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Nano Res

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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

Just Accepted

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Nano Research DOI 10.1007/s12274-014-0569-x

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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.

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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

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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 RGOs 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

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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 -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

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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

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

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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 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 on