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Fluorene−Perylene Diimide Arrays onto Graphene Sheets for Photocatalysis Anastasios Stergiou* and Nikos Tagmatarchis* Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece S Supporting Information *
ABSTRACT: A facile approach for introducing photoactive poly(fluorene−perylene diimide) arrays (PFPDI) onto graphene sheets was accomplished. Noncovalent PFPDI/graphene ensembles formed via π−π stacking interactions between the two components and covalent PFPDI−graphene hybrids realized upon a Stille polycondensation reaction between an iodobenzyl-functionalized graphene, a 9,9-dialkyl substituted fluorene diboronic acid, and a 1,7dibromo-PDI derivative were prepared. The morphology of PFPDI/graphene and PFPDI− graphene was evaluated by high-resolution transmission electron microscopy (HR-TEM), revealing the presence of even monolayered graphene sheets. Moreover, their photophysical and redox properties as assessed by electronic absorption spectroscopy and steady-state as well as time-resolved photoluminescence assays and electrochemistry, respectively, disclosed chargetransfer characteristics owing to the high photoluminescence quenching of PFPDI in the presence of graphene and the fast component attributed to the decay of the emission intensity of the singlet excited state of PFPDI in both PFPDI/graphene and PFPDI−graphene. Next, testing their ability to operate in energy conversion schemes, the PFPDI−graphene was successfully employed as catalyst for the reduction of 4-nitrophenol to 4-aminophenol. Notably, the kinetics for the reduction were enhanced by visible light photoirradiation as compared to dark conditions as well as the presence of PFPDI−graphene, contrasting the case where only PFPDI, in the absence of graphene, was employed. Finally, recycling of the catalyst PFPDI− graphene was achieved and reutilization in successive reduction reactions of 4-nitrophenol was found to proceed with the same efficiency. KEYWORDS: graphene, perylene, fluorene, donor−acceptor, charge transfer, photocatalysis
1. INTRODUCTION The progress in chemistry of graphene offers a series of facile modification protocols for the covalent grafting of photoactive organic molecules on its surface.1 Such strategies possess the advantage of incorporating photoactive units via robust chemical bonding with the graphene sheets, however, simultaneously disrupting the continuous electronic network by introducing sp3 defected sites. On the other hand, the exceptional electronic properties of undefected graphene sheets can be retained and combined with those of organic photoactive molecules by employing cheap and easy chemical protocols.2 Specifically, supramolecular functionalization, mainly realized by means of multiple van der Waals interactions for immobilizing photoactive moieties onto the basal plane of graphene sheets, is the method of choice for keeping the valence band of graphene unperturbed; however, it suffers from the weak anchorage of the dye components in wet conditions. Synergistic action between the two components in electron donor−acceptor systems incorporating graphene is beneficial in photocatalysis, particularly for hydrogen generation via water splitting.3 Aromatic nitro compounds, specifically nitrophenols, are highly toxic organic pollutants with minimum biodegrability. Undoubtedly, the most efficient pathway for removing these pollutants is via their hydrogenation and transformation © XXXX American Chemical Society
to the corresponding aromatic amines, which are less toxic and can be immediately utilized in pharmaceuticals. To this end, noble metal nanoparticles4,5 as well as inorganic semiconductors6,7 supported in diverse nanostructures were employed as photocatalysts for the production of hydrogen. However, considering the high cost and toxic nature of those inorganic-based systems, alternative catalysts, conceivably based on organic chromophores and/or macromolecular arrays acting as electron donors in combination with graphene sheets that can accept and transport efficiently electrons, are required. Among the most popular visible light absorbing compounds are perylene derivatives, which were employed in covalent functionalization of fullerene C60,8 azafullerene C59N,9 and carbon nanotubes10,11 and subsequently evaluated for chargetransfer phenomena under visible light irradiation. Especially perylene diimides (PDIs) due to their unique optical and electronic properties are interesting candidates in energy conversion schemes for targeting applications in organic electronics.12−18 In the frame of functional graphene-based materials, supramolecular nanoensembles employing perylene Received: June 7, 2016 Accepted: August 2, 2016
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DOI: 10.1021/acsami.6b06797 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Steady-state UV−vis electronic absorption spectra were recorded on a PerkinElmer (Lambda 19) UV−vis−NIR spectrophotometer. Steadystate emission spectra were recorded on a Fluorolog-3 Jobin YvonSpex spectrofluorometer (model GL3-21). Micro-Raman scattering measurements were performed at room temperature, in the backscattering geometry, using a RENISHAW inVia Raman microscope equipped with a CCD camera and a Leica microscope. A 2400 lines mm−1 grating was used for all measurements, providing a spectral resolution of ±1 cm−1. As an excitation source, the Ar+ laser (514 nm with less than 0.5 mW laser power) was used. Measurements were taken with 60 s of exposure times at varying numbers of accumulations. The laser spot was focused on the sample surface using a long working distance 50× objective. Raman spectra were collected on numerous spots on the sample and recorded with a Peltier cooled CCD camera. The data were collected and analyzed with Renishaw Wire and Origin software. Electrochemical studies were performed using a standard three-electrode cell. Glassy carbon was used as a working electrode, and platinum wires were used as counter and pseudoreference electrodes (ferrocene as an internal reference). TBAPF6 (98%) was recrystallized three times from acetone and dried in a vacuum at 100 °C before being used as an electrolyte. Before each experiment, the cell was purged with Ar for 120 s. Measurements were recorded using an EG&G Princeton Applied Research potentiostat/ galvanostat Model 2273A instrument connected to a personal computer running PowerSuite software. The working electrode was cleaned before each experiment through polishing with a cloth and 6, 3, and 1 mm diamond pastes. Mid-IR spectra in the region of 550− 4000 cm−1 were obtained on an FTIR spectrometer (Equinox 55 from Bruker Optics) equipped with a single reflection diamond ATR accessory (Dura-Samp1IR II by SensIR Technologies). 1H and 13C NMR spectra were recorded with a Varian 300 MHz spectrometer. High-resolution transmission electron microscopy (HR-TEM) measurements were carried out using a JEM-2100F (JEOL) high-resolution field-emission gun TEM operated at 80 keV at room temperature and under a pressure of 10−6 Pa. HR-TEM images were recorded with a charge-coupled device with an exposure time of typically 1 s. 2.2. Microwave-Assisted Synthesis of PFPDI. In a N2 purged flask, perylenediimide (20 mg, 0.0289 mmol), [9,9-bis(2-ethylhexyl)9H-fluorene-2,7-diyl]bisboronic acid (12 mg, 0.0289 mmol), CsF (13 mg, 0.085 mmol), CuI (1 mg, 0.005 mmol), and DMF (4 mL) were added. Then, Pd(PPh3)4 (1 mg, 0.0008 mmol) was added, and the mixture was heated in a microwave reactor to 80 °C at 50 W for 60 min. After that period, the reaction mixture was cooled to room temperature, and the copolymer was filtered to remove any solid traces from the catalysts, extracted with dichloromethane and water, dried and redissolved in DMF, cooled to −30 °C, and finally filtered to remove any decomposed catalyst left. Then, the filtrate was dried in a rotary evaporator to yield PFPDI (10 mg). 2.3. Liquid Exfoliation of Graphite. A mixture of 100 mg of graphite flakes (>75%, >150 mesh) in 50 mL of chlorosulfonic acid was sonicated for 8 h. The resulted black homogeneous solution quenched carefully (highly exothermic reaction) with distilled water. The mixture was filtered through a PTFE membrane filter (pore size of 0.1 μm) and washed with water, methanol, and dichloromethane. The filter cake was redispersed in NMP with the aid of bath sonication to give a black suspension. Then, the mixture was tip-sonicated (10% power) for 30 min, and the black suspension formed was left to stand for a week at room temperature. Two-thirds of the black supernatant was collected, filtered through a PTFE membrane filter (pore size of 0.1 μm), and washed with water, methanol, and dichloromethane. 2.4. Preparation of Iodobenzyl-Functionalized Graphene. In a flask purged with N2, 10 mg of exfoliated graphene and 300 mg of 2amino-5-iodobenzoic acid were mixed in NMP (10 mL) and sonicated for 30 min. Then, isoamyl nitrite (100 mL) was added, and the system was introduced in the microwave reactor and heated at 110 °C (100 W) for 1 h. After that period, the reaction mixture was cooled and purged again with nitrogen; isoamyl nitrite (100 mL) was added, and the system was heated at 80 °C for another 1 h. Then, the reaction mixture was cooled to room temperature and filtered via a PTFE membrane, and the solid iodobenzyl-functionalized graphene material
derivatives were examined, demonstrating strong photoinduced electronic communication that leads to energy and/or charge transfer from the PDI core to graphene.19,20 In addition, sulfonated PDIs were shown to increase the conductivity of graphite oxide (GO) under thermal treatment,21 while also perylene compounds were employed to identify the surface structure of CVD-grown graphene22 and self-assembly phenomena onto GO and reduced GO surfaces.23 Furthermore, recently, supramolecular ensembles of graphene nanoflakes and a bay-dicyanated perylene diimide derivative were also reported and revealed that the presence of graphene enhances n-type mobility within the supramolecular ensemble.24 On the other hand, single molecule perylene derivatives also stand along PDI-based macromolecular arrays, which have already been employed in bulk heterojunction solar cell devices.25 Low band gap p-conjugated polymers absorbing light in the visible region, so-called “third generation” semiconducting copolymers, attract scientific interest in energy conversion applications.26 The strong absorption of visible light by PDI derivatives can be combined with the electronic properties (i.e., charge transport) of other molecules, such as for example with fluorenes.27 In this context, PDI-based blocks, copolymerized with fluorene blocks yielding low band gap (i.e.,
