Electrodeposition of Polypyrenes with Tunable Hydrophobicity, Water

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Electrodeposition of Polypyrenes with Tunable Hydrophobicity, Water Adhesion and Fluorescence Properties Gabriela Ramos Chagas, Xiao Xie, Thierry Darmanin, Karine Steenkeste, Anne Gaucher, Damien Prim, Rachel Meallet-Renault, Sonia Amigoni, Guilhem Daniel Godeau, and Frederic Guittard J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b11586 • Publication Date (Web): 18 Mar 2016 Downloaded from http://pubs.acs.org on March 21, 2016

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The Journal of Physical Chemistry

Electrodeposition of Polypyrenes with Tunable Hydrophobicity, Water Adhesion and Fluorescence Properties Gabriela Ramos Chagas,a Xiao Xie,b,d Thierry Darmanin,a Karine Steenkeste,d Anne Gaucher,c Damien Prim,c Rachel Méallet-Renault,d Guilhem Godeau,a Sonia Amigoni,a Frédéric Guittarda,*

a

Univ. Nice Sophia Antipolis, CNRS, LPMC, UMR 7336, 06100 Nice, France [email protected]

b

c

ENS Cachan, Institut d'Alembert (FR3242), Laboratoire PPSM (UMR 8531), CNRS, Université Paris-Saclay, 61 av Président Wilson, 94230 Cachan, France

Université de Versailles St-Quentin, UMR CNRS 8180, Institut Lavoisier de Versailles, 45, avenue des Etats-Unis, 78035 Versailles Cedex, France [email protected]

d

Institut des Sciences Moléculaires d’Orsay (ISMO), CNRS, Univ. Paris-Sud, Université Paris-Saclay, F-91405 Orsay (France) [email protected]

1

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ABSTRACT: The control in surface hydrophobicity and water adhesion is extremely important for various applications in water harvesting, oil/water separation membrane, energy systems or biosensing, for example. Here, for the first time we show that the use of fluorescent monomers such as Pyrene with various substituents differing by their hydrophobicity, size or rigidity/flexibility can lead to surfaces with tunable hydrophobicity, water adhesion and fluorescence properties by a direct electropolymerization process. Seven original monomers with fluoroalkyl, alkyl, phenyl, adamantly and triethyleneglycol substituents were synthesized and studied. The surface roughness is highly dependent on the substituent and it seems that the fluorescence, although complex, correlates with the surface roughness. Superhydrophobic properties and highly oleophobic properties are obtained using fluoroalkyl chains due to the presence of nanostructured microparticles. In comparison to the structured absorption and emission bands of Pyrene monomers, the Pyrene polymers exhibit a broad structureless spectral shape both in absorption and emission. This work is a first tentative to combine superhydrophobic and fluorescent properties using an innovative strategy and opens new doors to explore in this domain.

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INTRODUCTION Today, the study of materials with superhydrophobic properties is extensively reported both for their various wetting theories and for the very wide range of potential applications such as in self-cleaning textiles, separation membranes, drag reduction for microfluidic devices, water harvesting, optical devices, anti-fogging windows, anti-corrosion or antibacterial coatings.1-5 The superhydrophobicity is characterized by both extremely high water apparent contact angle (θwater) and extremely low water adhesion or hysteresis (H), and was found to be relatively present in Nature.6,7 Superhydrophobic properties can be obtained by controlling surface roughening and surface energy.8 However, in Nature other species such as the geckos,9 red roses10 or the peaches11 have both extremely high θwater and H (parahydrophobic properties)12,13 while other species (cacti, desert plants and animals)14-16 can capture water droplets and as a consequence survive in extremely hot environment thanks to surfaces having different surface energies or surface structures/roughness. Hence, it is possible to control the surface hydrophobicity and water adhesion by adapting the geometrical parameters of surface structures and the surface energy.17,18 Conducting polymers are unique materials for the control of these two parameters19,20 for their opto-electronic properties,21,22 which is also extremely interesting to obtain smart materials, but also for the possibility to produce tunable nanostructured materials. These nanostructures can be obtained in solution by self-assembly during their formation23-25 but the growth is also possible directly on substrates using of templates26 or without templates by preferential growth,27 grafting,28 electrospinning,29 vapor phase polymerization,30 plasma polymerization31 or electropolymerization,32 for example. The growth of structured conducting polymers by electropolymerization is a choice method for the rapidity of 3



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implementation and the easy control in the surface structures.33 The surface energy of conducting polymers can be controlled using dopant agents34 or by grafting a hydrophobic substituent.35-37 The surface structures can be tuned either by electrochemical parameters38 or the used monomer.39,40 In this aim, the polymerizable core of the monomer plays a key role. Different polymerizable cores were tested in the literature such as pyrrole, thiophene or aniline.35-40 However, if aromatic rings such as pyrene, anthracene, naphthalene, phenanthrene or triphenylene can be used as polymerizable core,41-46 their use to obtain superhydrophobic properties remains a challenge. Moreover, the use of these aromatic molecules can have other advantages such as fluorescence properties. The choice of the aromatic ring is extremely important because both the monomer solubility, the polymerization capacity, the surface morphology and the fluorescence properties can be affected. Here, we report the first study of pyrene substituted with various substituents differing by their hydrophobicity, size or rigidity/flexibility (c.f. Scheme 1): fluorinated chains of different length (C4F9, C6F13 and C8F17), decyl, phenyl, triethylene glycol and adamantyl. All the monomers reported here are original molecules. The effect of the substituent on the surface morphology, surface hydrophobicity and fluorescence properties is discussed.

Scheme 1. Original monomers synthesized and studied in this work.

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EXPERIMENTAL SECTION Monomer Synthesis. The monomers were synthesized by esterification between 1pyreneacetic acid and the corresponding alcohols as shown in Scheme 2.

Scheme 2. Synthesis way to the monomers.

More precisely, in an ice bath, 1.9 mmol of pyrene acetic acid (0.5 g), 1.9 mmol of N-(3dimethylaminopropyl)-N′-ethylcarbodiimide

hydrochloride

(EDC),

1.9

mmol

of

4-

dimethylaminopyridine (DMAP) and 1.9 mmol of the corresponding alcohol were mixed to 20 mL of dichloromethane. Then, the mixture was stirred at room temperature for 48 hr. The crude products were purified by column chromatography on silica gel and using dichloromethane/methanol 9:1 as eluent for Pyrene-TEG and chloroform/cyclohexane 1:1 for the other monomers. Pyrene-F8:

3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluorodecyl

2-(pyren-1-

yl)acetate. Yield 30%; Yellow crystalline solid; m.p. 112.4 °C; δH(200 MHz, CDCl3): 8.06 (m, 9H), 4.41 (t, J = 6.5 Hz, 2H), 4.37 (s, 2H), 2.43 (tt, J = 18.3 Hz, J = 6.5 Hz, 2H); δF(188 MHz, CDCl3): -80.74 (m, 3H), -113.56 (m, 2H), -121.91 (m, 6H), -122.73 (m, 2H), -123.51 (m, 2H), -126.12 (m, 2H); δC(50 MHz, CDCl3): 171.17, 131.28, 130.95, 130.74, 129.41, 128.35, 128.04, 127.40, 126.02, 125.35, 125.19, 125.03, 124.86, 124.69, 123.00, 56.88 (t, J =

5



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3.0 Hz), 39.21, 30.47 (t, J = 21.9 Hz); FTIR (main vibrations): ν = 3044, 2975, 2920, 1738, 1239, 1203, 1148, 840; MS (70 eV): m/z 706 (M+, 2), 215 (C17H11+·, 100). Pyrene-F6: 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctyl 2-(pyren-1-yl)acetate. Yield 64.1%; Yellow crystalline solid; m.p. 82.0 °C; δH(200 MHz, CDCl3): 8.06 (m, 9H), 4.41 (t, J = 6.6 Hz, 2H), 4.37 (s, 2H), 2.47 (tt, J = 18.3 Hz, J = 6.6 Hz, 2H); δF(188 MHz, CDCl3): 80.77 (m, 3H), -113.60 (m, 2H), -121.95 (m, 2H), -122.94 (m, 2H), -123.62 (m, 2H), -126.18 (m, 2H); δC(50 MHz, CDCl3): 171.18, 131.28, 130.95, 130.75, 129.41, 128.35, 128.05, 127.40, 126.03, 125.36, 125.19, 125.03, 124.86, 124.70, 123.00, 56.88 (t, J = 5.0 Hz), 39.22, 30.46 (t, J = 22.2 Hz); FTIR (main vibrations): ν = 3044, 2975, 2920, 1738, 1238, 1206, 1144, 842; MS (70 eV): m/z (%): 606 (17) [M+], 215 (100) [C17H11+·]. Pyrene-F4:

3,3,4,4,5,5,6,6,6-Nonafluorohexyl

2-(pyren-1-yl)acetate.

Yield

19%;

Yellow crystalline solid; m.p. 39.0 °C; δH(200 MHz, CDCl3): 8.06 (m, 9H), 4.41 (t, J = 6.5 Hz, 2H), 4.37 (s, 2H), 2.45 (tt, J = 18.3 Hz, J = 6.5 Hz, 2H); δF(188 MHz, CDCl3): -81.047 (m, 3H), -113.78 (m, 2H), -124.52 (m, 2H), -126.06 (m, 2H); δC(200 MHz, CDCl3): 171.16, 131.26, 130.93, 130.73, 129.39, 128.33, 128.02, 127.38, 126.01, 125.34, 125.18, 125.00, 124.84, 124.67, 122.99, 56.83 (t, J = 4.8 Hz), 39.18, 30.35 (t, J = 22.7 Hz); FTIR (main vibrations): ν = 3045, 2975, 2924, 1742, 1228, 1202, 1134, 842; MS (70 eV): m/z (%): 506 (8) [M+], 215 (100) [C17H11+·]. Pyrene-H10: Decyl 2-(pyren-1-yl)acetate.Yield 67.5%; Yellow crystalline solid; m.p. 48.7 °C; δH(200 MHz, CDCl3): 8.06 (m, 9H), 4.35 (s, 2H), 4.09 (t, J =6.6 Hz, 2H), 1.55 (m, 2H), 1.13 (m, 14H), 0.88 (t, J = 6.6 Hz, 3H); δC(50 MHz, CDCl3): 171.67, 131.29, 130.78, 130.75, 129.41, 128.35, 127.83, 127.39, 127.22, 125.93, 125.21, 125.06, 125.00, 124.82, 124.73, 123.32, 65.18, 39.69, 31.84, 29.41, 29.24, 29.10, 28.51, 25.76, 22.66, 14.11; FTIR

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(main vibrations): ν = 3033, 2951, 2913, 2850, 1730, 1462, 1342, 1179, 838; MS (70 eV): m/z (%): 400 (27) [M+], 215 (100) [C17H11+·]. Pyrene-Ph: Phenyl 2-(pyren-1-yl)acetate. Yield 58%; Yellow crystalline solid; m.p. 116.4 °C; δH(200 MHz, CDCl3): 8.20 (m, 9H), 7.21 (m, 5H), 4.58 (s, 2H); δC(50 MHz, CDCl3): 170.05, 150.74, 131.29, 130.98, 130.76, 129.32, 128.43, 128.13, 127.48, 127.38, 126.03, 125.82, 125.37, 125.22, 125.08, 124.94, 124.71, 123.08, 121.39, 39.70; FTIR (main vibrations): ν = 3041, 2928, 1759, 1494, 1201, 1123, 847. Pyrene-TEG: 2-(2-(2-Hydroxyethoxy)ethoxy)ethyl 2-(pyren-1-yl)acetate. Yield 45%; Slightly yellow liquid; δH(200 MHz, CDCl3): 8.11 (m, 9H), 4.39 (s, 2H), 4.26 (m, 2H), 3.64 (m, 4H), 3.40 (m, 6H), 2.16 (s, 1H); δC(50 MHz, CDCl3): 171.51, 131.24, 130.77, 130.73, 129.43, 128.40, 128.00, 127.88, 127.35, 127.26, 125.97, 125.24, 125.08, 124.96, 124.81, 124.67, 123.27, 72.26, 70.39, 70.10, 68.96, 64.13, 61.64, 39.37; FTIR (main vibrations): ν = 3425, 3041, 2901, 2873, 1734, 1454, 1255, 1133, 847. Pyrene-Adam: (3r,5r,7r)-Adamantan-1-ylmethyl 2-(pyren-1-yl)acetate. Yield 60%; Slightly yellow liquid; δH(200 MHz, CDCl3): 8.11 (m, 9H), 4.37 (s, 2H), 3.69 (s, 2H), 1.49 (m, 15H); δC(50 MHz, CDCl3): 171.67, 131.29, 130.79, 130.73, 129.42, 128.46, 128.41, 127.80, 127.39, 127.20, 125.92, 125.18, 125.04, 124.81, 124.72, 123.50, 74.45, 39.66, 39.00, 36.78, 33.22, 27.87; FTIR (main vibrations): ν = 3041, 2901, 2846, 1734, 1452, 1257, 1144, 841. Monomer Characterization. UV-visible absorption spectra were measured with a Varian CARY 4000 double-beam spectrophotometer in quartz cells with path length 1cm, by using slit widths of 2 nm and scan rate of 600 nm/min. Excitation and fluorescence spectra were measured on a Horiba Fluorolog-3 (in quartz cells with path length 1 cm for liquid solution in right angle configuration), by using slit widths of 1.5 nm and an integration time of 0.1 s. The 7



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solvents were of spectrometric grade (DCM dichloromethane), optical density was adjusted below 0.1 to avoid reabsorption artifacts. Fluorescence quantum yield was measured relatively to 9,10-diphenylanthracene (DPA) in ethanol with ΦF= 95%.47 The polymer chain lengths were determined by gel permeation chromatography (GPC). The number average molar mass (Mn) and mass average molar mass (Mw) and as a consequence the polymerization degrees were determined by the society Specific Polymers with the method SP_RI_THF-PS (polystyrene calibration). Conducting Polymer Electrodeposition. The conducting polymer films were produced using a potentiostat of Metrohm (Autolab). An anhydrous acetonitrile containing 0.1 M of tetrabutylammonium (Bu4NClO4) solution was used as electrolyte. 10 mmol of monomer was added to this solution. The connection to the potentiostat was performed using 2 cm2 gold plates or ITO plates for fluorescence experiments as working electrodes, a carbon rod as counter-electrode and a saturated calomel electrode (SCE) as reference one. The polymer films were obtained by cyclic voltammetry using a scan rate of 20 mV/s. The scans were performed between -0.7 V to a potential slightly lower to the monomer oxidation potential. In order to compare the wettability results, “smooth” polymer surfaces with each monomer were produced using a two-step electrodeposition process. Indeed, because it is difficult to control the deposition with the cyclic voltammetry method, this method was changed by the deposition at constant potential. It was possible to induce the formation of a very thin and smooth surface with this method using an extremely low deposition charge of 1 mC/cm2. However, in order to have the same polymer, a second step was performed by cyclic voltammetry in the same electrolyte but without monomer from 1.5 V to 0 V vs SCE at 20 mV/s in order to reduce the polymer.

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Surface Characterization. The IR spectra were obtained using a Spectrum Spotlight 300 FT-IR microscope of Perkin Elmer. The spectra were collected using the Image mode and the in reflectance on gold plates. A goniometer was used for the contact angle measurements. The apparent contact angles (θ) were determined with 2 µL droplets of different surface tension (γLV): 72.8 mN/m for water, 50.0 mN/m for diiodomethane, 31 mN/m for sunflower oil and 27.6 mN/m for hexadecane. The dynamic contact angles were determined using the tilted-drop method. A 6 µL droplet was put on the substrate and this one is inclined until the droplet moves. The advanced (θadv) and receding (θrec) contact angles and as a consequence the hysteresis H = θadv – θrec are taken just before the droplet moving: θadv in the moving direction and θrec in the opposite direction. If the droplet does not move after an inclination of 90 °, the substrate is called sticky. The mean arithmetic (Ra) and quadratic (Rq) roughness were determined by optical profilometry (Wyko NT 1100 of Bruker). The measurements were realized with the High Mag Phase Shift Interference (PSI), the objective 50X and the field of view 0.5X. The scanning electron microscopy images were given by a 6700F microscope of JEOL. UV-visible absorption spectra of films on ITO surfaces were measured with a Varian CARY 4000 double-beam spectrophotometer, by using slit widths of 2 nm and scan rate of 600 nm/min. Excitation and fluorescence spectra were measured on a Horiba Fluorolog-3 in a front-face configuration, by using slit widths of 1.5 nm and an integration time of 0.1 s. Fluorescence images were acquired using Leica TCS SP5-AOBS confocal laser scanning microscope. The surfaces were rinsed by distilled water, and imaged using ×63-1.4 numerical aperture plan apochromat oil immersion objective. The size of the xy image was 512 × 512 pixels (image size 20 × 20 µm²) recorded on 8 bits. UV laser (364 nm) was used as the 9



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excitation source regardless of the fluorescent probes. The corresponding fluorescence was collected in the 380-750 nm spectral range. Each fluorescence intensity image corresponds to an average of 4 frames.

RESULTS AND DISCUSSION Spectroscopic Properties of Pyrene Derivatives Monomers. Absorption and emission fluorescence spectroscopy of Pyrene and its derivatives are shown in Figure 1, their main spectroscopic data are given in Table 1. The absorption spectrum of Pyrene monomer in aerated DCM solution is typical of what is expected.47 As described in Figure 1, whatever the derivative is, the UV−vis spectra showed similar spectral shape but with a bathochromic shift compared to parent Pyrene. The length of the grafted chains or structure does not affect the position or shape of the absorption bands. The maximum absorption wavelength was shifted by 7 nm whatever the functional group is. The shift does seem to depend upon the length or the nature of atoms present on the substituent. As shown by Konishi et al,48 it is likely that the methyl function which is connected to the aromatic ring controls the red shift absorption band. The excitation spectra for Pyrene monomer and its derivatives virtually superimpose with their respective absorption spectra between 260 nm to 380 nm, showing that only one species contributes to photophysics. The Pyrene monomer has a maximum luminescence emission band located at 372 nm, and a second less intense emission band at 393 nm. A red shift is observed for all the emission spectra of Pyrene derivatives which might come from the additional chains grafted on the Pyrene monomer (as mentioned for absorption).

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Pyrene Pyrene-F6

Normalized absorption

2,0

Pyrene-F4 Pyrene-F8

1,5

Pyrene-H10 Pyrene-Ph Pyrene-TEG Pyrene-Adam

1,0

0,5

0,0

240

260

280

300

320

340

360

380

400

Wavelength (nm) Pyrene Pyrene-F6 Pyrene-F4

Normalized excitation

1,2 1,0

Pyrene-F8 0,8

Pyrene-H10 Pyrene-Ph Pyrene-TEG Pyrene-Adam

0,6 0,4 0,2 0,0 260

280

300

320

340

360

380

Wavelength (nm) Pyrene Pyrene-F6

1,4

Normalized emission

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1,2

Pyrene-F4 Pyrene-F8

1,0

Pyrene-H10

0,8

Pyrene-Ph Pyrene-TEG Pyrene-Adam

0,6 0,4 0,2 0,0 360

380

400

420

440

460

480

500

520

540

Wavelength (nm)

Figure 1. Normalized absorption, excitation (λem = 395 nm, normalization to 1 at 343 nm) and emission (λex = 343 nm, normalization to 1 at 395 nm) spectra of pyrene compounds (monomers) in DCM 1.

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The fluorescence quantum yield for Pyrene monomer in aerated dichloromethane is 7%, which, as expected, is lower than under argon (28%). A small increase is observed for all the Pyrene derivatives (7%-12%): these results from the presence of C-C σ-orbital and C-H σorbital of methylene group which participate in the σ−π conjugation.48 However, the nature of substituent does not seem to affect the quantum yield of the Pyrene derivatives significantly.

Table 1. Spectroscopic Parameters of Pyrene Derivatives Monomers Monomer

λabs,max [nm]

λex,max [nm]

Pyrene

242, 263, 274, 308, 321, 337 244, 266, 277, 313, 327, 343 244, 266, 277, 313, 327, 343 244, 266, 277, 313, 327, 343 244, 266, 277, 314, 328, 344 244, 266, 277, 314, 328, 344 244, 266, 277, 314, 328, 344 244, 266, 277, 314, 328, 344

262, 273, 307, 320, 336 264, 276, 312, 327, 343 264, 276, 312, 327, 343 265, 275, 312, 326, 342 265, 276, 312, 327, 343 265, 276, 312, 327, 343 266, 277, 313, 327, 343 266, 277, 313, 327, 344

Pyrene-F4 Pyrene-F6 Pyrene-F8 PyreneH10 PyrenePh PyreneTEG PyreneAdam

372, 393

φF (DC M) 7%

375, 395

7%

375, 395

9%

375, 395

8%

376, 396

12%

376, 396

10%

377,397

9%

377,397

9%

λem,max [nm]

Deposition by Cyclic Voltammetry. In order to produce highly homogeneous polymer films, the cyclic voltammetry was used as deposition method. 1, 3 and 5 deposition scans were performed in order to study the influence of the polymer growth on the surface properties. The study of the growth of conducting polymers by cyclic voltammetry gives also extremely interesting information. Examples of cyclic voltammograms are displayed in Figure 2. The cyclic voltammograms of the monomers show that the oxidation and reduction potentials of the corresponding polymers are very close to that of the monomers. This 12


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indicates that the polymer chain lengths are very short, which is not surprising because they are many possible polymerization positions on the Pyrene unit. This is also in agreement with the literature,45 on which the authors showed that the electropolymerization of Pyrene gives oligomers of only several units.

Figure 2. Cyclic voltammograms (5 scans) of Pyrene-F6 and Pyrene-TEG recorded in 0.1 M Bu4NClO4 / acetonitrile; scan rate: 20 mV s-1.

Gel Permeation Chromatography. In order to confirm the results obtained by cyclic voltammetry, the polymer chain lengths were determined by (GPC). For this, the polymers were dissolved in chloroform. However, only PPyrene-F6, PPyrene-F8, PPyrene-H10, PPyrenePh and PPyrene-Adam were completely soluble and as a consequence suitable for these analyses. Moreover, because the mass of electrodeposited polymers is extremely low it was 13



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possible to detect the polymers but only after concentration in tetrahydrofuran (THF). Data obtained by GPC are given in Table 2. These data show that the polymerization degree of the polymers is extremely low (< 2.5), which means that the films are composed especially of monomers, dimers and trimers. Moreover, monomers and dimers are especially present with fluorinated chains and adamantyl while dimers and trimers are especially present with decyl and phenyl groups. However, due to the multiple polymerization sites of pyrene and the presence of a substituent, numerous different dimers and trimers can be formed during polymerization. An example of dimers is given in Scheme 3. These results are in complete agreement with the literature45 and with the cyclic voltammetry curves. Indeed, Lu and coworkers reported that the electropolymerization of non-substituted pyrene in the same conditions gives oligomers of only 6–11 units. Hence, it is not surprising to find that the substitution of pyrene with voluminous substituents reduces the polymer chain length to only some units.

Table 2. Data on Polymer Chain Length Obtained by GPC Polymerization Polymer Mn Mw Degree PPyrene-F6 814 1831 1.34 PPyrene-F8 894 1869 1.27 PPyrene-H10 953 2195 2.38 PPyrene-Ph 714 1665 2.12 PPyrene555 1079 1.41 Adam

Scheme 3. Example of dimers obtained during the polymerization of substituted pyrenes.

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Infrared Characterization. The polymers were characterized by infrared (IR) spectroscopy using an IR imaging and the reflectance mode (possible because the depositions were made on gold plate). The IR spectra are displayed in Figure 3. The IR spectrum of PPyrene displays several peaks including a peak at 1632 cm-1 attributed to the stretching of C=C in the benzene units. In all the substituted polymers a peak around 1740 cm-1 is present. This peak is characteristic of the presence of the ester group. Otherwise, peaks around 28003000 cm-1 attributed to the stretching of C–C are clearly present in the spectra of PPyrene-H10, PPyrene-Adam and PPyrene-TEG, while peaks around 1100-1300 cm-1 attributed to the stretching of C–F are present in the spectra of the fluorinated PPyrenes. A large peak around 3400 cm-1 attributed to the stretching of O–H is also present in the spectrum of PPyrene-TEG.

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Figure 3. IR spectra of the different polymers. Spectroscopic Properties of Pyrene Derivatives Polymers. The Pyrene polymers show a total different absorption and emission spectra compared with Pyrene monomers (Figure 4). Compared with the structured absorption of Pyrene monomers, the Pyrene polymers exhibit a broad structureless spectral shape and a large red-shift is observed. The pyrene monomer characteristic spectral shape is no longer seen. Such information tends to show that Pyrene oligomerization occurs. This confirms the GPC results shown previously. It is very difficult to correlate the oligomer nature and the position of the band (Table 3). The broadness of the band may suggest that several species such as aggregates exist. All the derivatives possess one 16


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single emission band (between 472 and 498 nm). This band is structureless and red shifted compared with their parent monomer spectra. This behaviour is likely to be characteristic of an excimer emission. Indeed usually the fluorescence spectrum of pyrene in concentrated solution consists of two distinct components, a band in the violet range with vibrational structure (monomer emission), and a blue-green band which is broad and structureless (excimer i.e excited state dimer formed by a monomer in the ground state and a monomer in the excited state - emission at 482 nm.49 At high concentration and in the crystalline state the fluorescence is almost exclusively from the excimer but no corresponding changes occur in the absorption spectrum with increase in concentration. In our case the absorption spectrum from monomer to polymer is drastically changed whereas the emission of monomer cannot be observed and an excimer-like emission is observed.

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Figure 4. Absorption and emission spectra (normalization to 1 at the maximum intensity of every spectrum). Above: absorption spectra of Pyrene monomer in DCM (black lines), emission spectra of Pyrene monomer in DCM with λex = 343 nm (red lines), absorption spectra of Pyrene polymer (blue lines)*, emission spectra of Pyrene polymer (scan 1) with λex = 343 nm (magenta lines); Below: same spectra of Pyrene-Ph monomer and polymer.

To better understand the photophysical properties of pyrene monomers and polymers, fluorescence decays were acquired (data not shown). The fluorescence lifetime of oligomers is significantly reduced by a factor of 30. No rise time was observable. A fit with a sum of exponentials could not be performed in a satisfying way (decays were too short and close to apparatus function). Nevertheless meanlifetimes were evaluated (from 200 ps to 1.2 ns). Short 18


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lifetime of pyrene excimer has been observed and reported by several research groups.50-52 In these examples, a lifetime (approximately 3 - 4 ns from a fit with a sum of exponential) accounts for a short-lived excimer. This species is either due to the self-quenching of some improperly stacked pyrenes or residual pyrene degradation. Although the measured lifetimes in our samples are much shorter compared to literature, improperly stacked pyrenes may account for such decay behaviour. Thus in films, movements of the pyrenes shall be greatly confined and pyrene molecules are likely to be poorly stacked. On the other hand the complex fluorescence behaviour might be attributed to ground state dimers or higher aggregation states. The extension of conjugation of the oligomer as well as the aggregation state might increase the deactivation through non-radiative pathways. Although fluorescence is reduced, we were able to measure fluorescence images see paragraph 3.3. Table 3. Spectroscopic Parameters of Pyrene Derivatives Polymers λabs,max [nm] Polymer scan 1 scan 2 scan 1 PPyrene 376 376 492 PPyrene-F4 359 360 487 PPyrene-F6 419 370 472 PPyrene-F8 409 349 475 PPyrene-H10 358 358 486 PPyrene-Ph 367 367 484 PPyrene358 358 X TEG

λem,max [nm] scan 2 498 487 478 475 479 488 X

Surface Morphology. The surface morphology of each polymer and after 3 deposition scans are given in Figure 5 and Figure 6 and the surface roughness are gathered in Table 4. The SEM images show that PPyrene is composed of flower-like microstructures inducing the higher roughness of all the studied polymers (Table 4).

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Figure 5. SEM images at two magnifications (scale bar = 1 µm) of PPyrene, PPyrene-F8, PPyrene-F6 and PPyrene-F4; number of depositions scans: 3. Otherwise, the electropolymerization of most of the substituted monomers induces the deposition of large spherical particles of micrometer and/or sub-micrometer size. The spherical particles were much defined in the case of PPyrene-Ph, PPyrene-Adam, PPyrene-F6 and PPyrrene-F4 than in the case of PPyrene-F8 resulting also in rougher surface (Table 3) and with large particle agglomerates. In the case of PPyrene-F6 and PPyrrene-F4, the spherical particles are not smooth, but nanostructured. Indeed, it is known that a dual-scale surface roughness can have a high impact on the superhydrophobic properties and can highly reduce 20


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the contact angle hysteresis.8 It is also important to notice that PPyrene-Adam is composed of raspberry particles. Otherwise, only some particles are present on PPyrene-TEG while PPyrene-H10 can be considered as relatively smooth. Usually, the morphology of electrodeposited was found to be highly dependent on the solubility of the oligomers formed in the first instance of the electropolymerization. Here, the polymer solubility if dependent on different parameters: the substituent hydrophobicity or polarity, the polymer chain lengths and the number of polymerization sites.38,39,53 Here, it seems that the more soluble in acetonitrile are PPyrene-H10, PPyrene-TEG and PPyrene-F8 (the smoother polymer films).

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Figure 6. SEM images at two magnifications (scale bar = 1 µm) of PPyrene-H10, PPyrenePh, PPyrene-TEG and PPyrene-Adam; number of depositions scans: 3.

Table 4. Surface Roughness as a Function of the Polymer and the Number of Deposition Scans Number of Ra Polymer Deposition Rq [nm] [nm] Scans 1 135 210 PPyrene 3 4000 5900 5 6200 8900 1 60 105 PPyrene-F8 3 180 290 5 250 365 1 45 115 3 580 1030 PPyrene-F6 5 1150 2000 1 65 180 PPyrene-F4 3 460 970 5 630 1080 1 135 210 PPyrene-H10 3 350 710 5 410 670 1 140 380 PPyrene-Ph 3 360 640 5 520 870 1 20 50 PPyrene3 35 60 TEG 5 40 70 1 75 160 PPyrene3 550 935 Adam 5 85 200

Moreover, fluorescent images (Figure 7 and Figure 8) show that the fluorescence intensity of these spherical particles formed on the surface are much stronger compared with places that have less material. As the number of scans increases the fluorescence intensity increases. Fluorescence lifetime is longer for the rougher surfaces such as PPyrene-Adam and PPyrene22


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F8. In spite of the low fluorescence intensity, it was possible to make fluorescence intensity image. No bleaching occurred. Fluorescence intensity images well corroborate the SEM images and similar topographies are observed whatever the sample is. Thus, fluorescence imaging could be an alternative method to evaluate the surface roughness.

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Figure 7. Confocal fluorescence microscopy of PPyrene, PPyrene-F4, PPyrene-F6 and PPyrene-F8 (scan 1) : fluorescence mode (left) and transmission mode (right).

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Figure 8. Confocal fluorescence microscopy of PPyrene-H10, PPyrene-Ph, PPyrene-TEG and PPyrene-Adam: fluorescence mode (left) and transmission mode (right).

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Surface Wettability. The summary of the mean apparent contact angles (θ) of liquids differing by their surface tension (water, diiodomethane, sunflower oil and hexadecane) is given in Table 4. The hysteresis (H) and sliding angles (α) can also be found in Table 5. From 3 deposition scans, it is possible to obtain superhydrophobic properties using PPyrene-F8, PPyrene-F6, PPyrrene-F4 and PPyrene, as shown in Figure 9. The highest θwater and the lowest H and α are obtained with PPyrene-F6 and PPyrene-F8, which can be explained by the presence of micro and nanostructures observed on this polymer film. As expected, these polymers are also the most oleophobic. The other polymers are much less hydrophobic because either they are much less structured (PPyrene-H10 and PPyrene-TEG) or the substituent is less hydrophobic (PPyrene-Ph, PPyrene-Adam, PPyrene-TEG).

Table 5. Apparent Contact Angles of Water (θwater), diiodomethane (θdiiodo), Sunflower Oil (θsunflower) and Hexadecane (θhexadecane) as a Function of the Polymer and the Number of Deposition Scans Number of θsunflo θhexad Polymer θwater Hwater αwater θdiiodo deposition wer ecane scans 1 133.5 Sticky 0 0 0 PPyrene 3 153.3 0.7 8.0 0 0 0 5 152.1 0.7 6.0 0 0 0 122. 1 125.0 Sticky 130.6 91.2 9 PPyrene116. 3 157.8 1.5 2.0 130.0 97.5 F8 5 5 156.3 16.0 15.2 131.0 92.0 85.6 1 136.5 Sticky 103.0 92.1 74.6 PPyrene111. 3 159.4 1.1 1.1 137.8 93.3 F6 2 5 160.0 0.9 1.0 135.8 98.5 95.0 1 116.4 Sticky 85.2 64.5 49.3 PPyrene3 157.6 4.1 3.8 105.7 64.7 43.2 F4 5 155.9 29.7 21.6 108.8 69.7 36.8 26


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PPyreneH10 PPyrenePh PPyreneTEG PPyreneAdam

1 3 5 1 3 5 1 3 5 1 3 5

88.1 91.0 99.7 85.2 91.2 105.7 57.4 61.5 62.8 98.1 113.2 94.2

/ / /

/ / /

/ / / / / / / /

/ / / / / / / /

33.5 34.1 39.2 0 0 0 28.5 27.6 30.6 0 13.2 0

24.6 22.8 21.8 0 0 0 0 17.0 17.1 0 0 0

0 0 0 0 0 0 0 0 0 0 0 0

Figure 9. Pictures of water droplets deposited on PPyrene-F4 (left) and PPyrene-TEG (right). To give a better explanation on the effects of the surface structures on the surface hydrophobicity and oleophobicity, it was first necessary to determine the apparent contact angles of the same polymer but “smooth” also called Young angles (θY).54 These θY are given in Table 6. These results show that as expected PPyrene-F8 is the most hydrophobic polymer but is only slightly hydrophobic (θYwater > 90 °). It is also the most oleophobic but the oil contact angles are not very important, especially with hexadecane (θYhexa < 20 °). Then, θY with each probe liquid decrease as the fluorinated chain length: PPyrene-F6 is at the limit between hydrophilicity and hydrophobicity and PPyrene-F4 is slightly hydrophilic (θYwater < 90 °) and similar to PPyrene-H10. The most hydrophilic polymers are PPyrene-Adam (θYwater = 73.0 °) and PPyrene-TEG (θYwater = 71.4 °). 27



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Table 6. Apparent Contact Angles of Water (Θywater), Diiodomethane (Θydiiodo), Sunflower Oil (Θysunflower) and Hexadecane (Θyhexadecane) for the “Smooth” Corresponding Polymers Polymer θYwater θYdiiodo θYsunflower θYhexadecane PPyrene 80.6 29.6 15.8 11.5 PPyrene-F8 92.8 59.2 40.8 18.9 90.2 43.4 22.7 15.8 PPyrene-F6 PPyrene-F4 85.7 36.7 23.2 9.9 PPyrene-H10 85.8 32.7 11.7 0 PPyrene-Ph PPyreneTEG PPyreneAdam

79.5

33.9

11.7

0

71.4

31.4

14.8

0

73.0

33.9

16.3

0

Hence, the wettability results can now be better explained. Indeed, two equations are very often used to explain the wetting properties of rough surfaces.55,56 The Wenzel equation (cos θ = rcos θY; r being a roughness parameter) gives the contact angle when a liquid droplet placed on it penetrates in all the surface roughness.55 Using the Wenzel equation, θ can be > θY only if θY > 90°, and reversely. Hence, the Wenzel equation cannot explain most of the results reported in this work, which indicates the presence of air between the droplet and the surface as described by the Cassie-Baxter equation.56 Using the Cassie-Baxter equation (cos θ = rffcos θY+ f – 1; rf being the roughness ratio of the substrate wetted by the liquid, f the solid fraction and (1 – f) the air fraction), the presence of air inside surface roughness can induce an increase of θ whatever θY. To better evaluate if our surfaces follow the Wenzel or the CassieBaxter equation, the Figure 10 shows the differences between θwater and θsunflower for the different smooth and structured (3 scans) polymers.

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Figure 10. Comparison between the apparent contact angles of water (θwater) and sunflower oil (θsunflower) for the different smooth and structured (3 scans) polymers.

Hence, most of the results obtained with water cannot be explained with the Wenzel equation indicating of the presence of air between the water droplet and the surfaces. In the case of oils such as sunflower oil, an extremely high increase of θsunflower (up to about 90 °) is observed for the fluorinated PPyrenes even if they are highly oleophilic. Marmur proposed to use the term of parahygrophobic to describe these kinds of surfaces.12 Indeed, whatever the surface tension of the probe liquids, a parahygrophobic surface is able to trap a high amount of air inducing a high increase in θ even if θY < 90 °. In the case of our surfaces PPyrene-F8 and PPyrene-F6, the presence of a dual-scale surface roughness allows to trap a high amount of air between the different liquids and the surface. This amount of air remains extremely 29



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important even if the liquid surface tension decreases from 72.8 mN/m (water) to 27.6 mN/m (hexadecane).

CONCLUSION Here, we showed that the electropolymerization of fluorescent monomers such as Pyrene with various substituents differing by their hydrophobicity, size or rigidity/flexibility can lead to surfaces with tunable hydrophobicity, water adhesion and fluorescence properties. The study of various substituents (fluoroalkyl, alkyl, phenyl, adamantly and triethyleneglycol) demonstrated to huge impact on surface roughening and the resulting fluorescence properties. Superhydrophobic properties and highly oleophobic properties were obtained using fluoroalkyl chains due to the presence of nanostructured microparticles. For the fluorescence properties, the Pyrene polymers displayed a broad structureless spectral shape, where the loss of vibronic structure may arise from the Pyrene oligomerization and aggregation. This work is a first tentative to combine superhydrophobic and fluorescent properties using an innovative strategy and opens new doors to explore in this domain, while the control in surface hydrophobicity and water adhesion is also extremely important for various applications in water harvesting, oil/water separation membrane, energy systems or biosensing, for example.

AUTHOR INFORMATION Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. These authors contributed equally.

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ACKNOWLEDGMENT This work was supported by CNPq, Conselho Nacional de Desenvolvimento Científico e Tecnológico - Brazil (Process Nº 202280/2014-4). The authors want to thank the Centre de Photonique Biomédicale (CPBM) of the Centre Laser de l’Université Paris-Sud (CLUPS/LUMAT FR2764, Orsay, France) for the confocal microscope facilities. AG DP RMR thank Labex Charmmmat for financial support. XX thanks Labex Charmmmat for postdoctoral fellowship.

Supporting Information Available: Fluorescence decays of monomers and polymers. This material is available free of charge via the Internet at http://pubs.acs.org

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TOC IMAGE

Surfaces with tunable hydrophobicity, water adhesion and fluorescence properties by a direct electropolymerization process using pyrene with various substituents differing by their hydrophobicity, size or rigidity/flexibility.

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Electrodeposition of Polypyrenes with Tunable Hydrophobicity, Water

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