Nano Res

1

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

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0569-x

http://www.thenanoresearch.com on Aughst 25, 2014

© Tsinghua University Press 2014

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Nano Research

DOI 10.1007/s12274-014-0569-x

1

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.

2

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, chizhang@jiangnan.edu.cn

3

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

4

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

5

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

6

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,

7

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

8

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

9

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

10

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

11

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

12

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

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

which arises from the first-order scattering of E2g phonons from sp2 carbon atoms (the G-band

or E2g mode).[43] 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

13

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

smaller intensity ratio ID/IG being ascribed to a larger average size and fewer sp3

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

14

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

the stacking direction,[46] 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.

15

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

16

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

17

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

18

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

19

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.

20

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

NLO performance of GO.[62] Fast relaxation from higher excited states Sn and slower

relaxation from the first singlet excited state S1 play an important factor in switching the

nonlinear optical absorption behavior from RSA to SA and back to RSA.[63] 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

21

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

22

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

the enhanced OL.[67,68] 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.

Thus, as in the C60- and MWCNT-porphyrin systems,[18,61] 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

23

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

24

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

25

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

analysis (TGA) was run on a Perkin-Elmer Pyris 1 system from 50 to 800 oC 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

26

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

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

27

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

28

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.

29

References

[1] Meyer, J. C.; Geim, A. K.; Katsnelson, M. I.; Novoselov, K. S.; Booth, T. J.; Roth, S. The

structure of suspended graphene sheets. Nature 2007, 446, 60-63.

[2] Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.;

Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon films.

Science 2004, 306, 666-669.

[3] Xu, Y. X.; Lin, Z. Y.; Huang, X. Q.; Liu, Y.; Huang, Y.; Duan, X. F. Flexible solid-state

supercapacitors based on three-dimensional graphene hydrogel films. ACS Nano 2013, 7,

4042-4049.

[4] Georgakilas, V.; Otyepka, M.; Bourlinos, A. B.; Chandra, V.; Kim, N.; Kemp, K. C.;

Hobza, P.; Zboril, R.; Kim, K. S. Functionalization of graphene: Covalent and

non-covalent approaches, derivatives and applications. Chem. Rev. 2012, 112, 6156-6214.

[5] Zhang, X.; Hou, L.; Cnossen, A.; Coleman, A. C.; Ivashenko, O.; Rudolf, P.; van Wees, B.

J.; Browne, W. R.; Feringa, B. L. One-pot functionalization of graphene with porphyrin

through cycloaddition reactions. Chem. Eur. J. 2011, 17, 8957-8964.

[6] Chua, C. K.; Pumera, M. Covalent chemistry on graphene. Chem. Soc. Rev. 2013, 42,

3222-3233.

[7] Ragoussi, M. -E.; Casado, S.; Ribeiro-Viana, R.; de la Torre, G.; Rojo, J.; Torres, T.

Selective carbohydrate-lectin interactions in covalent graphene- and SWCNT-based

molecular recognition systems. Chem. Sci. 2013, 4, 4035-4041.

[8] Wu, Q.; Xu, Y. X.; Yao, Z. Y.; Liu, A. R.; Shi, G. Q. Supercapacitors based on flexible

graphene/polyaniline nanofiber composite films. ACS Nano 2010, 4, 1963-1970.

[9] Xu, Y. X.; Zhao, L.; Bai, H.; Hong, W. J.; Li, C.; Shi, G. Q. Chemically converted

graphene induced molecular flattening of 5,10,15,20-tetrakis(1-methyl-4-

pyridinio)porphyrin and its application for optical detection of cadmium(II) ions. J. Am.

Chem. Soc. 2009, 131, 13490-13497.

[10] Li, Y. X.; Zhu, J. H.; Chen, Y.; Zhang, J. J.; Wang, J.; Zhang, B.; He, Y.; Blau, W. J.

Synthesis and strong optical limiting response of graphite oxide covalently functionalized

with gallium phthalocyanine. Nanotechnology 2011, 22, 205704.

30

[11] Westervelt, R. M. Graphene nanoelectronics. Science 2008, 320, 324-325.

[12] Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Surface transfer p-type doping of

epitaxial graphene. J. Am. Chem. Soc. 2007, 129, 10418-10422.

[13] Arnold, D. P.; Worthington, E. I.; Sakata, Y.; Sugiura, K. meso-ŋ1-Metalloporphyrins:

Preparation of palladio- and platinioporphyrins and the crystal structure of

5-[bromo-1,2-bis(diphenylphosphino)ethanepalladio(II)]-10,20-diphenylporphyrin. Chem.

Commun. 1998, 2331-2332.

[14] Merhi, A.; Drouet, S.; Kerisit, N.; Paul-Roth, C. O. Linear porphyrin dimers with

fluorenyl arms linked by an ethynyl bridge. Tetrahedron 2013, 69, 7112-7124.

[15] Saito, S.; Osuka, A. Expanded porphyrins: Intriguing structures, electronic properties, and

reactivities. Angew. Chem. Int. Ed. 2011, 50, 4342-4373.

[16] Esdaile, L. J.; Jensen, P.; McMurtrie, J. C.; Arnold, D. P. Azoporphyrin: the porphyrin

analogue of azobenzene. Angew. Chem. Int. Ed. 2007, 46, 2090-2093.

[17] Guldi, D. M.; Rahman, G. M. A.; Sqobba, V.; Ehli. C. Multifunctional molecular carbon

materials-from fullerenes to carbon nanotubes. Chem. Soc. Rev. 2006, 35, 471-487.

[18] Wang, A. J.; Fang, Y.; Long, L. L.; Song, Y. L.; Yu, W.; Zhao, W.; Cifuentes, M. P.;

Humphrey, M. G.; Zhang, C. Facile synthesis and enhanced nonlinear optical properties

of porphyrin-functionalized multi-walled carbon nanotubes. Chem. Eur. J. 2013, 19,

14159-14170.

[19] Wang, A. J.; Fang, Y.; Yu, W.; Long, L. L.; Song, Y. L.; Zhao, W.; Cifuentes, M. P.;

Humphrey, M. G.; Zhang, C. Allyoxyporphyrin-functionalized multiwalled carbon

nanotubes: Synthesis by radical polymerization and enhanced optical-limiting properties.

Chem. Asian J. 2014, 9, 639-648.

[20] Zhu, J. H.; Li, Y. X.; Chen, Y.; Wang, J.; Zhang, B.; Zhang, J. J.; Blau, W. J. Graphene

oxide covalently functionalized with zinc phthalocyanine for broadband optical limiting.

Carbon 2011, 1900-1905.

[21] Liu, Y. S.; Zhou, J. Y.; Zhang, X. L.; Liu, Z. B.; Wan, X. J.; Tian, J. G.; Wang, T.; Chen, Y.

S. Synthesis, characterization and optical limiting property of covalently

oligothiophene-functionalized graphene material. Carbon 2009, 47, 3113-3121.

[22] Nalla, V.; Polavarapu, L.; Manga, K. K.; Goh, B. M.; Loh, K. P.; Xu, Q. H.; Ji, W.

31

Transient photoconductivity and femtosecond nonlinear optical properties of a conjugated

polymer-graphene oxide composite. Nanotechnology 2010, 21, 415203.

[23] Wang, A. J.; Long, L. L.; Zhao, W.; Song, Y. L.; Humphrey, M. G.; Cifuentes, M. P.; Wu,

X.; Fu, Y.; Zhang, D. D.; Li, X.; Zhang, C. Increased optical nonlinearities of graphene

nanohybrids covalently functionalized by axially-coordinated porphyrins. Carbon 2013,

53, 327-338.

[24] Kamat, P. V. Graphene-based nanoassemblies. J. Phys. Chem. Lett. 2011, 2, 242-251.

[25] Zhang, B.; Chen, Y.; Liu, G.; Xu, L. Q.; Chen, J.; Zhu, X. X.; Neoh, K. G.; Kang, E. T.

Push-pull archetype of reducede graphene oxide functionalized with polyfluorene for

nonvolatile rewritable memory. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 378-387.

[26] Zhou, Y.; Bao, Q.; Tang, L. A. L.; Zhong, Y.; Loh, K. P. Hydrothermal dehydration for the

“green” reduction of exfoliated graphene oxide to graphene and demonstration of tunable

optical limiting properties. Chem. Mater. 2009, 21, 2950-2956.

[27] Li, P. P.; Chen, Y.; Zhu, J. H.; Feng, M.; Zhuang, X. D.; Lin, Y.; Zhan, H. B.

Charm-bracelet-type poly(N-vinylcarbazole) functionalized with reduced graphene oxide

for broadband optical limiting. Chem. Eur. J. 2011, 17, 780-785.

[28] Maggini, M.; Scorrano, G.; Prato, M. Addition of azomethine ylides to C60: Synthesis,

characterization, and functionalization of fullerene pyrrolidines. J. Am. Chem. Soc. 1993,

115, 9798-9799.

[29] Georgakilas, V.; Kordatos, K.; Prato, M.; Guldi, D. M.; Holzinger, M.; Hirsch, A. Organic

functionalization of carbon nanotubes. J. Am. Chem. Soc. 2002, 124, 760-761.

[30] Ballesteros, B.; de la Torre, G.; Ehli, C.; Rahman, G. M. A.; Agulló-Rueda, F.; Guldi, D.

M.; Torres, T. Single-wall carbon nanotubes bearing covalently linked phthalocyanines -

photoinduced electron transfer. J. Am. Chem. Soc. 2007, 129, 5061-5068.

[31] Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The chemistry of graphene oxide.

Chem. Soc. Rev. 2010, 39, 228-240.

[32] Segura, J. L.; Martin, N.; Guldi, D. M. Materials for organic solar cells: the

C60/π-conjugated oligomer approach. Chem. Soc. Rev. 2005, 34, 31-47.

[33] Cao, Y.; Houk, K. N. Computational assessment of 1,3-dipolar cycloadditions to graphene.

J. Mater. Chem. 2011, 21, 1503-1508.

32

[34] Ou, B.; Zhou, Z.; Liu, Q.; Liao, B.; Yi, S.; Ou, Y.; Zhang, X.; Li, D. Covalent

functionalization of graphene with poly(methyl methacrylate) by atom transfer

polymerization at room temperature. Polym. Chem. 2012, 3, 2768-2775.

[35] Quintana, M.; Spyrou, K.; Grzelczak, M.; Browne, W. R.; Rudolf, P.; Prato, M.

Functionalization of graphene via 1,3-dipolar cycloaddition. ACS Nano 2010, 4,

3527-3533.

[36] Quintana, M.; Vazquez, E.; Prato, M. Organic functionalization of graphene in

dispersions. Acc. Chem. Res. 2013, 46, 138-148.

[37] Quintana, M.; Montellano, A.; Castillo, A. E. D. R.; Tendeloo, G. V.; Bittencourt, C.;

Prato, M. Selective organic functionalization of graphene bulk or graphene edges. Chem.

Commun. 2011, 47, 9330-9332.

[38] Castelaín, M.; Martínez, G.; Merino, P.; Martín-Gago, J. Á.; Segura, J. L.; Ellis, G.;

Salavagione, H. J. Graphene functionalisation with a conjugated poly(fluorene) by click

coupling: Striking electronic properties in solution. Chem. Eur. J. 2012, 18, 4965-4973.

[39] Liu, Z. B.; Guo, Z.; Zhang, X. L.; Zheng, J. Y.; Tian, J. G. Increased optical nonlinearities

of multi-walled carbon nanotubes covalently functionalized with porphyrin. Carbon 2013,

51, 419-426.

[40] Balapanuru, J.; Yang, J. -X.; Xiao, S.; Bao, Q.; Jahan, M.; Polavarapu, L.; Wei, J.; Xu, Q.

-H.; Loh, K. P. A graphene oxide-organic dye ionic complex with DNA-sensing and

optical-limiting properties. Angew. Chem. Int. Ed. 2010, 49, 6549-6553.

[41] Li, Z.; Chen, Y. J.; Du, Y. K.; Wang, X. M.; Yang, P.; Zheng, J. W.

Triphenylamine-functionalized graphene decorated with Pt nanoparticles and its

application in photocatalytic hydrogen production. Int. J. Hydrogen Energy 2012, 37,

4880-4888.

[42] Fan, W.; Lai, Q.; Zhang, Q.; Wang, Y. Nanocomposites of TiO2 and reduced graphene

oxide as efficient photocatalysts for hydrogen evolution. J. Phys. Chem. C 2011, 115,

10694-10701.

[43] Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y.; Wu,

Y.; Nguyes, S. T.; Ruoff, R. S. Synthesis of graphene-based nanosheets via chemical

reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558-1565.

33

[44] Georgakilas, V.; Bourlinos, A. B.; Zboril, R.; Steriotis, T. A.; Dallas, P.; Stubos, A. K.;

Trapalis, C. Organic functionalisation of graphenes. Chem. Commun. 2010, 46,

1766-1768.

[45] Gong, F.; Xu, X.; Zhou, G.; Wang, Z. S. Enhanced charge transportation in a polypyrrole

counter electrode via incorporation of reduced graphene oxide sheets for dye-sensitized

solar cells. Phys. Chem. Chem. Phys. 2013, 15, 546-552.

[46] Hsiao, M. C.; Liao, S. H.; Yen, M. Y.; Liu, O. I.; Pu, N. W.; Wang, C. A.; Ma, C. C. M.

Preparation of covalently functionalized graphene using residual oxygen-containing

functional groups. ACS Appl. Mater. Inter. 2010, 2, 3092-3099.

[47] Jeong, H. K.; Lee, Y. P.; Lahaye, R. J. W. E.; Park, M. H.; An, K. H.; Kim, I. J.; Yang, C.

W.; Park, C. Y.; Ruoff, R. S.; Lee, Y. H. Evidence of graphitic AB stacking order of

graphite oxides. J. Am. Chem. Soc. 2008, 130, 1362-1366.

[48] Bourlinos, A. B.; Gournis, D.; Petridis, D.; Szabó, T.; Dékány, I. Graphite oxide: 

Chemical reduction to graphite and surface modification with primary aliphatic amines

and amino acids. Lamgmuir 2003, 19, 6050-6055.

[49] Han, Y.; Lu, Y. Preparation and characterization of graphite oxide/polypyrrole

composites. Carbon 2007, 45, 2394-2399.

[50] Guo, C. X.; Yang, H. B.; Sheng, Z. M.; Lu, Z. S.; Song, Q. L.; Li, C. M. Layered

graphene/quantum dots for photovoltaic devices. Angew. Chem. Int. Ed. 2010, 49,

3014-3017.

[51] Shen, J.; Li, N.; Shi, M.; Hu, Y.; Ye, M. Covalent synthesis of organophilic chemically

functionalized graphene sheets. J. Colloid. Interf. Sci 2010, 348, 377-383.

[52] Stankovich, S.; Piner, R. D.; Nguyen, S. T.; Ruoff, R. S. Synthesis and exfoliation of

isocyanate-treated graphene oxide nanoplatelets. Carbon 2006, 44, 3342-3347.

[53] Chang, C. M.; Liu, Y. L. Functionalization of multi-walled carbon nanotubes with furan

and maleimide compounds through Diels-Alder cycloaddition. Carbon 2009, 47,

3041-3049.

[54] Yang, K.; Gu, M. Y.; Guo, Y. P.; Pan, X. F.; Mu, G. H. Effects of carbon nanotube

functionalization on the mechanical and thermal properties of epoxy composites. Carbon

2009, 47, 1723-1737.

34

[55] Shao, H. Z.; Shi, Z. X.; Fang, J. H.; Yin, J. One pot synthesis of multiwalled carbon

nanotubes reinforced polybenzimidazole hybrids: Preparation, characterization and

properties. Polymer 2009, 50, 5987-5995.

[56] Wei, W.; He, T.; Teng, X.; Wu, S.; Ma, L.; Zhang, H.; Ma, J.; Yang, Y.; Chen, H.; Han, Y.;

Sun, H.; Huang, L. Nanocomposites of graphene oxide and upconversion rare-earth

nanocrystals with superior optical limiting performance. Small 2012, 8, 2271-2276.

[57] Shao, P.; Li, Y. J.; Sun, W. F. Cyclometalated platinum(II) complex with strong and

broadband nonlinear optical response. J. Phys. Chem. A 2008, 112, 1172-1179.

[58] Su, X.; Guang, S.; Li, C.; Xu, H.; Liu, X.; Wang, X.; Song, Y. Molecular hybrid optical

limiting materials from polyhedral oligomer silsequioxane: Preparation and relationship

between molecular structure and properties. Macromolecules 2010, 43, 2840-2845.

[59] Hou, H. W.; Song, Y. L.; Xu, H.; Wei, Y. L.; Fan, Y. T.; Zhu, Y.; Li, L. K.; Du, C. X.

Polymeric complexes with “piperazine-pyridine” building blocks:  Synthesis, network

structures, and third-order nonlinear optical properties. Macromolecules 2003, 36,

999-1008.

[60] Wang, J.; Blau,W. J. Solvent effect on optical limiting properties of single-walled carbon

nanotube dispersions. J. Phys. Chem. C 2008, 112, 2298-2303.

[61] Filidou, V.; Chatzikyriakos, G.; Lliopoulos, K.; Couris, S.; Bonifazi, D. The effect of

charge transfer on the NLO response of some porphyrin [60]fullerene dyads. AIP Conf.

Proc. 2010, 1288, 188-191.

[62] Argouarch, G.; Veillard, R.; Roisnel, T.; Amar, A.; Meghezzi, H.; Boucekkine, A.; Hugues,

V.; Mongin, O.; Blanchard-Desce, M.; Paul, F. Triaryl-1,3,5-triazinane-2,4,6-triones

(isocyanurates) peripherally functionalized by donor groups: Synthesis and study of their

linear and nonlinear optical properties. Chem. Eur. J. 2012, 18, 11811-11827.

[63] Liu, Z.; Wang, Y.; Zhang, X.; Xu, Y.; Chen, Y.; Tian, J. Nonlinear optical properties of

graphene oxide in nanosecond and picosecond regimes. Appl. Phys. Lett. 2009, 94,

021902.

[64] Krishna, M. B. M.; Venkatramaiah, N.; Venkatesan, R.; Rao, D. N. Nonlinear optical

properties of graphene-(OH, Sn) porphyrin composites in picosecond regime. AIP Conf.

35

Proc. 2011, 1391, 680-682.

[65] Wang, J.; Hernandez, Y.; Lotya, M.; Coleman, J. N.; Blau, W. J. Broadband nonlinear

optical response of graphene dispersions. Adv. Mater. 2009, 21, 2430-2435.

[66] Xu, X.; Chen, J.; Luo, X.; Lu, J.; Zhou, H.; Wu, W.; Zhan, H.; Dong, Y.; Yan, S.; Qin, J.;

Li, Z. Poly(9,9’-diheylfluorene carbazole) functionalized with reduced graphene oxide:

Convenient synthesis using nitrogen-based nucleophiles and potential applications in

optical limiting. Chem. Eur. J. 2012, 18, 14384-14391.

[67] Kavitha, M. K.; John, H.; Gopinath, P.; Philip, R. Synthesis of reduced graphene

oxide-ZnO hybrid with enhanced optical limiting properties. J. Mater. Chem. C 2013, 1,

3669-3676.

[68] Anand, B.; Kaniyoor, A.; Sai, S. S. S.; Philip, R.; Ramaprabhu, S. Enhanced optical

limiting in functionalized hydrogen exfoliated graphene and its metal hybrids. J. Mater.

Chem. C 2013, 1, 2773-2780.

[69] Liu, Z. B.; Xu, Y. F.; Zhang, X. Y.; Zhang, X. L.; Chen, Y. S.; Tian, J. G. Porphyrin and

fullerene covalently functionalized graphene hybrid materials with large nonlinear optical

properties. J. Phys. Chem. B 2009, 113, 9681-9686.

[70] Liu, Z. B.; Tian, J. G.; Guo, Z.; Du, F.; Zheng, J. Y.; Chen, Y. S. Enhanced optical limiting

effects in porphyrin-covalently functionalized single-walled carbon nanotubes. Adv.

Mater. 2008, 20, 511-515.

[71] Li, P. P.; Niu, L. J.; Chen, Y.; Wang, J.; Liu, Y.; Zhang, J. J.; Blau, W. J. In situ synthesis

and optical limiting response of poly(N-vinylcarbazole) functionalized single-walled

carbon nanotubes. Nanotechnology 2011, 22, 015204.

[72] D’Souza, F.; Gadde, S.; Zandler, M. E.; Arkady, K.; El-Khouly, M. E.; Fujitsuka, M.; Ito,

O. Studies on covalently linked porphyrin-C60 dyads: Stabilization of charge-separated

states by axial coordination. J. Phys. Chem. A 2002, 106, 12393-12404.

[73] Bhyrappa, P.; Krishnan, V. Covalently linked bisporphyrins bearing tetraphenylporphyrin

and perbromoporphyrin units: Synthesis and their properties. J. Chem. Sci. 2004, 116,

71-78.

[74] Shen, J. F.; Hu, Y. Z.; Li, C.; Qin, C.; Ye, M. X. Synthesis of amphiphilic graphene

nanoplatelets. Small 2009, 5, 82-85.

36

[75] Yoon, Z. S.; Cho, D. G.; Kim, K. S.; Sessler, J. L.; Kim, D. Nonlinear optical properties

as a guide to aromaticity in congeneric pentapyrrolic expanded porphyrins: Pentaphyrin,

sapphyrin, isosmaradyrin, and orangarin. J. Am. Chem. Soc. 2008, 130, 6930-6931.

37

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

38

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.

39

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

2 and RGO-TPP 2, in DMSO.

40

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.

41

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

42

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

43

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

44

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

45

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.

46

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

RGO-TPP 2.