Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (5): 677-683

#### The article information

Tian-yu Song, Yan-yang Zhu, Shuo-feng Liang, Gang Zou, Qi-jin Zhang

Coordinate Bond Breaking Induced by Collapse of Poly($N$-isopropyl acrylamide) as Ligands of a Rare Earth Complex

Chinese Journal of Chemical Physics, 2018, 31(5): 677-683

http://dx.doi.org/10.1063/1674-0068/31/cjcp1804059

### Article history

Accepted on: June 3, 2018
Coordinate Bond Breaking Induced by Collapse of Poly($N$-isopropyl acrylamide) as Ligands of a Rare Earth Complex
Tian-yu Song, Yan-yang Zhu, Shuo-feng Liang, Gang Zou, Qi-jin Zhang
Dated: Received on April 3, 2018; Accepted on June 3, 2018
CAS Key Laboratory of Soft Matter Chemistry, Key Laboratory of Optoelectronic Science and Technology, Innovation Centre of Chemistry for Energy Materials, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China
*Author to whom correspondence should be addressed. Gang Zou, E-mail: gangzou@ustc.edu.cn; Qi-jin Zhang, E-mail: zqjm@ustc.edu.cn
Abstract: A novel water-soluble luminescent complex consisting of Eu(ally-dbm)$_3$-2Tppo and poly(N-isopropyl acrylamide) (PNIPAM) is synthesized through a series of chemical reactions. The structure of the complex is characterized by TGA, GPC, HNMR, and the thermal-responsive fluorescence of the complex in aqueous solution is investigated. It is found that PNIPAM collapse above the lower critical solution temperature causes the coordination bond breaking, leading to weakening of the fluorescence from Eu$^{3+}$ and enhancing of the fluorescence from the ligands. When temperature decreases, the fluorescence from Eu$^{3+}$ is found to boost up and the fluorescence from ligands weakens accordingly. It is deduced from this phenomenon that the ligands re-coordinate with europium ions again along with the temperature decreasing, which is further confirmed by IR measurements. This thermal-responsive fluorescence is of reversibility, which can be used as molecular probes for biological imaging and collapse studying of PNIPAM.
Key words: Thermo-responsive polymer    Thermal quenching    Reversiblity
Ⅰ. INTRODUCTION

Water-soluble rare earth complexes have been a long-standing goal for many research teams. Various functional groups have been introduced into the rare earth complex in order to achieve peculiar properties that respond to external stimuli, such as photo, pH and thermal responsive, and so on, which are essential for applications in the biological system [1-3]. Rodrigues research group found that two water-soluble lanthanide luminescent complexes with ionophilic ligands can be used as a fluorescence imaging probe for invasive detection of mammal cancer cells [4].

It is well known that rare earth complexes have advantages of narrow band emission, good monochromatic character, strong UV absorption ability and long fluorescence life [5-10] and diketone is one kind of organic ligands that has been used in many complexes [11, 12]. Diketonate as ligands can sensitize Eu$^{3+}$ to emit fluorescence and the sensitization process is that the diketonate ligand absorbs energy, undergoes intersystem crossing into a triplet state, and transfers its energy to the Eu$^{3+}$ ion subsequently [13, 14]. Diketone chemical structure exists in two isomers: keto and enol types. Due to the activity of methylene hydrogen, the isomer is apt to enol structure. In general $\beta$-diketones can be coordinated with metal ions in the form of the chelating bidentates to form a six-membered chelate ring.

When water-soluble rare earth complexes are applied to ion detection by many research teams [15-17], it has been found that the fluorescence of the water-soluble rare earth complexes is easily quenched by water molecules. It is still a challenge to synthesize rare earth complexes that can both sensitize Eu$^{3+}$ emission and dissolve in water by improvement in molecular structure. Methods including grafting other functional groups on the complexes, blending semiconducting conjugated polymers with the complexes and introducing synergistic ligands to rare earth ions have been reported in many researches [18-20]. In these studies, hydrophilic segments are introduced into the complexes to enable fluorescence of the complexes in aqueous solution.

Poly($N$-isopropylacrylamide) (PNIPAM) represents one of the most widely investigated thermal-responsive polymers and has been utilized as drug controlled release material, enzyme solid material, dehydrating agent, and so on [21, 22]. Owing to its hydrophilic amide group, under the temperature below the lower critical solution temperature (LCST), the polymer is in swollen conformation, while above the LCST, due to hydrophobic isopropyl group, it is in collapsed state. The collapse behavior of PNIPAM chains is affected by the molecular structure of PNIPAM above the LCST. For instance, for highly cross-linked PNIPAM, the temperature range of the process broadens remarkably, which has been studied before [23, 24]. Besides, the effect of sodium salts on LCST of PNIPAM was investigated by Bergbreiter et al. [25]. In this work, PNIPAM is chosen to modify the diketone to achieve diketone sensitized Eu$^{3+}$ emission in aqueous solutions. Eu(ally-dbm)$_3$-2Tppo-6PNIPAM was synthesized via a multistep procedure outlined in Scheme 1. Thermal-responsive PNIPAM was prepared via reversible addition fragmentation chain transfer (RAFT) polymerization and PNIPAM chains were introduced into rare earth complexes by click reaction. Fluorescent and thermal-responsive properties of the complex were studied and some novel phenomena were explored in detail.

 Scheme 1 Synthetic routine of Eu(ally-dbm)$_3$-2Tppo- 6PNIPAM.
Ⅱ. EXPERIMENTS A. Materials

$N$-isopropylacrylamide was purchased from Aladdia and purified by recrystallization from $N$-hexane. Azobiisobutyronitrile (AIBN) was recrystallized from ethanol prior to use. 2-(Dodecylthiocarbonothioylthio)-2-methylpropionicacid (DDMAT) was previously prepared by our group by RAFT. Acetophenone, methyl benzoate, allyl bromide, triphenylphosphine oxide (TPPO) europium(Ⅲ) chloride hexahydrate, K$_2$CO$_3$, NaH, and isopropylamine were obtained from Aladdia$^\circledR$.

B. Synthesis of the compounds 1. Synthesis of (4, 4-diallyloxy)-dibenzoylmethane (ally-dbm)

(4-Allyloxy)-acetophenone (1.77 g, 0.01 mol) was dissolved in anhydrous THF (100 mL). After addition of sodium hydride (0.8 g, 0.02 mol), the mixture was stirred in an ice bath for 0.5 h. After addition of (4-allyloxy)-benzoic acid methyl ester, the mixture was slowly heated to 70 ℃ and reacted at a nitrogen atmosphere protection for 72 h. After solvent was evaporated, the mixture was dissolved in water and extracted with ethylacetate and the reaction was repeated twice. The crude product was purified by column chromatography on silica gel (ethyl acetate:petroleum ether=1:12 $V/V$) to obtain a yellow solid. Then the product was recrystallized from absolute ethanol (43.2%).

2. Synthesis of Eu(ally-dbm)$_3$-2Tppo

1.011 g, 0.003 mol (4, 4-di-allyloxy)-dibenzoylmethane and triphenylphosphine oxide (Tppo) (0.278 g, 0.001 mol) were dissolved in anhydrous THF (20 mL). Europium(Ⅲ) chloride hexahydrate (0.366 g, 0.001 mol) was dissolved in absolute ethanol (20 mL). The europium(Ⅲ) chloride hexahydrate solution was slowly added dropwise to the ally-dbm and Tppo mixture solution. After addition of sodium hydroxide (0.12 g, 0.003 mol) the mixture was stirred at 60 ℃ for 0.5 h. The solvent was evaporated, and the residue was purified by recrystallization from a mixed solution of ethyl acetate and petroleum ether (67.5%).

3. Synthesis of PNIPAM

$N$-isopropylacrylamide was purified by recrystallization from $N$-hexane. NIPAM (2.6 g, 0.02297 mol), DDMAT (0.1196 g, 0.328 mmol), and AIBN (0.5381 mg, 0.00328 mmol) were dissolved in anhydrous THF (20 mL) seal tube polymerization. The mixture was stirred at 65 ℃ for 72 h. The mixture was added dropwise to the anhydrous ether to precipitate three times (82.8%).

4. Synthesis of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM

PNIPAM-SH was easily obtained by reacting PNIPAM with isopropylamine. PNIPAM-SH (0.0344 g, 0.02 mmol), Eu(ally-dbm)$_3$-2Tppo (0.951 g, 0.12 mmol) and irgacure819 (10 mg, 1 wt%) were dissolved in anhydrous THF (20 mL). The mixture was exposed to 365 nm ultraviolet radiation for 1 h. The residue was purified by recrystallization from anhydrous ether two times (73.2%).

C. Characterization

Ally-dbm was characterized by 300 MHz $^1$H NMR (FIG. 1). Eu(ally-dbm)$_3$-2Tppo was characterized by TGA(Q5000IR) (FIG. 2). The residual oxide mass percentage is 18.35% after fully burning. The experimental calculated value is very close to the theoretical calculated value 17.10%. The phosphorus pentoxide hygroscopicity in the combustion ash resulted in a smaller theoretical value than actual value.

 FIG. 1 $^1$H NMR spectrum of ally-dbm in DMSO.
 FIG. 2 TGA of Eu(ally-dbm)$_3$-2Tppo.

The thermal-responsive polymer PNIPAM was prepared via reversible addition fragmentation chain transfer (RAFT) polymerization and characterized by $^1$H NMR (300 MHz) and GPC (waters 1515). The molecular weight and polydispersity index of the copolymer are 7200 and 1.175 resulting from the $^1$H NMR (FIG. 3) and GPC (FIG. 4), respectively.

 FIG. 3 $^1$H NMR spectrum of PNIPAM in CDCl$_3$.
 FIG. 4 GPC of PNIPAM.

Eu(ally-dbm)$_3$-2Tppo-6PNIPAM was characterized by 300 MHz $^1$H NMR and the $^1$H NMR analysis shows the disappearance of all the peaks of double bonds (FIG. 5).

 FIG. 5 $^1$H NMR spectrum of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM in CDCl$_3$.
Ⅲ. RESULTS AND DISCUSSION

Eu(ally-dbm)$_3$-2Tppo-6PNIPAM can be dissolved into water at room temperature. From absorptions shown in FIG. 6(a), it is easily found that two absorptions of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM in aqueous solution: a main absorption is located at about 365 nm and a shoulder peak appears at about 390 nm. The former is thought to be due to a $\pi$-$\pi^*$ type transition associated with the conjugated system of the complex benzene ring [26], and the latter is possibly from $n$-$\pi^*$ transition or singlet-triplet (1$\pi$-3$\pi^*$) transition. From excitation spectra shown in FIG. 6(b), it is seen that the excitation is located at 365 nm in aqueous solution, which is not distinguished from the absorption [27]. The similarity between two kind spectra shows that the energy transfer from ligands to rare earth ions is mainly influenced by the absorbance of the complex. It is worth pointing out that there are two characteristic emissions in FIG. 6(b), corresponding to fluorescence at 613 nm from Eu$^{3+}$ ion (FIG. 6(b)) and 455 nm from ligands (FIG. 6(b)). Because the emission of the europium complexes is easily quenched in an acid environment, these emissions was used to probe the tumoral acidic pH microenvironment [28, 29].

 FIG. 6 (a) Absorption spectrum of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM in aqueous solution. (b) Excitation ($\lambda_{\rm{em}}$=613 nm) and fluorescence ($\lambda_{\rm{ex}}$=365 nm) spectra of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM in aqueous solution ($c$=5$\times$10$^{-5}$ mol/L).

As we all know, PNIPAM is a thermal-responsive polymer with a low critical solution temperature (LCST) at around 32 ℃ due to its reversible hydrogen bonding with water in aqueous solution. When PNIPAM is introduced into the complex, such a thermal sensitive property would cause a change in the fluorescence of the complex. FIG. 7 shows a relationship between temperature and fluorescence intensity of the complex in aqueous solution ($\lambda_{\rm{ex}}$=365 nm). As shown in FIG. 6(b), there are two emission peaks in the fluorescence spectrum of the complex, one of which is at 613 nm from Eu$^{3+}$. In FIG. 7(a) the strength of the emission at 613 nm decreases along with temperature rising. Such a phenomenon called thermal quenching has been well realized in the previous reports [28, 30].

 FIG. 7 Temperature dependent fluorescence intensity of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM (a) at 613 nm from Eu$^{3+}$ and (b) at 455 nm from ligands in aqueous solution ($\lambda_{\rm{ex}}$=365 nm) ($c$=5$\times$10$^{-5}$ mol/L).

Fluorescence intensity at 455 nm from ligands was also measured at different temperatures as shown in FIG. 7(b). Fluorescence intensity ratios ($I_{40}/I_{20}$) of 40 ℃ to 20 ℃ are used to describe fluorescence quenching at 613 and 455 nm in FIG. 7(a). From results shown in FIG. 7, the ratio equals to 30.0% for the emission at 613 nm and 600% for the emission at 455 nm, respectively. Such a large difference is from the difference in two fluorescent processes of Eu$^{3+}$ and ligands. The former is an energy transfer process through intersystem crossing, and the latter is an intramolecular process. On the other hand, the large change in fluorescence intensity at 455 nm means that there is another mechanism responsible for the fluorescence quenching of the complex in aqueous solution along with the normal thermal quenching.

It is well known that, in fluorescent process of rare earth complexes, the ligand, which is PNIPAM modified-diketone in this work, is usually thought to be firstly excited into the singlet excited state of the ligand and generates a ligand-based triplet state through fast inter-system crossing. Then, this triplet state transfers its energy to the central rare earth ion to produce a luminescent f-f state through a Dexter double electron-exchange mechanism. The necessary condition for this mechanism is direct contact between aromatic ligand and rare earth ion, which facilitates the Dexter energy transfer process [31]. According to this realization, the result shown in FIG. 7(b) means part of the ligands can't sensitize europium ion when PNIPAM chains collapse in aqueous solution. This deduction makes us hypothesize that the intramolecular cohesion of PNIPAM chains collapse pulls the ligand to disassociate from the europium ions, and the ligand fails to transfer energy to the europium ions through Dexter energy transfer, as a result, the fluorescence of the ligards increases.

In order to confirm the hypothesis, the change of the coordinate bonding was investigated by IR spectrum measurements, and results are shown in FIG. 8. IR spectra of PNIPAM, Eu(ally-dbm)$_3$-2Tppo-6PNIPAM below LCST and above LCST show a peak at 1645 cm$^{-1}$, which is assigned to absorption of C=O groups in three compounds. It has been known that the absorption of the C=O group would split into a doublet peak when there was the coordination between the group and Eu$^{3+}$ in small molecule complexes (FIG. 9) [32]. The C=O of PNIPAM in three compounds is so strong that split peak is not observed in three IR spectra. Two differential spectra are made by subtracting the spectrum of PNIPAM in the spectra of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM below LCST and above LCST, which are shown in FIG. 8 (d) and (e), respectively. Results show that a split peak appears when the complex is at temperature below LCST, and above LCST, the split peak disappears, indicating the breaking of the coordinate bonding. As the discussion on result of FIG. 7, the breaking of the coordinate bonding means that the energy transfer from the ligand to Eu$^{3+}$ can't be carried on, resulting in a self-body fluorescence of ligands.

 FIG. 8 IR spectra of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM (a) above LCST and (b) below LCST, and (c) IR spectra of PNIPAM, IR differential spectra of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM (d) above LCST and (e) below LCST obtained by subtracting (c) in (a) and (b), respectively.
 FIG. 9 IR spectra of complex Eu(ally-dbm)$_3$-2Tppo and ligand ally-dbm.

Thermal-responsive fluorescence of the complex is dependent on the temperature because there is dynamic equilibrium between temperature and collapse of PNIPAM. Relationship between temperature and fluorescence intensity of the complex is shown in FIG. 10. For each measurement in FIG. 10, at five-minute time interval the temperature reached a stable value. From curves shown in FIG. 10, it is found that along with the increasing of the temperature, the florescence intensity decreases and there is a transition at about LCST of PNIPAM at 32 ℃. More interesting, the temperature-dependent process is reversible, and after a circulation fluorescence intensity is nearly restored with 96.5% of the initial value. This phenomenon means that the breaking of the coordinate bonding can be restored when temperature falls back to below LCST. This circulation has a potential application for the complex to be used as a probe in biological imaging and collapse studying of PNIPAM. It is worth exploring whether the circulation is reversible during repeatedly heating and cooling the aqueous solution of the complex.

 FIG. 10 Temperature dependent fluorescence intensity of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM in aqueous solution at 613 nm ($\lambda_{\rm{ex}}$=365 nm) during heating and cooling processes ($c$=5$\times$10$^{-5}$ mol/L).

A circulating experiment was designed as follows: the aqueous solution of the complex is first heated to 40 ℃, at which PNPAM chains are in collapse, for fluorescence measurement, and then, the solution is cooled down to 20 ℃, at which PNPAM chains are stretched, for fluorescence measurement again. From circulating experimental results shown in FIG. 11, it is found that the emission intensity at 613 nm from Eu$^{3+}$ is circulated along with temperature changing from 20 ℃ to 40 ℃, and at the same time, the emission at 455 nm from ligand has the same changing tendency with contrary values. From these experimental observation it is easily deduced that when temperature changes from one above LCST to another below LCST, coordinate bonding between rare earth ions and ligands has a reversible breaking and recovering process. The reversibility shown in FIG. 11 means that this thermal-responsive macromolecular complex has good thermal circulating stability in fluorescence intensity.

 FIG. 11 Fluorescence intensities of Eu(ally-dbm)$_3$-2Tppo-6PNIPAM in aqueous solution (a) at 613 nm and (b) at 455 nm circulating at 20 and 40 ℃, respectively. ($\lambda_{\rm{ex}}$=365 nm and $c$=5$\times$10$^{-5}$ mol/L).
Ⅳ. CONCLUSION

A new macromolecular rare earth complex is synthesized and characterized, in which thermal-responsive PNIPAM is used as a ligand. The fluorescent intensity from this complex shows a reversible changing along with temperature changing. Below LCST of PNIPAM, the fluorescent intensity from the ligand is found increasing along with the decreasing of the fluorescent intensity from rare earth ions. Analysis on IR spectra of complexes below and above LCST reveals that such a phenomenon is caused by breaking of coordinate bonding between ions and ligands. Circulate experiments show that there is a breaking-recovering process of the bond along with temperature changing from one above LCST to another below LCST. The complex with such a reversible thermal-responsive fluorescence can be used as a molecular probe not only for biological imaging, but also for collapse studying of PNIPAM.

Ⅴ. ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (No.51673178, No.51273186, No.21574120, and No.11404087), the Basic Research Fund for the Central Universities (WK2060200012), the Science and Technological Fund of Anhui Province for Outstanding Youth (No.1608085J01), and the Fundamental Research Funds for the Central Universities, China Postdoctoral Science Foundation (No.2015M571918 and No.2017T100442).

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