandpass filter. Temperature dependent dielectric constants of nano-spheres UCNPs@1 were measured on polycrystalline sample by a Tonghui TH2828A impedance analyzer in the frequency ranges of 0.5−1000 kHz at a heating/cooling rate of 5 Kmin-1. Synthesis of Core-Shell Nano-spheres UCNPs@1: The synthesis procedures of β-NaYF4:Yb3+, Er3+ UCNPs, ligand bipy-COOH and SCO complex

Fe(H2Bpz)2(bipy-COOH)

(1)

were

presented

in

Supporting

Information Section. UCNPs@1 was prepared by using ligand-exchange reaction. The as-prepared UCNPs in toluene (5 mg/mL, 30 mL) were added into a 100mL round-bottom flask at ambient condition with violently stirring, then 10 mL methanol solution containing 10 mg complex 1 (1 mg/mL) was added drop-wise within 1h. Then the mixture was stirred under ambient condition for about 10 h. Within that time, the purple-colored products of

21



UCNPs@1 were formed and precipitated. The solid of UCNPs@1 was separated by centrifugation at 1000 r/min and washed with methanol (20 mL) to completely remove free-standing unreacted complex 1. The final product of UCNPs@1 was successfully dispersed in chloroform. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The synthesis procedures of β-NaYF4:Yb3+, Er3+ UCNPs, ligand bipy-COOH and SCO complex 1, HNMR and PXRD pattern of the as-prepared UCNPs, additional TEM/SEM photographs and EDXA analysis of the as-prepared UCNPs and UCNPs@1, IR spectra of 1 and UCNPs@1, additional UV-vis absorption spectrum of UCNPs@1 irradiated with NIR light.

Notes The authors declare no competing financial interest.

ACHNOWLEDGEMENTS This research was supported by the Natural Science Foundation of China (Grant No. 21701023), Natural Science Foundation of Jiangsu Province (Grant

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No. BK20170660), Fundamental Research Funds for the Central Universities (No.3207048427) and PAPD of Jiangsu Higher Education Institutions.

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