Audrey Beillard

Thomas-Xavier Métro
email: thomas-xavier.metro@umontpellier.frxavier.bantreil@umontpellier.frfrederic.lamaty@umontpellier.fr

Xavier Bantreil

Jean Martinez

Frédéric Lamaty

A 3 -Coupling Reaction and [Ag(IPr) 2 ]PF 6 : A Successful Couple

Keywords: Alkynes, Amines, Carbenes, Silver, Multicomponent reactions, Continuous flow

The recently described homoleptic cationic [Ag(IPr) 2 ]PF 6 [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene] complex proved to be a versatile and highly efficient catalyst for the production of propargylamines by using the A 3coupling reaction. The reaction conditions were equally applicable to aliphatic and aromatic aldehydes and alkynes, including [

Introduction

Propargylamines are highly versatile building blocks that have been widely used for the construction of nitrogen-containing heterocycles, and they have found various applications ranging from ligands for catalysis to pharmaceuticals and agrochemicals. [1] The discovery that metals could catalyze the reaction between an aldehyde, an amine, and an acetylene to form propargylamines, also known as the A 3 -coupling reaction, has led research groups to develop various strategies aimed at producing propargylamines and related heterocycles under efficient, direct, and mild experimental conditions. [2] Many catalytic systems have been described, including copper, [3] gold, [4] silver, [5] magnesium, [6] indium, [7] rhodium, [8] iridium, [9] iron, [10] cobalt, [11] and zinc. [12] Whereas gold and copper appear to be the most frequently used catalysts for such transformations, silver presents some interesting advantages such as being greener and cheaper than gold, as well as generally requiring lower catalyst loadings and shorter reaction times than copper. [2b,13] Since the group of Li reported AgI as an efficient catalyst for the A 3 -coupling reaction in water, [5a] several studies described the catalytic activity of silver used in various forms, including salts, [14] nanoparticles, [15] nanocomposites, [16] metalorganic frameworks, [17] and polymeric [18] as well as discrete organometallic complexes. Lately, ligands such as imidazoles, [19] acridines, [20] pyridines, [5b,21] biarylylphosphanes, [22] and N-heterocyclic carbenes (NHCs) [23] have been studied, but to the best of our knowledge none of them were efficient for a wide range of substrates, including cyclic and linear secondary 4642 highly hindered aromatic aldehydes. Progargylamines were prepared in short reaction times with low catalyst loadings by using MeOH as a low-toxicity solvent. In addition, the catalyst was stable enough to support continuous-flow conditions, which showed that the reaction conditions are scalable. amines, aliphatic and aromatic aldehydes, as well as alkynes presenting varied electronic and steric properties.

We recently described a general, rapid, user-friendly, and solvent-free synthesis of NHC-Ag I complexes by using a ball mill. [24] In particular, homoleptic and cationic silver(I) complexes bearing noncoordinating tetrafluoroborate or hexafluorophosphate counteranions could be isolated in high yields (Figure 1). [24b] Figure 1. NHC-silver(I) complexes synthesized by ball milling and screened as catalysts in the A 3 -coupling reaction. Such complexes were rarely described because of their difficult synthesis, and to date, solely three examples of their use as catalysts were reported, including only one with N,N-diaryl-NHCs. [25] Hypothesizing that the high electrophilic nature of these cationic NHC-Ag I complexes would favor efficient coordination of the alkyne on the catalyst and subsequent formation of the silver alkynide, we envisioned studying the efficiency of these cationic NHC-Ag I complexes in catalyzing the A 3 -coupling reaction. Both the BF 4 and PF 6 salts of cationic silver complexes bearing the widely used IMes and IPr ligands [IMes = 1,3bis(2,4,6-trimethylphenyl)imidazolylidene; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene], their sterically more hindered homologues IMes Me and IPr Me having methyl groups on the imidazole backbone {IMes Me = [1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl]imidazolylidene; IPr Me = [1,3-bis(2,4,6-trimethylphenyl)-4,5-dimethyl]imidazolylidene}, as well as MeBnIm (1benzyl-3-methylimidazolylidene), Bn 2 Im 2 (1,3-dibenzylimidazolylidene) and TPT (1,3,4-triphenyl-1,2,4-triazolylidene) were evaluated.

Results and Discussion

Thus, benzaldehyde (1.0 equiv.), piperidine (1.2 equiv.), and phenylacetylene (1.5 equiv.) were treated in the presence of 3 mol-% of each of these complexes in MeOH at 110 °C under microwave irradiation for 1 h. Although enabling good conversion of the substrates, complexes with N,N-dialkylimidazolylidene ligands furnished propargylamine 1 in low to moderate yields (Table 1, Entries 1-4). Ag I complexes with the triazolylidene ligand did not lead to better results, supposedly as a result of an apparent low stability of the complex, as revealed by a relatively thick silver mirror observed on the reactor wall Table 1. Catalyst screening for the A 3 -coupling reaction. [a] Entry Catalyst Yield [%] [b] 1 [Ag(IPr) 2 ]PF 6 81 [c] (77) [d] 13

[Ag(IPr Me ) 2 ]BF 4 14 14

[Ag(IPr Me ) 2 ]PF 6 7

[a] Reaction conditions: benzaldehyde (1.0 mmol), piperidine (1.2 equiv.), phenylacetylene (1.5 equiv.), catalyst (3 mol-%), MeOH, microwave irradiation, 110 °C, 1 h. [b] Determined by HPLC analysis by using mesitylene as an internal standard. [c] Yield of isolated product. [d] Yield of isolated product upon using 1.1 equiv. of both piperidine and phenylacetylene along with 4 mol-% of [Ag(IPr) 2 ]PF 6 .

at the end of the reaction (Table 1, Entries 5 and 6). In contrast, Ag I complexes bearing N,N-diarylimidazolylidene ligands furnished more satisfying results. [Ag(IMes) 2 ]BF 4 and [Ag(IMes) 2 ]-PF 6 enabled the production of compound 1 in yields of 66 and 53 %, respectively (Table 1, Entries 7 and8). The use of sterically more hindered IMes Me ligands gave results similar to those obtained with the use of IMes ligands (Table 1, Entries 9 and10).

The best result was obtained with [Ag(IPr) 2 ]PF 6 , which led to the formation of 1 in 81 % yield (Table 1,Entry 12). Gratifyingly, the amounts of both piperidine and phenylacetylene could be reduced to 1.1 equiv. without dramatically affecting the yield (77 %), provided that 4 mol-% of [Ag(IPr) 2 ]PF 6 was engaged in the reaction. Unlike for IMes, the use of IPr Me instead of IPr was highly detrimental to the reaction, most probably because of the low solubility of [Ag(IPr Me ) 2 ] salts in MeOH (Table 1, Entries 13 and 14). Despite the presence of the two highly hindered IPr ligands, the efficiency of [Ag(IPr) 2 ]PF 6 could be explained by the strong Lewis acid character of the cationic silver atom, which favors coordination of the alkyne and subsequent formation of the silver alkynide intermediate.

In view of these encouraging results, the efficacy of [Ag(IPr) 2 ]PF 6 was evaluated for the synthesis of a wide range of propargylamines. Given that aromatic aldehydes are less reactive than aliphatic ones in the silver-catalyzed A 3 -coupling reaction, benzaldehyde was kept to screen the amine and alkyne partners. Other cyclic amines such as pyrrolidine and morpholine as well as linear dialkylamines led to the corresponding propargylamines in excellent yields (Table 2; compounds 2-5). Gratifyingly, dialkylated piperazine 6 was isolated in 86 % yield within 4 h upon using 0.55 equiv. of piperazine as the substrate (Table 2; compound 6), whereas a 24 h reaction was necessary if a gold catalyst was used. [4b] These reaction conditions proved to be also highly tolerant to the terminal alkyne. Propargylamines were obtained in good to excellent yields whether the alkyne was substituted by phenyl groups bearing various substituents (Table 2; compounds 7-11) or alkyl groups (Table 2; compounds 12-13). These reaction conditions were also applicable with equal efficiency toward a set of benzaldehydes substituted by iodo or methoxy groups in the para and meta positions to furnish the corresponding propargylamines 14-17 in good to excellent yields. Even more hindered ortho-substituted benzaldehydes reacted efficiently under the silver/microwave conditions to furnish amines 18 and 19 in good yields.

Pleasingly, [Ag(IPr) 2 ]PF 6 was able to convert highly encumbered 2,4,6-trimethoxybenzaldehyde into propargylamine 20 (90 % yield). Similarly, propargylamine 21 could be isolated in a rather good yield of 69 % upon starting from highly electrondeficient 4-(trifluoromethyl)benzaldehyde. Of note, 21 could also be isolated in 87 % yield after 10 h at room temperature. A distinctive feature of this [Ag(IPr) 2 ]PF 6 /microwave approach, if compared to most silver-catalyzed A 3 -coupling conditions, [5b,19,20,22,23] is its efficiency towards both variously substituted aromatic and aliphatic aldehydes. Indeed, cyclohexanecarbaldehyde and heptanal could be easily transformed into propargylamines 22-25 (86-92 % yield). Interestingly, propargylamines 22 and 23 could be isolated either at 110 °C under Table 2. Scope of [Ag(IPr) 2 ]PF 6 as a catalyst for the A 3 -coupling reaction. [a] [a] Reaction conditions: aldehyde (1.0 mmol), amine (1.1 equiv.), acetylene (1.1 equiv.), [Ag(IPr) 2 ]PF 6 (4 mol-%), MeOH (2 mL), microwave irradiation, 110 °C. [b] Piperazine (0.55 equiv.). [c] Aldehyde (1.0 mmol), piperidine (1.2 equiv.), phenylacetylene (1.5 equiv.). [d] Reaction was performed at room temperature.

microwave irradiation within 30 min or at room temperature for longer reaction times.

The efficient synthesis of propargylamines 24 and 25 by treatment of heptanal with 1,2,3,4-tetrahydroisoquinoline/ phenylacetylene and piperidine/2-ethynylpyridine, respectively, further extended the scope of this approach. Owing to their lower reactivity, ketones are less studied than aldehydes in the A 3 -coupling reaction. Delightfully, the [Ag(IPr) 2 ]PF 6 /microwave reaction conditions were also applicable to ketones, as treatment of 2-pentanone with pyrrolidine and phenylacetylene in the presence of [Ag(IPr) 2 ]PF 6 furnished propargylamine 26 in 88 % yield (Table 2). Of note, all of these propargylamines were synthesized under ambient atmosphere by using technicalgrade MeOH and without any purification of the reagents, giving additional ease to this user-friendly protocol.

To study the scalability of these reaction conditions, the synthesis of propargylamines 1 and 23 was tested under continuous flow. [4d,26] Indeed, as both microwave and continuous-flow synthesis allow efficient heating/cooling and generation of a superheated solvent, high-temperature microwave synthesis should be easily transferred to continuous-flow synthesis. [27] Thus, two separate solutions, one containing benzaldehyde (2 N) and [Ag(IPr) 2 ]PF 6 (0.08 N) in MeCN and the other containing piperidine (2.2 N) and phenylacetylene (2.2 N), were pumped in a 20 mL polytetrafluoroethylene (PTFE) coil heated up to 130 °C at a flow rate of 0.17 mL min -1 to ensure a 2 h residence time. An experimental setup with two pumps was necessary to avoid the fast formation and precipitation of the intermediate N-cyclohexylbenzaldimine. After classical workup and purification, propargylamine 1 was isolated in 80 % yield (Scheme 1), in the same range as that obtained for the reaction performed in MeOH at 110 °C under microwave irradiation (81 %; Table 1,Entry 12). On the other hand, pumping a single MeCN solution of heptanal (1 N), piperidine (1.1 N), phenylacetylene (1.1 N), and [Ag(IPr) 2 ]PF 6 (0.04 N) into a 10 mL PTFE coil heated up to 130 °C was possible and produced propargylamine 23 in 89 % yield (Scheme 1). Of note, MeCN was used instead of MeOH because of clogging and blockage of the system due to the poor solubility of [Ag(IPr) 2 ]PF 6 in MeOH. These results show that these reaction conditions are totally compatible with the production of propargylamines at a much higher scale.

Conclusions

The homoleptic and cationic [Ag(IPr) 2 ]PF 6 complex was found to be a highly versatile and efficient catalyst for the production of a wide range of propargylamines under microwave irradiation or continuous flow. This represents one of the rare examples of successful catalysis involving homoleptic silver-NHC complexes. Propargylamines were synthesized in short reaction times with low catalyst loadings and by using low-toxic, technical-grade MeOH, all of which make the methodology easy and user-friendly. This approach is versatile and compatible with both aliphatic and aromatic aldehydes and alkynes, including highly hindered aromatic aldehydes, and less reactive ketones reacted efficiently. Finally, a continuous-flow approach was developed, which thus allowed a facilitated scale-up.

Experimental Section

General Information: All reagents were purchased from Aldrich Chemical Co., Fluka, and Alfa Aesar and were used without further purification. Reactions performed under microwave irradiation were performed with a Biotage® Initiator+ microwave synthesizer. The temperature was measured with an IR sensor on the outer surface 4645 of the reaction vial. Continuous-flow experiments were performed by using Uniqsis Flowsyn Multi-X equipment. High-temperature PTFE coil reactors (10 or 20 mL) were set up. Flow rates were adapted to ensure an appropriate residence time in the reaction coil. The synthesis of the NHC-Ag I catalysts was performed with a Retsch MM200 or MM400 vibrating ball mill (vbm) operated at 25-30 Hz or with a Retsch PM100 planetary mill (pbm) operated at 450 rpm, according to a previously reported procedure. [24b] Analyses were performed at the "Plateforme Technologique Laboratoire de Mesures Physiques" (IBMM, Université de Montpellier). [Ag(IPr) 2 ]PF 6 : 1,3-Bis(2,6-diisopropylphenyl)imidazolium hexafluorophosphate (140.1 mg, 0.262 mmol, 1.00 equiv.), Ag 2 O (30.3 mg, 0.131 mmol, 0.50 equiv.), and NaOH (11.5 mg, 0.288 mmol, 1.10 equiv.) were introduced into a 10 mL stainless-steel grinding bowl with one stainless-steel ball (10 mm diameter). The bowl was closed and subjected to grinding in the vibrating ball mill operated at 30 Hz for 3 h. The black powder was recovered with CH 2 Cl 2 , and the suspension was filtered through Celite. The filtrate was concentrated under vacuum. The white solid was dissolved in a minimum amount of CH 2 Cl 2 , washed with water (3 ×), and concentrated under vacuum. The solid was washed with water and Et 2 O and dried under vacuum to afford bis[1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene]silver hexafluorophosphate (96.9 mg, 94.1 μmol, 72 %) as a white solid. 1 H NMR (400 MHz, [D 6 ]DMSO): δ = 7.77 (d, J = 0.9 Hz, 4 H), 7.51 (t, J = 7.8 Hz, 4 H), 7.18 (d, J = 7.8 Hz, 8 H), 2.20 (sept., J = 6.9 Hz, 8 H), 1.01 (d, J = 6.9 Hz, 24 H), 0.75 (d, J = 6.9 Hz, 24 H) ppm. [START_REF] Hashmi | Silver in Organic Chemistry[END_REF] C NMR (101 MHz, [D 6 ]DMSO): δ = 181.4 (dd, J = 201.6, 14.5 Hz), 144. 6, 134.5, 130.3, 125.7, 125.6, 124.0, 28.1, 24.0, 23.7

N-(1,3-Diphenylprop-2-yn-1-yl)piperidine (1). By Microwave Irradiation:

Benzaldehyde (103 μL, 1.00 mmol, 1.00 equiv.), piperidine (109 μL, 1.10 mmol, 1.10 equiv.), phenylacetylene (116 μL, 1.10 mmol, 1.10 equiv.), [Ag(IPr) 2 ]PF 6 (41.4 mg, 0.04 mmol, 0.04 equiv.), and MeOH (2 mL) were introduced into a sealed reactor. The suspension was heated under microwave irradiation at 110 °C for 1 h. After cooling to room temperature, the mixture was filtered through Celite and concentrated under reduced pressure. The crude mixture was purified by flash column chromatography (gradient cyclohexane/EtOAc) to afford N- (1,3-diphenyl-2propynyl)piperidine (211 mg, 0.77 mmol, 77 %) as a yellow oil. By Continuous Flow: A solution of benzaldehyde (103 μL, 1.00 mmol, 1.00 equiv.), [Ag(IPr) 2 ]PF 6 (41.4 mg, 0.04 mmol, 0.04 equiv.), and MeCN (500 μL) was prepared and connected to pump A. Similarly, a solution of piperidine (109 μL, 1.10 mmol, 1.10 equiv.), phenylacetylene (116 μL, 1.10 mmol, 1.10 equiv.), and MeCN (500 μL) was prepared and connected to pump B. After setting the tempera-ture to 130 °C, both valve positions were switched to place the mixture inline. Solutions A and B were pumped towards a T-shaped mixer, then into the 20 mL PTFE residence coil, and then through a back-pressure regulator (BPR) rated to 6.9 bar (100 psi) at a flow rate ensuring a 2 h residence time (0.17 mL min -1 ). The mixture was recovered and concentrated under reduced pressure. The crude mixture was purified by flash column chromatography (gradient cyclohexane/EtOAc) to afford N- (1,3-diphenyl-2-propynyl)piperidine (220 mg, 0.80 mmol, 80 %) as a yellow oil. 1 H NMR (300 MHz, CDCl 3 ): δ = 7.65 (d, J = 7.2 Hz, 2 H), 7.53 (dd, J = 6.6, 3.0 Hz, 2 H), 7.40-7.30 (m, 6 H), 4.82 (s, 1 H), 2.58 (t, J = 5.1 Hz, 4 H), 1.66-1.57 (m, 4 H), 1.46 (q, J = 5.7 Hz, 2 H) ppm. [START_REF] Hashmi | Silver in Organic Chemistry[END_REF] C NMR (101 MHz, CDCl 3 ): δ = 138. 8, 131.9, 128.7, 128.4, 128.2, 127.6, 123.5, 88.0, 86.2, 62.5, 50.8, 26.3, 24.9 ppm. 20): 2,4,6-Trimethoxybenzaldehyde (196 mg, 1.00 mmol, 1.0 equiv.), piperidine (119 μL, 1.20 mmol, 1.2 equiv.), phenylacetylene (158 μL, 1.50 mmol, 1.5 equiv.), [Ag(IPr) 2 ]PF 6 (41.4 mg, 0.04 mmol, 0.04 equiv.), and MeOH (2 mL) were introduced into a sealed reactor. The suspension was heated under microwave irradiation at 110 °C for 4 h. After cooling to room temperature, the mixture was filtered through Celite and concentrated under reduced pressure. The crude mixture was purified by flash column chromatography (gradient cyclohexane/EtOAc) to afford N- [1-(2,4,6-trimethoxyphenyl)-3-phenylprop-2-yn-1-yl]piperidine (330 mg, 0.90 mmol, 90 %) as a yellow oil. 1 H NMR (300 MHz, CDCl 3 ): δ = 7.47-7.41 (m, 2 H), 7.27-7.24 (m, 3 H), 6.17 (s, 2 H), 5.24 (s, 1 H), 3.84 (s, 6 H), 3.82 (s, 3 H), 2.70-2.57 (m, 4 H), 1.63-1.55 (m, 4 H), 1.42-1.34 (m, 2 H) ppm. [START_REF] Hashmi | Silver in Organic Chemistry[END_REF] C NMR (75 MHz, CDCl 3 ): δ = 160. 9, 160.0, 131.8, 128.2, 127.5, 124.6, 124.4, 108.0, 91.8, 89.7, 56.3, 55.4, 51.6, 26.3, 24.5

N-[1-(2,4,6-Trimethoxyphenyl)-3-phenylprop-2-yn-1-yl]piperidine (

N-[1-(2-Phenylethynyl)heptyl]piperidine (23)

. By Microwave Irradiation: Heptanal (140 μL, 1.00 mmol, 1.0 equiv.), piperidine (109 μL, 1.10 mmol, 1.10 equiv.), phenylacetylene (116 μL, 1.10 mmol, 1.1 equiv.), [Ag(IPr) 2 ]PF 6 (41.4 mg, 0.04 mmol, 0.04 equiv.), and MeOH (2 mL) were introduced into a sealed reactor. The suspension was heated under microwave irradiation at 110 °C for 30 min. After cooling to room temperature, the mixture was filtered through Celite and concentrated under reduced pressure. The crude mixture was purified by flash column chromatography (gradient cyclohexane/EtOAc) to afford N- [1-(2-phenylethynyl)heptyl]piperidine (261 mg, 0.92 mmol, 92 %) as a yellow oil. By Agitation at Room Temperature: Heptanal (140 μL, 1.00 mmol, 1.0 equiv.), piperidine (109 μL, 1.10 mmol, 1.10 equiv.), phenylacetylene (116 μL, 1.10 mmol, 1.1 equiv.), and [Ag(IPr) 2 ]PF 6 (41.4 mg, 0.04 mmol, 0.04 equiv.) were dissolved in MeOH (2 mL), and the mixture was agitated at room temperature for 12 h. The mixture was filtered through Celite and concentrated under reduced pressure. The crude mixture was purified by flash column chromatography (gradient cyclohexane/EtOAc) to afford N- [1-(2-phenylethynyl)heptyl]piperidine (223 mg, 0.79 mmol, 79 %) as a yellow oil. By Continuous Flow: Heptanal (140 μL, 1.00 mmol, 1.00 equiv.), piperidine (109 μL, 1.10 mmol, 1.10 equiv.), phenylacetylene (116 μL, 1.10 mmol, 1.10 equiv.), and [Ag(IPr) 2 ]PF 6 (41.4 mg, 0.04 mmol, 0.04 equiv.) were dissolved in MeCN (1 mL) and placed in a vial. After setting the temperature to 130 °C, the valve position was switched to place the mixture inline. The mixture then flowed into the 10 mL PTFE residence coil and then through a back-pressure regulator (BPR) rated to 6.9 bar (100 psi) at a flow rate ensuring a 30 min residence time (0.33 mL min -1 ). The mixture was then concentrated under reduced pressure and purified by flash column chromatography (gradient cyclohexane/EtOAc) to afford N-[1-(2-phenylethynyl)heptyl]piperidine (252 mg, 0.89 mmol, 89 %). 1 H NMR (300 MHz, CDCl 3 ): δ = 7. 48-7.45 (m, 2 H), 7.34-7.28 (m, 3 H), 3.48 (dd, J = 9.0, 5.8 Hz, 1 H), 2.77-2.62 (m, 2 H), 2.57-2.43 (m, 2 H), 1.77-1.69 (m, 3 H), 1.67-1.53 (m, 5 H), 1.51-1.42 (m, 3 H), 1.38-1.27 (m, 6 H), 0.94-0.86 (m, 3 H) ppm. [START_REF] Hashmi | Silver in Organic Chemistry[END_REF] C NMR (75 MHz, CDCl 3 ): δ = 131. 9, 128.3, 127.9, 123.8, 88.4, 85.7, 58.8, 50.7, 33.6, 31.9, 29.2, 27.0, 26.3, 24.7, 22.8, 14.2 ppm.

  Scheme 1. Continuous-flow synthesis of propargylamines 1 and 23.

  1 H NMR spectra were recorded in CDCl 3 or [D 6 ]DMSO with a Bruker Avance I 300 MHz or a Bruker Avance III HD 400 MHz spectrometer. Chemical shifts are reported in ppm and are referenced to the residual protio solvent signal (CHCl 3 at δ = 7.26 ppm or DMSO at δ = 2.50 ppm). Data are reported as s = singlet, d = doublet, t = triplet, q = quadruplet, quint = quintuplet, sept = septuplet, m = multiplet; coupling constant in Hz; integration. 13 C NMR spectra were recorded with a Bruker Avance I 75 MHz or a Bruker Avance III HD 101 MHz spectrometer. Chemical shifts are reported in ppm and are referenced to the solvent signal (CDCl 3 at δ = 77.2 ppm or [D 6 ]DMSO at δ = 39.5 ppm). 31 P NMR spectra were recorded with a Bruker Avance III HD 162 MHz spectrometer and 19 F NMR spectra with a Bruker Avance III HD 376 MHz instrument. HRMS analyses were performed with a UPLC Acquity H-Class from Waters hyphenated to a Synapt G2-S mass spectrometer with a dual ESI source from Waters.

  ppm. 31 P NMR (162 MHz, [D 6 ]DMSO): δ = -144.2 (sept, J = 711.3 Hz) ppm. 19 F NMR (376 MHz, [D 6 ]DMSO): δ = -70.2 (d, J = 711.3 Hz) ppm.

  ppm. HRMS: calcd. for C 23 H 28 NO 3 [M + H] + 366.2069; found 366.2064.

www.eurjoc.org © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Eur. J. Org. Chem. 2017, 4642-4647 www.eurjoc.org © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Acknowledgments

Université de Montpellier, Centre National de la Recherche Scientifique (CNRS), and French Ministry of Higher Education and Research (grant to A. B.) are acknowledged for funding.


Y Zhang

T Gard

C Theron

A Apostoluk

K Masenelli-Varlot

B Canut

S Daniele

B Masenelli
email: bruno.masenelli@insa-lyon.fr

Visible luminescence improvement of ZnO/PAA nano-hybrids by silica coating

Keywords: ZnO nanohybrid, PAA, visible luminescence, silica coating

The effect of a silica coating on the improvement of the visible light emission properties of ZnO/polyacrylic acid (PAA) nanohybrids is reported. The synthesized material consists of ZnO nanocrystals incorporated into the PAA mesospheres and then coated with silica. The silica amount can be controlled by the concentration of ammonia used in the sol-gel process as catalyst. The interaction between PAA and ammonia is crucial, the presence of the former tending to inhibit the catalytic action of the latter. We show that there is an optimum in the silica amount around the mesospheres, which leads to a drastic increase in the defect-related visible photoluminescence quantum yield of ZnO nanocrystals. A six-fold increase of the quantum yield can thus be achieved, reaching competitive values higher than 60%. This optimum is a compromise between a complete protective silica layer around the mesospheres and too thick a layer inducing inefficient absorption of excitation light by the coating.

Introduction

Crystalline defects are often luminescence quenching centers in semiconductors. However, some are also good emitters, such as NV centers in diamond or vacancies in SiC. These atomic defects can be so efficient that they can act as single photon sources [START_REF] Jelezko | Single defect centres in diamond : A review[END_REF][START_REF] Schirhagl | Nitrogen-Vacancy Centers in Diamond : Nanoscale Sensors for Physics and Biology[END_REF][START_REF] Aharonovich | Diamond-based single-photon emitters[END_REF][START_REF] Lohrmann | review on single photon sources in silicon carbide[END_REF]. This is the case in ZnO, which is known to exhibit a rich luminescence in the visible range due to the contribution of several types of defects.

Several studies have shown that this luminescence can indeed be very intense with a high quantum yield [START_REF] Tang | Synthesis of ZnO Nanoparticles with Tunable Emission Colors and Their Cell Labeling Applications[END_REF][START_REF] Felbier | Highly Luminescent ZnO Quantum Dots Made in a Nonthermal Plasma[END_REF][START_REF] Xiong | Photoluminescent ZnO nanoparticles synthesized at the interface between air and triethylene glycol[END_REF][START_REF] Fu | Stable aqueous dispersion of ZnO quantum dots with strong blue emission via simple solution route[END_REF][START_REF] Choi | Electroluminescence from isolated defects in zinc oxide -towards electrically triggered single photon sources at room temperature[END_REF][START_REF] Morfa | Singlephoton emission and quantum characterization of zinc oxide defects[END_REF]. Defect-based visible luminescence can therefore be foreseen as a potential alternative source of visible light. With a wide bandgap of 3.34 eV, ZnO can be promising for down-shifting of the UV light to visible one. ZnO is often characterized by deep defects such as oxygen vacancy (VO) and shallow defects such as zinc interstitial (Zni), which have been assigned as the main sources of the broadband emission in the visible range [START_REF] Mccluskey | Defects in ZnO[END_REF][START_REF] Kahn | Optical properties of zinc oxide nanoparticles and nanorods synthesized using an organometallic method[END_REF][START_REF] Wang | Green luminescence originates from surface defects in ZnO nanoparticles[END_REF][START_REF] Zeng | Blue luminescence of ZnO nanoparticles based on non-equilibrium processes: defect origins and emission controls[END_REF][START_REF] Reshchikov | Luminescence properties of defects in ZnO[END_REF]. The defect configuration is significantly modified when the size of the ZnO samples is reduced to the nanometer scale. In particular, defects at the surface of ZnO nanocrystals become the dominant factor in determining the optical properties of ZnO. Surface states, including defects and adsorbed groups (e.g., OH groups), are strongly dependent on synthesis (in case of chemical methods) [START_REF] Norbert | Influence of Surface Modification on the Luminescence of Colloidal ZnO Nanocrystals[END_REF][START_REF] Tainoff | Crystallinity, Stoichiometry and luminescence of high quality ZnO nanoclusters[END_REF]. Therefore, many researches on ZnO nanostructures have focused on the control and study of surface states. A core-shell structure offers multiple functions to nanoscale materials in terms of catalytic, optical and magnetic properties. The shell can change the charge, functionality and reactivity of the surface, or improve the stability and dispersibility of the core material [START_REF] Zhai | Core/shell structured ZnO/SiO2 nanoparticles: Preparation, characterization and photocatalytic property[END_REF]. Silica is one of the most common shell candidates due to its relative ease of preparation, good environmental stability and compatibility with other materials (allowing biocompatibility [START_REF] Babu | Controlling the size and optical properties of ZnO nanoparticles by capping with SiO2[END_REF]). Silica coating on ZnO is a feasible strategy for improving the optical properties of ZnO nanostructure.

The introduction of the silica coating on ZnO nanoparticles is however, rather difficult, due to the large difference in surface energy and the large surface area of ZnO nanoparticles [START_REF] El-Nahhal | Synthesis & characterization of silica coated and functionalized silica coated zinc oxide nanomaterials[END_REF]. Two strategies based on Stöber method can be used for the introduction of the silica coating: one during the growth process of ZnO, the other one after growth [START_REF] Ramasamy | Synthesis of silica coated zinc oxide-poly (ethylene-coacrylic acid) matrix and its UV shielding evaluation[END_REF][START_REF] Tang | Preparation of styrene polymer/ZnO nanocomposite latex via miniemulsion polymerization and its antibacterial property[END_REF]. In the first method, the growth of ZnO takes place simultaneously with the gelation of silica to form a composite. In the second route, previously prepared ZnO nanoparticles are added to the silica gel. Other techniques such as spray drying, magnetron sputtering, spin coating have also been utilized for silica coating [START_REF] Hagura | Enhanced photoluminescence of ZnO-SiO2 nanocomposite particles and the analyses of structure and composition[END_REF][START_REF] Peng | White-light emitting ZnO-SiO2 nanocomposite thin films prepared by the target-attached sputtering method[END_REF][START_REF] Singh | White light emission from chemically synthesized ZnO-porous silicon nanocomposite[END_REF]. Regardless of the synthesis process, the presence of silica can affect the shape, size and dispersion of ZnO nanoparticles, and eventually their optical properties.

We have previously developed a hybrid organic/inorganic material made of ZnO nanocrystals embedded in self-assembled polyacrylic acid (PAAH) 3D mesospheres (~ 50 to 100 nm). [START_REF] Zhu | Intense visible emission from ZnO/PAAX (X = H or Na) nanocomposite synthesized via a simple and scalable solgel method[END_REF] This material appears to be an efficient visible light emitter. In particular, we have shown that our self-assembled organic/inorganic system can enhance the visible luminescence of ZnO nanocrystals, resulting in a photoluminescence quantum efficiency (PL QY) of 20 % in routine [START_REF] Zhu | Intense visible emission from ZnO/PAAX (X = H or Na) nanocomposite synthesized via a simple and scalable solgel method[END_REF]. However, PL QY still needs to be improved and made robust over long a period of time. Moreover, one of the promising applications of such a material is its use as a rare earth-free phosphor for white light-emitting diodes (WLEDs), which also requires high performance for operating temperatures of up to 100-120 °C. For both challenges, in analogy with what has been observed for ZnO nanocrystals, an additional silica coating over the mesospheres embedding ZnO nanocrystals could prove to be beneficial too. In the present study, we aim at investigating the effects of a silica coating on the optical properties of ZnO/PAA nanohybrids as a function of the amount of silica. The latter is controlled essentially by the concentration of ammonia introduced during the coating synthesis as a catalyst. A new question arises concerning the influence of this catalyst on the self-assembled mesospheres. Indeed, the presence of numerous -COOH functions induces an additional difficulty, as they react with ammonia. Two previous articles from Fuji et al. [START_REF] Fuji | Synthesis and shell structure design of hollow silica nanoparticles using polyelectrolyte as template[END_REF] and Wan and Yu [START_REF] Wan | Polyelectrolyte controlled large-scale synthesis of hollow silica spheres with tunable sizes and wall thicknesses[END_REF] showed the elaboration of hollow silica nanospheres from tetraethyl orthosilicate (TEOS) in the presence of PAAH.

Experimental methods

ZnEt2 solution (15%wt in toluene), PAAH (MW = 2000 g.mol -1 , 63 wt% in water), tetraethyl orthosilicate (TEOS), NH4OH aqueous solution (32% wt) and anhydrous ethanol were purchased from Sigma-Aldrich and used as received. Deionized water was obtained from ion exchange resin (Lenntech, 1-10 M.cm). ZnO/PAA hybrid mesospheres of about 50-100 nm embedding wurtzite ZnO nanocrystals with a size of about 10 to 20 nm were synthesized as described in our previous works [START_REF] Zhu | Intense visible emission from ZnO/PAAX (X = H or Na) nanocomposite synthesized via a simple and scalable solgel method[END_REF][START_REF] Zhang | Doping of ZnO inorganicorganic nanohybrids with metal elements[END_REF]. Typically, the synthesis was performed at room temperature by reacting 1.8 mL of ZnEt2 and 25 mL of 0.63 wt% of polyacrylic acid (PAAH) aqueous solution. The obtained materials are composed roughly of 50 % PAAH and 50 % ZnO from TGA analysis.

Silica coating synthesis

ZnO/PAA samples were then coated by sol-gel process using TEOS and water as reactants and OH -as catalyst. In a typical coating process, 1.15 g (5.52 mmol) of TEOS was added to the suspension of 300 mg of ZnO/PAA powder (1.84 mmol of ZnO) in 1g of ethanol with constant stirring at 700 rpm at room temperature. The resulting molar ratio TEOS:ZnO being equal to 3, the sample will be denoted TEOS-3. After 30 min of stirring, 2 ml of a mixed solution of EtOH:H2O:NH4OH of mass ratio 1:0.93:0.027 were added to the suspension. The stirring continued at 700 rpm at room temperature for additional 4 hours to obtain the final suspension. The resulting white solid was recovered by centrifugation and then washed twice with ethanol and dried for 4 hours at 70 °C. The same experiments without ZnO/PAA and only PAAH or with different amount of NH4OH were also carried out. Samples with molar ratio TEOS:ZnO fixed at 3 but with different amounts of NH4OH will be referred to as TEOS-3-1, TEOS-3-2, etc., where the second numbers represent the multipliers of 0.027 NH4OH nominal mass ratio in the mixed solution of EtOH, H2O and NH4OH, while the amounts of EtOH and H2O are fixed.

For each sample of coated nanohybrids, the corresponding sample of identical uncoated nanohybrids was kept and used as a reference for characterizations. Since both samples contained identical nanohybrids, we can accurately probe the effect of the presence of silica alone in the coated sample, independent of any variation resulting from the initial mesosphere batches, by comparing both samples (reference and coated).

Characterizations

Structural and optical properties were examined for samples of both silica-coated and uncoated ZnO/PAA nanostructures. For structural characterization, X-ray diffraction (XRD) was performed on a Bruker AXS D8 diffractometer using Cu K radiation in the Bragg-Brentano configuration; transmission electron microscopy (TEM), high-resolution TEM (HRTEM), high angle annular dark-field (HAADF) imaging and energy dispersive X-ray spectroscopy (EDS) were performed on JEOL 2010F microscope operating at 200 kV. The samples were dry-deposited onto a holey carbon TEM grid. The experimental conditions (magnification, electron dose, dwell time in HAADF and EDS) were chosen in order to minimize irradiation damage in PAA and subsequent oriented attachment of ZnO nanocrystals. The crystal structure, size and morphology were revealed by XRD and TEM. The chemical composition of selected samples was analyzed by Fourier transform infrared spectroscopy (FTIR, Bruker Vertex 80) and Rutherford backscattering spectroscopy (RBS) using a 2 MeV 4 He + ion probe. For optical characterization, photoluminescence (PL) and PL quantum yield (PL QY) were measured at room temperature for each powder sample, which was manually pressed onto an indium foil. The excitation source was a continuous laser at 266 nm (8 mW, Crylas FQCW 266-10) and the emission was detected using a liquid N2-cooled Si CCD camera after dispersion in a spectrometer (iHR Triax 320 Jobin-Yvon, grating blazed at 500 nm, 600 grooves/mm, spectral accuracy ± 1 nm). PL QY measurements were performed using an integrating sphere, with an internal coating made of spectralon, according to the protocol developed by de Mello et al. [START_REF] De Mello | An improved experimental determination of external photoluminescence quantum efficiency[END_REF] PL excitation (PLE) was performed on a fluorescence spectrometer (FLSP920, Edinburgh Instruments) from 200 nm to 400 nm (with a spectral width of ± 1 nm) at emission wavelength of 550 nm ± 1 nm. The optical properties were also tested at higher temperatures (up to 100 °C) and the excitation wavelength was chosen at 340 nm ± 2 nm from a Xenon lamp. The emission was detected by a CCD camera (ANDOR NEWTON 970) after dispersion in a monochromator (Triax 320 Jobin-Yvon, 300 grooves/mm, precision ± 2 nm). The intensity variation was monitored by integrating the entire emitted spectrum. All the collected spectra were corrected by the corresponding instrumental function.

Results and discussion

Structural analyses

In the typical conditions and a molar ratio TEOS:ZnO of 3, gravimetric studies conducted without ZnO/PAAH, with the corresponding amount of PAAH (150 mg) and with ZnO/PAAH (300 mg) are summarized in table 1. The results show first that the kinetics is rather slow, however for practical reasons, in order to carry out all the operations in the same day, coating process should last only 4 hrs. It is obvious that the presence of PAAH inhibits the polycondensation reaction, however too much NH4OH might cause degradation of the self-assembly system. Our previous studies have shown that the use of an equimolar mixture of PAAH and PAANa or the addition of NaOH during synthesis leads to ZnO/PAA clear suspensions and not to self-assembled structures. [31] The coating process used here is not expected to affect the inorganic phase. show that the presence of the unassigned phase is sensitive to the amount of NH4OH (Figure 1). The greater the amount of NH4OH and the weaker the presence of the PAAH crystalline phase. This can be explained by the acid-base reaction of PAAH + NH4OH which transforms progressively the PAAH into its PAANH4 salt. After calcination at 400 °C, both impurity peaks disappear, confirming that these are most likely related to the organic polymer phase and independent of the introduction of TEOS. No crystalline phase related to silica is observed, suggesting that it probably exists in the amorphous state.

Indeed, at the highest ammonia concentration (TEOS-3-4), a broad peak is observed between 25° and 30° in Figure 1 consistent with the presence of amorphous silica. The presence of silica is evidenced by FTIR, as illustrated in Figure 2. For both TEOS-3-2 and uncoated ZnO/PAAH samples, the broad feature between 3000 cm -1 and 3500 cm -1 is associated with water or OH groups. Characteristic PAA bands are observed between 1219 cm -1 and 1800 cm -1 , including two strong bands at 1551 cm -1 and 1408 cm -1 , ascribed to the asymmetric and symmetric modes of carboxylate anion (COO -), respectively. Between 816 cm -1 and 1219 cm -1 , a broad band is observed for the TEOS-3-2 sample, while it is absent for the uncoated ZnO/PAA. The bands at 1018 cm -1 and 945 cm -1 are assigned to the asymmetric stretching modes of Si-O-Si vibrations and to the stretching modes of surface silanol groups (≡Si-OH) or even to the vibrational mode of Si-O-Zn, respectively [START_REF] Babu | Controlling the size and optical properties of ZnO nanoparticles by capping with SiO2[END_REF][START_REF] El-Kabbany | Thermal and spectroscopic properties of the nano-system (ZnO (1-x) SiO2 (x))[END_REF].

The band at 447 cm -1 in ZnO/PAA is attributed to the Zn-O stretching mode, which is a further evidence of the presence of ZnO. Interestingly, it is replaced with a band at 544 cm -1 in TEOS-3-2. The position of the Zn-O bond band depends on the size, morphology and axial ratio (c/a) of the ZnO nanocrystals [START_REF] Soni | Studies on ZnO nanorods synthesized by hydrothermal method and their characterization[END_REF]. Oxygen-related defect complexes are also associated with bands close to this range [START_REF] Xiong | Photoluminescence and FTIR study of ZnO nanoparticles: the impurity and defect perspective[END_REF]. In agreement with XRD, the band at 544 cm -1 is assigned to highly defective ZnO. This observation confirms that the light and heavy elements are segregated and in accordance with a coating scenario. Finally, the presence of Si is unambiguously confirmed by EDS (Figure 6). The amorphous coating tentatively assigned to silica contains Si and not Zn. It is difficult to get an absolute composition of the corresponding phases, i.e., to confirm that Si is present in a stoichiometric silica phase. Indeed, oxygen is mapped throughout the sample. Its presence is related to ZnO, the SiOx phase (with x possibly not equal to 2), PAAH and eventual contamination. However, C map shows significant variations, with higher concentration regions superimposed to Zn rich regions. Combining the elemental maps, the model we can construct is that of ZnO nanocrystals embedded in PAAH mesospheres, the latter being inhomogeneously coated with silica. Keeping the 3D nanostructures self-assembled while elaborating a SiO2 coating in basic medium proved to be very delicate and only a decoration of these objects could be carried out. At high base concentration, the loss of -COOH functions leads to the destruction of self-assembled nano-objects and the dispersion of ZnO nanocrystals that are gradually incorporated into a SiO2 matrix (Figure 7).

A key point concerns the role of exposed ZnO, both initially and following the action of ammonia. This exposed ZnO may well act as a nucleation seed for silica. Our observations do not provide definite proof of this hypothesis but will show that it could be so. Indeed, as we add more ammonium, the PAA spheres degrade (see figure 3), exposing more ZnO nanocrystals. At the same time, more silica is produced.

Therefore, the effect of ammonia could be two-fold: a direct catalytic effect and a redispersion of the ZnO crystallites, releasing more ZnO seeds. A series of samples with a tunable amount of ZnO nanocrystals may help to investigate this point and identify the important parameters needed to achieve high luminescence performance.

Photoluminescence properties

Effect of silica shell on the photoluminescence spectrum

Regarding the ammonia amount in the coating synthesis, thus the amount of silica in the final samples, the first effect to be probed was on the photoluminescence spectrum. Figure 8 shows the PL spectra of ZnO/PAA, TEOS-3-1 and TEOS-3-2 samples.

In the absence of silica, as already reported in our previous work [START_REF] Zhu | Intense visible emission from ZnO/PAAX (X = H or Na) nanocomposite synthesized via a simple and scalable solgel method[END_REF], the PL spectrum peaks at around 2.13 eV. With the addition of silica, the spectrum maximum gradually blue-shifts to blue up to 2.2 eV.

This could be due to a reduction of the yellow contribution at ~2 eV, related to the recombination of electrons trapped at zinc interstitials with the deeply trapped holes [START_REF] Zeng | blue luminescence of ZnO nanoparticles based on nonequilibrium processes: defect origins and emission controls[END_REF]. However, the main observation is that the silica coating does not significantly modify the spectrum, implying that the nature of the specific crystal defects of ZnO nanocrystals is not affected. The presence of some silica contribution could still be claimed. It is known that under UV excitation, silica can emit visible light in the 1.6 to 2.3 eV range because of defects such as non-bridging oxygen hole center, silylene, silanone or dioxasilyrane [START_REF] Vaccaro | Bright visible luminescence in silica nanoparticles[END_REF]. Some of these defects are not stable under ambient conditions while others are. At this stage, the contribution of silica defects cannot be totally ruled out but remains unlikely. Further evidence of the lack of effect of the silica coating on the PL spectra is given in the Supporting Information (Figure SI1).

Regarding the nature of these defects, although there has been a long debate on the defects that could lead to green emission (at or near 2.27 eV), there is now a consensus to consider singly ionized oxygen vacancies as the main cause. Two hypotheses for the corresponding electronic transition are being debated. For instance, Zeng et al. have considered that the green emission results from the de-excitation of electrons from the conduction band and their recombination with holes trapped in deep defects [START_REF] Zeng | blue luminescence of ZnO nanoparticles based on nonequilibrium processes: defect origins and emission controls[END_REF].

On the contrary, Camarda et al. pointed out that the green emission would rather result from the recombination between a hole of the valence band and an electron deeply trapped at singly-ionized oxygen vacancy [START_REF] Camarda | Luminescence mechanisms of defective znO nanoparticles[END_REF]. Based on the most recent analyses, we would favor the second hypothesis.

Whatever the exact transition, oxygen vacancies are intimately linked to the generation of a green visible emission in our nanohybrids. The fact that the silica coating does not modify the spectrum suggests that the PAA spheres, embedding the emitting ZnO nanocrystals, does not allow easy diffusion of oxygenrelated species from the reaction solution of directly from the silica coating. We then checked the effect of the silica coating on the excited states responsible for PL by performing PL excitation (PLE) experiments on samples prepared with different amounts of ammonia. The results, uncorrected for the absorption profile, are presented in Figure 9 for ZnO/PAA, TEOS-3-1 and TEOS-3-2 samples. The PLE spectrum of ZnO nanocrystals embedded in PAA, in the absence of silica, reproduces the one we measured in our previous work [START_REF] Zhu | Intense visible emission from ZnO/PAAX (X = H or Na) nanocomposite synthesized via a simple and scalable solgel method[END_REF] and is consistent with PLE spectra reported for other ZnO nanocrystals [START_REF] Zeng | blue luminescence of ZnO nanoparticles based on nonequilibrium processes: defect origins and emission controls[END_REF][START_REF] Camarda | Luminescence mechanisms of defective znO nanoparticles[END_REF]. The visible emission is essentially generated by absorption across the band gap for wavelengths above 365 nm. A first optimum is obtained at 330 nm, comparable to the maximum at 335 nm observed in [START_REF] Zeng | blue luminescence of ZnO nanoparticles based on nonequilibrium processes: defect origins and emission controls[END_REF]. A second optimum is obtained at 255 nm. The latter could be partially related to absorption by the surrounding PAA followed by charge transfer to the embedded ZnO nanocrystals.

No modification in the PLE spectra is observed for once or twice the nominal amount of ammonia, meaning that for such ammonia concentrations no change in the PL path is induced. Hence, neither the nature of the defects responsible for visible luminescence nor the way energy is transferred to these emitting defects in these cases are affected. In particular, we can rule out any energy transfer from silica coating to ZnO nanocrystals. It is likely that the excitation energy is partially absorbed directly by the ZnO nanocrystals and partially absorbed by the PAA spheres. In the first case, the absorption leads to direct visible emission while in the second case, the emission is mediated by energy or charge transfer from the PAA matrix to the ZnO nanocrystals.

Effect of silica shell on the photoluminescence quantum yield

We studied the PL QY variation of silica-coated nanohybrids with respect to their uncoated counterparts, synthesized in the same way at the initial step of synthesis. The only difference is thus due to the silica coating. Figure 10 shows PL QY for all silica-coated samples with an optimum for TEOS-3-2 (see also figure SI2). The PL QY is considerably improved by almost six-fold with the absolute value of PL QY increasing from 8.9 % to 64.0 %. We checked that the presence of ammonia (at twice the nominal concentration), during the process without TEOS injection, does not modify the PL QY of the initial samples and thus that ammonia is not responsible for any PL QY modification. This enables us to completely rule out the possible contribution of silica defects to the visible emission. A miscellaneous effect of silica on PL QY improvement could be that the initial samples contain ZnO nanocrystals that are not embedded in the PAA mesospheres, either initially or as a result of the action of ammonia. These non-passivated nanocrystals are mostly ineffective for PL. After coating with silica, they could be passivated and begin to emit visible light more efficiently, along with the ZnO nanocrystals embedded in silica-coated PAA mesospheres. Although we expect this effect to be minor and despite the fact that the silica coating on bare ZnO nanocrystals often quenches the visible luminescence, we cannot entirely rule it out.

In Figure 10, two antagonistic effects of silica coating can be noticed. As the silica amount increases, the coverage of the mesosphere increases while preserving the integrity of the mesospheres that are beneficial for light trapping and scattering (see previous sections). This improves the protection of the nanohybrids (possibly from moisture or contaminant). At large ammonia concentrations, the detrimental effect is twofold. On the one hand, ammonia degrades the integrity of the mesospheres while on the other hand the silica coating grows thicker. This induces some absorption of the excitation light by the silica coating and less by the ZnO/PAA nanohybrids. Consequently, the PL QY of the overall system

decreases. An optimum in silica coating is thus to be achieved.

The exact mechanism responsible for the PL QY improvement remains elusive. However, two complementary hypotheses can be put forward. The first one consists in the feeling?? of pores present in the PAA spheres. The mesospheres would therefore no longer be permeable to the ambient contaminant, or to the contaminant of the synthesis solution which could quench some of the defects (i.e. essentially oxygen vacancies) necessary for visible emission of ZnO. The second hypothesis is based on an electronic argument. The visible emission in ZnO nanocrystals is essentially due to singly ionized oxygen vacancies (Vo). We have previously stated that the nature of the defects responsible for the visible emission is not disturbed by the introduction of TEOS. However, either the overall concentration of VO, or more likely, the concentration of singly ionized VO could be increased by the introduction of TEOS and the resulting synthesis of silica. It seems difficult to imagine that TEOS would actually induce the formation of oxygen vacancies with ZnO nanocrystals embedded in PAA mesospheres. However, since the PAA mesospheres are not impermeable to charge migration, a change of the charge state of oxygen vacancies of some of the ZnO nanocrystals can be considered, which would lead to an increase in the singly ionized Vo concentration.

Effect of silica shell on the photoluminescence performances as function of temperature

In the perspective of using such coated nanohybrid mesospheres in WLED applications, their PL performance has been studied as a function of operating temperature. The spectra evolutions are presented in Supporting Information (figure SI3) and summed up in Figure 11.

The first observation is that increasing the temperature to 100 °C, a typical operation temperature in WLEDs, leads to a reduction in luminescence intensity by a factor of about five for the uncoated sample and by a factor of three for the coated samples. Second, when the sample is cooled to room temperature (in cycles), the recovered PL intensity amounts to approximately 75 % of the initial one. This value is independent of the coating as can be observed from the temperature evolution of the normalized PL intensity in Figure 11. The main difference between the samples is the absolute value of the intensity at room temperature, the PL QY of the TEOS-3-2 sample being substantially higher than that of the uncoated one as shown in figure 10. Therefore, the thermal quenching of the PL, which would otherwise be detrimental to the application of uncoated mesosphere samples in devices operating at high temperature, has a reduced impact for the application of coated samples, leaving sufficient light produced at 100 °C.

Conclusions

In conclusion, we have demonstrated that the silica coating of our ZnO/PAA nanohybrids is strongly affected by the amount of NH4OH used. This reagent has two antagonistic effects since it improves the polycondensation kinetics of silica but at high concentration, NH4OH can disrupt the stability of the initial ZnO/PAA mesospheres, by degrading their integrity. At the same time, the resulting silica layer thickens and partly absorbs the UV excitation light, preventing it from exciting the ZnO nanocrystals embedded in the PAA mesospheres. As a result, part of the excitation light is lost in the coating, resulting in a decrease in PL QY. However, whatever the ammonia content in the synthesis solution, the PL QY of the resulting silica coated samples is always improved as compared to the uncoated counterparts.

Therefore, a silica shell around the nanohybrid mesospheres is beneficial, leading, in the optimal case, to significant, robust and competitive PL QY over 60%. The mesospheres have a beneficial effect on the scattering and trapping of light in the sample thus improving the light absorption. An intermediate situation that is more akin to a surface decoration of these objects seems to be the best compromise between maintaining self-assembly and improving optical performances. However, this coating does not make it possible to improve the thermal performance of these nano-objects, which is an area for further improvement.

Figure 1

 1 presents the XRD diffractograms of uncoated ZnO/PAA and TEOS-3 nanohybrids. The diffraction peaks of the asprepared material correspond well to ZnO wurtzite with two impurity peaks at 2θ = 33.4° and 59.4°, related to the polymer (PAAH) as shown in uncoated ZnO/PAA starting materials. The diffractograms

Figure 1 :

 1 Figure 1: XRD diffractograms of (from bottom to top) uncoated ZnO/PAA, TEOS-3-1, TEOS-3-2, TEOS-3-4 and TEOS-3-1 calcined at 400 °C. The symbols ⧫ mark the peaks related to PAA.

Figure 2 :

 2 Figure 2: FTIR spectra of the ZnO/PAA and TEOS-3-2 samples.

Figure 3 shows

 3 Figure 3 shows TEM observations of TEOS-3 samples synthesized with different amounts of NH4OH.For TEOS-3-1 (Figure3(a) and (b)), the presence of an amorphous Si-based shell is confirmed around ZnO nanocrystals (Figure3 (a)). Similarly, it must also be present around PAA mesospheres containing ZnO nanocrystals (Figure3 (b)), which make the majority of the sample, even though it is hard to obtain an image of a few-nanometer thick silica shell around a 100 nm-thick hybrid structure. The same applies to TEOS-3-2 samples (Figure3 (c) and (d)). In Figure3 (d), the ZnO-containing PAA mesospheres are distinguished along with unembedded ZnO nanocrystals around them. For the highest ammonia concentration (four times the nominal amount), the Si-based shell is thicker, which results in a weaker contrast of the ZnO nanocrystals (Figure3 (e)). More unembedded ZnO nanocrystals can also be observed, suggesting that ammonia at high concentrations disrupts the PAA mesospheres. The effect of ammonia thus appears to be twofold: as its content in solution increases, the successful deposition of silica increases while the mesosphere structures are gradually degraded.

Figure 3 :

 3 Figure 3: TEM images of TEOS-3 samples with different amount of NH4OH, (a) (b) TEOS-3-1, (c) (d) TEOS-3-2 and (e) (f) TEOS-3-4.

Figure 4 :

 4 Figure 4: Atomic ratio of Si to Zn in TEOS-3-y samples measured with RBS.

Figure 5

 5 Figure5shows representative TEM images of silica-coated ZnO TEOS-3-2 nanostructures. Figure5 (a)exhibits ZnO/PAA mesospheres decorated with SiO2 nanoparticles. We intentionally focused our observations on a few ZnO nanocrystals that were not fully embedded in PAA mesospheres. This enabled us to obtain an unambiguous image of the presence of Si-based matrix, since the sample in these regions is much thinner than PAA mesospheres containing ZnO nanocrystals. In the latter case, it would be more difficult to distinguish the Si-based phase from that of the PAA since the PAA mesospheres are rather thick (~ 100 nm) for a high-resolution TEM analysis. It can be clearly seen in Figure5(b) and (c) that wurtzite ZnO nanocrystals of ~ 5 nm in size are uniformly dispersed in an amorphous matrix. The main statement is therefore that Zn and Si domains are clearly separated. The results obtained by HAADF in a similar region are presented in the inset of Figure 5 (b). The brightest areas of the HAADF image are the signals from the heavier phase (Zn-rich) and the dim areas from lighter ones (Si-rich).

Figure 5 :

 5 Figure 5: TEM images of TEOS-3-2 (a) silica-decorated ZnO/PAA mesosphere and SiO2 nanoparticles images (inset, the scale-bar represents 10 nm), (b) TEM showing ZnO nanocrystals in amorphous matrix and HAADF image (inset) evidencing the presence of Si (dim area) and Zn (bright area), and (c) HRTEM image with the inset showing fast Fourier transformed (FFT) image of the red square area of a single ZnO nanocrystal.

Figure 6 :

 6 Figure 6: Elemental local analysis of a cluster of ZnO nanocrystals coated with silica in TEOS-3-2 sample. Top left: HAADF image. The other three panels display the elemental maps for Si, C and Zn, respectively.

Figure 7 :

 7 Figure 7: Effect of the [NH4OH] onto the silica coating of ZnO-PAA mesospheres.

Figure 8 :

 8 Figure 8: Normalized PL spectra of ZnO/PAA, TEOS-3-1 and TEOS-3-2 samples.

2 Figure 9 :

 29 Figure 9: PLE spectra at λem = 550 nm of ZnO/PAA, TEOS-3-1 and TEOS-3-2 samples with different amounts of NH4OH. The spectra are normalized to the intensity at λex = 266 nm.

Figure 10 :

 10 Figure 10: PL QY of TEOS-3 samples with different amount of NH4OH before (blue circles) and after (red squares) the deposition of the silica coating.

Figure 11 :

 11 Figure 11: Evolution of the normalized PL intensity ZnO/PAA, TEOS-3-1 and TEOS-3-2 at different temperatures from 25 °C to 100 °C. The circled labels refer to the values reached after cooling back to room temperature.

Table 1 :

 1 SiO2 yield (%) depending on the synthesis conditions

	NH4OH mass (g)	0.027	0.054	0.081	0.108
	without ZnO/PAAH	46 (75*)	73 (100*)	95	100
	With PAAH	< 1	< 1	< 1	3
	With ZnO/PAAH	< 1	12	27	48
	* after 24 hrs of reaction

Acknowledgements

Y. Z. is grateful to the CSC for Ph-D scholarship. The authors acknowledge the Consortium Lyon Saint-Etienne de Microscopie (CLYM) for TEM measurements, the IP2I laboratory (Université de Lyon) for RBS measurements and Prof. C. Dujardin's team (ILM, Université de Lyon) for technical assistance in the temperature PL measurements.