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

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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 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 This is a “Just Accepted” manuscript, which has been examined by the peer-review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these “Just Accepted” manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI®), which is identical for all formats of publication. 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. 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 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] 2 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 3 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 4 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 5 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, 6 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]
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