Enhancement of Two-Photon Initiating Efficiency ... - ACS Publications

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Enhancement of Two-Photon Initiating Efficiency of a 4,4′Diaminostyryl-2,2′-bipyridine Derivative Promoted by Complexation with Silver Ions Arnaud Spangenberg,*,† Jean-Pierre Malval,*,† Huriye Akdas-Kilig,‡ Jean-Luc Fillaut,‡ Fabrice Stehlin,† Nelly Hobeika,† Fabrice Morlet-Savary,† and Olivier Soppera† †

Institut de Science des Matériaux de Mulhouse, LRC CNRS 7228, Université de Haute Alsace, 15 rue Jean Starcky, 68057 Mulhouse, France ‡ Sciences chimiques de Rennes, UMR 6226, CNRS - Université Rennes I, Campus de Beaulieu, 263 av. du Général Leclerc, 35042 Rennes, France S Supporting Information *

ABSTRACT: We report on the two-photon-induced polymerization (TPIP) ability of a new class of free radical two-photon initiator based on a cationic silver(I) complex incorporating 4,4′-diaminostyryl-2,2′bipyridine (DES) derivatives as ligands. Coordination with Ag+ induces a strong increase of the charge transfer character at excited state, which enhances the two-photon absorption properties of the complex with respect to that of the free ligand. A comparative analysis of the photophysical properties of DES and DES2Ag+ shows that the presence of silver cation increases the efficiency for the generation of radical cations (DES•+), which can be used as hydrogen abstractor in free radical photopolymerization. We show that the addition of an aliphatic amine used as hydrogen donor also opens a parallel route for the regeneration of DES. The improvement of the two-photon polymerization efficiency is then evidenced by the fabrication of microstructures. We finally demonstrate that the use of DES2Ag+ as two-photon initiator offers new opportunities for the fabrication of functional nanostructures composed of metal−polymer nanocomposites.

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INTRODUCTION Since the past decade, two-photon-induced polymerization (TPIP) has been extensively studied due to the growing need of three-dimensional (3D) micro/nanofabrication techniques in many fields such as nanophotonics, nanoplasmonics, microfluidics, or nano- and microelectromechanical systems (NEMS/ MEMS).1−11 Among the numerous approaches for fabricating 3D microstructures, direct laser writing processes based on multiphoton polymerization provide a unique opportunity to create complex microstructures by combining both sub-100 nm resolution and relatively high throughput. In addition, various engineering materials such as polymers, ceramics, metals, and hybrid materials12−16 have been successfully used for 3D microfabrication of micromodels,17 micromachines,8,18 and microdevices.1−5 Nowadays, many efforts are focused on the development of super-resolution TPIP and on the design of new materials which can simultaneously respond to the need of higher resolution and to specific criteria such as biocompatibility for emerging fields like biosciences.19,20 Hence, the development of new two-photon-induced initiating systems appears fundamental so as to improve the sensitivity of the materials and consequently allows the use of lower laser power and less time-consuming processes.21−24 In this context, an elegant approach of increasing the two-photon absorption © 2012 American Chemical Society

efficiency of photoinitiator (PI) is based on the complexation of PI dyes around a metal ion core. Such a geometrical organization leads to important photophysical changes, particularly a strong enhancement of the two-photon absorption cross section. Recently, we applied this strategy with 4,4′-diaminostyryl-2,2′-bipyridine series. Indeed, we showed that these “push−pull” chromophores exhibited a strong enhancement of their TPA cross sections when chelating a metal cation.25 Several groups also demonstrated that the coordination of bypiridine ligands with transition metal results in significant enhancement of their TPA cross section,26−31 but so far, no application to microfabrication has been attempted. It should be emphasized that even though the two-photon absorption mechanism can also refer to stepwise processes involving the absorption of an intermediate excited state, in this article the two-photon excitation of our metal-based complex leads to a simultaneous absorption of two photons.25 In the present study, we focus on a silver-based complex with 4,4′-bis(diethylaminostyryl)-6,6′diphenyl-2,2′-bipyridine (DES, Scheme 1) which showed very promising two-photon Received: October 4, 2011 Revised: December 21, 2011 Published: January 25, 2012 1262

dx.doi.org/10.1021/ma202224c | Macromolecules 2012, 45, 1262−1269

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Phosphorescence measurements were performed in ethanol at 77 K. The samples were placed in a 5 mm diameter quartz tube inside a Dewar filled with liquid nitrogen. Phosphorescence lifetimes were measured using the FluoroMax-4 spectrofluorometer which was also equipped with a Xe-pulsed lamp operating at up to 25 Hz. The phosphorescence time decays were obtained according to a time-gated method. The emission was recorded using a control module that includes a gate-and-delay generator which allows the integration of the signal during a specific period after a flash (delay) and for a predetermined time window. The total signal was accumulated for a large number of exciting pulses. Photolysis of photoinitiators and subsequently photoreduction of silver ions in solution were carried out under UV irradiation (P = 100 mW) via a medium-pressure mercury arc lamp (Oriel, main radiation at 365 nm) and monitored by UV−vis absorption spectroscopy. Solutions were N2-degassed prior irradiation. The two-photon excitable resin formulation used for microfabrication, typically DES (0.3 wt %), MDEA (1 wt %), and AgSbF6 (1 wt %) in monomer, was dropped on a glass coverslip. The 3D lithographic microfabrication was performed using a Zeiss Axio Observer D1 inverted microscope equipped with a frequency-doubled Nd:YAG microlaser Nanolase from JDS Uniphase (λexc = 532 nm, pulse duration 0.61 ns, maximum pulse energy 2 μJ, repetition 11.8 kHz). The sample was mounted on a computer-controlled 3D piezoelectric stage allowing the translation relative to the laser focal point. The incident beam was focused through a 1.21 NA objective (100×) which leads to 300 nm radial spot size (1/e Gaussian). In the presence of silver ions, the average laser power and scan speeds were fixed at 130 μW and 40 μm/s, respectively. In the absence of silver ions, due to higher polymerization threshold, the average laser power was set to 732 μW. This entire lithography setup was purchased from Teemphotonics. Transmission electron microscopy (TEM) was used to characterize the size and shape of Ag nanoparticles. Before getting their TEM images, the microgrids were cut by means of a microtome (LKB model 8800). Transmission electron microscopy measurement was carried out at 200 kV using a Philips CM20 instrument with Lab6 cathode. Energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (SEM) were performed using a FEI-Quanta 400 model. AFM images were acquired using atomic force microscopy (PicoPlus model, Molecular Imaging AFM) in an ambient atmosphere. The characterization was obtained in resonant mode using a silicon noncontact probe (Arrow Silicon SPM Sensor; force constant: 42 N/ m; resonance frequency: 285 kHz).

Scheme 1. Molecular Structure of DES

absorption properties. In addition, Ag+ precursors have been used recently in our group for simultaneous photoreduction and photopolymerization.32−34 Although our first goal here is to enhance the two-photon initiating efficiency, such a strategy can be advantageously used for the production of nanocomposites in a single step by TPIP. After a short introduction about the photophysical properties of DES, the effects of complexation with Ag+ are discussed. Through a comparative study of the photolysis of the dye and its complex, we then propose a sequential mechanism that leads to photoinitiating reaction. Finally, we demonstrate the interest of such molecular systems for TPA microfabrication by using low-power microlaser to produce microstructures by TPIP.

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EXPERIMENTAL SECTION

Materials. All the solvents employed for photophysical analysis were Aldrich or Fluka spectroscopic grade. Antimony silver fluoride (AgSbF6), 4-diethylaminobenzaldehyde, and N-methyldiethanolamine (MDEA) were purchased from Aldrich. The monomer used, namely Eb605 (bisphenol A epoxy diacrylate diluted with 25% tripropylene glycol diacrylate), was kindly provided by Sartomer. All materials were used without further purification. Synthesis of DES. The synthesis of DES (4,4′-bis(diethylaminostyryl)-6,6′diphenyl-2,2′-bipyridine) was already described in previous work25 and results of a double Knoevenagel condensation between 6,6′-diphenyl-4,4′-dimethyl-2,2′-bipyridine and 4-diethylaminobenzaldehyde in dimethylformamide and tBuOK. Recrystallization from a dichloromethane−pentane mixture gave DES as an orange microcrystalline powder (230 mg, 79%). 1H NMR (CD2Cl2, 500 MHz): δ ppm 8.68 (s, 2H, H6,6′), 8.31 (d, J = 8.3 Hz, 4H, 6H4-), 7.92 (s, 2H, H3,3′), 7.58 (m, 12H, C6H5− + −CH CH−), 7.10 (d, J = 16.3 Hz, 4H, −CHCH−), 6.75 (d, J = 9.0 Hz, 4H, C6H5−), 3.46 (q, J = 7.1 Hz, 8H, CH3CH2−), 1.24 (t, J = 7.1 Hz, 12H, CH3CH2−). 13C NMR (CDCl3, 400 MHz): δ ppm 156.8, 156.5, 148.1, 147.3, 140.0, 133.1, 128.7, 128.68, 128.64, 127.1, 123.7, 121.6, 117.0, 116.8, 111.6, 44.4, 12.6. Anal. found: C, 83.09; H, 7.05; N, 8.72. C46H46N4·0.5H2O. Calcd: C, 83.22; H, 7.14; N, 8.44. m/z (MicroTOF-Q II) 655.3793; ([M + H]+, C46H47N4 requires 655.3795). General Techniques. The absorption measurements were carried out with a Perkin-Elmer Lambda 2 spectrometer. Steady-state fluorescence and phosphorescence spectra were collected from a FluoroMax-4 spectrofluorometer. Emission spectra were spectrally corrected. Fluorescence quantum yields include the correction due to the solvent refractive index and were determined relative to quinine bisulfate in 0.05 M sulfuric acid (Φ = 0.52). Quantum yields of trans → cis photoisomerization were carried out under irradiation at 370 nm. The progress of the reaction was monitored via UV−vis absorption spectra. The absorbance at excitation wavelength was greater than 2 to assume a total absorption of the incident photons. The incident light intensity was measured by ferrioxalate actinometry.35

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RESULTS AND DISCUSSION Photophysical Properties of DES. Figure 1 depicts the absorption and the fluorescence spectra of DES in acetonitrile and hexane. The low-energy side of DES absorption spectrum is dominated by an intensive band located in the 350−450 nm

Figure 1. Normalized absorption (solid lines) and fluorescence (dashed lines) spectra of DES in acetonitrile (gray) and hexane (black). λexc = 370 nm. 1263


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Figure 2. (A) Absorption and (B) fluorescence spectra obtained upon increasing amount of AgSbF6 (from 0 to 3300 equiv). Inset: absorption− concentration profile recorded at 445 nm (solvent: acetonitrile, [DES] = 1 × 10−6 M, λexc = 415 nm).

range. This band mainly corresponds to a π−π electronic transition (S0−S1) centered on the stilbene moieties of the chromophore and exhibits a charge transfer (CT) character from the amine (donor) to the bipyridine (acceptor) groups. At the blue side of the CT band a shoulder located at 320 nm is observed which typically corresponds to the π−π transition centered in the 2,2′-bipyridine moiety.36,37 The longest wavelength band undergoes a significant red-shift upon increasing solvent polarity. For instance, from hexane (apolar solvent) to acetonitrile (highly polar solvent), the band shifts about 20 nm. This effect clearly confirms the CT character of the S0−S1 transition. Such bathochromic effects are drastically enhanced when considering the fluorescence band of DES. In this case, the emission band shift is about 120 nm from apolar to polar solvent. Moreover, the vibronic structurations observed for the absorption and fluorescence bands in hexane completely disappear in polar medium. For this push−pull stilbene derivative, we have therefore a clear indication of a strong electronic relaxation at excited state which leads to the enhancement of the CT character of the emitting excited state with respect to the ground state. Even if the fluorescence spectrum of DES undergoes a strong solvent-induced spectral shift, the polarity of the solvent hardly affects the fluorescence quantum yield (Φfluo). For instance, Φfluo of DES exhibits low values of ca. 0.05 and 0.12 in hexane and acetonitrile, respectively. This weak emissivity indicates that the excited S1 state of DES mainly undergoes nonradiative deactivation processes irrespective of the polarity of the solvent. Among these multiple deactivation channels, the photoinduced transto-cis isomerization should be considered as minor route since the corresponding quantum yield is very low with values of ca. 0.09 and 0.03 in hexane and acetonitrile, respectively. Hence, we propose that both S1−S0 internal conversion and S1−Tn intersystem crossing constitute the main deactivation pathways of the lowest singlet excited state of DES. Figure S1 shows the emission spectrum of DES in glassy ethanol. The fluorescence band is located in the 425−550 nm range with a maximum at 455 nm. In the low-energy side of spectrum, the phosphorescence band of DES exhibits a maximum at 620 nm which leads to a T1 energy of ca. 2.00 eV. This low T1 state energy is similar to those previously measured for D−A stilbene derivatives.38 Moreover, the phosphorescence lifetime has a value of 64 ms, which suggests a π−π* character for T1 state.39 Such an electronic configuration is in agreement with the weak reactivity of the T1 state of DES as hydrogen abstractor which will be underlined hereafter.

Formation of the Complex DES2Ag+ in Solution. The formation of a DES complex with Ag+ in acetonitrile is evidenced by both absorption and fluorescence spectroscopies. Upon gradual addition of Ag+, the CT band of DES centered at 400 nm is progressively red-shifted (Figure 2A). This change is in agreement with a stronger electron-withdrawing character of the bipyridine group after coordination with Ag+ cations. The complexation is confirmed by the growth of the shoulder located at 320 nm which is typical of coordination of a metal ion to the bipyridine subunit.36,40 Three isosbestic points at 277, 342, and 408 nm confirm the occurrence of an equilibrium reaction. In order to determine the reaction constant, the absorption changes as a function of the concentration of Ag+ have been analyzed according to the model proposed by Valeur et al.41 The best fit is obtained with the assumption of a main formation of a 1:2 complex (metal:ligand) which is the typical complex stoichiometry observed for such bidentate chromophores.25 The inset Figure 2A depicts the best-fitting curve of the experimental data using the method of least squares, with a calculated log K1:2 value of ca. 9.14 ± 0.03. Figure 2B depicts the effect of gradual addition of Ag+ on the fluorescence spectrum of DES. The presence of silver cations both induces a strong decrease of the fluorescence band of the free ligand and the progressive growth of a weak band in the 550−750 nm range. The excitation spectrum at the longer wavelength and collected at the maximum emission wavelength at 650 nm matches perfectly the longest wavelength absorption band of the silver complex which confirms that the emission at the lowenergy region stems from the excited DES2Ag+. Upon quantitative complexation, the fluorescence quantum yield of DES is divided by a factor 8 (from 0.12 to 0.015) in acetonitrile. The very low emissivity of DES2Ag+ with respect to that of the free ligand should be ascribed to the occurrence of a new nonradiative deactivation channel connected to a strong increase of the CT character at S1 state which promotes the twist of the dialkylaniline group at excited state. Such a largeamplitude motion at S1 state has been recently proposed as the major deactivation channel in the excited-state dynamics of dialkylaminostyrylpyridinium derivatives leading to the formation of low emissive twisted internal charge transfer (TICT) state.42 The significant increase of the charge transfer character at S1 state is also consistent with the strong enhancement of the TPA cross section of the complex by a factor 4 as compared to that of DES.25 This latter effect will have a strong consequence on the two-photon induced polymerization threshold as described hereafter. 1264


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Figure 3. Absorption spectra as a function of irradiation time in different acetonitrile mixtures. Solutions were N2-degassed prior irradiation except in case D. (A) [DES] = 1.5 × 10−5 M; (B) [DES] = 1.5 × 10−5 M and [MDEA] = 3 × 10−2 M; (C, D) [DES] = 1.5 × 10−5 M, [Ag+] = 2 × 10−2 M and [MDEA] = 3 × 10−2 M. Insets show kinetics at λmax (400 nm in A, B cases and 417 nm in C, D cases) and at 555 nm in the presence of Ag+ upon UV irradiation of mixture. Irradiation conditions were identical in both cases (P = 100 mW).

Figure 4. TEM images of Ag nanoparticles: (A) obtained by UV irradiation of acetonitrile solution N2-degassed containing 1.5 μM DES, 20 mM Ag+, and 30 mM MDEA (P = 100 mW); (B) embedded in microstructures obtained by 532 nm laser pulse excitation of Ebecryl 605 formulation containing 0.3 wt % DES, 1 wt % MDEA, and 1 wt % AgSbF6 (power was measured after the ×100 objective (N.A. = 1.21): P = 130 μW).

(MDEA) is added (i.e., 3 × 10−2 M). In this case, the decrease of absorption band during irradiation is drastically slowed down, which indicates that the photobleaching reaction is strongly reduced in the presence of MDEA. Finally, addition of silver cations (i.e., 2 × 10−2 M) to a solution which both contains the chromophore and MDEA leads to the growth of a new band in the 350−700 nm range (Figure 3C). This large band should be assigned to the surface plasmon resonance of silver nanoparticles (NPs) that progressively aggregates during irradiation. The production of metal nanoparticles is also confirmed by TEM analysis of the sample as shown in Figure 4A. The TEM image clearly indicates a polydispersed distribution of Ag NPs with an average diameter of 50 nm. Hence, a photoreduction of the silver cation is demonstrated. Interestingly, this photoreaction is inhibited by oxygen since

Study of the Photolysis of the DES2Ag+ Complex. Figure 3 shows the evolution of the absorption spectra of different solutions of DES in acetonitrile in presence of Ag+ and N-methyldiethanolamine (MDEA) as a function of irradiation time. Solutions are N2-flushed prior irradiation to prevent inhibiting effect of oxygen, except in case of Figure 3D. It should be noted all these reactants will be used for TPIP; therefore, their respective role in the photoinitiating reaction should be first clarified. Figure 3A corresponds to a 1.5 × 10−5 M DES solution in acetonitrile. The main band located at 400 nm drastically decreases during irradiation process and fully collapses after 180 s of irradiation. In parallel to this photobleaching, an absorption band at 335 nm arises underlining the formation of a photoproduct. In the second solution (Figure 3B), a large excess of N-methyldiethanolamine 1265


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the photoreduction of Ag+ is hardly observed when the same solution is not N2-degassed as shown in Figure 3D. Moreover, in this case, the absorption spectrum of DES2Ag+ remains nearly invariant during irradiation which is ascribable to the presence of the amine as previously observed for the absorption spectrum of the free ligand in Figure 3B. Such an enhancement of the photostability observed for the dye or for its metal complex should be explained according to a regenerative pathway of the DES promoted by MDEA. In order to advance the understanding of this complex process, we propose a general mechanism susceptible to describe the photoreduction of silver ions and the regenerative pathway of DES as observed experimentally. As previously demonstrated for diaminofluorene derivatives,33 we propose that the major route for the photobleaching of DES stems from the production of the corresponding radical cation (DES•+) and anion (DES•−). These radicals result from a self-quenching reaction of DES (eq 1) between its excited triplet state (3DES) and its ground state (DES) as follows: 3

•+

DES + DES → DES

DES•+ + MDEA → DESH+ + MDEA• (H abstraction) −H +

DESH+ Xooooo+Y DES

(6)

The first reaction step corresponds to the hydrogen transfer reaction which also produces the acid form of DES (eq 5). The second reaction step is a rapid acid−base equilibrium (eq 6) that regenerates DES due to the strong basicity character of MDEA which is present in large excess (i.e., 3 × 10−2 M). Finally, the presence of silver cation leads to an efficient redox reaction with MDEA•:

Ag + + MDEA• → Ag 0 + MDEA+ (Ag + reduction)

(7)

The efficiency of this process is strongly affected by the presence of oxygen. This effect can be correlated with the sensitivity of α-aminoalkyl radicals toward oxygen49 and is consistent with the slight increase of the surface plasmon resonance band during the irradiation, as shown in Figure 3D. Hence, a regenerative process of the chromophore and its metal complex is observed in presence of MDEA which undergoes a hydrogen abstraction and produces a highly reducible αaminoalkyl radical. The proposed mechanism is summarized in Scheme 2. When reactions are performed in monomer medium, photopolymerization occurs simultaneously with the

+ DES

(1)

DES•+ + DES•− → DES + DES (ions recombination)

(acid−base equilibrium)

+H

•−

(self‐quenching)

(5)

(2)

Both radical ions can either recombine (eq 2) or initiate the photobleaching process.43 Similar reactions should also occur for the metal complex as

(4)

Scheme 2. Proposed Photoinitiation Mechanism of Simultaneous Photopolymerization and Photoreduction via a Regenerative Process Leading to Higher Formation of Radical Cations DES•+

The self-quenching reaction for DES2Ag is not controlled by diffusion since both reactants are initially in close proximity (eq 3). In contrast with the mechanism of photobleaching involving the free ligand, the radicals recombination step for the complex should compete with the concomitant reduction of Ag+ (eq 4). Therefore, the presence of the silver cation should indirectly contribute to the increase of the quantum yield for the generation of radical cation DES•+. This latter effect is illustrated by Figure S8 (see Supporting Information) which displays the changes of the absorption spectrum of DES2Ag+ upon irradiation. While the main absorption band undergoes a strong decrease with a slight red-shift, a new weakly intensive band is clearly developing in the 590−790 nm region. This band is similar to that obtain when mixing DES with Cu(NO3)2 and should be ascribed to the formation of DES•+. Indeed, it has been previously reported a simple chemical method to obtain radical cation from aromatic amine. The reaction involves the mixing of aromatic amine with Cu(NO3)2 in acetonitrile solution.44−46 Finally, it is noteworthy that the reduction of Ag+ by DES•− (eq 4) constitutes a minor route for the production of NPs as compared to the major channel opened in presence of MDEA. MDEA is usually used as a coinitiator which undergoes a hydrogen abstraction from excited triplet species or radical cations.33,47,48 This reaction then produces α-aminoalkyl radicals (MDEA•) which can both initiate free radical photopolymerization49 and reduce silver cations.32,33,50−52 Therefore, the sequential steps for the dye regeneration should be described as follows:

photoreduction. Therefore, FTIR experiments have been performed under one photon condition to test the photoinitiation ability of DES and DES2Ag+. Surprisingly, no photoinitiation activity was observed. However, since these photoinitiators are specifically dedicated to TPIP, their reactivity will be probed via two-photon microfabrication. This last step will be the object of the next section. Microfabrication by Two-Photon-Induced Polymerization (TPIP) and Structural Characterizations. The potential of DES and DES2Ag+ as novel two-photon activable photoinitiators has been evaluated for the fabrication of

3

DES2Ag + → DES•+ + DES•− + Ag +

DES•− + Ag + → DES + Ag 0

(3)

+

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in order to form the complex. It has to be noted that DES alone in solution can initiate the two-photon polymerization reaction, but owing to a lack of reactivity compare to that in presence of MDEA, we focused on formulations containing MDEA for microfabrication (see Figure S4). As shown in Figure 5, the energy threshold in formulation A (Figure 5A) is found to be ca. 3 times higher than that relative to formulation B (Figure 5B). Indeed, the formation of voxel induced by a minimum power is 326 and 130 μW for formulation A and B, respectively. Owing to the competition between the photoreduction and the photopolymerization in the presence of Ag+, we would have expected to a much higher energy threshold in favor of the formulation B as demonstrated and described by Prasad et al. in the case of gold nanoparticles-doped polymeric structures.15 In the frame of this work, this opposite effect can be explained by two distinct phenomena both resulting from the complexation of DES with Ag+. The first point corresponds to an increase of the TPA cross section. In short, this consists in taking advantage of the complexing properties of the bipyridine functions to bond the PI around a metal ion. Indeed, metal cations can be used as 3D templates31 which organize a set of organic ligands into a multipolar arrangement for the promotion of TPA properties. In the case of the formation of the complex DES2Ag+, a strong enhancement of the TPA cross section by a factor 4 was measured in ref 25. The second point concerns the formation of radical cations DES•+ which is a key step since it will affect the production of α-aminoalkyl radicals

microstructures by using a 532 nm laser pulse excitation since these photoinitiator systems do not absorb at this wavelength. For each system, arrays of dots are produced to determine the energy threshold defined here as the minimum energy to observe microdots inside the photopolymer matrix (Figure 5). The microdots represent polymerized regions which can be observed by classical optical microscopy due to the difference

Figure 5. Determination of energy threshold by decreasing gradually the power of 532 nm laser pulse excitation source (from 1140 to 187 μW) obtained with (A) formulation A containing 0.3 wt % DES and 1 wt % MDEA in Ebecryl 605 monomer (EB605) and (B) formulation B containing 0.3 wt % DES, 1 wt % MDEA, and 1 wt % AgSbF6 in Ebecryl 605 monomer. Exposure time was set to 10 ms. Scale bar: 6 μm.

of refractive index between the monomer and the polymer. The writing process is described in detail in the Experimental Section. Two types of formulation A and B have been used; both contain 0.3 wt % DES and 1 wt % MDEA in Ebecryl 605 monomer (EB605). In formulation B, 1 wt % AgSbF6 is added

Figure 6. SEM images of the polymeric structures designed by CAD (computer-aided design). (A) and (B) correspond to a microgrid with spacing between two lines of 5 μm. (C) and (D) correspond to a microgrid with spacing between two lines of 2 μm. (A, C) EB605/DES microgrids produced at 732 μW. (B, D) EB605/DES/Ag NPs microgrids produced at 130 μW. Insets show the close-up view of the grids. 1267


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(MDEA•) responsible of the photopolymerization and the photoreduction. On the basis of our experimental observations and the above proposed mechanism (see photolysis section), a more important yield of radical cation DES•+ is expected with DES2Ag+. Using this data, more complex shapes are generated, consisting in microgrids of 100 μm × 100 μm: Figures 6A and 6C exhibit the fabrication of microgrids in absence of Ag+ with respectively a spacing between two lines of 5 and 2 μm. In order to avoid washing off the structures from the glass substrate during the development step, the power is set to 732 μW, which is almost 2 times the threshold value determined previously. The structures present submicrometric features: lines widths are measured to be ca. 780 nm. Images profiles obtained by AFM provide accurate measurements of their sizes in z dimension (1.1 μm, see Figure S2A). Similar structures based on identical computer model have been fabricated in the presence of Ag+. In this case, the power is set to 130 μW (Figure 6B,D), with no observation of sample delamination. The required power to write microstructures is thus divided by a factor of 5 in the presence of Ag+. Interestingly, in this case, the value used for the polymerization threshold measurement can be used as a reference for microstructuring. This fact shows that the polymerization process initiated by the DES2Ag+ complex is efficient and produces a highly cross-linked polymer network compatible with applications in microfabrication. Moreover, measurements of lines widths by SEM give a value ca. 270 nm, and AFM measurement values of the heights are ca. 750 nm. The ratio between lateral and vertical size is in agreement with expected values observed in TPIP. Energy-dispersive X-ray spectroscopy (EDX) is also performed to detect the presence of Ag NPs inside the microgrids (Figure S3 in Supporting Information). According to EDX spectra, 0.3% of the material probed is composed of silver. No Ag+ counterions, such as antimony or fluoride, have been detected by EDX, suggesting that all silver in the structure is totally reduced. Finally, the microstructure is cut by means of a microtome, and TEM image are recorded showing clearly well-isolated and spherical Ag NPs embedded in polymer matrix (Figure 4B). As depicted in inset of Figure 4B, the distribution of NPs’ size exhibits wide dispersion (from 5 to 40 nm), but no noticeable aggregation effect is observed. Such a system is really interesting since it opens the doors for the fabrication of metal−polymer nanocomposites by TPA microfabrication which are highly promising materials for optical, electrical, and medical applications.

photoinitiator can play a crucial role in the emerging research area of 2D and 3D microfabrication of nanocomposites and metallic structures. This role is illustrated by the TPIP fabrication of microgrids with an inexpensive and low-power microlaser, leading to the observation of nanofeatures and Ag NPs embedded in a polymer matrix.

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ASSOCIATED CONTENT

S Supporting Information *

Emission spectrum of DES in glassy ethanol; structural characterization and composition analysis of microgrids using atomic force microscopy (AFM); energy-dispersive X-ray spectroscopy (EDX); experimental evidence of DES’s TPA properties; 1H and 13C NMR spectra; output spectrum of the UV irradiation source; experimental evidence of existence of DES•+. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (A.S.), jean-pierre. [email protected] (J.-P.M.).

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ACKNOWLEDGMENTS The authors gratefully acknowledge Loic̈ Vidal and Stéphan Knopf for help and discussion. The authors also acknowledge financial support by Agence Nationale de la Recherche (ANR Nanoquenching, Contract 10PDOC-009 01).

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REFERENCES

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CONCLUSION The two-photon-induced polymerization properties of a cationic silver(I) complex have been reported. We demonstrated that the presence of silver ions in the formulation influences strongly and positively the energy threshold of photopolymerization despite a potential competition with the photoreduction step of Ag+. This paradox was explained by the formation of a strong complex between the dye DES and the silver ions which exhibits ca. 4 times higher TPA cross section and leads to an increase of the yield in radical cations DES•+. Besides, in the presence of MDEA, a regenerative pathway of the complex during photoirradiation experiments was evidenced. Our present observations allow us to propose a mechanism involving several sequential reactions. By taking benefits from a remarkable compromise between the efficiencies of the photoreduction process and the twophoton-induced polymerization, we show that this type of 1268


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Enhancement of Two-Photon Initiating Efficiency ... - ACS Publications

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