Yiran Zhao

Jingyao Ma

Xia Yu

Min-Hui Li
email: min-hui.li@chimieparistech.psl.eu

Jun Hu

Tunable UCST thermoresponsive copolymers based on natural glycyrrhetinic acid

Keywords:

Water-soluble thermoresponsive polymers present either upper critical solution temperature (UCST) or lower critical solution temperature (LCST) depending on the location of their miscibility range with water at high temperatures or at low temperatures. Compared with LCST polymers, the water-soluble UCST polymers are still less explored until now. In this work three copolymers of P(AAm-co-GAA) were synthesized by copolymerizing two acrylamide monomers, acrylamide (AAm) and acrylamide functionalized with natural glycyrrhetinic acid (GAA), using reversible addition-fragmentation chain transfer (RAFT) polymerization. These copolymers exhibited the typical UCST thermoresponsive behavior, and their phase transition temperatures could be easily tuned to around 37 o C for potential biological applications. Moreover, the UCST of P(AAm-co-GAA) can be adjusted not only by the content of glycyrrhetinic acid (GA) and polymer concentrations, but also by the host-guest interactions between GA and cyclodextrins (β-and γ-CD). The suitable value of UCST and the biocompatible nature of GA and CDs may endow these copolymers with practical applications in biomedical chemistry.

Water-soluble thermoresponsive polymers have been widely studied in polymer and material science due to the drastic hydrophilicity/hydrophobicity transformation upon changes of temperature [1,2]. They can be divided into lower critical solution temperature (LCST)-type and upper critical solution temperature (UCST)-type, depending on the drop of miscibility at low temperatures or at high temperatures [3]. After the discovery of LCST behavior of poly(N-isopropylacrylamide) by Heskins and co-workers in 1968 [4], numerous water-soluble LCST polymers have been reported [5,6]. Compared with LCST polymers, the UCST polymers are less explored since they were discovered later and the number of examples was still limited [7,8]. The UCST behavior stems from either Coulomb interactions (the effect of amphoteric ions) or hydrogen bonding, namely C-UCST or HB-UCST, respectively [9]. As C-UCST behavior depends heavily on ionic strength, type of ion and pH value, the potentials of C-UCST polymers are restricted because in most applications mild ionic strength is required [9,10]. Thus, HB-UCST polymers which are less sensitive to electrolytes would be more suitable for preparing thermosensitive materials.

In 2012, Seuring and co-workers copolymerized acrylamide and acrylonitrile to afford poly(acrylamide-co-acrylonitrile) with the typical UCST behavior in water [11]. Later, researchers used additional comonomers to copolymerize with acrylamide (AAm) and acrylonitrile (AN), and applied them in various fields [12][13][14]. For example, Zhao and co-workers reported an ultrasensitive pH-induced water-soluble switch by incorporating either acrylic acid or 4-vinylpyridine comonomers in the random copolymer of acrylamide and acrylonitrile [12]. Sukhishvili and co-workers have synthesized a HB-UCST block copolymer, poly(acrylamide-co-acrylonitrile)-b-polyvinylpyrrolidone, and prepared its self-assembled films with thermoresponsive properties [13]. Apparently, incorporating a suitable hydrophobic unit into the water-soluble polymer chain would be an effective strategy to obtain HB-UCST polymers.

Glycyrrhetinic acid (GA) is one of the physiologically natural triterpenoids in vivo and plays key roles in a variety of remarkable biological activities [15]. In addition, GA has the rigid hydrophobic scaffold, great biocompatibility, multiple reactive sites and chiral centers. These characteristics can be used to make low molecular weight gels [16,17], supramolecular chiral structures [18], and functional polymers [19,20]. In our previous works, GA has been incorporated into LCST-based polymers, "necklace"-like polypseudorotaxanes, and selfhealable hydrogels [21][22][23]. In this work, we proposed to take advantage of the hydrophobic backbone of GA to construct new biocompatible HB-UCST polymers. Three copolymers of P(AAm-co-GAA-x%) (x = 2, 3, 4) were designed and synthesized (Scheme 1), where an acrylamide monomer bearing GA (GAA) copolymerized with acrylamide by reversible addition-fragmentation chain transfer (RAFT) polymerization (synthetic details see Supporting Information). The results showed that P(AAm-co-GAA) exhibited the typical HB-UCST behavior with a phase transition temperature around 37 o C. Moreover, their thermoresponsive behavior can be adjusted not only by the content of GA and polymer concentrations, but also the host-guest interactions between GA and β-/γ-cyclodextrin (CD). The suitable UCST and the biocompatible nature of GA and CDs may allow these copolymers to be useful in biomedical field.

The copolymers with different molar ratios of GAA and AAm were characterized by size exclusion chromatography (SEC) and 1 H NMR spectra (Fig. S1 andS2). The 1 H NMR spectra of P(AAm-co-GAA-x%) (x = 2, 3, 4) displayed the characteristic peaks corresponding to protons carried by GA (δ = 5.49 ppm, assigned to Ha; 1.35, 1.05, 1.02, 0.92, 0.74, 0.71 ppm, assigned to methyl groups), and the content of GA unit in copolymers was calculated from the proton integration ratio between the peak at 5.49 ppm (Ha) versus backbone protons at 2.13 ppm (Hb, Hc) (Fig. S2). Meanwhile, in order to explore the UCST mechanism, two control copolymers, P(AAm-co-HAA-3%) and P(MMA-co-GAA-3%), were synthesized as shown in Scheme S1 (synthetic details see Supporting Information). Compared with P(AAm-co-GAA-3%), the rigid hydrophobic backbone of GA was replaced with a flexible alkyl chain (15-hydroxypentadecanoic acid, HA) in P(AAm-co-HAA-3%), while the hydrophilic AAm was substituted by the H-bonding-free methyl methacrylate (MMA) in P(MMA-co-GAA-3%). All the information on compositions of these copolymers was illustrated in Table 1.

The thermal response of polymer aqueous solutions was recorded by UV-Vis spectrometer. As shown in Fig. 1A, the phase transition with a UCST of 32 o C of P(AAm-co-GAA-3%) at the concentration of 15 mg/mL was observed upon cooling, strongly revealing the UCST behavior of P(AAm-co-GAA-3%) in the range from 10 to 60 o C. At a high temperature of 40 o C, a clear solution formed, while at 20 o C it appeared a cloudy suspension (inset in Fig. 1A). Either increasing or decreasing the molar fraction of GAA to 4 mol% or 2 mol% led to the UCST higher than 60 o C or lower than 10 o C. It was because P(AAmco-GAA-2%) was less hydrophobic in comparison with P(AAmco-GAA-3%), while P(AAm-co-GAA-4%) was more hydrophobic. With increasing the GA content, the hydration shell of hydrophobic segments of copolymers became more ordered [24]. In this case, a higher temperature was needed to dissolve the aggregates, i.e. a high phase transition temperature. Apparently, the content of hydrophobic GA unit had a significant effect on UCST behavior. Notably, compared with the reported P(AAm-co-AN) which exhibited the UCST behavior only when AN content was more than 10 mol% [11], P(AAm-co-GAA) would show UCST behavior with the content of GAA as low as 3 mol%. Such difference was mainly due to GAA had a larger hydrophobic rigid backbone than AN moiety.

In addition, there was a significant concentration dependence of UCST. As shown in Fig. 1B, the UCST of P(AAm-co-GAA-3%) decreased from 32, to 21 o C, and to < 10 o C when the concentration decreased from 15, to 10 and to 5 mg/mL. This observation can be explained by the strong inter-and intramolecular H-bonding between polymer chains at the high concentration, as solubilizing copolymers required the disruption of polymer-polymer interactions by heat in favor of polymerwater interactions.

Moreover, this phase transition was reversible in multiple heating-cooling processes (3 cycles, Fig. 1C), indicating that no chemical change happened in the system. It should be noted that there was a certain hysteresis which was attributed to the chain mobility of P(AAm-co-GAA-3%) below or above UCST. Above UCST, GA pendants was involved in the aggregates companied by the weakening of inter-and intra-molecular H-bonding, and these aggregates were stable enough to delay the generation of large clusters during the cooling process, thus causing the hysteresis [11]. Besides, the thermoresponsive experiments of control polymers (P(AAm-co-HAA-3%) and P(MMA-co-GAA-3%) were performed. As shown in Fig. S3, the transmittance of P(AAm-co-HAA-3%) aqueous solution maintained approximate 90% within the test temperature range from 10 to 60 o C, and did not exhibit any UCST property. It strongly revealed that only the rigid scaffold of GA can afford the suitable face-to-face hydrophobic stacking effect in this UCST-type phase transition, since the better mobility of the flexible alkyl chain HA tended to form chain entanglements, thus restricting their supramolecular complexation and resulting in no phase transition [24]. For the H-bonding-free P(MMA-co-GAA-3%) aqueous solution, its transmittance was less than 1% from 10 to 60 o C, and no UCST behavior was observed, which indicated that the H-bonding monomer AAm was necessary in this UCST system. Based on the results of control experiments, it was proposed that the synergistic effect of inter-and intra-molecular H-bonding of amide groups and the face-to-face hydrophobic stacking of GA moieties promoted the appearance of UCST phenomenon.

More evidences for the UCST behavior of P(AAm-co-GAA-3%) were revealed by dynamic light scattering (DLS) measurements, 1 H NMR spectra, and transmission electron microscope (TEM) images. As shown in Fig. 2A, the average particle size of P(AAm-co-GAA-3%) in water was approximately 30 nm when the temperature was above 40 o C. Once cooling to 20 o C, some large clusters around 1000 nm appeared rapidly. With the continuous decrease of temperature, more and more large clusters formed with the disappearance of small particles, eventually a turbid solution was obtained at 10 o C. These observations can be further confirmed by TEM images, where the small particles around 40 nm were observed at 40 o C (Fig. 2B), while only irregular large precipitates were observed at 20 o C (Fig. 2C). The temperature-dependent 1 H NMR experiments showed that all proton peaks of P(AAm-co-GAA-3%) in D2O became broad with the decrease of temperature from 60 to 25 o C, especially the signals of methyl groups on GA (Fig. S4). The explanation was that at high temperature above UCST the GA side chain was exposed to D2O and presented good mobility, while at low temperature the intra-and inter-molecular H-bonding caused the GA side chain to be wrapped by polyacrylamide chains and to be less mobile. In conclusion, all results from the experiments of 1 H NMR, UV-Vis, DLS and TEM were totally consistent.

Our previous work has revealed that GA (molecular size 0.58-0.60 nm) could form the host-guest inclusion complex with β-cyclodextrin (β-CD) (cavity diameter 0.60-0.65 nm, binding constant Ka = 1.59 × 10 3 M -1 ) and with γ-cyclodextrin (γ-CD) (cavity diameter 0.70-0.83 nm, Ka = 4.41 × 10 5 M -1 ), except α-CD (cavity diameter ~0.47 nm) [22,25]. Inspired by the sizematch effect between GA and CDs, we studied here the role of CDs in adjusting the USCT behavior of P(AAm-co-GAA-3%). The 1 H NMR spectrum was firstly used to confirm the complexation of CDs with GA units in P(AAm-co-GAA-3%). As shown in Fig. S5, no signal of GA was observed in D2O at room temperature because of the poor solubility of P(AAm-co-GAA-3%). Upon addition of β-CD and γ-CD, respectively, the characteristic methyl peaks of GA units appeared, while no change was observed upon α-CD addition. Moreover, cross correlation signals were observed clearly in 2D NOESY 1 H NMR spectrum (rectangles in Fig. 3 andS6), indicating the close contact between GA methyl protons and β-/γ-CD protons (limit of detection for NOE signal: <5 Å). As in the aqueous environment GA and CD were phase-separated, these crosscorrelation signals were attributed to the formation of inclusion complex by positioning GA moiety in the cavity of β-/γ-CD.

In the UV-Vis absorption experiments, α-CD had no effect on the UCST behavior because of its small cavity diameter (Fig. 4A) [25]. The change in the curve shape was mainly due to the Hbonding between hydroxyl groups of α-CD and amide groups in polymers. In contrast, the significant changes were observed both for β-CD and γ-CD. In the presence of β-CD, P(AAm-co-GAA-3%) still exhibited the UCST property, while its UCST decreased gradually with the increase of molar ratio of [β-CD]/[GA] (Fig. 4B). When [β-CD]/[GA] increased from 0 to 0.03, 0.06, and 0.10, the UCST decreased from 32 to 27, 24, and 19 o C. Such results were mainly due to the hydrophilic exterior of β-CD, which enhanced the hydrophilicity of polymers, thus resulting in the decrease of UCST. Note that the UCST was extremely sensitive to the molar ratio of [β-CD]/[GA]. As shown in Fig. 4B, a tiny increase of concentration ratio from 0 to 0.03 would result in a decrease of 5 o C in UCST. This was totally different from the behavior of LCST polymers in the presence of β-CD as reported in our previous work, where an increase of [β-CD]/[GA] ratio from 0 to 5 only resulted in an increase of 13 o C for the cloud point [21]. As γ-CD had a stronger binding ability with GA and a better hydrophilicity compared with β-CD, it was more effective in regulating the USCT behavior of P(AAm-co-GAA-3%). As shown in Fig. 4C, a decrease of 13 o C for UCST was achieved upon addition of 0.03 equiv. γ-CD, and the UCST decreased further to lower than 10 o C when 0.06 equiv. of γ-CD was added. Obviously, the host-guest interactions between GA and β/γ-CD would be an effective strategy to regulate the UCST behavior of polymers, and the UCST of P(AAm-co-GAA-3%) was more sensitive to γ-CD than to β-CD.

In summary, by incorporating acrylamide monomer functionalized by natural glycyrrhetinic acid into the polyacrylamide chains, three copolymers of P(AAm-co-GAA-x%) (x = 2, 3, 4) were synthesized, which exhibited the typical HB-UCST behavior in water promoted by the synergistic effect of the intra-and inter-molecular H-bonding of amide groups and the face-to-face hydrophobic stacking of GA units. Their phase transition temperatures could be easily adjusted to the temperature range from below 10 to above 40 o C by the GA content and polymer concentrations. Moreover, the UCST can be further tuned by β-CD or γ-CD effectively due to host-guest interactions between GA and CDs. The suitable UCST ranges and the biocompatible features of GA and CDs are particularly interesting for thermoresponsive polymers applied in biomedical field. Synthesis of raft agent CMDT. 1-Dodecanethiol (1.20 mL, 5.04 mmol) and potassium tert-butoxide (566 mg, 5.04 mmol) were dissolved in 10 mL THF. After stirring at room temperature for 15 min, carbon disulfide (0.61 mL, 10.08 mmol) was added and stirred for 30 min. Then chloroacetonitrile (0.32 mL, 5.04 mmol) was added and the solution was stirred at room temperature for another 24 h. The resultant solution was evaporated to remove THF and re-dissolved by ethyl acetate. After washing with water, the organic layer was dried by anhydrous Na2SO4 and evaporated under reduced pressure. The crude was further purified by chromatography (hexane: ethyl acetate = 80:1, v/v) to afford CMDT as a yellow powder (1.55 g, yield 97% The content of GAA units in P(MMA-co-GAA-3%) was calculated from the proton integration ratio between the peak at 5.52 ppm (Ha) versus methyl protons at 3.56 ppm (Hb).

AScheme 1 .

 1 Scheme 1. Illustration of copolymerization of natural glycyrrhetinic acid-derived acrylamide (GAA) with acrylamide (AAm).

Fig. 1 .

 1 Fig. 1. Transmittance of the copolymer aqueous solutions as a function of temperature at a wavelength of 450 nm: (A) with different GAA molar ratios of P(AAm-co-GAA-x%) (x = 2, 3, 4) under the concentration of 15 mg/mL. Inset: digital photos of P(AAm-co-GAA-3%) at 20 o C (left) and 40 o C (right); (B) at different concentrations of P(AAm-co-GAA-3%); (C) the aqueous solution of P(AAm-co-GAA-3%) with three cooling-heating cycles at the concentration of 15 mg/mL. Cooling and heating rate is 1.0 o C/min.

Fig. 2 .

 2 Fig. 2. (A) Temperature-dependent size variation of P(AAm-co-GAA-3%) aqueous solution. TEM images of P(AAm-co-GAA-3%) aqueous solution at (B) 40 o C and (C) 20 o C without stain. Concentration is 15 mg/mL.

Fig. 3 .

 3 Fig. 3. (A) 1 H NMR spectrum of P(AAm-co-GAA-3%) in D2O (400 MHz, 25 o C, concentration = 15 mg/mL). (B) 2D NOESY 1 H NMR spectrum of P(AAm-co-GAA-3%) with γ-CD in D2O (600 MHz, 25 o C, [GA]/[γ-CD] = 1:1, concentration = 15 mg/mL). "*" means residual H signal in D2O.

Fig. 4 .

 4 Fig. 4. Transmittance of P(AAm-co-GAA-3%) aqueous solution as a function of temperature at a wavelength of 450 nm in the presence of (A) α-CD, (B) β-CD, and (C) γ-CD. Cooling rate is 1.0 o C/min. Concentration is 15 mg/mL.

  Scheme S1. Synthetic route of copolymers of P(AAm-co-GAA-x%) (x = 2, 3, 4), P(AAm-co-HAA-3%), and P(MMA-co-GAA-3%).

Fig. S1 .

 S1 Fig. S1. SEC spectra of P(AAm-co-GAA-x%) (x = 2, 3, 4) with refractometer as detector (DMSO as eluent, dextran standards for molecular weight calibration). The right shoulder peak of P(AAm-co-GAA-4%) was mainly due to the formation of trace oligomers.

Fig. S2. 1 H

 1 Fig. S2. 1 H NMR spectra (400 MHz, d6-DMSO) of P(AAm-co-GAA-x%) in the molar fraction of GAA (A) 2%, (B) 3%, and (C) 4%. "*" means DMSO and water.

Fig. S5. 1 H

 1 Fig. S5. 1 H NMR spectra of P(AAm-co-GAA-3%) in D2O at the concentration of 15 mg/mL upon addition of α-, β-, or γ-CD, respectively ([GA]/[CD] = 1:1). "*" means D2O.

Fig. S6 .

 S6 Fig. S6. 2D NOESY 1 H NMR spectrum of P(AAm-co-GAA-3%) with β-CD in D2O (600 MHz). "*" means D2O.

Fig. S7. 1 HFig.

 1 Fig. S7. 1 H NMR spectrum of CMDT (400 MHz, CDCl3)

Fig. S24 .

 S24 Fig. S24. SEC spectrum of P(MMA-co-GAA-3%) with refractometer as detector (DMAC as eluent, polystyrene standards for molecular weight calibration).

Table 1

 1 Characteristics of these copolymers.

	Copolymer	GAA or HAA in feed (mol%)	GAA or HAA in product (mol%) a	Mn (g/mol)	Đ b	UCST ( o C) c
	P(AAm-co-GAA-2%)	2.0	1.7	17100 b	2.07	< 10
	P(AAm-co-GAA-3%)	3.0	2.5	12700 b	1.77	32
	P(AAm-co-GAA-4%)	4.0	3.6	6700 b	2.19	> 60
	P(AAm-co-HAA-3%)	3.0	2.4	14800 a	-	-
	P(MMA-co-GAA-3%)	3.0	2.6	11800 b	1.23	-

a Determined by 1 H NMR spectrum; b determined by SEC spectrum; c determined by UV-Vis spectrum at the concentration of 15 mg/mL.

  The mixture was stirred at room temperature for 12 h and the precipitates was removed by filtration. The resultant solution was evaporated to remove THF and re-dissolved by ethyl acetate. After washing with water, the organic layer was dried by anhydrous Na2SO4 and evaporated under reduced pressure to afford intermediate GA active ester. This intermediate was dissolved by 50 mL dichloromethane (DCM), and then added dropwise to a DCM solution (150 mL) of 1,6-hexylenediamine (10 g, 86.06 mmol) at room temperature. After stirring for 18 h, the solvent was removed and the crude was re-dissolved by ethyl acetate, washed with water, dried by anhydrous Na2SO4, and evaporated under reduced pressure. To a solution of GANH2 (1.40 g, 2.46 mmol) and triethylamine (0.34 mL, 2.46 mmol) in DCM (60 mL), acryloyl chloride (0.24 mL, 2.95 mmol) in DCM (10 mL) was added dropwise at ice bath. The mixture was stirred at room temperature for 6 h and then the resultant solution was evaporated to remove DCM. The crude was re-dissolved by ethyl acetate, washed with water, and dried by anhydrous Na2SO4. After removing the solvent, the crude further was purified by chromatography (dichloromethane: methanol = 10:1, v/v) to afford GAA as a

	The crude
	product was further purified by chromatography (dichloromethane: methanol = 2:1, v/v) to afford
	GANH2 as a white powder (1.40 g, yield 29%). ESI-MS (+): m/z 569 [M+H] + ; 1 H NMR (400
	MHz, CDCl3, ppm): δ 5.75 (t, 1H, J = 5.48 Hz, O=C-NHCH2-), 5.64 (s, 1H, 12-H), 3.17-3.34 (m,
	3H, O=C-NHCH2-, 3-H), 2.70 (t, 2H, J = 6.84 Hz, NH2CH2CH2-), 1.36, 1.12, 0.99, 0.80, 0.79 (s, 7
	× 3H, 7 × CH3); 13 C NMR (100 MHz, CDCl3, ppm): δ 200.3, 175.6, 169.5, 128.4, 78.7, 61.9, 55.0,
	48.2, 45.4, 43.6, 43.3, 41.9, 41.8, 39.3, 39.2, 37.5, 37.1, 33.0, 32.8, 31.9, 31.5, 29.7, 28.5, 28.1,
	27.3, 26.6, 26.5, 26.4, 26.3, 23.4, 18.7, 17.5, 16.4, 15.6.
	Synthesis of GAA. white powder (1.34 g, yield 87%). HRMS-ESI (+): m/z calcd for C39H62N2O4: 622.4710, found:
	623.4786 [M+H] + ; 1 H NMR (400 MHz, CDCl3, ppm): δ 6.29 (m, 1H, CONH), 6.25 (d, 1H, J =
	16.92 Hz, -CH=CH2), 6.15 (q, 1H, J1= 18.92 Hz, J2 = 9.96 Hz, -CH=CH2), 5.73 (m, 1H, CONH),
	5.61 (s, 1H, 12-H), 5.59 (d, 1H, J = 9.96 Hz, -CH=CH2), 3.14-3.33 (m, 5H, -C=C-CO-NHCH2,
	C-CO-NHCH2, 3-H), 1.37, 1.12, 1.10, 1.00, 0.80 (s, 7 × 3H, 7×CH3); 13 C NMR (100 MHz, CDCl3,
	ppm): δ 200.3, 175.7, 169.7, 165.7, 131.2, 128.2, 125.9, 78.7, 61.9, 55.0, 53.4, 48.4, 45.5, 43.6,
	43.3, 42.2, 39.2, 39.1, 38.9, 38.8, 37.4, 37.1, 32.8, 32.0, 31.4, 31.2, 29.7, 29.6, 29.4, 28.5, 28.1,
	27.3, 26.5, 26.4, 26.1, 25.9, 23.3, 18.7, 17.5, 16.4, 15.6.
	Synthesis of HANH2. To a solution of HA (2.00 g, 7.74 mmol) and N-hydroxysuccinimide (1.20
	g, 10.43 mmol) in 40 mL dry THF, N,N'-dicyclohexylcarbodiimide (2.08 g, 10.08 mmol) in 20 mL
	dry THF was added dropwise at ice bath. The mixture was stirred at room temperature for 10 h
	and the precipitates was removed by filtration. The resultant solution was evaporated to remove
	THF and re-dissolved by ethyl acetate. After washing with water, the organic layer was dried by
	anhydrous Na2SO4 and evaporated under reduced pressure to afford active ester intermediate. This
	intermediate was dissolved by 200 mL dichloromethane (DCM), and then added dropwise to a ). HRMS-ESI (+): m/z calcd for C15H27NS3: 317.1306, found: 318.1382 [M+H] + ; 1 H DCM solution (250 mL) of 1,6-hexylenediamine (18 g, 154.89 mmol) at room temperature. After NMR (400 MHz, CDCl3, ppm): δ 4.15 (s, 2H, -SCH2CN), 3.41 (t, 2H, J = 6.96 Hz, stirring for 18 h, the solvent was removed and the crude was re-dissolved by ethyl acetate, washed -CH2CH2S(C=S)S), 1.72 (m, 2H, -CH2CH2S(C=S)S), 1.5-1.2 (m, 18H, CH3(CH2)9CH2-), 0.88 (t, with water, dried by anhydrous Na2SO4, and evaporated under reduced pressure. The crude 3H, J = 6.64 Hz, CH3(CH2)9CH2-); 13 C NMR (100 MHz, CDCl3, ppm): δ 218.8, 114.8, 38.0, 31.9, product was purified by chromatography (dichloromethane: methanol = 2:1, v/v) to afford 29.6, 29.5, 29.4, 29.3, 29.1, 28.9, 27.8, 22.7, 21.4, 14.1. HANH2 as a white powder (1.11 g, yield 40%). HRMS-ESI (+): m/z calcd for C21H44N2O2:
	Synthesis of GANH2. To a solution of GA (4.00 g, 8.50 mmol) and N-hydroxysuccinimide (1.27 356.3403, found: 357.3491 [M+H] + ; 1 H NMR (400 MHz, CDCl3, ppm): δ 5.43 (s, 1H, O=C-NH-),
	3.63 (

g, 11.05 mmol) in 30 mL dry THF, N,N'-dicyclohexylcarbodiimide (2.28 g, 11.05 mmol) in 10 mL dry THF was added dropwise at ice bath. t, 2H, J = 6.64 Hz, OH-CH2-), 3.21-3.26 (m, 2H, O=C-NHCH2-), 2.68 (t, 2H, J = 6.80 Hz, NH2CH2CH2-), 2.14 (t, 2H, J = 7.40 Hz, O=C-CH2-);

13

C NMR (100 MHz, CDCl3, ppm): δ 173.2, 63.1, 37.1, 33.8, 33.0, 29.8, 29.7, 29.6, 29.5, 29.4, 26.9, 26.7, 26.0, 25.9.

Acknowledgment

This work is supported by National Key R&D Program of China (no. 2017YFD0200302) and NSFC (no. 21604085).

Supporting Information

Tunable UCST thermoresponsive copolymers based on natural glycyrrhetinic acid Yiran Zhao a , Jingyao Ma a , Xia Yu a , Min-Hui Li a,b,* , Jun Hu a,* a Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China b Chimie ParisTech, PSL University Paris, CNRS, Institut de Recherche de ChimieParis, 75005 Paris, France Email: jhu@mail.buct.edu.cn; min-hui.li@chimieparistech.psl.eu 19. Fig. S11. 13

S4

Materials.

Glycyrrhetinic acid (GA), 15-hydroxypentadecanoic acid (HA), N-hydroxysuccinimide (NHS), N,N'-dicyclohexylcarbodiimide (DCC), 1,6-hexylenediamine, acryloyl chloride, triethylamine, potassium tert-butoxide, 1-dodecanethiol, carbon disulfide, chloroacetonitrile, α-/β-/γ-cyclodextrin (CD) and other reagents were local commercial products and used as received. 2,2'-Azobis(isobutyronitrile) (AIBN) was recrystallized twice from methanol. Acrylamide (AAm) was recrystallized twice from chloroform and stored at -10 o C. Methyl methacrylate (MMA) was passed through a neutral alumina column to remove the inhibitor and stored at -10 o C. Milli-Q water was used throughout the experiments. Molecular weight and molecular weight distribution (Mw/Mn) of P(AAm-co-GAA-x%) (x = 2, 3, 4) were determined on a gel permeation chromatograph (PL-GPC 50 Plus, Varian) with DMSO containing 0.05 M LiBr as eluent (flow rate: 0.6 mL/min, at 50 o C) against dextran standards. Molecular weight and Mw/Mn of P(MMA-co-GAA-3%) were analyzed by using a Shimadzu gel permeation chromatography system equipped with a SIL-20A autosampler, a refractive index detector, three Shodex KF-805L columns (8 × 300 mm 2 , 10 μm, 5000 Å), and one Shodex KF-801 column (8 × 300 mm 2 , 6 μm, 50 Å) using DMAC containing 0.05 M LiCl as the eluent (flow rate: 1.0 mL/min, at 50 °C) against polystyrene standards.

Methods

Phase transition temperature was determined on a Jasco ETC-505S UV-vis spectrophotometer equipped with a Jasco ETC-505T temperature controller. Polymer aqueous solutions were normally heated and cooled at a rate of 1.0 o C/min. The phase transition temperature was taken as the 50% transmittance change at 450 nm.

Dynamic light scattering (DLS) measurements were performed at different temperatures using Zetasizer Nano-ZS90 instrument equipped with a monochromatic coherent He-Ne laser (633 nm).

Transmission electron microscopy (TEM) images were obtained using Hitachi HT7700 with a maximum accelerating voltage of 120 kV. The samples were prepared as follows: 1) for the sample at 20 o C, a drop of polymer solution (15 mg/mL) at 20 o C was dropped on 300-mesh carbon-coated copper grids which was placed on a hot stage of 20 o C, followed by the removal of water through filter paper after 30 s; 2) for the sample at 40 o C, a drop of polymer solution (15 mg/mL) at 40 o C was dropped on 300-mesh carbon-coated copper grids which was placed on a hot stage of 40 o C, followed by the removal of water through filter paper after 30 s.

Synthesis of HAA.

To a solution of HANH2 (530 mg, 1.49 mmol) and triethylamine (0.27 mL, 1.93 mmol) in DCM (120 mL), acryloyl chloride (0.12 mL, 1.49 mmol) in DCM (5 mL) was added dropwise at ice bath. The mixture was stirred at room temperature for 6 h and then the resultant solution was evaporated to remove DCM. The crude was re-dissolved by ethyl acetate, washed with water, and dried by anhydrous Na2SO4. After removing the solvent, the crude further was purified by chromatography (dichloromethane: methanol = 10:1, v/v) to afford HAA as a white powder (0.35 g, yield 57%). HRMS-ESI (+): m/z calcd for C24H46N2O3: 410.3508, found: 411.3605 [M+H] + ; 1 H NMR (400 MHz, d6-DMSO, ppm): δ 8.04, 7.70 (s, 2 × 1H, -CONH), 6.19 (q, 1H, J1= 17.08 Hz, J2 = 10.04 Hz, -CH=CH2), 6.03 (q, 1H, J1= 17.08 Hz, J2 = 2.32 Hz, -CH=CH2), 5.54 (q, 1H, J1= 10.04 Hz, J2 = 2.32 Hz, -CH=CH2), 3.36 (t, 2H, J = 6.48 Hz, OH-CH2-), 3.09, 3.00 (m, 2 × 2H, -CONHCH2-), 2.02 (t, 2H, J = 7.28 Hz, -CH2-CONHCH2-); 13 C NMR (100 MHz, CDCl3/CD3OD, v/v = 15:1, ppm): δ 174.3, 166.4, 130.8, 126.3, 62.7, 39.0, 36.7, 32.6, 29.6, 29.5, 29.3, 29.2, 29.0, 25.9, 25.8.

Polymerization of P(AAm-co-GAA-x%) (x = 2, 3, 4), P(AAm-co-HAA-3%), and P(MMA-co-GAA-3%). Copolymerization was conducted in DMSO at 70 o C with a [comonomers]/[CMDT]/[AIBN] = 340:1:0.2. For example, GAA (80 mg, 0.13 mmol), AAm (445 mg, 6.27 mmol), CMDT (6.0 mg, 0.019 mmol) and AIBN (0.6 mg, 0.004 mmol) were dissolved in 1.5 mL DMSO in Schlenk tube, and the mixture was purged with Ar for 40 min prior to its immersion in a preheated oil bath. The copolymerization was allowed to proceed for 20 h before being quenched by immersion into ice water. After that, 5 mL DMSO was added to dissolve the copolymer, and the precipitate was collected after pouring the mixture into 200 mL methanol. The copolymer was dried in vacuo to yield 466 mg of P(AAm-co-GAA-2%). The polymers with different GA contents, P(AAm-co-HAA-3%), and P(MMA-co-GAA-3%) were synthesized using the similar method. The content of HAA units in P(AAm-co-HAA-3%) was calculated from the proton integration ratio between the peaks at 2.98-3.02 ppm (Hd and He) versus backone protons at 2.10 ppm (Hb and Hc). In addition, the molar mass of P(AAm-co-HAA-3%) was determined by the integral values of protons in HAA (Hd and He), protons in AAm (Hc), and protons of end groups in the main chain (Ha, 0.85 ppm). The molar mass was calculated as follows:


Hao Zhang

Jie Hao

Xia Yu

Jianqiao Wu

Jiawei Li

Min-Hui Li

Jun Hu

A Simple PVA/Cu(OAc) 2 Thermogel with Inherent Near-Infrared Light Response and Its Applications in Smart Window and Photoresistor

A simple PVA/Cu(OAc) 2 thermogel with an inherent near-infrared light response and its applications in smart windows and photoresistors

Introduction

As one of the most promising smart materials, stimuliresponsive hydrogels are characterized by their abilities to respond to external stimuli like temperature, 1,2 ions strength, 3,4 light, 5,6 pH, 7,8 electric/magnetic field [9][10][11][12] and chemical composition. 13,14 Among them, thermogel has attracted considerable interest because it can undergo physical sol-gel transition once temperature increases over critical point. [15][16][17] At present, the most frequently adopted strategy to construct thermogel is to incorporate the thermosensitive polymers into the system. For example, poly(N-isopropylacrylamide)-derived copolymers have been widely used toward the fabrication of thermogels because they exhibit lower critical solution temperatures (LCST); [18][19][20] aqueous solutions of amphiphilic block copolymers consisting of hydrophilic poly(ethylene glycol) and hydrophobic poly(propylene oxide), poly(glycolide), poly(lactide), or poly(ε-caprolactone) can self-assemble into polymeric micelles upon heating, which also induce the reversible sol-gel transition. [21][22][23] Nevertheless, this approach has two intrinsic drawbacks: (1) it often requires the tedious synthesis of polymers which is time consuming; and (2) the solgel transition is hardly induced by remote stimuli like light, electricity, and magnetism, which makes it difficult to control the properties of thermogel accurately and remotely.

In order to achieve the controllability by remote stimuli, one of the feasible approaches is to introduce additives with inherent energy transformation abilities into polymeric matrices as they can convert remote energy into local heat. [24][25][26][27] Particularly, near-infrared (NIR) light-induced photothermal conversion reagents such as metal nanoparticles, 28,29 carbon nanotubes, 30,31 graphene, 32,33 and Fe3O4 nanoparticles 34,35 have attracted much attention, because NIR light can be regulated remotely and easily by adjustable wavelength and intensity. 24,36- 38 For example, Liu and co-workers developed a hybrid hydrogel via doping 131 I-labeled copper sulfide (CuS/ 131 I) nanoparticles, which allowed in vivo gelation remotely by NIR light with an excellent photothermal conversion performance. 39 Similarly, Zhang et al. have prepared an analogous hydrogel for tumor therapy by using TiO2@MWCNT nanocomposite as the photoinitiator and photosensitizer-photothermal agent. 40 However, due to the poor solubility of photothermal conversion reagents in polymeric matrices, this physical doping strategy suffers from a dilemma that the increase of doping percentages normally causes the phase segregation, whereas lowering the doping percentages would reduce the photothermal conversion efficiency. Thus, the fabrication of a thermogel with inherent NIR light response without the tedious polymer synthesis still remains a challenge.

Herein, we discovered a novel NIR light-responsive thermogel by simply mixing hydrophilic polyvinyl alcohol (PVA) and copper acetate [Cu(OAc)2] in deionized water (Scheme 1). Upon irradiation of NIR light (808 nm), the blue PVA/Cu(OAc)2 [41][42][43][44] Consequently, these clathrate complexes gradually formed the three-dimensional (3D) thermogel network. On account of the absorption of NIR light and the intrinsic temperature sensitivity, this PVA/Cu(OAc)2 thermogel showed a promising application in the fields of smart window and photoresistor (Scheme 1). Notably, different from other reported PVA-based hydrogels which either required the tedious modification of PVA or the extra harsh basic condition, [45][46][47] our PVA/Cu(OAc)2 thermogel was induced by NIR light directly without additional chemical modification of PVA or incorporation of extra reagents. In this system, Cu 2+ acted as a photothermal reagent and a crosslinker simultaneously. To our best knowledge, this is the first PVA-tailored inherent NIR lightresponsive thermogel without the tedious chemical synthesis.

Experimental section

Materials

Polyvinyl alcohol (PVA, Mw: 67,000 Da, hydrolysis degree: 87-89%,) and copper acetate monohydrate [Cu(OAc)2•H2O, AR, 99%] were purchased from Shanghai Aladdin Bio-Chem Technology Co. Other reagents were purchased and used without further purification.

Fabrication of PVA/Cu(OAc)2 window

Firstly, a hollow double-layer PMMA window with a gap of 5 mm was prepared by using PMMA plates with a thickness of 5 mm. After that, the PVA/Cu(OAc)2 solution was injected and sealed by glue to afford a PVA/Cu(OAc)2 window. Similarly, the Cu(OAc)2 window as a control was prepared by sealing pure Cu(OAc)2 solution in the same way.

Fabrication of PVA/Cu(OAc)2 photoresistor

Firstly, one end of a transparent PMMA tube (length 5 mm, inner diameter 3 mm) was sealed with a metal wire, and then the PVA/Cu(OAc)2 solution was injected from the other end of the tube. After sealing with the same metal wire, the photoresistor was obtained.

Characterization methods

UV-vis spectra were recorded on a G-9 UV-vis spectrophotometer (Nanjing PKILES Instrument Co., Ltd) from 190 to 1100 nm. The test specimens were kept at corresponding temperature (25 and 80°C) for 5 min before testing. The optical path of quartz cuvette was 1 cm. Electron spin resonance (ESR) spectrometer (JES-FA200, JEOL Ltd.) with a 100 kHz field modulation was used to measure ESR spectra of PVA/Cu(OAc)2 and pure Cu(OAc)2 solutions at 25 and 80°C, respectively. All ESR samples were kept at the testing temperature for 5 min before testing. The center field of magnet field was 300 mT. The sweep time and sweep width (+/-) were 5 min and 150 mT, respectively.

Temperature-dependent 1 H NMR spectra were recorded on a Bruker AV400 spectrometer (400 MHz) operating at the temperature from 25 to 80°C. D2O was used as solvent, and DMSO-d6 was added as an internal standard.

Hydrodynamic sizes of samples were investigated by dynamic light scattering (DLS) on a Zeta sizer Nano-ZS90 (Malvern Instruments, UK). The concentration of PVA in all samples was 1.0 wt%. Molar ratio of Cu 2+ to hydroxyl was 3/20.

Morphologies were studied by scanning electron microscopy (SEM, JSM-7610F with 5 kV acceleration voltage) and transmission electron microscopy (TEM, Hitachi HC-1 with 80 kV accelerating voltage). To obtain SEM samples of PVA/Cu(OAc)2 thermogel, 2 mL of PVA/Cu(OAc)2 solution was first placed in an oven at 80°C for 10 min to promote the formation of thermogel, and then quickly transferred into to liquid nitrogen for rapid freezing. After that, the frozen sample was freeze-dried by a freeze dryer. The sample was coated with gold before the test. For preparing TEM samples, the original solution was diluted 50 Scheme 1 Schematic illustration of the sol-gel transition of PVA/Cu(OAc)2 thermogel and its applications in smart window and photoresistor.

Please do not adjust margins Please do not adjust margins times with deionized water and heated at 80°C for 15 min. Then, the liquid was adsorbed on copper grids in an oven at 80°C until it was completely dry.

The pH variation of the solution was recorded with a pH meter (PHS-3E, INESA Analytical Instrument Co., Ltd) at the range of 25 to 80°C. Before the test, the solution (concentration of PVA was 10 wt% and the molar ratio of hydroxyl group/metal ion was 20/3) was diluted 2 times with deionized water, and then 40 mL of the diluent was taken and placed into a beaker. The solution was heated by a magnetic agitator (Dragon Laboratory Instruments Ltd.).

Viscoelasticity was explored by rheological testing via a Thermo HAAKE MARS 60 with a 16 mm coaxial cylinder in the oscillation mode. The temperature sweep was performed from 25 to 80°C (3 °C min -1 ) at a frequency of 1 Hz and a strain of 5%. Dynamic temperature sweep rheological measurements were measured from 25 to 80°C to gain insights into the reversible sol-gel transition of PVA/Cu(OAc)2 thermogel. Storage modulus (G') and loss modulus (G'') were recorded during the cyclic temperature changes between 25 and 80°C at a strain of 5% and a frequency of 1 Hz.

Nitrogen physisorption measurements were carried out on a Micromeritics ASAP 2020 apparatus. Before measurements, the freeze-dried samples were degassed at 60°C under vacuum. The porosity was determined from the adsorption isotherms by the Brunauer-Emmett-Teller equation.

The simulated sunlight irradiation was provided by a Solar Simulator (SS-F5-3A, Enlitech). A thermocouple thermometer (UT320A, UNI-T) with a K-type thermocouple sensor was used to monitor simulated indoor temperature. The top surface temperature of window was recorded by IR camera (FLIR ONE Pro).

The electrical resistance was measured by using a digital display multimeter (B35T+, OWON) at a working voltage of 3 V. The relative resistance changes of PVA/Cu(OAc) 2 solution was calculated using ∆R = R -R0, where R and R0 represented the real-time resistance and the initial resistance, respectively.

Results and discussion

NIR light-induced PVA/Cu(OAc)2 thermogel

Initially, PVA was added into deionized water and continuously stirred at 95°C for 2 h to give a homogeneous transparent solution (10 wt%). After cooling to room temperature, Cu(OAc)2 was added and then a blue solution formed (hydroxyl group/cooper ion = 20/3, molar ratio). Upon remote irradiation by NIR light device (VCL808N1-4.0W) for 300 s, a stable green thermogel of PVA/Cu(OAc)2 was obtained with a rapid rise of temperature from 27 to 80°C in Fig. 1a. Moreover, this green thermogel gradually returned to its initial blue sol-state again in absence of irradiation. Notably, the direct heating above 70°C also resulted in the formation of PVA/Cu(OAc)2 thermogel as shown in Fig. S1. After 100 cycles of heating/cooling, PVA/Cu(OAc)2 thermogel could still recover to its initial state with the good thermal reversibility (Fig. S2). In order to illustrate the importance of Cu 2+ in the photothermal conversion performance, other metal acetates (Li + , Na + , Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ ) with equal molar number of Cu(OAc)2 were added into PVA solution, respectively. Their temperature variations after NIR irradiation were recorded by IR camera (FLIR ONE Pro), as shown in Fig. 1b andS3. Compared with the other PVA/metal acetates, only PVA/Cu(OAc)2 formed the thermogel with a generation of the largest temperature increase from 33.3 to 76.4°C, which indicated that Cu 2+ played an irreplaceable role as the photothermal conversion reagent and crosslinker. UV-vis spectra in Fig. 1c revealed that PVA/Cu(OAc)2 showed a broad absorption feature in NIR window from 650 to 1100 nm, opposite the negligible absorption for the other PVA/metal acetates. It is known that d orbit of Cu 2+ can split into two different degenerate orbits under a specific ligand field, namely eg and t2g orbit. Once the energy gap between eg and t2g (splitting energy) matches the energy of NIR region, the photothermal conversion occurs. 48 For PVA/Cu(OAc)2, the Cu 2+ complex was able to absorb NIR light to induce the electron transition between eg and t2g orbits, and then the electrons in the excited state could be restored to the ground state by non-radiative decay process, thus realizing the transformation of light energy into heat and generating the thermogel.

In addition, it was found that not all PVA/Cu(II) anion solutions can form thermogel upon NIR light irradiation. As This journal is © The Royal Society of Chemistry 20xx

Please do not adjust margins Please do not adjust margins shown in Fig. S4, the thermal-induced gelation behavior only occurred in PVA/Cu(OAc)2 solution rather than PVA/Cu(NO3)2, PVA/CuCl2, and PVA/CuSO4 solutions, although these four solutions exhibited the similar temperature change tendency upon NIR light irradiation as shown in Fig. 1d. Note that the unique difference between these PVA/Cu(II) anion solutions was the pH. As shown in Fig. 1e, the pH value of PVA/Cu(OAc)2, PVA/Cu(NO3)2, PVA/CuCl2, and PVA/CuSO4 declined from 5.4, 4.7, 4.6 and 4.0 at 25°C to 4.8, 4.1, 3.8 and 3.6 at 80°C, respectively, while that of pure PVA solution remained unchanged. The decrease of pH can be reasonably explained by the fact that additional H + was produced by the hydrolysis of Cu 2+ with the rise of temperature. Since the pH value of PVA/Cu(OAc)2 was higher than those of other PVA/Cu(II) anion solutions at any given temperature from 25 to 80°C, the failure of PVA/Cu(NO3)2, PVA/CuSO4, and PVA/CuCl2 solutions to form thermogel was mainly attributed to their relative strong acidity.

Microstructure, rheological behavior and photothermal performance of PVA/Cu(OAc)2 thermogel

Scanning electron microscope (SEM) was applied to explore the microstructures of PVA/Cu(OAc)2 thermogel. As shown in Fig. 2a, the dense micro-porous structure was observed in thermogel. It was consistent with the results of Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption isotherms, where a porosity of PVA/Cu(OAc)2 thermogel was calculated to be 37.9 m 2 /g as shown in Fig. 2b.

To further investigate the rheological behavior of sol-gel transition, the temperature-dependent storage modulus (G′) and loss modulus (G′′) of PVA/Cu(OAc)2 solution were measured. As shown in Fig. 2c, the plot of G′′ remained higher than that of G′ from 25 to 70°C, which accorded with sol-state property. The slight decline of G′ and G′′ from 25 to 45°C was caused by the fact that the gelation force at low temperature was insufficient to offset the negative effect on viscosity arised from temperature rising. From 55°C, G′ and G′′ began to increase until they coincided with each other at around 70°C, indicating the appearance of sol-gel transition. At 80°C, G′ was in the order of 10 2 Pa, higher than G′′ (10 1 Pa), corresponding to the gel-state of PVA/Cu(OAc)2. Dynamic temperature sweep rheological measurements in Fig. 2d showed that after four cycle tests between 25 and 80°C, the system was still stable and kept its original rheological properties, further confirming that PVA/Cu(OAc)2 owned excellent and repeatable sol-gel transition performance.

For assessing the photothermal conversion capability of PVA/Cu(OAc)2, conversion efficiency η was quantified by eq (1): [49][50][51] ƞ = hA(∆T max, mix -∆T max, water )

I(1-10 -A λ ) (1)
In this equation, ΔTmax,mix and ∆Tmax,water are the maximum temperature variation of PVA/Cu(OAc)2 solution and water, respectively. I is the laser power and Aλ is the absorbance of PVA/Cu(OAc)2 solution at 808 nm. h is the heat transfer coefficient and A is the surface area of the container. Unknown hA is calculated from eq (2):

𝜏𝜏 𝑠𝑠

= m D c D ℎ𝐴𝐴 ( 2
)
where mD is the mass of water in PVA/Cu(OAc)2 solution, and cD is the heat capacity of water. By making a linear fit of cooling time (t) and -ln(ΔTt, mix/∆Tmax,mix), time constant τs can be obtained as the curve slope.

According to the temperature curve of PVA/Cu(OAc)2 solution in Fig. 2e, τs was determined to be 275 s with R 2 = 0.994 (inset). Given the mass of water (mD) was 0.5 g and the heat capacity of water (cD) was 4.2 J g -1 K -1 , hA was calculated to be 7.63 × 10 -3 W K -1 by using eq (2). In this case, the η of PVA/Cu(OAc)2 solution was quantified to be 37.8% according to eq (1), where ΔTmax,mix value was 59.8°C, ∆Tmax,water value was 2.7°C (Fig. S5), and Aλ was 0.638 determined from UV-vis spectrum (Fig. 1c). Note that this η value was higher than other reported photothermal agents like Au nanorods (21%), 52 Cu2-xSe (22%), 53 and Cu9S5 (26%). 54 Furthermore, the photothermal stability of PVA/Cu(OAc)2 was evaluated by the repeated irradiation on/off cycles. As shown in Fig. 2f, no change in the thermal generation ability of PVA/Cu(OAc)2 was observed within three on/off processes, implying a good photothermal stability. Obviously, PVA/Cu(OAc)2 possessed a great capability to convert NIR irradiation to heat, which easily facilitated the remote control of reversible sol-gel transition.

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Mechanism of NIR light-induced PVA/Cu(OAc)2 thermogel

From the standpoint of coordination chemistry, it seems unlikely that aliphatic hydroxyl groups deprotonate at pH < 7 and coordinate with free Cu 2+ , because of its high pKa ∼16. 41 For PVA/Cu(OAc)2 solution, its pH value was around 5.4 and would be further decreased with the rise of temperature as shown in Fig. 1e. Therefore, the coordination between free Cu 2+ and hydroxyl groups cannot occur in PVA/Cu(OAc)2 solution, which supported the phenomenon that the addition of Cu(OAc)2 into PVA solution did not induce the gel formation at room temperature unless extra strong alkaline reagent was added (Fig. S6).

In order to explore the mechanism in depth, temperaturedependent experiments of dynamic light scattering (DLS), electron spin resonance (ESR), UV-vis, 1 H NMR and complex viscosity were carried out. A large number of light-blue suspended particles formed in diluted pure Cu(OAc)2 at 80°C, and gradually turned into black with the prolongation of heating time (Fig. S7a). During this process, its average particle diameter determined by DLS increased from 50 to 240 nm (Fig. 3a). Since the high temperature can accelerate the hydrolysis of Cu 2+ into Cu(OH)2, it was assumed that these particles should be Cu(OH)2 crystal clusters, which subsequently thermally decomposed to the black copper oxide. Conversely, no precipitation was observed in PVA/Cu(OAc)2 solution except the color change from blue to green upon heating at 80°C (Fig. S7b). Once cooling to the room temperature, it could further return to its initial blue again. Their average particle sizes ranged from 10 to 40 nm at the corresponding temperatures, much smaller than that in pure Cu(OAc)2 solution, and the transition was reversible (Fig. 3a). This process could be directly revealed by TEM images. As shown in Fig. 3b, a large number of clathrate complexes formed in PVA/Cu(OAc)2 solution, whereas only film structures and needle-shaped assemblies were observed for PVA and Cu(OAc)2 solutions, respectively (Fig. S8). The above results strongly indicated that PVA can stabilize and co-aggregate with Cu(OH)2 clusters, and further prevent the formation of larger aggregates.

More evidence came from ESR spectroscopy in Fig. 3c. With temperature rising from 25 to 80°C, a decrease in ESR intensity was observed for Cu(OAc)2 solution (brown and purple dashed lines), which is due to the reduction of Cu 2+ caused by the aggravation of hydrolysis. Nevertheless, the addition of PVA into Cu(OAc)2 solution at 25°C also resulted in a slight decrease in ESR intensity (blue solid line), which further declined to a lower level at 80°C (red solid line). Given that no coordination between hydroxyl group and free Cu 2+ existed in PVA/Cu(OAc)2 solution, this slight change was probably due to PVA bonded with trace Cu(OH)2 in PVA/Cu(OAc)2 solution at 25°C, thus promoting the hydrolysis reaction and the decrease of Cu 2+ .

In addition, UV-vis and 1 H NMR spectra were further used to demonstrate the interaction between PVA and Cu(OH)2 clusters. Please do not adjust margins Please do not adjust margins As shown in Fig. 3d, Cu(OAc)2 solution showed a broad absorption peak at 770 nm due to the d-d electron transition of hydrated Cu 2+ (brown dashed line), and red-shifted to 773 nm resulting from the formation of Cu(OH)2 clusters at 80°C (purple dashed line). However, upon addition of PVA, the absorption peak underwent a slight blue-shift from 770 to 766 nm (blue solid line), and further blue-shifted to 758 nm at 80°C (red solid line). This implied that there was an interaction between PVA and Cu(OH)2 clusters which caused the environment change around Cu(OH)2 clusters. Through a continuous heating process from 25 to 80°C, the absorption peak in PVA/Cu(OAc)2 solution showed a continuous blue-shift and would return to the initial position after cooling (Fig. S9), which illustrated that the interaction between PVA and Cu(OH)2 clusters was reversible and heavily dependent on temperature. Moreover, the interaction also can be reflected from the results of 1 H NMR. As shown in Fig. 3e and Fig. S10, proton peaks of methine (CH-) and methylene (CH2-) in PVA chains broadened either with increasing the temperature or the molar ratio of Cu 2+ /hydroxyl group. On the contrary, proton peaks of pure PVA solution became gradually sharped with the raise of temperature (Fig. S11). It is known that the broadening of proton peaks implies that insoluble polymer segments form or polymer chains are kinetically limited in solution, as 1 H NMR signals arise only from protons in a liquid-like fraction. 45 Therefore, it was deduced that the generated Cu(OH)2 clusters can be surrounded by PVA hydrophobic backbones to form a green clathrate complexes [Cu(OH)2-PVA] via hydrophobic forces, similar to the report by Yokoi. 41,42 In this case, PVA chains fixed by Cu(OH)2 clusters had undetectable 1 H NMR signals while Cu(OH)2 clusters surrounded by PVA chains were difficult to aggregate into larger-sized precipitates. This deduction can be further confirmed by temperature-dependent viscosity of PVA/Cu(OAc)2 solution from 25 to 80°C. As shown in Fig. 3f, the viscosity underwent a slight decrease from 25 to 45°C due to the weak interactions between PVA and Cu(OH)2 clusters, whereas began to rise dramatically after 45°C, which revealed the monotonic enhancement of hydrophobic interactions between PVA and Cu(OH)2 clusters as a function of the concentration for Cu(OH)2 clusters.

Taking all above results together, the formation process of PVA/Cu(OAc)2 thermogel was proposed in Fig. 3g. Initially, upon addition of Cu(OAc)2, PVA could stabilize the trace hydrolyzed Cu(OH)2 clusters in PVA/Cu(OAc)2 solution at room temperature. But because of the low degree of hydrolysis, the amount of Cu(OH)2 clusters was insufficient to form strong hydrophobic interactions with PVA chains, which would not induce significant changes in the solution state (Fig. 3g1 and3g2). Subsequently, NIR light irradiation induced the rise of solution temperature that accelerated the hydrolysis of Cu 2+ with generating more Cu(OH)2 clusters. These clusters were surrounded by PVA hydrophobic backbones to form a green clathrate complexes [Cu(OH)2-PVA] via hydrophobic forces, where the hydrophilic hydroxyl groups in PVA chains toward the outside and Cu(OH)2 clusters located in the center of the tangled chains. Consequently, the 3D cross-linked network of thermogel formed (Fig. 3g2 and3g3). While, once NIR irradiation for PVA/Cu(OAc)2 solution was removed, the reverse reaction of Cu 2+ hydrolysis would be enhanced, which gave rise to the dissolution of Cu(OH)2 clusters. As a result, the PVA chains fixed by Cu(OH)2 clusters previously were restored to the original free state, resulting in the collapse of gel structures (Fig. 3g3 and3g2).

Application on smart window and photoresistor.

Considering that PVA/Cu(OAc)2 solution possessed high absorption performance in UV and NIR light range (190-380 nm and 650-1100 nm) and allowed for the high visible light transmission in the visible region as shown in Fig. 4a, its potential application as the smart window material was investigated. With the temperature rising from 25 to 80°C, the light transmittance of PVA/Cu(OAc)2 solution dramatically declined from 85% to 32% at 485 nm with the gradual color change from blue to green (Fig. 4a andS12). Moreover, the higher the solution temperature, the faster the color change. As shown in Fig. 4b, the light transmittance of PVA/Cu(OAc)2 solution at 80°C sharply decreased from 85% to 32% within 300 s, strongly revealing the good optical conversion performance of PVA/Cu(OAc)2. In order to simulate the effect of PVA/Cu(OAc)2 to reduce the indoor temperature under sunlight, a PVA/Cu(OAc)2 window was made by injecting PVA/Cu(OAc)2 solution into a hollow double-layer PMMA window (Fig. S13), and then installed it on a model house as shown in Fig. 4c, where a thermocouple thermometer was used to monitor the change of indoor temperature upon sunlight irradiation. As shown in Fig. 4d, after irradiating for 3600 s, the steady-state indoor temperature with PMMA window was 53.0°C because of its high light transmittance, while 36.9°C for Cu(OAc)2 window. Conversely, the steady-state indoor temperature with PVA/Cu(OAc)2 window was much lower (∼33.5°C) accompanying with a top surface temperature around 54.3°C

Please do not adjust margins Please do not adjust margins (Fig. S14), which was contributed to its NIR absorption property as well as the decrease in transmittance. The above results indicated the PVA/Cu(OAc)2 window with high NIR shielding and reversible optical conversion property can reduce the indoor temperature effectively under sunlight irradiation by blocking the sunlight both in the NIR and visible region.

Besides, benefiting from intrinsic conductive quality of free Cu 2+ and photothermal ability of PVA/Cu(OAc)2 solution, the conductivity of PVA/Cu(OAc)2 solution was deeply affected by sunlight irradiation. As shown in Fig. 5a, the conductivity of PVA/Cu(OAc)2 solution reached 2.84 mS/cm at 70°C from 2.18 mS/cm at 25°C, revealing that a higher temperature of solution corresponding to a lower resistivity due to the accelerated migration rate of Cu 2+ with the increase of temperature. Based on this change, a PVA/Cu(OAc)2 photoresistor was fabricated by encapsulating PVA/Cu(OAc)2 solution into a PMMA tube and then connected to a circuit for adjusting the current (Fig. S15). When the photoresistor was irradiated by sunlight, the current rapidly flowed across the PVA/Cu(OAc)2 solution and the lightemitting diode (LED) gradually brightened (Fig. 5b). In addition, the photoresistor was connected with a multimeter to record the changes of resistance upon sunlight irradiation (1000 W m - 2 ). As shown in Figure 5c, the resistance signal of the PVA/Cu(OAc)2 photoresistor showed repeatable variations during the treatment with irradiation on/off cycles, which exhibited a great potential application prospect in optical control devices.

Conclusions

In summary, we have prepared a novel PVA/Cu(OAc)2 thermogel with inherent NIR light response via a simple yet cost-effective method. At room temperature, PVA/Cu(OAc)2 existed in a blue sol-state, while gradually changed into a green gel-state upon NIR light irradiation. During this process, the clathrate complexes [Cu(OH)2-PVA] were formed mainly promoted by the hydrophobic interactions between Cu(OH)2 clusters and PVA chains, where the hydrophilic hydroxyl groups in PVA chains toward the outside and Cu(OH)2 clusters located in the center of the tangled chains, thus resulting in the sol-gel transition. Because of the good inherent photothermal conversion (∼37.8%) and high photothermal stability of PVA/Cu(OAc)2 thermogel, its potential applications have been explored in the fields of smart window and photoresistor. The results showed that the lab-made PVA/Cu(OAc)2 window was able to effectively maintain the indoor temperature below 35°C under sunlight irradiation, while the as-prepared PVA/Cu(OAc)2 photoresistor exhibited a sensitive and repeatable responses to sunlight irradiation. In brief, our simple PVA/Cu(OAc)2 thermogel with inherent NIR light response will be a promising candidate for the future manufacture industry of smart materials. This journal is © The Royal Society of Chemistry 20xx

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Fig. 1

 1 Fig. 1 (a) Thermal images of PVA/Cu(OAc)2 solution treated with NIR light irradiation. (b) Photothermal effect of different PVA/metal acetate solutions upon NIR light irradiation. (c) UV-vis spectra of different PVA/metal acetate solutions at room temperature. (d) Temperature variation of different PVA/Cu(II) anion solutions after NIR light irradiation for 420 s. (e) The pH plots of different PVA/Cu(II) anion solutions from 25 to 80°C. Concentration of PVA was 10 wt% in (a), (b), and (d), while 1.25 wt% in (c) and 5 wt% in (e). Molar ratio of hydroxyl group/metal ion was 20/3. Wavelength of NIR light was 808 nm. Laser power was 1.0 W.

Fig. 2

 2 Fig. 2 (a) SEM image and (b) BET nitrogen adsorption-desorption isotherm of PVA/Cu(OAc)2 thermogel after freeze-drying. (c) Temperature-dependent G′ and G′′ of PVA/Cu(OAc)2 solution from 25 to 80°C with a heating rate of 3 °C min -1 . Frequency = 1 Hz and strain = 5%. (d) G′ and G′′ of PVA/Cu(OAc)2 solution during heating-cooling cycles between 25 and 80°C with a heating/cooling rate of 3 °C min -1 . Frequency = 1 Hz and strain = 5%. (e) Temperature variation of PVA/Cu(OAc)2 solution after irradiating for 660 s (red points) followed by a cooling period from 660 to 1380 s (blue points). Inset showed the time constant (τs) for heat transfer, determined by the linear time-dependent data in the cooling period. (f) Temperature curves of PVA/Cu(OAc)2 solution with repeated irradiation on/off cycles. Concentration of PVA was 10 wt% in (a), (b), (c), and (d), while 1.25 wt% in (e) and (f). Molar ratio of hydroxyl group/metal ion was 20/3. Wavelength of NIR light was 808 nm. Laser power was 1.5 W.

Fig. 3

 3 Fig. 3 (a) DLS profiles of Cu(OAc)2 and PVA/Cu(OAc)2 solutions at different temperatures.(b) TEM image of PVA/Cu(OAc)2 after heating at 80°C for 15 min. (c) ESR spectra of Cu(OAc)2 and PVA/Cu(OAc)2 solutions at different temperature. (d) UV-vis spectra of Cu(OAc)2 and PVA/Cu(OAc)2 solutions at 25 and 80°C. (e) Temperature-dependent 1 H NMR spectra (400 MHz, D2O) of PVA/Cu(OAc)2 solution with a molar ratio of 20/3. * and ∆ represent D2O and DMSO-d6, respectively. (f) Temperature-dependent complex viscosity of PVA/Cu(OAc)2 solution. (g) Schematic illustration for the formation of PVA/Cu(OAc)2 thermogel. (g1) PVA solution, (g2) PVA/Cu(OAc)2 solution, and (g3) PVA/Cu(OAc)2 thermogel.

Fig. 4

 4 Fig. 4 (a) Transmission spectra of PVA/Cu(OAc)2 solution at 25 and 80°C. Insets are digital photos of PVA/Cu(OAc)2 solution at 25°C (left) and 80°C (right). (b) Time-dependent transmittance of PVA/Cu(OAc)2 solution at 485 nm under different temperatures. (c) Schematic illustration of the lab-made model house used for the measurement of indoor temperature upon sunlight irradiation. (d) Time-dependent indoor temperature in model house covered by PMMA window, Cu(OAc)2 window and PVA/Cu(OAc)2 window upon one sun irradiation (1000 W m -2 ).

Fig. 5

 5 Fig. 5 (a) Conductivity of PVA/Cu(OAc)2 solution at different temperatures. (b) Photographs of an LED light connected with PVA/Cu(OAc)2 photoresistor upon sunlight irradiation. (c) Variation in the resistivity of PVA/Cu(OAc)2 photoresistor after repeated sunlight irradiation on/off cycles.

Fig. S1

 S1 Fig. S1 Optical photographs of PVA/Cu(OAc)2 thermogel at 70, 80, 90 and 100 o C, respectively, as

Fig. S2

 S2 Fig. S2 Optical photographs of gel-sol transitions of PVA/Cu(OAc)2 upon 100 heating/cooling

Fig. S4

 S4 Fig. S4 Optical photographs of PVA/Cu(OAc)2, PVA/Cu(NO3)2, PVA/CuCl2, and PVA/CuSO4

Fig. S6

 S6 Fig. S6 Optical photograph of PVA/Cu(OAc)2 hydrogel with addition of NaOH (concentration was

Fig. S7

 S7 Fig. S7 Optical photographs of (a) Cu(OAc)2 and (b) PVA/Cu(OAc)2 solution upon heating at 80 o C

Fig. S9

 S9 Fig. S9 Temperature-dependent UV-vis spectra of PVA/Cu(OAc)2 solution (concentration of PVA

Fig. S10 1 H

 1 Fig. S10 1 H NMR spectra (400 MHz, D2O) of PVA/Cu(OAc)2 solution as increasing the molar ratio

Fig. S12

 S12 Fig. S12 Transmittance variation of PVA/Cu(OAc)2 solution as a function of temperature at 485

Fig. S14

 S14 Fig. S14 Time-dependent top surface temperatures of PVA/Cu(OAc)2 window. Inset is the IR

Fig. S15

 S15 Fig. S15 (a) Schematic illustration and (b) photograph of the PVA/Cu(OAc)2 photoresistor

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 3

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 5

This journal is © The Royal Society of Chemistry 20xx J. Name., 2013, 00, 1-3 | 7

Acknowledgements

This work is supported by National Key R&D Program of China (no. 2017YFD0200302) and NSFC (no. 21604085).

Conflicts of interest

There are no conflicts to declare.

Electronic Supplementary

*Corresponding Authors

Email: jhu@mail.buct.edu.cn (Jun Hu)

The supporting information includes 8 pages and 15 figures.


Mostafa Sabra
email: mostafa.sabra@xlim.fr

Baptiste Leconte

Romain Dauliat

Dia Darwich

Tobias Tiess

Anka Schwuchow

Raphaël Jamier

Georges Humbert

Katrin Wondraczek

Matthias Jäger

Raphael Jamier

Philippe Roy

Widely tunable Q-switched dual-wavelength synchronous-pulsed Tm-doped fiber laser emitting in the 2 µm region

teaching and research institutions in France or abroad, or from public or private research centers.

The development of tunable dual-wavelength laser sources has recently attracted much attention in a broad range of application, such as nonlinear biomedical applications [1], atmospheric sensing [START_REF] Sharma | Laser Applications to Chemical and Environmental Analysis[END_REF], spectroscopic measurements [START_REF] Gaulton | [END_REF] and distance measurements [4]. Such sources have also drawn some interest for realizing tunable terahertz (THz) sources, by difference frequency generation (DFG) in a nonlinear crystal [5,6,7].

The needs of dual-wavelength laser associating a wide tunability range of the spectral separation between both wavelengths (Dl), a high peak power in the kW range, and narrow spectral laser peaks is thus triggered by these applications. All those properties are simultaneously accessible in the ns-regime by Q-switched laser.

Moreover, since fiber lasers offer numerous advantages compared to solid-state lasers, as excellent beam quality, high conversion efficiency, high average power, high thermal dissipation, and thermal stability, it stands out as an ideal candidate. Dualwavelength fiber laser with fixed Dl have been realized by using two fiber lasers based on a pair of fiber Bragg gratings (FBGs) [8], or by associating two single-frequency fiber lasers [9].

A dual-wavelength fiber laser with a Dl tunability of 25 nm, around the wavelength of 1.07 µm, has been recently demonstrated by Tiess et al. [10]. This system is based on a delay line composed of a discretely chirped FBG array inserted in a theta cavity configuration. An AOM is used for generating synchronous pulses and selecting a specific couple of wavelengths by controlling the pulses delay in the FBG array. However, as others all-fiber dualwavelength sources, the output power is limited by parasitic nonlinear effects and requires further amplification stages to obtain high peak powers in the kW range.

Laser configurations based on an active fiber as gain medium and free space AOM, offer possibility to broader the tunability range by using free-space components and reduce the parasitic nonlinear effects by using large mode area active-fiber. Kadwani et al. [11] have used a diffraction grating associated to a Tm-doped photonic crystal fiber for demonstrating a broad tunability of 200 nm (around 2 µm) into a single-wavelength laser with a maximum peak power of 6 kW (in nanosecond pulse regime). However, a diffraction grating is not suitable for tunable dual-wavelength laser operation since it is difficult to set two diffraction gratings in cascade. VBG component reflect the beam only at the Bragg wavelength enabling dual-wavelength reflections by cascading two VBGs. Furthermore, VBGs offer the advantages of narrow linewidth reflection, high reflectivity, high damage threshold, low insertion losses, and continuous selection of the reflected wavelength by rotation. Dual-wavelength fiber laser with a tunability range of Dl from 1 nm to 144 nm has recently been demonstrated by cascading a pair of VBGs [12].

In this letter, a widely tunable Q-switched dual-wavelength pulsed fiber laser based on the association of two VBGs, a free space AOM and a rod-type Tm-doped FA-LPF is achieved. The laser emission in the 2 µm range is selected for this demonstration since this domain is more favorable for applications based on nonlinear crystals (as DFG of THz waves [5,7]), due to a larger quantum efficiency in this frequency range. The tuning range of dual wavelength separation from Dl = 1 nm to 120 nm is to the best of our knowledge the widest reported in such a fiber-based laser system. Synchronous pulses with duration of 26 ns and peak power > 8 kW, for a repetition rate of 1 kHz have been generated over the whole tuning range. Such kW peak power level has been made accessible by the first use of a Tm-doped rod-type FA-LPF.

The experimental setup is illustrated in Fig. 1. A novel 62 cm long Tm-doped rod-type FA-LPF, angle cleaved end facet (5°) on the pumping side to avoid parasitic optical feedback, is used. The fiber was placed on a metal plate with thermal paste applied all along the fiber to dissipate the heat. An AOM (Gooch & Housego) controlled by a radio frequency signal was used to modulate the losses in the laser cavity, with > 80% of diffraction efficiency on the first order and a rise time of 40 ns. Two VBGs (VBG1 and VBG2, from OptiGrate Corp., dimensions of 5x5x5 mm) with high reflectivity (>90%), centered at 1984 nm and 1970 nm respectively, were used as spectral filters to select the emitting wavelengths of the laser. The laser cavity is formed on one side by the two VBGs, and on the other side by the 4% Fresnel reflection of the perpendicularly cleaved fiber end facet, corresponding to the output coupler. The fiber was pumped at 790 nm by a multimode fibred laser diode (core diameter of 105 μm and NA of 0.22), providing up to 31 W output power. Two dichroic mirrors with high reflectivity at 2 µm and high transmission at 790 nm, were used to reflect the signal in the direction of the VBGs (while transmitting the pump), and to separate the residual pump power from the output signal. A lens with a focal length of 150 mm is used to properly collimate the beam onto the VBGs, essential to benefit from their maximum reflectivity.

The Tm-doped rod-type FA-LPF has been specially devised to reduce nonlinear disturbances in pulsed operation and to increase the peak power. The design of this original fiber, shown in Fig. 2(a), is composed of a large Tm-doped core (in red) surrounded by an aperiodic pattern of low-index inclusions (in yellow) embedded into an index-matched passively doped silica cladding. This specific pattern ensures the confinement of the fundamental mode into the fiber core and the delocalization of high-order modes, even for very large core diameters [13]. A microscopic image of the fabricated fiber is presented in Fig. 2 First tests have been realized for Dl = 3.8 nm at different Repetition Rate (RR) from 1 kHz to 30 kHz, and at fixed pump power of 26.4 W. The output spectrum, temporal signal and average power of the laser at RR = 1 kHz have been measured respectively with an optical spectrum analyzer (OSA), a photodiode and a power-meter after a wedge, as shown in Fig. 1. The output spectrum is composed of two main laser peaks and two parasitic side peaks, each ones spaced by 3.8 nm (Fig. 3(a)). The linewidth at the full widths at half-maximum (FWHM) of each main laser peak is around 0.3 nm (the resolution of the OSA being 0.05 nm), and the signal contrast to the side lobes is about 21 dB. These parasitic peaks are the signature of the χ3 nonlinear four-wave-mixing (FWM) effect that appears when the phase of both laser peaks are nearly phase matched (i.e. at low Dl) [10,[START_REF] Song | [END_REF]. As shown in Fig. 3(b), the pulse shape is Gaussian with a pulse duration of 25 ns. This temporal trace confirms the synchronization of the dual-wavelength pulses, without competition between the two pulses of the laser. It is worth noting that the distance between VBG2 and the high reflection mirror used to reflect the wavelength of VBG1 is shorter than 15 cm, thus yielding into negligible temporal difference between both pulses (<1 ns).

Intensity (a.u) Time (ns)

The evolution of the output average power and peak power versus the RR is shown in Fig. 4. The peak power was calculated for each RR value, from the measured average power, the pulse width, and by subtracting the ASE power (7 mW) emitted by the Tmdoped rod-type FA-LPF. This former value was obtained by measuring the ASE power emitted by the rod-type fiber, pumped at 26.4 W in absence of seed power. As expected in Q-switched operation, lower RR increases the peak power and reduces the average power due to the high population inversion in the gain medium of the laser [16]. A maximum peak power of 11.9 kW and an average power of 0.3 W was obtained for RR = 1 kHz. The peak power was limited by the saturation of the population inversion for RR<1 kHz. The tunability of Dl was realized by changing the angle between the VBG1 and the signal beam, referring to the Bragg law: 𝜆 (#) = 𝜆 (#%&) cos(θ) Where l(θ=0) is the reflected wavelength at normal incidence (i.e. 1984 nm and 1970 nm, respectively for VBG1 and VBG2), and θ is the angle of incidence. VBG2 was set at normal angle of incidence. VGB1 was rotated to tune the reflected wavelength toward smaller wavelengths down to 1850 nm, leading to a wavelength separation up to 120 nm. By rotating VBG1 the signal beam was reflected with an angle depending of the incidence angle (θ). A high reflection mirror at 2 µm was used and rotated jointly with VBG1 to close the laser cavity over the reflected wavelength (l(θ)). Output spectra obtained at different wavelength separation from Dl = 15 nm to Dl =120 nm are shown in Fig. 5. The signal to noise ratio is higher than 30 dB and the parasitic peaks from FWM effect are suppressed above Dl = 45 nm. It is worth noting that Dl was limited to 120 nm by the power drop due to low emission cross section for smaller wavelength (l<1850 nm, i.e. Dl > 120 nm). The pulse durations and peak powers, measured at RR = 1 kHz, are below 30 ns and above 8 kW, respectively (Fig. 6). The maximum peak power of 13.5 kW was obtained at Dl = 15 nm with a pulse duration of 24.5 ns, corresponding to a pulse energy of 0.33 mJ. Although spectrum clearly show laser lines due to FWM, their contribution is neglected as their level is 21dB smaller than the main laser lines. The variations in peak power and pulse duration are logically influenced by the spectral profile of the gain of the Tm-doped fiber and the slight reduction of the VBG reflectivity with the rotating angle (θ). Measurements at RR = 1 kHz of different Δλ showed no significant power difference between both wavelengths (as shown in Fig. 6 (b)), without temporal instability. Besides, as for Δλ = 3.8 nm (fig. 3(a)), no competition between both pulses was observed for larger Δλ, over the whole tuning range. For example, the measured pulse profiles at Δλ = 85 nm and Δλ = 115 nm (Fig. 7) have a Gaussian shape with a FWHM of 26 ns and 29 ns, respectively. Nevertheless, parasitic instabilities appear at RR > 10 kHz and Δλ > 65 nm, when competitions between both wavelengths appears due to a weaker population inversion at high RR and to a large difference of emission cross-sections at large Δλ. It is worth noting that the optimization of the position of the whole tuning range on the gain spectrum would limit further this regime of instabilities. The measured slope efficiencies of the average output power of the laser were 7.15%, 12%, and 13%, at RR = 3 kHz, 7.5 kHz, and 30 kHz, respectively (Fig. 8). The wavelength separation was set to Δλ = 25 nm. As expected, the average power increases with RR from 0.97 W at 3 kHz to 1.87 W at 30 kHz due to the high overall inversion. A picture of the signal beam profile measured at the output of the laser cavity after the wedge (Fig. 1) presented in the inset of Fig. 8 has a Gaussian shape. The beam profile was slightly distorted due to parasitic astigmatism in the collimation system. Fig. 8. Evolution of the average power versus the pump power for different RR of 3 kHz, 7.5 kHz, and 30 kHz. Insert, measured power distribution of the signal beam (at RR = 7.5 kHz and pump power = 26.4 W, corresponding to an output power of 1.9 W).

In conclusion, a wide continuously tunable Q-switched dualwavelength thulium-doped fiber laser is demonstrated, using two VBGs and a free space AOM together for the first time to our knowledge. A novel design of thulium doped fiber (FA-LPF) was presented and used, to reduce the nonlinear effects in the laser cavity. A peak power higher than 8 kW, with a signal to noise contrast >20 dB, covering the tuning range from 1nm to 120 nm of wavelength separation have been achieved. The highest peak power was obtained at 15 nm of wavelength separation (13.5 kW), corresponding to a pulse duration of 24.5 ns at 1 kHz of RR. This configuration is well adapted to realize a tunable THz source by DFG by using a nonlinear crystal directly after the output signal, or by boosting the signal in an amplification stage to increase the peak power levels.

Fig. 1 .

 1 Fig. 1. Experimental setup of the Q-switched tunable dual-wavelength Tm-doped fiber.

Fig. 2 .

 2 Fig. 2. (a) Schematic cross section of the reduced cladding FA-LPF design. The Tm-doped core is represented in red, the clear blue region corresponds to the index matched aluminum-doped silica material, the yellow hexagons stand for the low-index inclusions, the dark blue area is made of pure silica, and the green ring represents the air-cladding ensuring a high pump coupling efficiency. (b) Cross section picture taken with an optical microscope of the fabricated Tm-doped rod-type FA-LPF [14].

Fig. 3 .

 3 Fig. 3. (a) Output spectrum and (b) temporal trace measured at Δλ = 3.8 nm, pump power = 26.4 W and RR = 1 kHz.

Fig. 4 .

 4 Fig. 4. Evolution of the average power and peak power (for Δλ = 3.8 nm and a pump power = 26.4 W) versus RR. Pulse duration values (tp) are written in red.

Fig. 5 .

 5 Fig. 5. Spectra of Q-switched dual-wavelength emissions at different wavelength separation (Δλ) from 15 nm to 120 nm.

Fig. 6 .

 6 Fig. 6. (a) Measured output peak power and pulse duration, and (b) the output power difference of both wavelengths emitted [P(λ1 -P(λ2)], versus Dl from 15 nm to 120 nm (RR = 1 kHz, pump power = 26.4 W)

Fig. 7 .

 7 Fig. 7. Temporal traces measured at 26.4 W of pump power for different wavelength separation of Δλ = 85 nm (a), and Δλ = 115 nm (b).

Funding.

  French Research Agency (ANR) (ANR-15 CE24-0031-01); German Research Foundation (DFG) (DFG GZ JA 2611/1-1).

Acknowledgment. We thank the COST MP1401 for supporting this work, and Kay Schuster for his technical support of the project.