Emission-Tunable Multicolor Graphene Molecules with Controllable

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Emission-Tunable Multicolor Graphene Molecules with Controllable Synthesis, Excellent Optical Properties, and Specific Applications Jun Wei Yang, Yu Li Huang, Haoyun Zhu, Wei Huang, and Weizhi Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b00832 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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ACS Applied Materials & Interfaces

Emission-Tunable Multicolor Graphene Molecules with Controllable Synthesis, Excellent Optical Properties, and Specific Applications Junwei Yang, † Yuli Huang, † Haoyun Zhu, † Wei Huang† and Weizhi Wang*,† †State Key Laboratory of Molecular Engineering of Polymers, Collaborative Innovation Center of Polymers and Polymer Composite Materials, Department of Macromolecular Science, Fudan University, Shanghai 200433, P. R. China ABSTRACT: Series of graphene molecules with varied emission colors have been prepared by oxidative cyclodehydrogenation using anhydrous ferric chloride (FeCl3) as the catalyst under mild conditions. By controlling the oxidation time in the initial step only, molecules with different fluorescence colors are conveniently obtained. New colors can be recorded evidently because of the stepwise and controllable process, which highly related to the conjugation length. Blue emissive starting compounds in the solid state can be transformed into orange upon brief oxidation, while green emissive oligomers are varied to red with an emission wavelength redshift about 123 nm. Cyclic voltammetry measurements performed can give the corresponding data, which verify the results drawn from the UV and PL spectroscope. The gradual change of conjugation length with tunable emission is confirmed in the MALDI-TOF study as well. Further characterizations indicate that the graphene molecules possess satisfactory optical properties, which are highly emissive both in solution and in the solid state because of the alkyl group. In addition, the good thermal stability and the self-assembly of graphene molecules suggest that they are promising candidates for high-tech applications. Furthermore, the fabricated field-effect transistors possess the nice performance, whose mobilities are about 0.57 cm2 V-1s-1 with an on-off ratio of 104 and 0.81 cm2 V-1s-1 with an on-off ratio of 103, respectively.

KEYWORDS: graphene molecules, color tuning, tetraphenylethene, Scholl reaction, field-effect transistors

INTRODUCTION Bright and colorful fluorophores, which cover the visible wavelength range from 400 to 780 nm, play an important role in color displays, waveguides,1-2 light emission devices,3-6 as well as chemical and biosensing sensors.7-10 Particularly, blue-, greenand red-light-emitting materials are desirable for application in the white-light-emitting system.11-15 Compared with their inorganic counterparts, plentiful ways to fine-tune optical properties of organic materials by varying their structures allow one to tailor materials. Color tuning conjugated molecules and polymers, such as benzothiadiazole,16 poly(phenylenevinylene) (PPV) and polyfluorene, have drawn considerable attention due to the excellent optical and electronic behavior. Alkyl or alkoxy substitution of PPV changes the fluorescence light from green to orange to red,17 while the phenylene linked at the meta-position along the main chain tunes the color to blue.18 Introduction of kinky π-conjugated linkages or saturated carbons along the PPV chain changes conjugation length, thereby tuning the emission color in a foreseeable manner.19-22 However, the vulnerability of PPVs to rapid oxidative degradation makes them become the secondary choice.23 Yoshina and co-workers24 reported the first example of the blue-emitting polymer LED using poly(9,9dihexylfluorene) by coupling of the fluorene monomer with FeCl3 due to the thermal, oxidative, photochemical stability and high efficiency.25 Since then, many groups have worked on the design

and synthesis of polyfluorene derivatives to achieve efficient red, green and blue emission in high purity for full color displays.26-30 The emission color of polyfluorene can be tuned by insertion of functional groups at the position C9,28 by attaching dye molecules as endcapping groups to the end or side chains,17, 31 by copolymerization with special comonomers.26, 32 Furthermore, blending a dye chromophore into a polymer matrix33-34 or polymer blend systems35-37 can change the emission built on the intermolecular interaction as well. Predictably, problems of the interface contact and phase separation limit the application of this method. In general, a new emission color turned up accompanying with the synthesis of the new materials as noted above. That is to say, if we want to obtain the different color fluorescence, a series of materials should be prepared in turn in multiple systems. Is it possible to prepare the emission tunable molecules from one structure by a simple method? Tetraphenylethylene derivatives (TPEs) have emerged as the important compounds that have been widely used in optoelectronic devices and chemo-/biosensors.38-42 Among their outstanding properties, the exceptional phenomenon called aggregation induced emission (AIE) is most attractive to solve the intractable aggregation quenching, which was reported by Tang’s group in 2001.43 Various TPE derivatives, consisting of olefinic stators and multiple phenyl rotors, are highly emissive in aggregated in comparison to the weak emission in solution owing

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Scheme 1. Synthetic Routes towards Compounds 5-8 and 5c-8c a

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Reagents and conditions: i. Pt(PPh3)4, DMF, 90oC, 24h; ii. Pd(PPh3)4, toluene, 2mol/L K2CO3/H2O, 87oC, 48h; iii. FeCl3, CH3NO2, CH2Cl2, room temperature. R: C4H9 Other solvents were of analytical grade and were purified using to the rapid internal rotations deactivate the excited state via the standard methods. 1,4-Diphenylethyne was purchased from nonradiative decay channels. Müllen and co-workers have Tokyo Chemical Industry. The method of preparing boronate described an efficient approach for preparing well-defined derivatives was reported by our previously published polycyclic aromatic hydrocarbons by mild oxidative procedures.47 cyclodehydrogenation of hexaphenylbenzene precursors.44 1 Similarly, TPEs can be oxidatively converted to dibenzochrysenes H and 13C nuclear magnetic resonance (NMR) spectra were using the DDQ/H+ system, which was published by Rathore’ s measured on a Bruker AVANCE Ⅲ HD 400MHz spectrometer group in 2011.45 In our previous work, graphene-like molecules using deuterated chloroform (CDCl3), dichloromethane (CD2Cl2) based on TPEs have been synthesized by the Scholl reaction.46 It or 1,1,2,2-tetrachloroethane (C2D2Cl4) as the solvent and is a facile and straightforward approach to lock the phenyl rotors tetramethylsilane (TMS; δ = 0 ppm) as the internal standard. of the twisted TPEs using anhydrous iron chloride (FeCl3) as the Matrix assisted laser desorption time of flight (MALDI-TOF) oxidant. The fluorescent properties can be tuned by varying their mass spectrometry was recorded on the AB SCIEX TOF/TOFTM conjugation length. 5800 Analyzer (AB Sciex, Framingham, MA, USA). Gel Herein, we describe a convenient and rapid method to prepare a permeation chromatography (GPC) was performed on an series of colorful molecules with tunable light emission through Agilent/Wyatt 1260 gel permeation chromatography. The the FeCl3/CH3NO2 oxidative system from the same starting calibration was managed by employing commercially available compound. Scheme 1 shows the detailed steps of our preparative Polystyrene. Raman spectra were performed on the XploRA strategy. First, the starting compounds, based on TPEs, were (HORIBA JobinYvon) spectrometer. Thermogravimetric analysis obtained by the facile Suzuki reaction. Boronate derivatives 1-4, (TGA) of the oligomers was evaluated on a PerkinElmer Pyris 1 the effective and versatile platform, made it easy to prepare a TGA instrument under nitrogen at a heating rate of 10oC/min. series of TPE-based oligomers with 1-Bromo-4-butylbenzene. Differential scanning calorimetry (DSC) analysis was undertaken The thienyl group has been taken as an example to prove the high with a Q2000 DSC (TA Instrument LLC). UV/Vis absorption effectiveness of the platform in our previous report.47 Finally, the (UV) spectra were obtained on a Perkin-Elmer Lambda 750 UVnew emission color appeared in succession by controlling the visible spectrophotometer. Fluorescence (PL) spectra were oxidation time, which actually controlled the number of linked measured on an Edinburgh FLS920 spectrometer at ambient phenyl stators. Accordingly, the conjugation length thus increases, temperature. PL lifetimes were recorded on a Photo Technology which leads to the low band gap and red emission. From another International, Inc. QM40 by means of a time-corrected single perspective, multicolor tunable emissions from the same structrue photon counting system at room temperature. The PL quantum were rarely reported. In this contribution, a simple and convenient yields (ΦF) were estimated using 9, 10-diphenylanthracene in organic synthetic method is proposed to prepare well-defined and cyclohexane (ΦF = 90%) and fluorescein (ΦF = 92%) in 0.1 M color-tuning graphene molecules by appropriate structures. NaOH as standard. Atomic force microscopy (AFM) studies were performed with a Digital Instruments NanoScope IV (Bruker Multimode 8, America) operating in the ScanAsyst mode. And the EXPERIMENTAL SECTION polarizing microscope (POM) was carried out on a DM2500P Materials and Instruments. Chemicals and reagents were polarizing microscope at room temperature. Cyclic voltammetry mostly obtained from Aldrich or Sinopharm Chemical Reagent (CV) was performed on a CHI 600E electrochemical workstation. Company and used as received. Toluene was purified by The oligomer films were coated on a glass-carbon electrode about distillation by sodium wires/benzophenone under nitrogen (N2) 0.08 cm2. The measurements were performed in an electrolyte of prior to use, and N,N-dimethylformamide (DMF) used without 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in purification was of the high pressure liquid chromatography grade. acetonitrile, and used ferrocene (4.8 eV under vacuum) as the a


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internal standard with a scan rate of 100 mV/s under a constant N2 flow. The two-dimensional gazing-incidence X-ray diffraction (2D GIXRD) patterns were obtained at BL14B1 at Shanghai Synchrotron Radiation Facility (SSRF) (λ = 1.24 Å). The incidence angle is 0.25° and the exposure time is 60 seconds. Samples were spin-coated on SiO2/Si wafers at 3000 r/min from 0.1 wt % chloroform solution of the polymer PB. Field-effect transistors (FETs) fabrication. Commercially available SiO2 (300 nm) /Si (sufficiently p-doped) wafers were washed by deionized water, acetone and methanol. The graphene molecules in THF solution (5 mg/mL) were spin-coated on the clean SiO2/Si wafers with the rotating speed of 6000 r/min. The thin films to be the semiconductors were exposed to air for a while to evaporate the surplus solvent. Then, the 50 nm thick gold drain (Au) source and drain electrodes were vapor-deposited on the prepared thin film. One kind of ion gel, composed of a triblock copolymer, poly(styreneblock-methyl methacrylate-block styrene) (PS-PMMA-PS; MPS =4.3 kg/mol, MPMMA = 12.5 kg/mol, MW = 21.3 kg/mol), and a sort of ionic liquid, 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide in an ethyl propionate solution, was used as the dielectrics inserted into the space between the thin film and the Al gate. The weight ratio of ionic liquid, the polymer and the solvent was confected at 9.3:0.7:20. These ionic solutions were drop-cast to cover the surfaces of the graphene molecule film as well as the drain and source electrodes.

Figure 1. (a) Photographs of compounds 5-8 in the stepwise oxidation process. (b) The trajectory to tune colors by changing the oxidation time of 8 shown in the CIE coordinate diagram (λex = 365 nm). Taking the compound 8 as an example to analyze the mixtures of Scholl reaction as a function of oxidation time and better understand the relative rates of ring closing (Figure 2). An isotropically resolved MALDI-TOF mass spectrum of the mixture recorded 14 min after adding the oxidant demonstrates the formation of eight new C-C bonds in the majority. The most intense peak of the completely formed 8c appears until 108 min. These features can further support a stepwise ring-closing mechanism, as put forward by our previous publication.46 Besides, pairs of adjacent phenyl rings at different positions take the dissimilar time to form new C-C bonds.

RESULTS AND DISCUSSION Synthesis. The boronate derivatives 1-4 and polymer 2 provide an effective and versatile platform for analogous compounds. The compounds 5-8 as well as polymers PB and PBB were readily prepared via the Suzuki reaction from these important building blocks (Scheme 2). Scheme 2. Synthetic Routes for Polymers

To obtain elaborate graphene molecules 5c-8c, the FeCl3/CH3NO2 system was used for the transformations. Intriguingly, the cyclodehydrogenation-triggered structures emit obviously diverse color in different oxidation time (Figure 1). Particularly, the fluorescence emission of 8 changes from cyan to red, with an increased emission range compared to the ring closing reactions of 5-7. In other words, a simple and convenient approach was successfully proposed to prepare colorful molecules from one starting compound by controlling the number of linked phenyl rings.

Figure 2. MALDI-TOF mass spectra of 8 at different oxidation time. 8 (20mg) dissolved in CH2Cl2 (20 mL) reacted with saturated FeCl3/CH3NO2 system (0.5 mL) according to Supporting Information. Each mixture extracted from the bottle was quenched by methanol before the test. Characterization. All the intermediates and products were confirmed by standard spectroscopic methods, including MALDITOF, 1H NMR and 13C NMR spectra, and provided reasonable analysis data corresponding to their expected structures (more details in Figure S1-S28, Supporting Information). To confirm the synthesis of polymers at each step, the gel permeation chromatography (GPC) measurements were employed. GPC data for polymers obtained with different elution times are shown in Table 1 and Figure S29. After repeatedly precipitating in methanol, the orange polymer 2 has a number-average molecular weight (Mn) of 1.63×104 g/mol with a narrow molecular weight distribution. The polydispersity index (PDI) of



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the PB and PBB have slightly increased to 1.34 and 1.20, respectively, after the Suzuki reaction. Importantly, the numberaverage molecular weight measured by GPC analysis is consistent with the corresponding results calculated based on polymer 1. These suggest that the new approach proposed is effective and straightforward even throughout the whole polymerization. Table 1.Molecular Weights of the Polymers. Polymer

Yield (%)

Mn,calc (g/mol)

Mnb (g/mol)

Mwb (g/mol)

PDIb

Polymer 1 Polymer 2 PB PBB

81.5 72.2 85.0 87.5

7300a 16,500 12,500 16,500

7300 16,300 12,100 16,000

10,800 17,900 16,200 19,200

1.47 1.10 1.34 1.20

The data taken from analysis of polymer 1 by GPC. Determined by GPC in THF on the basis of a polystyrene calibration. For 1H NMR spectra of PB, the aliphatic region showed the main chain methyl (0.8ppm) and methylene (1.24-1.25, 1.68-1.74, 2.30) of the octyl group, while the range of 7.46-7.70 ppm represented the aromatic protons. The peaks corresponding to PB at aliphatic and aromatic regions were in an appropriate ratio. The polymer PBB showed a proper ratio as well. Meantime, the introduction of the n-butyl made the peaks of aliphatic area more visible. Obviously, PBB showed doublets at 7.03 ppm and 7.35 ppm representing the o-H and m-H of butylphenyl, respectively. The protons shifted due to the electron-donating group, further verifying the conclusion drawn from the 1H-1H correlation spectroscopy (Figure S30). The features discussed above further demonstrated the achievement of the expected polymers. The 13C NMR spectra (Figure 3) also showed that the pinacolborane group carbon atoms of Polymer 2 resonated at δ 25.03 and 83.45 completely disappeared after Suzuki reaction, and new peaks emerged at δ 130.20, 131.28 and 141.80, which are assigned to the resonance of carbon from the butylphenyl segment. Meanwhile, the peak associated with the internal double bond carbons shifted to 119.30 ppm, owing to the change of the pendant groups. These results confirmed again that the PBB has been successfully prepared via the Suzuki coupling reaction. a

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electronic information of the graphene molecule, because of the high resolution, non-destruction and speediness.48 It can provide proofs for the efficient graphitization of 6, 7 and 8. The powder compounds were subjected to Raman spectroscopy on microslides with the laser excitation at a wavelength of 532 nm. The resulting first-order Raman spectrum of 7c displays two important lines: a first band (D band) located at 1340 cm-1 with a shoulder at 1385 cm-1 and a second band (G band) located at 1604 cm-1 (Figure 4a). Additionally, the main D bands of 6c and 8c in Figure S31 and Figure 4b were located at 1314 cm-1 and 1344 cm-1, respectively, while the corresponding appearance of G bands at 1585 cm-1and 1604 cm-1. The D and G bands are related to the carbon hexatomic rings breathing mode and the stretching of C=C, respectively, the former requiring a defect for activation.48 These peaks are typical of other structurally well-defined graphene molecules in the literature.49-50

b

Figure 3. 13C NMR spectra of (a) polymer 2 and (b) PBB in CD3Cl. Raman Characterization. Raman spectroscopy is an ideal characterization tool to investigate the structural property and

Figure 4. Raman spectra of (a) 7c and (b) 8c in the solid state. (c) The TGA data and (d) DSC thermograms of the powder of 7, 8, 7c and 8c. Thermal Properties. For the primary testing of the thermal stability after the cyclodehydrogenation for 7 and 8, the thermogravimetric analysis and diﬀerential scanning calorimetry were performed under a nitrogen atmosphere (Figure 4). As shown in Figure 4c, 7c and 8c have the high thermal stability without loss of weight below 352 °C and 396 oC, respectively. The total weight losses were around 52 and 45% even at 800oC, vastly different from corresponding compounds 7 and 8, which owing to the extended conjugated chains and more rigid structures. The glass transition temperatures (Tg) of 7 and 8 were clearly detected at 40 and 47 oC, respectively, while such thermal transitions were not detected in oxidized compounds (Figure 4d). In addition, no transition peaks were observed when 7c and 8c were heated up to 300 °C. Oxidized luminogens with high thermal and morphological stabilities are beneficial for the processing of the electrical devices.51 Optical Properties. Photoluminescence and ultraviolet/visible data of oligomers (5-8) as well as polymers (PB, PBB) in CH2Cl2 are shown in Figure 5 and Table S1. Absorption peaks of the oligomer 5-8 were located in close proximity, about 308 nm. The TPE molecular chain continue to increase will extend the conjugation, and in turn red-shift the absorption. Indeed, from oligomers (5-8) to PB and PBB, enlarging the conjugated system have red-shifted absorption from 308 to 327 nm. It is wellknown that TPE is a typical AIE molecule.38-39, 52 Obviously, all the oligomers (5-8) were faintly emissive when dissolved in the solutions but became bright emitters when aggregated in the solid state. Thus, the PL spectra of 5-8 and PB in their solid state are given in Figure 5b.


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Figure 5. (a) Normalized UV spectra of compounds 5-8 as well as polymers PB and PBB in CH2Cl2 solutions. (b) Normalized PL spectra of compounds 5-8 and PB in the solid state. (c) Fluorescence images of 5-8 in THF/water mixtures with different water fractions taken under 365 nm UV irradiation. (d) The PL spectra of solutions of 7 in THF/water mixtures. Normalized (e) UV and (f) PL spectra of 5c-8c in the solid state. (g) Fluorescence images of 5c-8c in THF/water mixtures. (h) The PL spectra of solutions of 7c in THF/water mixtures. In the solid state, oligomers 5-8 emit strong blue and green locked graphene molecules 5c-8c were respectively located at 346, photoluminescence at 493, 497, 508 and 522 nm, respectively, 417, 452 and 467 nm. As expected, the emission peaks showed a while the PL spectrum of PBB is centered in the longer wavelength bathochromic shift upon extending the effective conjugation region at 559 nm. length from 5 (493 nm) to 5c (529 nm), 6 (497 nm) to 6c (549 nm), 7 (508 nm) to 7c (619 nm) and 8 (522 nm) to 8c (645 nm). Under UV irradiation, the emission images of the THF/H2O On the other hand, the emission spectra displayed a similar mixtures of 5-8 are shown in Figure 5c. The THF solutions of 5-8 redshift from monomer 5c to dimer 6c to trimer 7c to tetramer 8c. are all aphotic. When a certain content of water was added to The optical properties of the oligomers 5-8 and 5c-8c were induce the aggregation, which boosted their emission intensity investigated and compared (Table S1). Distinct from the AIEand displayed the similar green solutions. Instead of visual active 7, the final oxidized compound 7c can emit a bright yellow observation, the PL properties of oligomers in the mixture light in solution under UV irradiation (Figure 5g). To further solutions were also measured quantitatively using a quantitatively study and confirm the changes brought from spectrofluorometer (Figure 5d and Figure S32). Figure 5d shows different structures, the quantum yield and PL lifetime needed to the PL spectra of 7 in THF–H2O mixtures with different water be measured. Its solution ΦF rises dramatically to 68.66%, in fraction (fw) as an example. With the increase of a small amount comparison to 7 with a negligible ΦF. By locking the phenyl rings, of water (fw< 60%), its PL intensity was slightly weakened. the covalent bonds restricted the rotation. Thus, the nonradiative However, the emission peak had red-shifted to 502 nm with more pathway caused by intermolecular rotation of the phenyl blades water added. Afterwards, the emission intensified swiftly at 80% may be blocked.54 Accordingly, the excited species of 7c can water content. Further increment of the water fraction, however, relax much more slowly with PL lifetime lengthened from 2.82 ns weakened the emission. At 90% water content, the PL intensity is to 3.55 ns. Internally halting the motions of the phenyl rotors at merely 15% of that at 80%. The large-sized aggregates formed the molecular structure level also offered direct support to the and precipitated at the high water content, which decreased the 53 restriction of intramolecular rotation hypothesis. effective concentration in the solution. Besides this, the formation of the amorphous aggregates might trap the solvent As shown in Figure 5g, the 7c emits brightly in the pure THF molecules inside. In these loose aggregates, the rotation of the solution. When its poor solvent, water, was added, the molecule phenyl blades against the ethylene core also decreased the 7c tended to aggregate which impeded the emission emission intensity.39 accompanying with a clear redshift. It was nicely correlated with the changed intensity at different water contents, which To further compare the AIE phenomenon of different oligomers, monotonously decreased with the gradual addition of water the quantum yields (ΦF) of 5-8 were conducted, using 9,10(Figure 5h). Evidently, hooking up the phenyl rings of 7c has diphenylanthracene (DPA) as the standard (Figure S32). The ΦF turned it to a conventional fluorophore, whose conformation of 5 in the pure THF solution was very low (0.13%), and changed favors π−π interaction. Such effect was not an isolated case slightly when water was added. However, 6 and 7 have a little appeared only in 7c but was also found in other graphene high quantum yields (ΦF) in the starting solution because of the molecules (Figure S33). The solution of 7c emits yellow light crowded enough structures. The intramolecular motions of which changes to weak magenta when 50 vol % or more water is adjacent phenyl groups were not totally free. added. Similarly, the appearance of the solutions of 8c changes Interestingly, locking the phenyl rings significantly broadened from yellow to jacinth and then to dark red. Intriguingly, the emission region. New colors appeared successively with the compound 8c has a slight blueshift at “medium” water contents, increase of the locked phenyl ring number (Figure 1). Graphene which may be due to the formation of ordered aggregates. molecules 5c-8c were facilely obtained after the full Nevertheless, fully oxidized compounds 5c-8c still emitted cyclodehydrogenation, which can be testified clearly by UV and obvious lights even in the solid state. This is because their PL spectra in their solid state (Figure 5e, 5f). Absorption peaks of



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molecules cannot be closely packed to form harmful species and the butyl will expand the distance between molecular aggregates. Electrochemical Properties. To provide more insights into the electronic structures and optical properties and further to explain the discussed fluorescence redshift, cyclic voltammetry (CV) measurements for thin films of oxidized oligomers were carried out. A Pt wire used as the counter electrode was coated with compounds 5-8 and 5c-8c, while an Ag/AgNO3 electrode was used as the reference electrode. The relevant data are displayed in Table 2 and Figure S34-S35. The oxidation potential values of oxidized oligomers follow the order 5c > 6c > 7c > 8c, which shows the same trend with the values of pre-oxidative compounds from 5 to 8. Extended conjugation length has been proposed to be the main cause for the decreased HOMO from 5 to 8 as well as from 5c to 8c. The HOMO energy levels of compounds 5c-8c were estimated to be -5.64, -5.44, -5.40 and -5.33 eV, respectively. The energy band gaps of 5c-8c were calculated to be 2.40, 2.24, 2.19 and 2.12 eV, which determined from the onset wavelength of their UV absorptions. As shown in Figure S34-S35, some CV curves are not perfect because of the activity of some molecules. The data obtained from the onset potential of the CV curves are used for reference and comparison. It is obvious that oxidized compounds 5c-8c exhibit lower HOMO energy and narrower ∆Eg in comparison with those of corresponding compounds 5-8. The above results match well with the measurement of optical properties and the molecular incremental conjugated degree. Table 2. Electrochemical Properties of 5-8 and 5c-8ca Compound 5 5c 6 6c 7 7c 8 8c

Eonsetox (V) 0.99 0.89 0.79 0.69 0.76 0.65 0.72 0.58

HOMO (eV)

λonset (nm)

∆Eg (eV)

LUMO (eV)

-5.74 -5.64 -5.54 -5.44 -5.51 -5.40 -5.47 -5.33

394 517 428 554 442 565 450 584

3.15 2.40 2.90 2.24 2.81 2.19 2.76 2.12

-2.59 -3.24 -2.64 -3.20 -2.70 -3.21 -2.71 -3.21

Abbreviations: Eonset-ox is the onset potential for oxidation. HOMO is calculated by the equation: HOMO = − e(Eonsetox – 0.0468 V) − 4.8 eV. λonset is the onset wavelength of UV absorptions in the solid state. LUMO = ∆Eg + HOMO. Self-assembly and Morphology Characterization. Precious evidence suggested that the strong π-π stacking could be an effective method to assemble into the one-dimensional macrostructure for planar, rigid organic molecules.55 As detailed above, a group of oligomers with twisty, flexible structures were readily locked by the covalent linkage. a

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Figure 6. The image of the aggregate formation of (a) 6c (Inset: the whole structure of 6c), (b) 7c and (c) 8c under natural light. Images of (d) 6c (Inset: the whole structure of 6c), (e) 7c and (f) 8c under 365 nm UV light. Strikingly, the near-planar molecule 6c tended to evidently aggregate at the bottom of the beaker, which readily be seen with the naked eye, after the solvent (CH2Cl2) was slowly evaporated at room temperature. The legible structure of the whole aggregate can be obtained by a camera (Figure 6a, 6d), while the polarizing microscope image offers the information on microscopic structures. As shown in Figure 6a, the lamellar structure was observed and the typical lengths of the lamellar were several millimeters. And the lamellar structure was gathered together to form the whole areatus structure. The strong π-π interaction between the planar aromatic skeletons makes it an ideal candidate for assembly. Accordingly, the self-assembly behavior of 7c and 8c also turned up at the same conditions (Figure 6b-c, 6e-f ). Intriguingly, the unoxidized polymer PB exhibited a good selfassembly ability as well. In Figure 7a, the shape of the flowerlike was clearly observed by using a polarizing microscope. The assembly ability of the polymer PB was ascribed to the extended conjugation length and the considerable length-diameter ratio. Furthermore, we can clearly observe fiber-like structures of PB films in Figure 7b. From the pictures, the widths of these fibers are approximately 30 nm. To gain more information on the polymer PB packing arrangements in the solid state, Grazingincidence Xray diffraction measurements were applied (Figure 7c). Specifically, the clear (100) and (200) reflection arches appeared along the qz direction, which related to the lamellar ordering. And the maximum intensity of the (100) arch exists at about 90o, which expresses that the edge-on orientations with the backbones parallel to the substrate are predominant over other directions between the lamellas.56 Furthermore, an obvious (010) arch appeared at qxy=12.01 nm-1 and was associated by in-plane ππ stacking distance, which was 0.52 nm (Figure 7d).


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ACS Applied Materials & Interfaces different gate voltages (VG), affirming the clear p-channel FET characteristics.

Figure 7. (a) The polarizing microscope image of PB. (b) ScanAsyst-mode AFM height image of PB film spin-coated from toluene solutions (1 mg/mL). The inset shows a higher magnified image. (c) The 2D-GIXRD pattern of PB. (d) Schematic polymer chain arrangement. FETs Characterization. As noted above, the oxidized oligomers showed the self-assembly behaviors, which have tremendous potential to enhance the properties.57-58 Furthermore, the graphene molecules are of interest and their well-defined structures make them great candidates for electrical devices, such as photovoltaics and organic field effect transistors.59-61 To investigate the charge transporting characteristic of oxidized molecules, 7c and 8c were utilized as active layers in FETs. Fascinatingly, different from the conventional inorganic oxides like Al2O3, SiO2 and ZrO2, FETs with 7c and 8c semiconductors were fabricated using the ion gel, which would effectively diminish the heat generated by working devices.62-64 Ion-gel gated conjugated polymer65-66 transistors with high capacitance under low voltage operation have attracted more attention. Furthermore, ion-gel gated graphene FETs fabricated on plastic substrates show very good mechanical flexibility. In general, the choice of the ion gel as the dielectric layer for the FETs was deliberate. Figure 8a displays the images of the thin-film top-gate FET for the oxidized compounds (refer to the Supporting Information for fabrication details). The compounds to be tested as the solution processing thin-film semiconductors were spin-coated on the Si/SiO2 substrates. The representative transfer curve (ID-VG) of the FET with 7c is shown in Figure 8b and that of the 8c device is displayed in Figure 8d, where the drain voltage (VD) is fixed at 3V. From the saturation regime of the │ID│1/2 vs.│VG│curves, the mobility µ and threshold voltage were calculated according to the metal-oxide semiconductor FET formula:

2 Where Vth is the threshold voltage, Ci is the specific capacitance of the dielectric, estimated about 20 µF/cm2 in this work. The channel width (W) and length (L) were 400 and 1000 µm, respectively. The mobility of the 7c-based device was evaluated to be 0.57 cm2V-1s-1 with a high on/off current ratio of 104. Nevertheless, the mobility for 8c was up to 0.81 cm2V-1s-1 because of the lengthy π–conjugated system. The mobility showed no significant disparity compared with the reported ion gel gated thin-film FETs, which possessed the high performance.65 In addition, Figure 8c and Figure 8e show the output characteristics (ID-VD) of the aluminum-gated graphene molecule FETs at five

Figure 8. (a) The geometry of top-gated FETs fabricated using ion gel dielectrics. (b) Transfer and (c) output characteristics of the 7c devices at a drain source voltage (VDS) = 3V. (d) Transfer and (e) output characteristics of the 8c thin-film FETs. CONCLUSIONS In summary, multicolor graphene molecules with tunable emission were successfully designed and synthesized via an efficient and convenient route using the Scholl reaction. Most important of all, the facile color tuning of the molecule to obtain a full range of fluorescent colors was achieved from the same structure. By adjusting the oxidation time of a single system only, the conjugation length regularly extended, thus obtaining the new emitting color. Characterization of the starting oligomers 5-8 and graphene molecules 5c-8c by MALDI-TOF, 1H NMR and 13C NMR demonstrated the successful transformation from 5-8. Furthermore, the satisfactory emission both in solution and solid state as well as the good thermal stability of 5c-8c suggested that these color tuning oligomers would be suitable for optical and electrical devices. Subsequently, the 7c and 8c based FET devices possess good performance at low-voltage operation, whose mobilities were 0.57 and 0.81cm-2V-1s-1, respectively. In addition, the synthesis of the TPE-based oligomers 5-8 as well as PB and PBB can further prove the practicability and versatility of the boronate derivatives, an important platform.

ASSOCIATED CONTENT Supporting Information Synthetic routes and structure characterization (MALDI-TOF, GPC, 1H NMR, 13C NMR); Raman spectrum of 6c in the solid state; optical properties of compounds 5-8 and 5c-8c; emission spectra of 5, 6, 8 and 8c in THF/water mixtures with different water; cyclic voltammograms of compounds 5-8 and 5c-8c; POM images of 5c for detailed structures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION


ACS Applied Materials & Interfaces Corresponding Author

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*E-mail: [email protected]. Tel: +86 21-65643836 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (21274027 and 20974022) and the Innovation Program of Shanghai Municipal Education Commission (15ZZ002). The synchrotron-based 2D-GIXRD measurement was supported by Shanghai Synchrotron Radiation Facility (15ssrf00474). The FET devices have been carried out in Fudan Nanofabrication Laboratory.

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