A 1,3-Dipolar Cycloaddition Protocol to Porphyrin-Functionalized Reduced Graphene Oxide with a Push-Pull Motif

Nano Research 1 DOINano 10.1007/s12274Res -014-0569-x

Aijian Wang,1 Wang Yu,1 Zhengguo Xiao,2 Yinglin Song,2 Marie P. Cifuentes,3 Mark G. Humphrey,3 and Chi Zhang*1

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0569-x http://www.thenanoresearch.com on Aughst 25, 2014

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Graphical Table of Contents

Porphyrin-functionalized reduced graphene oxide with a push-pull motif has been prepared following two different approaches: a straightforward Prato reaction with sarcosine and a formyl-containing porphyrin, and a stepwise approach that involved a former Prato cycloaddition followed by nucleophilic substitution with an appropriate porphyrin.

A 1,3-Dipolar Cycloaddition Protocol to Porphyrin-Functionalized

Reduced Graphene Oxide with a Push-Pull Motif

Aijian Wang,1 Wang Yu,1 Zhengguo Xiao,2 Yinglin Song,2 Marie P. Cifuentes,3 Mark G. Humphrey,3 and Chi Zhang*1

1 China-Australia Joint Research Center for Functional Molecular Materials, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, P.R. China

2 School of Physical Science and Technology, Soochow University, Suzhou 215006, P.R. China

3 Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia

Address correspondence to Chi Zhang, [email protected]

Abstract: Reduced graphene oxide (RGO) has been covalently functionalized with porphyrin moieties by two methods: a straightforward Prato reaction (i.e. a 1,3-dipolar cycloaddition) with sarcosine and a formyl-containing porphyrin, and a stepwise method that involves a 1,3-dipolar cycloaddition to the RGO surface using 4-hydroxybenzaldehyde, followed by nucleophilic substitution with an appropriate porphyrin. The chemical bonding of porphyrins to the RGO’s surface has been confirmed by ultraviolet/visible absorption, fluorescence, Fourier-transform infrared, and Raman spectroscopies, X-ray powder diffraction and X-ray photoelectron spectroscopy, transmission electron and atomic force microscopies, and thermogravimetric analysis; this chemical attachment assures efficient electron/energy transfer between RGO and the porphyrin, and affords improved optical nonlinearities compared to those of the RGO precursor and the pristine porphyrin.

Keywords: porphyrin ∙ cycloaddition ∙ reduced graphene oxide ∙ nonlinear optics

1. Introduction

Graphene is a single layer two-dimensional planar sheet of sp2 hybridized carbon atoms arranged in a hexagonal lattice, the basic structural element for graphite and carbon nanotubes.[1] First produced in the lab by Novoselov et al. in 2004,[2] graphene is considered to be one of the most robust nano-scale materials. The novel and unique electronic properties exhibited by graphene and its derivatives that result from the presence of extended and delocalized π-electron systems make them excellent candidates for applications in the area of optoelectronics, energy storage and photovoltaic devices.[3-5] The easy of processing of graphene is of critical importance in facilitating its integration with substrates and materials.[6,7] Many reports have focused on the chemical modification of graphene with specific functionalities via covalent or non-covalent methods for tuning its chemical and physical properties,[8] while the resultant graphene materials can facilitate charge transfer when graphene is combined with electron donors, such as porphyrin[9] or phthalocyanine;[10] graphene is a particularly efficient electron acceptor. Modification of the carbon network by grafting organic moieties is important in the design of graphene-based nanoelectronics due to the fact that this may provide a means to dope the material.[11,12]

Porphyrins have many potential uses in optoelectronics, nonlinear optics, solar cell applications, and photodynamic therapies, because of strong excited-state absorption, high triplet yields, long excited-state lifetimes, and delocalizable electron density.[13-16]

Carbon-nanotube-porphyrin and C60-porphyrin nanohybrids have attracted widespread attention and have been explored in a number of potential applications.[17-19] Because of these precedents, and the similarity of graphene, carbon nanotubes and C60, nanohybrids combining

graphene with porphyrin may be useful for a diverse range of potential applications in biology, catalysis, sensors and solar cells, etc.[5] Thus far, reports of graphene-based hybrid nanomaterials are largely restricted to graphene oxide (GO), which has various chemically reactive oxygen-containing functionalities (e.g. carboxyl, epoxy, and hydroxyl groups).[20-23]

In contrast to GO, the electrical conductivity and electron/hole transporting properties can be recovered following the reduction step to afford reduced graphene oxide (RGO),[24,25] and organic moieties can be chemically grafted to the surface of RGO with retention of the structural integrity and electronic structure of the RGO framework. As such, chemical functionalization could potentially pave the way towards the use of RGO in practical applications. However, reports on functionalized RGO systems are scarce, especially for those involving covalent attachment;[26,27] to some extent, this can be attributed to the irreversible aggregation of RGO which ensues in the absence of electrostatic or steric protection, rendering further processing more difficult. It is therefore critically important to design and prepare RGO-based readily-processed nanohybrid materials for optoelectronic and photonic devices.

Encouraged by these considerations, we wondered if the combination of RGO and optoelectronic porphyrin molecules would afford species that possess not only the intrinsic properties of RGO and porphyrins, but also novel functions resulting from the mutual π interaction between the RGO and the porphyrins; multifunctional nanometer-scale systems for optical and optoelectronic applications may thereby be generated. However, to the best of our knowledge, there is no report in the literature of the fabrication of RGO-porphyrin conjugates.

In this contribution, we present the first study of the preparation of dispersible

RGO-porphyrin nanohybrids through 1,3-dipolar cycloaddition reaction of RGO, sarcosine and appropriately functionalized porphyrin; this reaction has been previously used for the chemical modification of carbon nanotubes and fullerenes.[28,29] A stepwise approach to achieve the porphyrin functionalized RGO at minimum synthetic cost has also been explored; this widely applicable approach affords functionalized RGO in which the electronic structure is preserved. The hybrid materials thus prepared are stable in solution and have been characterized by a number of spectroscopic and microscopy techniques. In particular, we complement our work with a detailed photophysical investigation on ground- and excited-state RGO-porphyrin interactions, as well as the third-order nonlinear optical (NLO) performance of these nanohybrids in the nanosecond regime at 532 nm; the hybrids exhibit enhanced NLO responses in comparison with the individual RGO and porphyrins.

2. Results and Discussion

2.1. Syntheses

1,3-Dipolar cycloaddition has proven to be an effective method for functionalizing conjugated π systems: convenient synthetic applications of 1,3-dipolar cycloadditions to fullerenes, carbon nanotubes, onions and nanohorns have led to many applications in areas such as drug delivery, nanoelectronic devices, solar cells and biotechnology.[30-32] The carbon-based nanohybrids usually act as electron acceptors when they are appropriately interfaced with an electron donor moiety. Although the reactivity of graphene differs from that of fullerenes and carbon nanotubes, the 1,3-dipolar cycloaddition can be efficiently performed and affords a highly functionalized hybrid with reaction taking place not only at the edges, but also at the C=C bonds in the center of graphene sheets.[33-37] Inspired by these considerations,

we decided to explore this class of reactions with graphene surfaces. In the present study,

RGO was successfully functionalized with porphyrin molecules by 1,3-dipolar cycloaddition reactions. Although functionalization of graphene employing unstable 1,3-dipolar azomethine ylides (obtained from condensation of an aldehyde with an α-amino acid) has been reported previously,[33,36] the functionalization of RGO sheets with porphyrin units is not straightforward. Two possible routes toward functionalizing RGO with porphyrin were envisaged. Scheme 1 summarizes these procedures, employed for the preparation of the nanohybrids RGO-TPP 1 and RGO-TPP 2. The first route (Route 1, Scheme 1) involves initial reaction of sarcosine and 4-hydroxybenzaldehyde with RGO, followed by nucleophilic substitution of 5-[4-(2-bromoethoxy)phenyl]-10,15,20-triphenylporphyrin (TPP 1) by the derivatized RGO. This method is advantageous in that chemical modification of graphene usually requires a large excess of the reactants;[34] in this particular case, one reactant is the inexpensive 4-hydroxybenzaldehyde. Another important consideration is that hydroxyl groups can be attached to the RGO surfaces by a single reaction without degrading the RGO electronic properties, permitting a high grafting density of organic units and potentially a wider application as an optoelectronic nanohybrid material. The subsequent nucleophilic substitution reaction of TPP 1 and the functionalized RGO bearing pendent OH groups can, in principle, proceed under nearly stoichiometric conditions. A potential setback of this route lies in the fact that it is very difficult to control the number of OH units that are used in nucleophilic substitution with TPP 1. As a consequence, we explored a second method (Route

2 in Scheme 1) that involves preparing a formyl-containing porphyrin

5-[4-(2-(4-formylphenoxy)ethyloxy)phenyl]-10,15,20-triphenylporphyrin (TPP 2). The

advantage of this method is that the formyl-containing TPP 2 can be attached to RGO in a straightforward fashion, through the formation of the corresponding azomethine ylide

(Scheme 1). After only a few hours, the color of the solution had changed to dark brown, implying that reaction had occurred. The crude product was isolated by filtration and washed with deionized water and several organic solvents; UV-Vis spectroscopy and thin layer chromatography (TLC) were then used to confirm that the supernatant layer contained no unreacted TPP 2 following the final washing.

(Please insert Scheme 1 here)

2.2. Linear Optical Absorption and Fluorescence Spectroscopy Analysis

The fingerprint for porphyrin-functionalized RGO nanohybrids is their electronic behavior; the linear absorption and fluorescence spectra are sensitive to the presence of donor/acceptor units that can influence such spectra by photo-induced energy and/or electron transfer.[38] DMSO was used to achieve RGO dispersion, and thereby facilitate comparison of the electronic properties of RGO-TPP 1 and RGO-TPP 2 with those of TPP 1 and TPP 2, respectively. RGO exhibits a broad absorption with continuously decreasing intensity extending to 700 nm, while the electronic absorption spectrum of TPP 1 is characterized by a strong Soret absorption at 420 nm, together with four relatively weaker Q bands between 500 and 700 nm (Figure 1a). Covalent grafting of TPP 1 onto RGO results in the reduction in intensity of the absorption of the porphyrin moieties in the RGO-TPP 1 nanohybrid compared to those in the absorption spectrum of TPP 1 in DMSO, although no obvious spectral shift is observed. A noteworthy broadening of the Soret band is observed compared with that of the free porphyrin moiety, which may be due to electronic interactions between the RGO and

porphyrin units in this system (RGO is usually regarded as an electron acceptor and TPP 1 as an electron donor). For RGO-TPP 2, a broad absorption at 423 nm, corresponding to a small red shift of 3 nm compared to TPP 2 (Figure 1b), is observed. . Upon comparing the spectra of the two porphyrin-functionalized RGOs (RGO-TPP 1 and RGO-TPP 2), three observations can be made (Figure 1). First, the overall intensity of the porphyrin-centered transitions increases on proceeding from RGO-TPP 2 to RGO-TPP 1. The intensity of the Soret band of

RGO-TPP 2 is diminished significantly, while the G-bands in RGO-TPP 2 are not as prominent as those in RGO-TPP 1. This suggests that the porphyrin content in the two

RGO-TPP nanohybrids declines on proceeding from RGO-TPP 1 to RGO-TPP 2 due to the different functionalization procedures. Secondly, a red-shift of 3 nm (i.e. from 420 to 423 nm) in the absorption maximum is seen for RGO-TPP 2 in comparison with that of RGO-TPP 1, indicating the effect of different reaction conditions on the absorption spectra. Thirdly, the

RGO characteristics in both RGO-TPP nanohybrids are concomitantly reduced in absorption intensity. The ground-state absorption spectra of RGO-TPP 1 and RGO-TPP 2 contain features of both the RGO and porphyrin, confirming the formation of RGO-TPP nanohybrids.

Remarkably, similar effects rules have also been observed in the UV/Vis absorption spectra of porphyrin-functionalized multi-walled carbon nanotubes (MWCNTs),[18] corresponding to the different degree of functionalization.[39]

(Please insert Figure 1 here)

A more compelling test of the interactions between the porphyrins and RGO is available from fluorescence experiments focusing on the fluorescence features of the porphyrin moieties. An intramolecular donor-acceptor structure usually allows charge-transfer

interaction or the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) transitions to occur,[24,40] and this is evident from the fluorescence spectra of

RGO-TPP 1 and RGO-TPP 2, the results being shown in Figure 2. Upon excitation at 420 nm,

RGO-TPP 1 displays fluorescence features with a maximum and a shoulder at 654 and 719 nm, respectively. Because RGO does not show a fluorescence signal, the fluorescence of

RGO-TPP 1 must be from the porphyrin moieties, which unequivocally confirms the presence of the porphyrin units in RGO-TPP 1. Following excitation at the same wavelength, a physically blended sample of RGO and TPP 1 (1:1 weight ratio, a control sample with the same absorbance value as that of the nanohybrids) exhibits similar emission bands with 15% quenching of fluorescence emission in comparison with TPP 1. However, compared to TPP 1, a significant fluorescence quenching (58%) is observed for RGO-TPP 1, a result likely to arise from a through-bond mechanism because of the unique direct linkage of the porphyrin moieties and the RGO, suggesting the presence of photo-induced electron/energy transfer between the excited states of the TPP unit and the RGO moiety. The physically blended sample of RGO with TPP 2 (1:1 weight ratio) shows similar fluorescence quenching (13%) to the mixture of RGO and TPP 1. In contrast, RGO-TPP 2 exhibits a vastly different fluorescence quantum yield from RGO-TPP 1. A quenching factor of 133 was determined for

RGO-TPP 2 (compared with TPP 2), which is much larger than the value of 2.3 for RGO-TPP

1 (compared with TPP 1). In addition, the weak fluorescence emission for RGO-TPP 2 was observed at 656 nm, corresponding to a red shift of 2 nm and a significantly decrease in intensity compared with that of RGO-TPP 1. These observations are consistent with both the formation of RGO-TPP nanohybrids and the tunability of optical properties through different

covalent functionalization methods. Similar fluorescence quenching has been observed in the

MWCNT-TPP hybrids, for which photo-induced energy/electron transfer was suggested to explain the mechanism of their fluorescence quenching.[18] The functionalized porphyrin moieties presumably act as an electron transporting antenna when covalently linked to RGO, while the RGO acts as an electron acceptor unit, leading to the observed fluorescence quenching and energy release. In the present RGO-TPP nanohybrids, the RGO significantly quenches the photoluminescence of the porphyrin moieties, which confirms the close proximity of RGO and porphyrin, and their strong interaction. Both nanohybrids have potential as the active materials for various optoelectronic applications, such as the sources for solar energy harvesting in solar cells.

(Please insert Figure 2 here)

2.3. FTIR Spectroscopy Analysis

Further evidence in support of covalent functionalization of RGO is available from FTIR spectroscopy. Figure 3 displays FTIR spectra of RGO, TPP 1, TPP 2 and the nanohybrid materials (RGO-TPP 1 and RGO-TPP 2). The FTIR spectrum of RGO is almost featureless with weak skeletal vibration of the aromatic domains around 1634 cm-1, which is consistent with previous observations[41,42] and with removal or transformation of much of the oxygen-containing functional groups in GO on its conversion into RGO. For the RGO-TPP 1 and RGO-TPP 2 nanohybrids, some bands that are characteristic of porphyrin are observed, which are coincident with those displayed by TPP 1 and TPP 2. The disappearance of the aldehyde stretching-band centred at 1705 cm-1, in proceeding from RGO-TPP 1 to RGO-TPP

2, is consistent with reaction of the aldehyde units and thereby covalent attachment of the

porphyrin to the RGO. The absorption bands in the region 2850-2960 cm-1, attributed to the

C-H stretching vibrations of the aromatic rings, further confirm the existence of the porphyrin moieties on RGO. All features in the FTIR spectra are consistent with the formation of

RGO-TPP nanohybrids. Although some absorption peaks characteristic of porphyrins were observed in the FTIR spectra, it should be noted that some difference was found for the

RGO-TPP nanohybrids, ascribed to the presence of residual phenol units in RGO-TPP 1 resulting from incomplete reaction of hydroxyl groups in the

4-hydroxybenzaldehyde-functionalized RGO hybrid with TPP 1. Consistent with the clear distinctions apparent in the UV/Vis and fluorescence measurements, these observations illustrate the effect of the reaction conditions on the spectroscopic properties of the two nanohybrids prepared via the different routes.

(Please insert Figure 3 here)

2.4. Raman Spectroscopy Analysis

The significant structural changes in proceeding from GO to RGO, and then to

RGO-TPP 1 and RGO-TPP 2, are also reflected in their Raman spectra (Figure 4). GO exhibits a prominent band at 1370 cm-1, corresponding to the breathing mode of κ-point

-1 phonons of A1g symmetry (the D-band). There is also an intense tangential mode at 1602 cm ,

2 which arises from the first-order scattering of E2g phonons from sp carbon atoms (the G-band

[43] or E2g mode). After treatment by NaBH4, the D- and G-bands of RGO are located at 1343 and 1577 cm-1, respectively, confirming the reduction of GO during the chemical treatment.

The Raman spectrum of RGO-TPP 1 displays two bands at 1341 and 1584 cm-1. The down-shift of the D-band and the up-shift of the G-band for RGO-TPP 1, in comparison to

RGO, are consistent with previously reported functionalization of RGO.[25] However, the D and G bands of the RGO-TPP 2 hybrid, which appear at 1348 and 1587 cm-1, respectively, are both up-shifted compared to those of RGO. These results suggest a strong interaction between porphyrin and the RGO sheets, and are consistent with the fluorescence emission results. The intensity ratio of the D/G bands provides a measure of the disorder/defects in graphene, a

3 smaller intensity ratio ID/IG being ascribed to a larger average size and fewer sp defects/disorders of the in-plane graphitic crystallite sp2 domains. The D-band is a measure of the degree of covalent functionalization, which transforms sp2 to sp3 sites, whereas the

G-band has been used to estimate the distribution of this modification.[44-46] We have not found clear evidence of any peaks characteristic of the functionalizing moieties, due to the fact that functionalization is weak. However, the intensity ratio of the D band to the corresponding G band increased from 0.98 for RGO to 1.03 for RGO-TPP 1 and 1.02 for

RGO-TPP 2, which is consistent with the introduction of a considerable amount of structural defects on the lattice following functionalization, suggesting that the newly formed graphitic domains are smaller in size, but more numerous in number. Reduction in the sp2 hybridization is expected following the successful cycloaddition reactions, consistent with the results of

FTIR spectroscopy discussed above.

(Please insert Figure 4 here)

2.5. XRD Studies

The attachment of porphyrins to RGO is also supported by powder XRD patterns, the results being shown in Figure S4. The inter-planar d-spacing of GO is 0.78 nm (2θ = 11.2°), consistent with the presence of oxygen-containing functional groups attached to the surface of

GO and the atomic-scale roughness arising from structural defects.[47,48] A shoulder with a small broad peak was observed at around 22o, indicating that a fraction of GO was not fully intercalated. After reduction of GO by NaBH4, the (002) peak of GO disappears, whereas a broad diffraction peak was observed extending from 15o to 30o, assigned to exfoliation of

RGO into monolayers or several-layer species. The decreased interlayer spacing can be attributed to the removal of oxygen-based functional groups on the basal plane following reduction of the GO, resulting in a tighter RGO. After functionalization, as depicted in Figure

S4, the (002) peaks of RGO-TPP 1 and RGO-TPP 2 were shifted to smaller 2 values compared to RGO, which is indicative of the grafting of porphyrin moieties onto the surface of the RGO, and their functions as “spacers” for the RGO platelets, resulting in increased interlayer spacing.[49] The broad peaks imply that the samples are very poorly ordered along

[46] the stacking direction, while the increased intensity ratio (ID/IG) of the D-band intensity (ID) compared to the G-band intensity (IG) is consistent with a lower degree of crystallinity of these RGO materials (Figure 4). Both nanohybrids have a pattern similar to RGO, so the functionalization process does not destroy the layered structure of RGO.

2.6. AFM and TEM Studies

The distribution of porphyrin on the RGO nanosheets of the RGO-TPP nanohybrids and the height profile in selected locations were examined by AFM. As shown in Figure 5a, the average thickness of the RGO aggregations was determined to be about 3.6 nm because of the poor dispersion of the RGO nanosheets in methanol and π-π stacking during reduction, resulting in the formation of several-layer graphene. Considering that the thickness of a clean graphene nanosheet is approximately 0.95 nm,[50] this suggests ca. 4 layers of RGO nanosheet.

By comparison, when RGO was functionalized with TPP 1, the average thickness of a

RGO-TPP 1 aggregation was measured to be about 2.1 nm (Figure 5b), a ca. 1.15 nm increase over the thickness of a graphene sheet. As the thickness of one porphyrin molecule is about

0.5 nm,[9] it can be concluded that porphyrin molecules are most likely to be grafted onto the

RGO surface of both sides. Moreover, as shown in Figure 5b, the distribution line of the height profile of RGO-TPP 1 is relatively flat, suggesting that the porphyrin molecules should be uniformly distributed on the surface of RGO. Similar results were also observed for

RGO-TPP 2 (Figure 5c). The structural change of RGO before and after functionalization was further investigated by TEM. In the case of RGO, the lack of functional groups in the centre of the sheets induces significant aggregation (Figure S5a), consistent with a flake-like shape of RGO.[34] From the TEM images of RGO-TPP 1 and RGO-TPP 2 (Figures S5b and S5c), the presence of transparent single- and several-layer RGO flakes can be observed due to their thin nature, which indicates that the compatibility between the RGO and organic solvents was significantly improved following functionalization,[51] and that the porphyrin-functionalized

RGO can be uniformly dispersed in DMSO.

(Please insert Figure 5 here)

2.7. Thermogravimetric Analysis

The presence of organic groups on the RGO sheets was further confirmed by TGA, which has been widely used to characterize the covalent chemical functionalization of graphene.[52] Figure S6 displays TGA thermograms of weight loss as a function of temperature for RGO, RGO-TPP 1 and RGO-TPP 2, as measured under a nitrogen atmosphere. The TGA curve of RGO reveals significant thermal stability following the

removal of the labile oxygen functional groups by the chemical reduction. Apart from a slight mass loss below 150 oC, which can be attributed to the loss of adsorbed water and the organic impurities, no significant mass loss (approximately 5.2%) was detected when the material was heated up to 800 oC. RGO-TPP 1 and RGO-TPP 2 display approximately 27% and 30% weight loss, respectively, between 50 and 800 oC. It is likely that the significantly poorer thermal stability of RGO-TPP 1 and RGO-TPP 2 nanohybrids over the investigated temperature range is due to the loss of the porphyrin units, which therefore affords further evidence of the success in incorporating porphyrin moieties onto the RGO surfaces. The weight loss that occurred above 500 oC can be ascribed to the thermal decomposition of defects created at sites where the RGO functionalization occurred. The degradation behavior of RGO-TPP 1 and RGO-TPP 2 is complicated in comparison with that of RGO because porphyrin is a char-forming substance, which leaves a certain amount of residual material at high temperature in TGA measurements;[18] thus, the exact porphyrin content of both hybrids is likely to be larger than that derived from the weight loss in the TGA measurements, and consistent with their solubility in common organic solvents such as DMF and DMSO.[53]

2.8. X-ray Photoelectron Spectroscopy

XPS is a surface analysis technique that determines relative atomic composition,[54] in the present case providing confirmation of the covalent attachment of the porphyrin moieties onto the surface of the RGO. From Figure 6, it can be clearly seen that after reduction by

NaBH4, a weak O1s signal is observed in the XPS spectrum of RGO, which is indicative of the presence of residual oxygen functionalities on the RGO. After functionalization with

porphyrin, the spectra of RGO-TPP 1 and RGO-TPP 2 show a new peak that is ascribed to N

1s, confirming the formation of porphyrin grafted RGO.

(Please insert Figure 6 here)

Detailed analysis of the XPS spectra provides clear evidence that the grafting of porphyrin to RGO has been achieved. The C 1s XPS spectrum of RGO (Figure S7a) displays three peaks at around 284.5, 285.3 and 286.0 eV, corresponding to a strong sp2 C signal from the C=C bond and a weak sp3 C signal from the C-O bond,[25] and consistent with the absence of significant oxygen-containing functional groups, i.e. most of the oxygen functionalities in the GO have been removed via reduction. After coupling porphyrin to the RGO, new contributions appear in the C 1s spectra of RGO-TPP 1 and RGO-TPP 2 (Figures S7b and

S7c), corresponding to the functionalized porphyrins. For example, Figures S7b and S7c include two new peaks, located at around 285.8 and 288.2 eV, which were attributed to the

C-N and C=N units, respectively.[55] The N 1s core-level XPS spectra afford further information about the presence of the porphyrin moieties on the functionalized RGO (Figures

S7d and S7e). The N 1s spectrum of RGO-TPP 1 displays three peaks at 398.1 (the N atoms in the C-N bonds), 399.3 (the N atoms in the N-H bonds), and 400.2 eV (the N atoms in the

C=N bonds).[18] In contrast to RGO-TPP 1, the binding energies of the N 1s of RGO-TPP 2 are 398.8, 399.7 and 400.7 eV, and there is a considerable intensity increase of the N 1s peak due to the different chemical environments. From analysis of the various spectroscopic and structural data above, we conclude that (i) the 1,3-dipolar cycloaddition reaction has efficiently introduced porphyrin moieties onto the surface of RGO; and (ii) the peak separation of the N 1s spectra of RGO-TPP 1 and RGO-TPP 2 confirms the different nitrogen 17

species on the surface of both nanohybrids, and thereby the success of the porphyrin grafting process.

2.9. Optical Nonlinearities Studies

Because of their ultrafast carrier dynamics and good incident light absorption capabilities, graphene and its derivatives have become the benchmark standards for optical limiting studies,[56] so it was of significant interest to assess the optical nonlinearities of the RGO-TPP

1 and RGO-TPP 2 nanohybrids. The RGO, TPP 1, TPP 2, RGO-TPP 1, and RGO-TPP 2 samples were prepared by dissolution in DMSO, followed by 0.5 h of ultrasonic processing.

Z-scan is a simple technique that is used to measure the on-axis phase change of a laser beam as the beam propagates through nonlinear media.[57] The nonlinear media experience the maximum laser light intensity at the focal point, with the intensity decreasing on moving away from the focal point. The open-aperture Z-scan results of the tested samples at 532 nm are presented in Figure 7a. As shown in Figure 1, the linear absorption spectra of RGO-TPP 1 and RGO-TPP 2 display very low linear absorption at 532 nm, suggesting low intensity loss and little temperature change by photon absorption during the nonlinear optical measurements.[58] A broad transparency range is of significant importance for NLO applications, especially those in waveguide form.[59] For ease of comparison of the Z-scan results, all samples were adjusted to have the same linear transmittance (71% at 532 nm) by varying their concentration in DMSO. For an optical limiter, the depth of the valley in the open-aperture Z-scan curves is related to the extent of OL.[60] As shown in Figure 7a, both

RGO-TPP nanohybrids exhibit better NLO response than RGO or the porphyrin precursors

(TPP 1 and TPP 2), suggesting a significant effect from covalent grafting. A similar

observation was made for the porphyrin-covalently-functionalized C60 and MWCNTs, which exhibited enhanced NLO properties compared with their precursors.[18,39,61] The valley-shaped curves are indicative of the occurrence of positive nonlinear absorption, and an OL behavior.[60] The optical nonlinearities of the solvent DMSO and the empty quartz cells were also measured under the same experimental conditions, but no NLO response was detected at the measurement wavelength, so their effects on the experimental results can be neglected.

Comparing the nonlinear absorption performance of the RGO-TPP nanohybrids and the

C60-TPP system in Ref. [61], it is seen that both RGO-TPP nanohybrids (RGO-TPP 1 and

RGO-TPP 2) possess larger effective nonlinear absorption responses. However, different phenomena are believed responsible for the observed NLO behavior of the RGO-TPP nanohybrids and the MWCNT-TPP systems,[18] rendering further comment unwarranted. The normalized transmittance of RGO-TPP 2 is very similar to that of the previously reported

ZnTPP-functionalized MWCNTs (ZnTPP = 5-[4-{(4-formylphenyl)ethynyl}phenyl]-

10,15,20-triphenyl-porphinatozinc(II)) upon excitation at 532 nm with 4 ns pulses, with the largest dip of normalized transmittance valley observed at the focal point, while the changes of normalized transmittance of both RGO-TPP nanohybrids are clearly greater than the

MWCNT-TPP system (TPP = 5-[4-{2-(4-formylphenoxy)ethyloxy}phenyl]-10,15,20- triphenylporphyrin). The tetraphenylporphyrin-functionalized MWCNTs prepared by the reaction of MWCNTs with in situ generated porphyrin diazonium compounds exhibit a weaker nonlinear absorption performance at 532 nm with 5.6 ns pulse duration compared with those of both RGO-TPP nanohybrids in the present case,[39] though the differing experimental geometries render such comparison necessarily cautious.

The curve for RGO clearly shows reverse saturable absorption (RSA) behavior followed by saturable absorption (SA) and then a reversion to RSA (Figure 7a), consistent with the

[62] NLO performance of GO. Fast relaxation from higher excited states Sn and slower relaxation from the first singlet excited state S1 play an important factor in switching the

[63] nonlinear optical absorption behavior from RSA to SA and back to RSA. The S1 state is populated both by relaxation from the higher excited states and by ground-state absorption.

The longer lifetime of the S1 state results in RGO exhibiting saturation in the intensity region

0.58-0.92 μJ. The Sn states are populated by the ESA corresponding to the S1 to Sn transition as well as two-photon absorption, but these relax to the lower excited state very quickly. At still higher intensities, two-photon absorption may dominate for RGO, resulting in the observed RSA effect.[26] The intensity-dependent switching of nonlinear absorption implies that materials such as RGO are potential candidates for ultrafast nonlinear optical switching applications.

Figure S8 displays closed-aperture Z-scan curves of dispersions of RGO, TPP 1, and TPP

2, measured with nanosecond pulses. As depicted in Figure S8a, the DMSO suspension of the

RGO nanosheets was found to exhibit a valley-peak transmission configuration, indicative of positive nonlinear refraction, and corresponding to self-focusing behavior. In contrast, the solutions of TPP 1 and TPP 2 exhibited a peak-valley transmission configuration under visible excitation (Figures S8b and S8c), suggesting negative nonlinear refraction and corresponding to self-defocusing behavior. Similar to RGO, the nonlinear refraction Z-scan curves of both nanohybrids (RGO-TPP 1 and RGO-TPP 2) were also characterized by a valley followed by a peak as shown in Figures 8a and 8b, implying that the behavior of the RGO nanosheets is

dominant in determining the nonlinear refraction performance of the RGO-TPP nanohybrids.

However, both RGO-TPP nanohybrids exhibit different normalized transmittance values and peak-valley positions in comparison with those of RGO, TPP 1, and TPP 2, presumably a result of their covalent linkage.

Optical limiting, the optical analogy of an electrical surge protector, is of significant importance for protecting human eyes and optical sensors against damaging sources of light.[64] Any materials (optical limiters) which have high transmittance to low-intensity light but low transmittance to intense light, and that thereby possess an efficient nonlinear absorption effect as a function of input fluence, are of great interest for OL. With the aim of investigating the OL behavior of all samples, input fluence versus output fluence plots were constructed as given in Figure 7b. At low input fluence, there is no appreciable change in the output fluence, the linear relationship obeying Beer’s Law. As the incident fluence increases, the output fluence decreases and deviates from linearity, consistent with an OL response. The decrease in output fluence of RGO-TPP 1 and RGO-TPP 2 with increasing fluence exceeds that of the other three samples in the present study. The extent of the decrease in output fluence (RGO-TPP 2 > RGO-TPP 1 > TPP 1 > TPP 2 > RGO) suggests that the OL performance of the RGO-TPP nanohybrids is better than RGO, TPP 1 and TPP 2, consistent with the results of open-aperture Z-scan. It should be pointed out that the NLO response of

RGO may partially arise from nonlinear scattering (NLS), which is consistent with the behaviour of graphene dispersions,[65] while that of porphyrin is dominated by RSA.

Accordingly, the improved OL effect of both RGO-TPP nanohybrids can be partially attributed to the combination of NLS and RSA.[66] The defect-induced states present in both

nanohybrids, which contribute to interband transitions through ESA, may also play a role in

[67,68] the enhanced OL. The presence of defects has been confirmed by the increased ID/IG ratio observed following covalent linkage between porphyrin and RGO, which implies an increase in the number of defects in the samples. In the RGO-TPP system, porphyrin is a favorable electron donor and RGO is an electron acceptor when the two moieties are connected directly.

[18,61] Thus, as in the C60- and MWCNT-porphyrin systems, the intramolecular donor-acceptor interaction between the two moieties of TPP and RGO in our RGO-TPP nanohybrids may result in charge transfer from the photo-excited singlet TPP to the RGO moiety, resulting in fluorescence quenching and energy release. Similarly, photo-induced electron transfer has also been observed in the GO-TPP and MWCNT-TPP systems.[23,39] It is well-known that photo-induced electron transfer can lead to an increased NLO response,[69] as observed in the single-walled carbon nanotube (SWCNT)-TPP system.[70] Therefore, the photo-induced energy/electron transfer between RGO and the porphyrin moieties may also be responsible for the increased OL response. Finally, the porphyrin units may assist the homogeneous dispersion of the RGO-TPP nanohybrids in organic solvents such as DMF and DMSO, which will improve the utility of these OL effects, and consistent with the results of OL properties of functionalized SWCNT.[71]

(Please insert Figure 7 here)

3. Conclusions

Porphyrin-RGO hybrids with a push-pull motif have been satisfactorily prepared following the Prato protocol, via a 1,3-dipolar cycloaddition reaction of appropriate formyl derivatives with sarcosine. Stepwise and “one-pot” procedures have been explored. The

former involves 1,3-dipolar cycloaddition of a large excess of 4-hydroxybenzaldehyde and the sarcosine with RGO, affording the OH functionalized RGO, followed by a nucleophilic substitution reaction between the functional RGO and TPP 1, giving the nanohybrid material

RGO-TPP 1. The latter affords RGO-TPP 2 by means of a cycloaddition reaction of TPP 2 bearing a formyl group. This method solves the difficulty in controlling the degree of nucleophilic substitution and is presented as a one-step reaction that affords the same nanohybrid systems. UV/Vis, fluorescence, FTIR, and Raman spectroscopies, XRD, XPS,

TEM, AFM and TGA clearly confirm that the porphyrins are covalently grafted to the RGO, affording hybrid materials that combine the advantages of both the RGO and the porphyrin in the OL response. The interaction between RGO and the porphyrins was followed by fluorescence spectroscopy, and significant fluorescence quenching was observed for both nanohybrids, indicating the presence of efficient electron/energy transfer. When compared to

RGO and porphyrins (TPP 1 and TPP 2), both hybrids display better OL performance, implying a synergistic effect between two components arising from the covalent linkage. In concert with the unique structures and excellent electronic features of the nanohybrids, the approach toward versatile nanoconjugates with new and improved properties described herein opens the way to novel chemical and optoelectronic systems.

4. Experimental Section

4.1 Materials and Reagents

All reactions were carried out under a nitrogen (N2) atmosphere with the use of standard

Schlenk techniques. All reagents were of chemical or analytical grade. Dimethyl formamide

(DMF) was dried over CaH2 and distilled before use. Purified natural graphite was purchased

from Qingdao Zhongtian Co. Ltd. Other chemicals were purchased from commercial suppliers and used as received unless otherwise stated. Reactions that required anhydrous conditions were carried out under N2 in oven-dried glassware.

5-[4-(2-(4-Formylphenoxy)ethyloxy)-phenyl]-10,15,20-triphenylporphyrin (TPP 2),

5-[4-(2-bromoethoxy)phenyl]-10,15,20-triphenylporphyrin (TPP 1), and RGO were obtained according to literature procedures.[72-74]

4.2 Instruments and Measurements

Fourier transform infrared (FTIR) spectra were measured with a MB154S-FTIR spectrometer (Canada) between 400 and 4000 cm-1. The powdered samples were mixed with

KBr and pressed into thin pellets for FTIR study. All spectra were recorded by accumulating

32 scans at a spectral resolution of 4 cm-1 at room temperature. The UV/Vis absorption spectra were recorded in the range between 200 and 700 nm by using a JASCO V-570 spectrophotometer. Steady-state fluorescence spectra were acquired using a Fluoro-Max-P instrument; samples were dissolved in dry dimethylsulfoxide (DMSO), and the resultant solutions were filtered, transferred to a long quartz cell, and then capped and deoxygenated by bubbling with N2 before measurement. Raman spectra were performed on a Renishaw inVia

Raman Microscope using the 532 nm line of an Ar+ ion laser for excitation with a backscattering geometry. X-ray powder diffraction (XRD) studies of all samples were carried out at room temperature on a XD-3 diffractometer (Beijing Purkinje General Instrument Co.

Ltd, China) by using Cu Kα radiation (λ = 0.15418 nm). Atomic force microscopy (AFM) measurements were carried out with an AFM XE-100 (Park System) in the tapping mode of dropping the sample solution onto the freshly exfoliated mica substrate. Transmission electron

microscopy (TEM) experiments were conducted using a JEM-2100 (JEOL) instrument working at 200 kV. Samples for TEM imaging were prepared by placing a drop of a dilute dispersion of the as-prepared products on amorphous carbon-coated copper grids and then drying in air before transfer to the TEM sample chamber. The chemical nature and elemental composition of RGO and its composites were characterized by X-ray photoelectron spectroscopy (XPS), which was performed on a RBD upgraded PHI-5000C ESCA

(Perkin-Elmer) electron spectrometer with a Mg Kα line at 280 eV. Thermogravimetric

o analysis (TGA) was run on a Perkin-Elmer Pyris 1 system from 50 to 800 C under a N2 purge and with a heating rate of 10 °C/min. Samples of about 1.5 mg were measured in an alumina crucible.

4.3 Nonlinear Optical Measurements

The nonlinear absorption responses of all samples (including RGO, TPP 1, TPP 2,

RGO-TPP 1 and RGO-TPP 2) were measured by using open-aperture Z-scan; nonlinear refraction Z-scan curves of the samples were obtained by dividing the closed-aperture curves by the corresponding open-aperture Z-scan curves. The Z-scan experimental setup has been described previously.[75] Linearly polarized 4 ns pulsed 532 nm light generated from a mode-locked Nd:YAG laser with a repetition rate of 2 Hz was used as the light source for the nanosecond experiments of all samples. The laser beam was focused by a 40 cm focal length plano-convex lens. DMSO solutions of all samples were placed in a quartz cell of 2 mm thickness, which was controlled by a computer, and then moved along the z-axis of the incident beam. The incident and transmitted laser pulses were recorded by two energy detectors (Rjp-765 energy probe), which were linked to an energy meter (Rj-7620 ENERGY

RATIOMETER, Laserprobe).

The nonlinear transmission measurements, i.e. the OL performances of all samples, were performed by the Z-scan technique with the same laser system as in the nonlinear absorption experiments. A variable beam splitter was used to vary the intensity of the incident energy.

The input laser intensity was varied systematically and the whole output energy was captured by a large aperture photodetector maintained at a 10 cm distance from the OL materials. The output fluence transmitted by the sample was measured as a function of the input fluence. In order to avoid the influence of cumulative thermal effects, the laser pulses were fired at a low frequency of 2 Hz, to confirm that each pulse of light encountered fresh sample.

4.4 Preparation of Nanohybrid RGO-TPP 1

The previously-prepared purified RGO (40 mg) was sonicated for 0.5 h in DMF (40 mL).

4-Hydroxybenzaldehyde (500 mg) was added to the suspension, and the mixture was heated at 145 oC. Sarcosine (900 mg) was added in portions (3 × 300 mg every 24 h) over a period of

6 days. After this period, 100 mL of deionized water was added to the mixture. The crude product was filtered through a 0.45 μm nylon membrane to isolate the carbon-based material, which was washed with deionized water, methanol and then ethanol. The filtrate was sonicated in DMF for 2 h and then centrifuged. The supernatant was separated and the solvent was evaporated to afford the 4-hydroxybenzaldehyde-functionalized RGO hybrid as a black solid, which was thoroughly vacuum-dried at room temperature for 24 h. The procedure for the preparation of RGO-TPP 1 is shown in Scheme 1 (Route 1). A 50 mL flask was charged with anhydrous DMF (25 mL), 4-hydroxybenzaldehyde-functionalized RGO (25 mg), TPP 1

o (30 mg), and K2CO3 (100 mg). The mixture was stirred at 80 C under N2 for 4 days. The

resulting product was isolated by filtration, and the black solid was thoroughly washed with deionized water, CH2Cl2, methanol and ethanol, and then dried under vacuum overnight to give 30 mg of RGO-TPP 1.

4.5 Functionalization of RGO with TPP 2 (RGO-TPP 2)

A 1,3-dipolar cycloaddition reaction was carried out with the sarcosine and TPP 2 (Route

2 in Scheme 1), the experimental procedure being as follows: In a typical experiment, RGO

(30 mg) was added to DMF (30 mL) in a 100 mL round-bottomed flask. The resulting solution was sonicated in an ultrasonic bath for 0.5 h. Sarcosine (50 mg) and TPP 2 (40 mg) were then added, and the mixture was stirred at 145 °C under N2 for 6 days. After the reaction was finished, it was allowed to cool to room temperature over a 2 h period while stirring was maintained. The resultant solution was poured into iced water (100 mL) and then filtered through a 0.45 μm nylon membrane. The black solid on the nylon film was collected and washed with water, methanol, CH2Cl2 and anhydrous diethyl ether, after which the filtrate became colorless. The crude product was subsequently sonicated in DMF and centrifuged.

The supernatant was separated, the solvent evaporated, and the resultant solid residue was washed with methanol to give 26 mg of the desired RGO-TPP 2 hybrid material as a black powder.

Acknowledgements

Financial support from the National Natural Science Foundation of China (50925207,

51172100), the Ministry of Science and Technology of China for the International Science

Linkages Program (2009DFA50620, 2011DFG52970), the Ministry of Education of China for

the Changjiang Innovation Research Team (IRT1064), the Ministry of Education and the State

Administration of Foreign Experts Affairs for the 111 Project (B13025), and Jiangsu

Innovation Research Team are gratefully acknowledged. M.G.H and M.P.C. thank the

Australian Research Council (ARC) for support.

Electronic Supplementary Material: Supplementary material (details of ultraviolet/visible absorption, fluorescence, and Fourier-transform infrared spectra of GO, XRD spectra, TEM images, normalized TGA plots, deconvoluted experimental XPS spectra and closed-aperture

Z-scan curves of related materials) is available in the online version of this article at http://dx.doi.org/10.1007/****** and is accessible free of charge.

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Scheme 1. Chemical routes to functionalize RGO with porphyrin molecules: Preparation of RGO-TPP 1 (Route 1) and RGO-TPP 2 (Route 2).

Figure Captions

Figure 1. Absorption spectra of (a) RGO, TPP 1 and RGO-TPP 1, and (b) RGO, TPP 2 and RGO-TPP 2, in DMSO. Figure 2. Fluorescence spectra of (a) RGO, TPP 1, RGO-TPP 1 and blended RGO and TPP 1, and (b) TPP 2, RGO-TPP 2 and blended RGO and TPP 2, in DMSO. The curve (purple line) in Figure 2b for RGO-TPP 2 has been amplified by a factor of 30. Figure 3. FTIR spectra of RGO, TPP 1, RGO-TPP 1, TPP 2 and RGO-TPP 2. Figure 4. Raman spectra of GO, RGO, RGO-TPP 1, and RGO-TPP 2. Figure 5. Tapping mode AFM images of (a) RGO, (b) RGO-TPP 1, and (c) RGO-TPP 2 on mica. Figure 6. XPS wide-scan spectra of the samples. Figure 7. (a) Normalized open-aperture Z-scan data of RGO, TPP 1, TPP 2, RGO-TPP 1, and RGO-TPP 2 at 532 nm with 4 ns pulse durations. Shown in the inset is the open-aperture data of RGO. (b) Optical limiting responses (to 4 ns 532 nm optical pulses) of RGO, TPP 1, TPP 2, RGO-TPP 1, and RGO-TPP 2 in DMSO. Figure 8. Normalized closed-aperture Z-scan curves of (a) RGO-TPP 1 and (b) RGO-TPP 2.

Figure 1. Absorption spectra of (a) RGO, TPP 1 and RGO-TPP 1, and (b) RGO, TPP

2 and RGO-TPP 2, in DMSO.

Figure 2. Fluorescence spectra of (a) RGO, TPP 1, RGO-TPP 1 and blended RGO and TPP 1, and (b) TPP 2, RGO-TPP 2 and blended RGO and TPP 2, in DMSO. The curve (purple line) in Figure 2b for RGO-TPP 2 has been amplified by a factor of 30.

Figure 3. FTIR spectra of RGO, TPP 1, RGO-TPP 1, TPP 2 and RGO-TPP 2.

Figure 4. Raman spectra of GO, RGO, RGO-TPP 1, and RGO-TPP 2.

Figure 5. Tapping mode AFM images of (a) RGO, (b) RGO-TPP 1, and (c) RGO-TPP 2 on mica.

Figure 6. XPS wide-scan spectra of the samples.

Figure 7. (a) Normalized open-aperture Z-scan data of RGO, TPP 1, TPP 2, RGO-TPP 1, and RGO-TPP 2 at 532 nm with 4 ns pulse durations. Shown in the inset is the open-aperture data of RGO. (b) Optical limiting responses (to 4 ns 532 nm optical pulses) of RGO, TPP 1, TPP 2, RGO-TPP 1, and RGO-TPP 2 in DMSO.

Figure 8. Normalized closed-aperture Z-scan curves of (a) RGO-TPP 1 and (b) RGO-TPP 2.