Controlled Functionalization of Reduced Graphene Oxide Enabled by

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Controlled functionalization of reduced graphene oxide enabled by microfluidic reactors Simone Silvestrini, Christian C. De Filippo, Nicola Vicentini, Enzo Menna, Raffaello Mazzaro, Vittorio Morandi, Luca Ravotto, Paola Ceroni, and Michele Maggini Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04740 • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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Chemistry of Materials

Controlled functionalization of reduced graphene oxide enabled by microfluidic reactors Simone Silvestrinia†, Christian C. De Filippoa‡, Nicola Vicentinia, Enzo Mennaa, Raffaello Mazzarob, Vittorio Morandic, Luca Ravottob, Paola Ceroni*b, Michele Maggini*a a

Department of Chemical Sciences, University of Padova, via Marzolo 1, 35131 Padova, Italy. Department of Chemistry "Giacomo Ciamician", University of Bologna, via Selmi 2, 40126 Bologna, Italy. c IMM-CNR, via Gobetti 101, 40129 Bologna, Italy. b

ABSTRACT: We report the use of microfluidics to functionalize suspended reduced graphene oxide flakes through the addition of aryl radical, generated in situ by reaction between aryl amines and isopentyl nitrite. Microfluidic enabled a tight control of temperature, reaction times and stoichiometric ratios, making it possible to tune the growth of oligomers on the surface of the flakes, which in turn affects the interactions of the functional material with the surrounding environment. The results suggest that shear stress phenomena within the reactor may play a role in the chemistry of graphene materials, by providing a constant driving force towards exfoliation of the layered structures. Scale up of the functionalization process is also reported, along with the grafting of dyes based on squaric acid cores. Photophysical characterization of the dye-modified flakes proves that the microfluidic approach is a viable method towards the development of new materials with tailored properties.

Introduction Reduced graphene oxide (rGO) is a layered carbon nanostructured material obtained from the reduction of oxidized graphite. rGO has been developed as an alternative to singlelayer graphene that is processable in bulk quantities, being produced by a top-down approach from graphite, rather than grown on a layer-by-layer basis.1 Thanks to their properties, rGO and its derivatives have been proposed for the development of flexible electronics and molecular sensors, as fillers in polymer composites and scaffolds for the support of catalysts.2-7 The chemistry of rGO is closely related to that of graphene, but a higher reactivity can be expected due to the presence of defects, such as unreduced oxygen-bearing moieties and vacancies in the basal plane left over by the reduction process. 5 In order to overcome the relative inertness of the sp2 honeycomb, typical of graphene, highly reactive species, such as radicals generated in situ from the corresponding aryl-diazonium salts, have found widespread use for the functionalization of these substrates.8-11 This aryl radical approach affords functionalized materials in shorter times and with a larger variety of functional groups if compared to other strategies based on cycloaddition 12 or addition reactions.13 The reactivity of diazonium salts and radical species plays a pivotal role for the success of this reaction, but it also presents the potential risks associated to the manipulation of unstable species and the need to control multiple additions to the graphene surfaces, that reportedly result in the formation of polymer “brushes”.14 We have studied extensively the use of flow reactors for efficient and continuous functionalization of fullerenes15,16 and carbon nanotubes (CNTs),17-20 demonstrating how microfluidic technology represents a powerful tool to control their chemistry. The use of continuous processing techniques brings about

important advantages such as increased safety and higher process intensity.21 In addition, the flow approach allows for a direct scale up of the functionalization processes, as highlighted by Vázquez and coworkers on their review on non-conventional methods for the manipulation of carbon nanoforms.22 In this work we introduce the continuous-flow functionalization of rGO in a microreactor and a first, proof-of-principle scale up to larger flow setups. We employ microfluidics to control the addition of aryl radicals on rGO flakes, affording functionalized derivatives (frGO) with tailored properties. We start by reporting on the functionalization with 4-methoxyphenyl groups, which offer the possibility to study the growth of multiple layers of small aryl radicals. Next, the grafting of squaraine chromophores will be instrumental to showcase the potential of rGO to act as a scaffold for the preparation of new materials for light harvesting. Grafting of 4-methoxyphenyl moieties on rGO. 4-methoxyphenyl radicals can be generated in situ by the reaction between 4-methoxyaniline and isopentyl nitrite.23 The addition of aryl radicals to graphite and graphene yields monolayers, in case of sterically hindered building blocks such as 3,4,5-trimethoxyaniline. However, the use of unhindered 4-methoxyaniline favors the growth of oligomers with a lower surface coverage.14 This can take place on our samples, even though rGO is intrinsically more reactive than pristine graphene and, therefore, the difference in the reactivity of the basal plane and that of the attached moieties should be less pronounced, possibly favoring denser surface coverages. The properties of the functionalized materials are known to depend on the reaction parameters (temperature, number of equivalents, reaction time, solvent).24,25 Covalent attachment of organic groups has a strong

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impact on the interaction of carbon nanomaterials with the surrounding environment, affecting the stability of their suspensions26 and their electronic properties by disrupting its π-conjugated system. This prompts for a systematic exploration of the variables controlling the reaction, in order to rationalize their effect on the properties of frGO. We identify key observables to describe our frGO samples. The first is the concentration C (in mg∙ml-1) of the suspensions extracted according to a standardized procedure from a set amount of washed, dried products, by sonication in DMF followed by mild centrifugation that removes coarse powders. We have proposed an analogue observable for the rationalization of the properties of CNTs,26 as the closest analogue to solubility we can report for an heterogeneous material, such as the frGO flakes described herein. Thermogravimetric analysis can be used to assess the degree of functionalization, DF, by evaluating the mass loss due to the degradation of the functionalization layer and the total mass of the sample.27 DF is reported as the moles of functional groups per moles of carbon in the rGO substrate (nC), in a 1/nC format. The degradation temperatures of the rGO substrate (T2) and the organic layer on its surface (T1) hold a wealth of information as well, since, upon oligomerization of the aryl radicals, we expect a gradual increase in the degradation temperature of the functionalization layer. Flow chemistry setups. Throughout this work we employ microfluidic technology control the addition of aryl radical to rGO, moving around the variable space controlling the reaction to functionalize rGO in a controlled fashion. To this end, we use a commercially-available microfluidic platform, where constant flow rates for the streams of reagents are provided by syringe pumps. The streams are heated up by a Peltier system and then merge in a glass microfluidic chip, where a passive mixer ensures quasi-ideal mixing in a timeframe much shorter than the following residence time. Reaction time is set by the ratio between the channel's volume and the total flow rate of the streams and, as such, one may be tempted to control it by varying the flow rates rather than by changing the microfluidic chip. However, flow velocities control Reynolds number and the shear rates within the channel, that have been reported to promote exfoliation of layered materials.28 By using two microfluidic chips with different internal volumes (and the same channel section), experiments can be carried out at varying reaction times while keeping the flow regime constant, thus cancelling out a hard-tocontrol variable. This setup allows to carry out many experiments over small quantities of rGO substrate, but scale up is needed in order to produce enough sample for practical applications. For this reason we tested a different continuous flow setup, consisting in a PTFE tube with a hydraulic radius about 6 times larger than that of the glass microchip, heated by immersion in an oil bath. In experiments carried out with this “meso” reactor, shear rates are lower, but can still be precisely controlled. The two systems are shown in the SI (figure S1). Production of relatively large quantities of frGO materials allows to test them in practical applications and to compare their performances with those of frGO prepared by traditional, batchwise processing. In this regard, the quenching of a benchmark fluorescent polymer such as poly(3-hexylthiophene) (P3HT) can be used to assess the quenching ability of different frGOs, thus comparing different samples on the basis of an important practical application such as light harvesting.

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Grafting of squaric acid derivatives on rGO We can exploit microreactor processing to covalently attach chromophores on the surface of rGO to develop light harvesting systems. Among the dyes employed as antennae systems, squaraine derivatives offer many advantages in terms of stability, tunable absorption and synthetic accessibility.29 Chart 1 show derivatives 1 and 2, which we prepared according to the procedures reported in the SI (schemes S2 and S3, respectively). They represent two classes of squaraines, named 1,2- and 1,3after the relative positions of the substituents around the squaric acid core. They are both asymmetric, bearing two different substituents, one of which contains the aromatic amino group necessary to generate the reactive aryl radical that adds to rGO. We carried out the photophysical and electrochemical characterization of the materials obtained by reaction of 1 and 2 with rGO in the presence of isopentyl nitrite (samples 1-frGO and 2frGO), to assess their features. Finally, we carried out a co-functionalization experiment with both dyes, to evaluate the interactions between the two chromophores once they are attached on the carbon backbone of rGO. Chart 1. Structures of squaric acid derivatives 1 and 2.


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Table 1. Properties of frGO samples functionalized in microfluidic reactors. Variable space Entry no

Reaction time (s)

Temp (°C)

1

30

2

Properties Eq. of aniline

C (μg/mL)

DF

T1 (°C)

T2 (°C)

80

1

372

1/7

330

510

3

80

5

407

1/8

290

476

3

3

80

1

270

1/8

190

455

4

30

80

0.01

350

1/9

290

471

5

3

80

0.01

169

1/14

178

458

6

30

25

1

232

1/4

345

486

7

3

25

1

144

1/18

178

474

8

30

25

0.01

122

1/77

222

316

Results and discussion Grafting of 4-methoxyphenyl moieties. In the first part of this discussion we report on the growth of 4-methoxyphenyl layers on rGO, by correlating reaction parameters (the position in the time-temperature-stoichiometry variable space) with the value of key observables describing the materials afforded by the reaction. Table 1 shows the experimental data for the functionalization of rGO in microfluidic reactors. The first three columns locate the sample within the variable space, whereas the latter ones describe the properties of the samples. In addition to the characterization techniques described in the introduction, Raman spectra and TEM images were collected to account for the effects of the functionalization reaction on the morphology and spectroscopic characteristics of the basal plane of the parent material. The results of these analyses are reported in the SI for the rGO starting material (figure S2) and for the sample corresponding to entry no 1 in table 1 (figure S3). Individual thin flakes can be seen together with aggregates, with lateral dimensions up to a few micrometers. Electron diffraction performed on the thin flakes confirms the presence of the typical hexagonal pattern of the graphitic lattice. Nevertheless, the spots are heavily broadened in both the radial and tangent directions, due to crystal defects locally modifying the lattice orientation and interatomic distances on the short scale, more so for the functionalized samples. Both materials are characterized by an elevated number of curved wrinkles and folds typical of graphene materials with many defects in their sp2 lattice. The folds and wrinkles are deformed due to the low crystallinity and thus the number of layers is hard to determine, but it is possible to measure the thickness of the sheet, which in most cases is between 2.5 and 4 nm. Raman spectra of both samples (figure S4) present the typical features of rGO, with a broad, intense D band at 1335 cm-1. Neither the position nor the intensity of the D band relative to the G one appear to be affected by the functionalization process, due to the high number of basal plane defects that are already present in the starting material. The graph in figure 1 shows the effect of process variables on C, the concentration of the material extracted in DMF (reported as the size of the datapoint in the three-dimensional variable space). For carbon nanotubes, C has been reported to grow with increasing loading of organic moieties up to a point, and then to drop when the oligomerization of 4-methoxyphenyl radicals yields very long chains.26 Figure 1 proves that we are probing a

portion of the reaction space where the increase of C is monotonic with respect to temperature, reaction time and number of equivalent of aniline and, ultimately, with the growth of the oligomers on the surface of the flakes. The effect of the aforementioned variables is quite straightforward, albeit a numerical correlation between the data is far beyond the scope of this paper (and very difficult to generalize given the heterogeneous nature of the starting materials). TGA analysis was performed to assay the extent of functionalization of the material with layers of aryl moieties. Figure 2a compares the thermograms of samples functionalized at 80 °C with varying rGO-to-aniline stoichiometric ratios and reaction times. The thermal degradation of these materials under inert atmosphere occurs in two steps, with a first weight loss due to the grafted moieties (characterized by the peak temperature T1 in the weight derivative plot) and a second one due to the rGO substrate (peak temperature T2). T1 varies widely among the samples. At the extremes of the range, the sample functionalized for 3 s with 0.01 eq of aniline (the mildest conditions) shows a peak at 178 °C, while the functionalization for 30 s and 1 eq of aniline resulted in a shift of the same peak to 330 °C.

Figure 1. Effect of process parameters on the concentration of the material extracted in DMF. The size of each datapoint is proportional to the observable C.



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Figure 2. Comparisons between the first derivative traces of the thermograms of samples functionalized with the microreactor setup. Complete TGA traces are reported in the SI (figures S7-S9)

Figure 3. Comparisons between thermograms of samples functionalized with different reactor setups.

We relate this property to the formation of aryl oligomers on the substrate, whose thermal inertia increases with the number of monomer units (i.e. longer oligomer results in higher degradation temperature). Comparison within the subset of samples functionalized for 3 s (black, red and purple traces, for 0.01, 1 and 5 eq of aniline, respectively) highlights the expected effect of stoichiometry. Interestingly, we achieved very similar results by working at 5 eq for 3 s and 1 eq for 30 s. Similar comparisons can be made for the samples functionalized with 1 eq of aniline (figure 2b) and a reaction time of 30 s (figure 2c). The data reported in figure 2b, in particular, indicate that T1 depends strongly on the reaction time, rather than the reaction temperature. Indeed, in figure 2c, where samples prepared with the same reaction time are compared, all traces show very similar T1. The case of the sample obtained at 0.01 eq, 25 °C and 30 s is worth

mentioning, however, since it doesn’t seem to present a peak for the degradation of the carbon backbone. This being the least functionalized sample, we attributed the peak/shoulder setting on at ca 175 °C (we indicated a peak temperature at 222 °C, but it is bound to be strongly affected by the next, more intense peak) to the functionalization layer and the following peak at 316 °C to the sparsely functionalized rGO. T2 lies in the 450-480 °C range for all but two samples. A slight broadening of the peak at increasing stoichiometric ratio is observed. Only the sample functionalized for 30 s with 1 eq of aniline shows a marked increase in T2, which is still well below the degradation temperature of pristine rGO powder (vide infra, figure 3), that displays an onset at about 500 °C and peaks at 700 °C. It is typical for exfoliated materials to degrade at a lower temperature than their packed counterparts and our


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flow setup provides the shear rates for this phenomenon to take place, as neatly described by Coleman et al.28 All our experiments were carried out at constant flow rates, within channels with identical section and shape. In such conditions, shear rates depend on the viscosity of the solvent, which is in turn affected by the temperature of the process (for liquids, viscosity tends to decreases with increasing temperature). Indeed, T2 values for frGO samples functionalized at 25 °C are lower than their 80 °C counterparts, except for the one case of the red and blue traces in figure 2b. As described in the SI, the degree of functionalization has been reported as the ratio between the moles of attached aryl moieties and those of carbon in the substrate. As for the case of the observable C, the effect of reaction parameters on DF is quite expectable. We note that our values for DF are somewhat higher than those typically reported by Tour and coworkers, 8 which we relate to the higher reactivity of rGO with respect to other sp2 carbon nanostructures. The data reported for the sample functionalized with 0.01 eq of aniline may look downright awkward, since they always report DF values way higher than 1/100 that would be achieved if the aniline reacted quantitatively with the sample. It should be considered, however, that our purification and extraction protocol, based on sonication in DMF and removal of coarse powders would select the most soluble fraction within a sample. The values of C account for this effect, suggesting that the observables we used cannot really stand alone to describe the series of samples and should be considered as a whole. Even though the description of four properties in a three-dimensional space is a complex task, important trends can be highlighted and can shed new light on the process parameters that affect the properties of frGO materials. For example, we correlate T1 with the length of the poly(4methoxyphenyl) chains grown on the substrate and DF with total number of loaded aryl units. Entries 2 and 5 display similar T1 and T2 values, but different DF, indicating a polymer chains with similar lengths but a different density on the carbon substrate. Entries 2 and 3, on the other hand, bear a similar loading of aryl groups that are organized into polymer chains of different lengths. frGO samples reported as entries 1 and 6 share the same stoichiometric conditions and reaction times. Sample 6 shows a higher DF, but a lower C, indicating that a smaller fraction of the rGO substrate was functionalized with a higher amount of aniline. This is counterintuitive if we consider that higher temperatures usually go along higher reaction rates, but our considerations on T2 and the aforementioned works published on the exfoliation of layered materials suggest that shear stress can play a pivotal role in the chemical modification of rGO. These findings highlight the full potential of microreactor technology in the handling of materials as well as molecules, showing how it can be used to tune the features of functional rGO. Batch versus flow comparison and scale up. The properties of samples functionalized in microfluidic conditions are markedly different from those of the samples functionalized with different setups. In figure 3, the TGA traces of three samples functionalized at 80 °C with 1 eq of aniline in different reactors are compared. It should be noted that all starting materials and analytical samples were prepared in the same way throughout this study, so we deemed the differences in their properties to depend on the functionalization process alone.

The round bottomed flask experiment (blue trace) corresponds to the “traditional” way used to carry out the Tour reaction, where the substrate (be it carbon nanotubes, exfoliated graphite or graphene) is suspended in the solvent within a round bottomed flask and magnetically stirred, providing only small shear rates during reaction. Under this regime, exfoliation depends on the reduction of surface energy of rGO brought about by the growing functionalization layer, rather than the presence of strong forces tangent to the surface. This results in a poor exfoliation of the substrate, whose degradation temperature (T2 = 639 °C) is the most similar to that of pristine rGO powders. In the previous section we have observed that, among the variables considered, reaction time plays an important role in determining T1. In this case we don’t recognize a degradation peak for the functionalization layer. Rather, there is a progressive mass loss starting at temperatures as low as 200 °C and carrying on without noticeable peaks till the degradation of the carbon backbone. Our explanation is that in this case the two degradation temperatures came to match, so that the two peaks effectively became one. The broad mass loss at lower temperatures seems to indicate a wide distribution of aryl oligomers lengths, which may result from the combination of poor exfoliation of the starting material and long reaction times. The “meso” reactor we employed consisted in a PTFE tube with 0.5 mm inner diameter, heated in an oil bath and operated in continuous flow conditions thanks to syringe pumps. With a residence time of 15 minutes and the shear stress imposed by the flow in a relatively narrow conduit (the hydraulic radius of the PTFE tube is only about 6 times larger than that of the microfluidic chip), the thermal properties of the product highlighted by TGA analysis fit in between those of the samples functionalized in the microreactor setup and the round bottomed flask. In order to prove the importance of the morphological properties considered insofar for the development of applications, we employed our functional rGO materials as quencher of the fluorescence of P3HT, the benchmark electron donor conjugated polymer typically employed in bulk heterojunction solar cells. Therefore, we quantified the rate of fluorescence quenching of P3HT at increasing concentration of different frGO materials. Figure 4 shows the ratio of fluorescence intensities of a pristine sample of P3HT (F0) and upon addition of increasing amounts of rGO materials (F). Based on the very short lifetime of the P3HT fluorescent excited state (τ0= 223 ps), the quenching mechanism is mainly due to a static process, i.e. P3HT and rGO material are associated in the ground state. The plot for the batch-functionalized frGO (red plot) is linear and the slope is related to the association constant, while a non-linear plot (in black) resulted for frGO prepared in the scaled-up flow reactor. Experimental data show that the material functionalized in a moderate, controlled manner with the flow reactor quenches the fluorescence of P3HT to a greater extent. This does not come at the expense of the suspension stability, as it was previously reported by our group for CNTs.30 Grafting of squaric acid derivatives. Table 2 shows the properties of the samples functionalized with squaric acid derivatives, 1-frGO, 2-frGO and 1-2-frGO. Even though DF values are lower than that of most samples functionalized with 4methoxyaniline, the concentration obtained in DMF is higher, due to the different nature of the appended molecules. T2 values are very similar for all the samples and we found T1 to be close



to the value observed for the free chromophore. We do not report a DF value for 1-2-frGO, because the degradation of the organic layer takes place over a very broad temperature range and there is no clear point of minimum in the derivative plot, to be taken as a reference. Table 2. Properties of frGO samples functionalized with squaraine dyes 1 and 2.

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(Δλ = 12 nm) and from 652 to 669 nm for 2 (Δλ = 17 nm). The shift is compatible with the extension of their π-conjugated system to the underlying material. The entity of this extension depends on the angle between the two π systems and the chemical whereabouts of the bond, hence the broadening of the absorption peak, since not all the squaraine units may present the same degree of conjugation due to different bonding angles and the presence of defects on the sp2 lattice of rGO.

Properties Sample name

C (μg/mL)

DF

T1 (°C)

T2 (°C)

1-frGO

552

1/36

337

542

2-frGO

482

1/20

214

557

1-2-frGO

568

-

Broad peaks

551

Figures 5a and 5c show the UV-Vis absorption spectra of 1frGO and 2-frGO in DMF, together with that of the physical mixture of two free analogues to chromophores 1 and 2 (not bearing the amino group) with rGO. Figures 5b and 5d report the absorption spectrum obtained by subtraction of the rGO contribution from those of 1-frGO and 2-frGO, respectively, to compare the normalized absorption spectra of the bound and free squaraines. Both chromophores undergo a red shift in their absorption spectra as a result of the covalent bond on the rGO scaffold, together with a strong broadening. Absorption maxima shifted from 523 to 535 nm for squaraine derivative 1

Figure 4. comparison between fluorescence quenching of P3HT by using rGO functionalized in batch conditions (red plot) and by using the flow setup (black plot).

Figure 5. UV-Vis absorption spectra of (a) 1-frGO and (c) 2-frGO in DMF, plot together with a physical mixture of the free chromophore analogue (without the amino group) and rGO. In (b) and (d) the contribution from rGO was subtracted and both signals normalized to better compare the samples. Emission spectra of (e) 1-frGO and (f) 2-frGO in DMF are also plot together with the emission of the physical mixture of the free chromophore and rGO.


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The emission spectra of 1-frGO and 2-frGO in DMF are shown in figures 5e and 5f, respectively. Again, the emission spectra obtained by the physical mixture of the free chromophore analogues and rGO are compared to those of the covalently bound dyes. Normalized spectra (not shown) highlight a red shift in the emission as in the case of the absorption spectra. We evaluated the quantum yield of the frGO samples in respect to that of their relative free chromophores as the ratio of the integrals of their emission spectra. This proves that when the bound to the rGO a strong quenching occurs, with ca 70% efficiency for 1-frGO and 90% for 2-frGO. We were unable to resolve the fluorescence spectra of 1-frGO over time, due to the limitations of our setup in observing very fast processes, but we were able to do so for 2-frGO. Emission intensity of free 2 under 650 nm excitation decays exponentially with a lifetime of 1.1 ns. 2-frGO, on the other hand, shows a double-exponential 𝑡 𝑡 decay according to 𝐼 = 𝐴1 𝑒 − ⁄𝜏1 + 𝐴2 𝑒 − ⁄𝜏2 + 𝐵, with lifetimes 𝜏1 = 1.1 ns, 𝜏2 = 0.1 ns and preexponential factors 𝐴1 = 0.01, 𝐴2 = 0.11. The shorter lifetime is assigned to the squaraine chromophores linked to rGO, while the longer one is due to a very small amount (less than 10%) of free squaraine impurities present in solution. Cyclic voltammetry measurements were carried out in acetonitrile, with methylviologen as a reference. Both materials showed a marked tendency to detach from the glassy carbon working electrode they were deposited on, after few scans. The rGO layer also increased the resistance of the circuit, disturbing the measurement and leading to somewhat weak peaks, reported in the SI (figure S5 and S6). Both samples showed an

increase in the absolute values of their redox potential, compared to those of the free dyes. Actual values are reported in table 3. In the case of 1-frGO no reduction process was observed within the experimental potential window. Functionalization with a mixture of the two chromophores (sample 1-2-frGO) provided a material that sums up the absorption and emission characteristics of the former materials. The absorption spectrum of 1-2-frGO in DMF (figure 6a-b) appears to be the sum of those of 1-frGO and 2-frGO. The emission spectra upon selective excitation of chromophore 1 or 2 show the emission bands typical of 1 and 2, respectively (figure 6cd). As expected, no energy transfer process occurs between 1 and 2 bound to the rGO, since the average distance between the two chromophores is quite large (given the low degree of functionalization of 1-2-frGO) and the quenching process by rGO is very fast (see above). Table 3. Redox potentials of 1, 2, 1-frGO and 2-frGO Redox potential (V)

Sample name

Reduction

First oxidation

Second oxidation

1

-1.80

+0.28

+0.75

1-frGO

-

+0.48

+0.94

2

-1.28

+0.34

+0.76

2-frGO

-1.30

+0.43

+0.88

Figure 6. (a) Absorption spectrum of 1-2-frGO and pristine rGO in DMF. (b) Superimposition of the absorption spectra of 1-2-frGO (violet), 1-frGO (red) 2-frGO (blue), each with subtracted contribution from pristine rGO. Emission spectra of 1-2-frGO in DMF, while selectively exciting (c) chromophore 1 and (d) 2.



Conclusions The strict control over reaction parameters enabled by microfluidic techniques allowed us to rationalize the effects of reaction time, temperature and stoichiometry on the addition to reduced graphene oxide of aryl radicals generated in situ by treatment of aromatic amines with isopentyl nitrite. Each variable proved to affect the growth of a poly(aryl) layer which, in turn, has a strong impact on the interactions of functional reduced graphene oxide flakes with the surrounding environment. We highlighted how an educated control over the shear rate of the system can help to control the addition of organic moieties on this kind of layered material. Our findings prompt for further exploration of this aspect, which is peculiar to materials chemistry (even though, we note, has been reported to affect chemistry on the molecular level in some cases31) and represents powerful tool towards the engineering of 2D-layered materials. Besides scouting reaction conditions, we were able to scale up the production of functional reduced graphene oxide flakes, in order to use them as quenchers of the fluorescence of poly(3hexylthiophene). The P3HT fluorescence quenching properties of the material produced under microfluidic conditions was found to be increased with respect to the material prepared under the more traditional, discontinuous protocol. This proves that our functionalization approach is instrumental to produce materials with tailored properties, suitable for practical application such as light harvesting. Finally, we functionalized reduced graphene oxide with two chromophores derived from squaric acid. The absorption and emission features of the dyes experienced a red shift compared to that of the free molecules, as expected with the extension of their π-conjugation. Mixtures of the two chromophores could also be bound to the substrate, to yield materials with the spectral characteristics of both molecular structures.

Experimental part Materials. All the reagents and solvents were purchased from Sigma-Aldrich and were used as received if not otherwise specified. 1-Cyclohexyl-2-pyrrolidone (CHP) was purified by distillation at reduced pressure (0.05 mbar, bp 104 °C) prior to use. rGO, produced by reduction of graphene oxide, was acquired from ACS Materials, LLC under the name of “single layer graphene” and was used as received. Thermogravimetric analyses of the samples were carried out within platinum pans with a Q5000IR TGA (TA Instruments). Raman spectra were recorded with the 633 nm line of a He–Ne laser at room temperature on an Invia Renishaw Raman microspectrometer equipped with a 50× objective. Transmission Electron Microscopy (TEM) characterization were carried out with a FEI Tecnai F20 instrument, equipped with a Schottky emitter and operated at 120 kV, to minimize the electron beam damage. The samples were drop cast onto lacey carbon coated Cu grids and dried under vacuum before analysis. UV-visible absorbance spectra were recorded with a Perkin Elmer λ650 spectrophotometer or a Varian Cary 5000 spectrophotometer, using quartz cells with 1.0 cm path length. Emission spectra were obtained with a Perkin Elmer LS-50 spectrofluorometer, equipped with a Hamamatsu R928 phototube. Luminescent excited state lifetimes were measured by an Edinburgh FLS920 spectrofluorometer equipped with a TCC900

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card for data acquisition in time-correlated single-photon counting experiments (0.2 ns time resolution) with a LDH-P-C-405 pulsed diode laser. Dispersion of rGO. rGO was suspended in 10 mg portions, each in 7 ml CHP within a glass test tube, by ultrasonication with a Misonix Sonicator 3000 operating with a titanium microtip for 1 h. Set parameters: time on = 3 sec, time off = 3 sec, power level = 2, resulting in 0.6-0.9 W/ml. Grafting of 4-methoxyaniline on rGO (batch conditions). In a dry, 250 ml round bottom flask equipped with a magnetic stirrer, held under dry nitrogen flow behind a protective screen due to the potential explosion risk, an aliquot of rGO suspension in CHP (7 ml, corresponding to 10 mg of C, 0.83 mmol) was loaded and heated up at 80 °C under stirring. Then, a solution of 4-methoxyaniline (102 mg, 1 eq) in 3.0 ml of CHP and isopentylnitrite (97 mg, 1 eq) were added to start the reaction. The resulting mixture was kept at 80 °C under stirring for 4 hours, after which it was diluted with methanol (100 ml) and filtered on a Millipore membrane (VCTP, 0.1 µm pore size). The filtered solid was washed with methanol (3 x 100 ml) to remove unreacted reagents and other side products, then extracted and characterized (vide infra). Grafting of 4-methoxyaniline on rGO (microfluidic conditions). A Labtrix Start Standard (Chemtrix BV, The Netherlands) was used for all the experiments. Chemtrix BV proprietary glass chips with different volumes were used (SOR3221, with a volume of 1 µl and SOR3223, with a volume of 10 µl) to control the reaction time. The reagents were pumped by syringe pumps (Fusion 100, by Chemyx Inc.), fitted with 2.5 ml SGE glass syringes, PEEK fittings and 1/16’’ od PEEK tubings. No check valves were mounted on the flow paths of rGO, to avoid the risk of clogging. Four connections are available for the microfluidic chips used in this work. The first two (A and B) were used as inlets for the starting materials, which were mixed by the passive mixer and transported along the microchannel held at constant temperature by the Peltier unit during the reaction. After the channel, a third connector (C) was used to mix products with a quencher and stop the reaction. The crude mixture was then collected through the last connection (D), into a magnetically-stirred vial filled with more quencher to further dilute the material. To an aliquot of rGO suspension in CHP (3.5 ml, corresponding to 5 mg of C, 0.42 mmol), 4-methoxyaniline (0.01, 1 or 5 eq, depending on the experimental conditions) was added and dissolved by stirring. The resulting solution was pumped through inlet A (14 µl/min). Isopentyl nitrite (0.01, 1 or 5 eq) was dissolved in 1.5 ml CHP and pumped through inlet B (6 µl/min). Methanol was used as a quencher in inlet C (20 µl/min). The resulting molar flow rate of rGO was 2.37 µmol/min, corresponding to 1.71 mg/h. Under these conditions, different reaction times were selected by choosing the volume of the glass microreactor and the temperature was set by the built-in controller of the Peltier cell heater. In a typical experiment, the whole setup was run for 5 minutes while discarding the products, in order for the system to reach a steady state condition, then the products were collected for 1 h in a vial loaded with 4 ml of methanol. The products were filtered on Millipore membranes (VCTP, 0.1 µm pore size). The filtered solids were washed with methanol (3 x 20 ml) to remove unreacted reagents and other side products, then extracted and characterized (vide infra).


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Grafting of 4-methoxyaniline on rGO (scaled up, continuous flow conditions). The reactor used to scale up the continuous-flow processing of rGO consisted in a 5.50 m long PTFE tube (0.5 mm id, by Deutsch&Neumann GmbH). It was connected with PEEK fittings and PTFE tubing to the syringe pumps dispensing the reagents, through a stainless steel tee junction. A section of the reactor (5.1 m long, corresponding to an internal volume of 1.0 ml) was coiled and immersed in an oil bath held at 80 °C. The remaining length of the tube brought the stream of products to a magnetically stirred round bottom flask filled with quencher. The starting materials were prepared as reported in the previous paragraph (microfluidic conditions) but on a larger scale (14 ml of rGO suspension were prepared) and the flow rates for the two streams were 2.8 and 1.2 ml/h, resulting in an rGO molar flow rate of 8 µmol/min, or 5.7 mg/h. The scaled-up continuous flow experiment was run for 5 hours, then the products were filtered on Millipore membranes (VCTP, 0.1 µm pore size) and washed with methanol (3 x 100 ml) to remove unreacted reagents and other side products before extraction and characterization (vide infra). Grafting of squaric acid derivatives. The synthesis of derivatives 1 and 2, is detaileded in the SI. Grafting of the derivatives on rGO was carried out with the microfluidic setup and conditions described above. The 10 µl glass chip was used and the stream pumped through connection A was prepared by dissolving the derivatives, either 1 or 2 (0.1 eq), in 3.5 ml of an rGO suspension in CHP. Likewise, isopentyl nitrite (0.1 eq) was dissolved in 1.5 ml CHP for use in inlet B. Co-functionalization with both squaraines was carried out by using 0.05 eq of each chromophore in stream A. Extraction protocol. To ensure the reproducibility of the experiments and allow for a meaningful comparison of the products of different reactions, the same extraction protocol was used to recover functionalized rGO from crude reaction mixtures throughout this work. The dried material resulting from functionalization reactions was sonicated for 2 minutes in 7 ml DMF (0.6-0.9 W/ml) and centrifuged in an MR23i Jouan ultracentrifuge equipped with a SWM 180.5 swinging bucket rotor (Thermo electron corporation), at 3000 rpm (rcf = 1430 g) for 5 minutes. Finally, the supernatant was transferred in a clean vial for characterization. Characterization of frGO. TGA samples were prepared by slowly drying five 100 µl aliquots of DMF suspensions of frGO on a platinum pan. The temperature of the sample was stabilized at 110 °C for 20 minutes, then increased to 900 °C at 10 °C/min under nitrogen atmosphere. The weight measured at the end of the isotherm at 110 °C was used to calculate the frGO content of the samples. T1 and T2 temperatures were evaluated as the maxima in the weight vs temperature derivative plot. The point of minimum between T1 and T2, Tmin, was used to evaluate the degree of functionalization DF, as the ratio between the number of moles of organic groups attached to the rGO backbone and that of rGO itself. The former was calculated by dividing the mass loss between 110 °C and Tmin by the molecular weight of the aryl radical. The weight loss between Tmin and 900 °C, divided by the atomic weight of carbon yielded the number of moles of rGO. Samples for Raman spectroscopy were prepared by dropcasting DMF suspensions on pre-cleaned glass micro slides (Corning) and annealing at 110 °C.

Fluorescence quenching experiments. The fluorescence emission at 575 nm of a solution of P3HT in DMF was measured upon excitation at 460 nm (slits were set at 5 nm for excitation and 7.5 nm for emission). frGO suspensions were added in 50 µl aliquots and the fluorescence spectra recorded after each addition, after mixing with a magnetic stirrer for 15 minutes to stabilize the system. UV-Vis spectra of the starting P3HT solution and of the mixture with 15 aliquots of frGO suspension were also recorded. Fluorescence intensities were corrected for the dilution induced by the addition of the frGO suspension. Photophysical characterization of 1-frGO, 2-frGO and 12-frGO. The UV-Vis spectra of frGO samples were recorded in air-equilibrated DMF solution. To compare them to the physical mixture of either 1 or 2 with rGO, two samples were prepared, by first diluting dispersions of rGO, to the point at which they displayed the same optical density of the aforementioned samples at 800 nm, to ensure an equal carbon content. 32 The free dye was then added in small aliquots till the mixture displayed the same absorbance as the relative frGO sample, at 492 nm for chromophore 1 and at 589 nm for chromophore 2. The UV-Vis spectra of the pristine rGO suspensions were subtracted from the spectra of both the chromophore-rGO physical mixture and the frGO samples, for comparison, to observe the effects of the covalent bonds on the absorption properties of squaraine dyes. The fluorescence intensity of all the samples was also recorded in air-equilibrated DMF solution: λexc = 492 nm for 1-frGO; λexc = 589 nm for 2-frGO; λexc = 450 - 625 nm (in 25 nm steps) for 1-2-frGO. Electrochemical characterization of 1-frGO, 2-frGO and 1-2-frGO. Cyclic voltammetry traces of 1 and 2 were recorded in acetonitrile using 0.1 M tetraethylammonium hexafluorophosphate as the supporting electrolyte, glassy carbon WE, platinum CE and an silver QRE. A 1 V/s scanning rate was employed and either methylviologen or decamethylferrocene provided internal reference for the redox potentials. The same conditions were used for 1-frGO and 2-frGO, that were drop cast on the WE prior to analysis. For 1-frGO, potentials in the -2.00 to +1.50 V were scanned, while for 2-frGO the window was narrowed to the -1.30 to +1.50 V range.

ASSOCIATED CONTENT Supporting Information. Continuous flow chemistry setups, transmission electron microscope images, Raman spectroscopy spectra, in-depth information on the calculation of the degree of functionalization of samples, cyclic voltammetry and thermogravimetric traces, detailed synthesis and characterization of the squaric acid derivatives 1 and 2. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]; [email protected]

Present Addresses † Institut de Science et Ingénierie Supramoléculaires, Université de Strasbourg. 8 allée Gaspard Monge 67083 Strasbourg Cedex France ‡ FIS – Fabbrica Italiana Sintetici S.p.A., viale Milano 26, 36075 Montecchio Maggiore (VI) - Italy

Author Contributions



The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors would like to thank P. Salice for technical support and fruitful discussions. Financial support from MIUR through FIRB NANOSOLAR (RBAP11C58Y) and PRIN HiPHUTURE (2010N3T9M4) is gratefully acknowledged. PC, LR and RM gratefully acknowledge financial support by the European Commission ERC Starting Grant (PhotoSi, 278912)

ABBREVIATIONS rGO, reduced graphene oxide; frGO, functionalized rGO; CNTs, carbon nanotubes; P3HT, poly(3-hexylthiophene); TGA, termogravimetric analysis; TEM, transmission electron microscopy.


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Chemistry of Materials Functionalized

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Controlled Functionalization of Reduced Graphene Oxide Enabled by

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