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On the basis of AFM (atomic force microscopy) images, the surface of the phthalocyanine-modified glass substrate has uneven structures on the nanomete...
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A wisely designed phthalocyanine derivative for convenient molecular fabrication on a substrate wataru harada, mana Hirahara, Takanari Togashi, Manabu Ishizaki, Masato Kurihara, Masa-aki Haga, and Katsuhiko Kanaizuka Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03466 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017

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A wisely designed phthalocyanine derivative for convenient molecular fabrication on a substrate Wataru Harada,1 Mana Hirahara,1 Takanari Togashi,1 Manabu Ishizaki,1 Masato Kurihara,1 Masa-aki Haga,2 and Katsuhiko Kanaizuka*1 1. Department of Material and Biological Chemistry, Faculty of Science, Yamagata University,14-12 Kojirakawa-machi, Yamagata 990-8560, Japan 2. Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 113-27 Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan KEYWORDS: phthalocyanine, silane coupling reaction, molecular fabrication, bottom up, photocurrent Supporting Information ABSTRACT: An axial substituted silicon phthalocyanine derivative, SiPc(OR)2 (R = C4H9), that is soluble in organic solvent is conveniently synthesized. This silicon phthalocyanine derivative reacts with a hydroxyl group on a substrate and then with another phthalocyanine derivative under mild conditions. The accumulation number of the phthalocyanine molecules on the substrates is easily controlled by the immersion time. Based on AFM (atomic force microscopy) images, the surface of the phthalocyanine-modified glass substrate has uneven structures in nano-meter scale. ITO electrodes modified with the composition of the phthalocyanine derivative and PCBM show stable cathodic photocurrent generation upon light irradiation.

INTRODUCTION The phthalocyanine derivatives have been widely used not only as pigment materials but also as devices such as FET1-4 and solar cells5-8 because of their unique physical and photophysical properties.9-13 Oriented phthalocyanine films have been focused on recently for the construction of smart electronic devices because of their high hole mobility due to the stacked phthalocyanines;14 various phthalocyanine derivatives have been explored on the orientation via their strong π–π interaction and the compatibility with substrates.15,16 The convenient solution processes have facilitated exhaustive research in a large number of fields; however, the solubility of metal and metal-free unsubstituted phthalocyanine is generally poor.17 Therefore, these insoluble phthalocyanines deposited on substrates by the vacuum deposition methods have been widely employed to date.18-19 In order to overcome the dilemma of the solution-processed technologies, functional groups and/or long alkyl chains have been induced onto the phthalocyanine’s backbone for improving the solubility.20

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We have focused on the axial exchange reactions of unsubstituted silicon phthalocyanines (Figure 1).21-23 In 1963, Kenny and his co-workers reported on the convenient conversion from an insoluble silicon unsubstituted phthalocyanine, SiPc(OH)2, to soluble SiPc(OR)2 (R = n-C2H5, nC8H17, and so on) under heating in the corresponding alcohols (R-OH).21 In this study, we note that the a series of SiPc(OR)2 is an advantageous candidate to prompt the solution-processed technologies for unsubstituted phthalocyanine-based electronic devices.

We have adopted

SiPc(OBu)2 by the refluxing of SiPc(OH)2 in 1-buthanol in reference to demonstrate the advantage: unsubstituted phthalocyanine-modified substrates can be easily fabricated via the spin-coating method or a molecular assembling technique using soluble SiPc(OBu)2. We have developed the construction of uneven film (nano-scale bamboo shoots structures) composed of substituted phthalocyanine derivatives via an one-pot spontaneous silane coupling reaction.24,25 The silane coupling reactions have been widely employed for the construction of 3D materials and electronic devices.26-28 We have previously developed on solution-processed molecular assembling methods using such a soluble silanol-induced substituted

phthalocyanine,

silicon

2,9,16,23-tetra-tert-butyl-29H,31H-phthalocyanine

dihydroxide tBuSiPc(OH)2.29 This continuous polymerization (Si-O-Si network) of the molecules occurs on a substrate30 and forms their nanoscale towers.16 Although tetra-tert-butyl groups are important for the solubility of tBuSiPc(OH)2, this polymerization has a limitation because the tetra-tert-butyl group is too bulky to construct higher tower structures due to the strong π–π interaction on the substrates. As a result, the polymerization of tBuSiPc(OH)2 stops even in a longer period.29 In this paper, highly soluble SiPc(OBu)2 i.e., π-planar molecule without any peripheral bulky substituents can be continuously polymerized on a transparent glass or an indium-tin-oxide (ITO) electrode (Figure 1). This π-planar molecular design makes it possible to construct longer polymer chains on a substrate. This SiPc(OBu)2-modifed ITO electrode shows stable cathodic photocurrent upon light irradiation. EXPERIMENTAL SECTION Materials. Reagents of silicon phthalocyanine dihydroxide (SiPc(OH)2) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) were purchased from Sigma-Aldrich; SiCl4 was purchased from TCI. 1-Buthanol was purchased from Kanto Chemical Co. Na2SO4 was purchased from Nacalai Tesque. These reagents were used without further purification. A glass plate (25 mm x 15 mm) and ITO (10 ohms/cm2, Furuuchi Kagaku) were washed with 2-propanol, methanol, and then pure water before use. Organic solvents were purchased from Kanto Chemical Co. and used without further purification. TBAPF6 was purchased from TCI. Methods. AFM images were observed using Shimadzu SPM-9700. UV-vis absorption spectra were monitored using Shimadzu UV-3150 and UV-2600. Photo-electrochemical measurement

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was carried out in 0.1 M Na2SO4 aqueous solution under various potentials (voltages). The photocurrent was measured using BAS 1100. PCBM was overlaid on the silicon phthalocyanine bamboo shoots by spin coating (KYOWARIKEN K-359S1). Thermogravimetric analysis measurement (TG) was carried out using Thermo plus EV02 (Rigaku); the temperature was increased at a rate of 10 oC/min to 800 oC. A cross-section SEM image was taken by using JEOL JSM-7600F. Synthesis of SiPc(OBu)2. Silicon phthalocyanine dihydroxide, SiPc(OH)2 (17.3 mg, 3.00 × 10 mol), was added to 1-buthanol (5 mL), and the mixture was heated to reflux for 12 hours. -5

After cooling to room temperature, the reaction mixture was filtered, and the solvent was evaporated to dryness. The blue powdery residue was collected (19.6 mg, 94.5%). 1H NMR (500 MHz, CDCl3): δ = -2.12 (t, J = 6.5 Hz, 4H), -1.72 (t, J = 7.3 Hz, 4H), -1.35 (d, J = 7.5 Hz, 4H), -0.68 (t, J = 7.5 Hz, 6H), 8.32 (m, J = 2.8 Hz, 8H), 9.63 (m, J = 2.8 Hz, 8H).21 Formation of phthalocyanine bamboo shoots (SiPc) on substrates A small excess amount of SiCl4 was added to a 0.1 mM chloroform solution containing SiPc(OBu)2 at room temperature under air. A cleaned glass or ITO substrate was immersed in the solution mixture for various periods of time at room temperature. The plate was thoroughly washed with chloroform by ultrasonic cleaning for 1 min to remove the physically attached molecules, and then it was dried by a stream of nitrogen gas. In this reaction, 1-buthanol forms in the mixture, and it has been detected by 1H NMR. Preparation of composite film of SiPc bamboo shoots and PCBM on ITO electrode (ITO/SiPc/PCBM) A 13.2 mM PCBM solution of chloroform (0.25 mL) was spin-coated (1000 rpm for 5 sec and then 1500 rpm for 10 sec) on a plate with ITO/SiPc and then heated at 100 oC for 1 hour. This ITO/SiPc/PCBM was used for photocurrent measurements. RESULTS AND DISCUSSION Synthesis and characterizations of SiPc(OBu)2 The concept of molecular fabrication of SiPc is shown in Figure 1. This spontaneous molecular fabrication has been observed for the first time. By using the axial exchange reactions of unsubstituted silicon phthalocyanines (silicon phthalocyanine dihydroxide) with 1-buthanol, highly soluble phthalocyanine (SiPc(OBu)2) was easily synthesized (Figure 1). The weight loss of 18.5 % is observed in TG spectrum (Figure S1) at 314 oC, and this loss is probably based on the two buthanol (calculated to be 16.5%). Cyclic voltamogram (CV) of SiPc(OBu)2 in 0.1 M TBAPF6CH2Cl2 shows reversible redox waves at 0.65, -1.19 V vs. Fc+/Fc, which corresponds to HOMO

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and LUMO, respectively (Figure S2). This highly soluble and redox-active phthalocyanine has a large advantage of solution process device constructions. Formation of phthalocyanine bamboo shoots (SiPc) on substrates Figure 2 shows the UV-vis absorption spectra of SiPc(OBu)2 in a chloroform solution (inset) and a glass plate after immersion in a mixed solution of SiPc(OBu)2 and SiCl4. A small amount of SiCl4 was added for the acceleration of polymerization. The Si-O-Si bond formation rate depends on the acidity of the solution.31 When the SiPc(OBu)2 solution is kept in inert condition such as nitrogen a polymerization does not occur. Water molecules in air are important to form second layer.30 Strong peaks at 357 and 681 nm in a chloroform solution are based on the Soret and Q bands of typical phthalocyanine, respectively (Figure 2). After the immersion of glass in the solution mixture, these Soret and Q bands were observed, and the peaks are red-shifted in the initial step. After a few hours, the large peak shift was not observed. We have considered that the molecular density is high in the initial step, therefore intermolecular wire interaction is predominant. On the other hand, interaction between SiPc(OBu)2 in the solution and SiPc on the substrate also occurs in the longer immersion time.32,33 The peak of Q bands is shifted to blue after longer immersion time. This behaviour was not observed in our previous study, when we employed phthalocyanine having bulky tert-butyl groups, Si(OH)2-ttbPc. In the case of ttbSiPc(OH)2, the maximum absorbance based on the Q band is less than 0.1. On the other hand, in the case of SiPc(OBu)2, the growth reached the limit at the absorbance of ca. 0.45 (at 712 nm). We consider that flat (coin-shaped) molecules without bulky functional groups can be easily connected on the substrate. As we note above, the polymerization has a limitation when tetratert-butyl groups are induced onto Pc backbone because the tetra-tert-butyl group is too bulky to form a stacking structure. Thus, we consider that a simple molecular structure (without huge functional groups onto phthalocyanine backbone) is the key to fix a large number of molecules on a substrate. These films are redox-active (cyclic voltammograms of the films are shown in Figure S3). Morphology of SiPc bamboo shoots on substrates The growth of SiPc was monitored by using AFM. Figure 3 shows AFM images of glass/SiPc substrates (immersion times are 10 min (a, b) and 60 min (c, d)). The number of white dots increased with the immersion time, and the height of the white dots is ca. 20 nm. Clear XRD pattern has not been observed in the films, however, from the shape of Q-bands in electronic spectra, the orientation of SiPc is probably parallel to the substrates.34 Figures S4 show chemical structures of SiPc dimer calculated by MM2 minimized energy (chem 3D). The distance between two Si atoms (PcSi-O-SiPc) is estimated to be 0.3344 nm. From the consideration of the distance of Si-O-Si is about 0.33 nm,24 the number of SiPc unit is ca. 70 on the surface.

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Figure S5 shows an AFM image of the plate of longer immersion time (glass/SiPc(180min)); the uneven structure is similar to glass/SiPc(60min). This indicates that same amount of SiPc is connected onto both top and bottom of bamboo shoots after the immersion time of 60 min. This result supports the UV-vis absorption data that the peaks of Soret and Q bands are red-shifted in the initial step because of higher density of SiPc at the substrate. Photocurrent generation of ITO/SiPc/PCBM Figure 4 shows the spin-coated film of PCBM on the ITO/SiPc(60min) (that is, ITO/SiPc/PCBM). An image of the flat surface (less than a few nanometer) was observed after deposition of PCBM on the SiPc bamboo shoots. This PN organic molecular junction of SiPc and PCBM was constructed. A cross-section SEM image of the ITO/SiPc/PCBMP plate is shown in Figure S6. The thickness of SiPc/PCBM is estimated to be 60 nm. Photocurrent measurement of ITO/SiPc (immersion time is 60 min)/PCBM was carried out in 0.1 M Na2SO4 aqueous solution. Figure 5 shows on–off responses of the film at a bias of -0.3 V vs. Ag/AgCl. Stable cathodic photocurrents were observed upon light irradiation (λex = 710 nm). The action spectrum (plots of photocurrent vs. wavelength) of the film is shown in Figure 6a. The action spectrum is similar to the absorption spectra of ITO/SiPc/PCBM, which indicates that the photocurrent generation sensitizers are both PCBM and the phthalocyanine derivative (Figure S7). Figure 6(a) also shows the increase (15min-green, 30min-black, 60min-red) and decrease (120min-blue) of photocurrent generation efficiency with the increase of immersion time (plots of peak efficiency around at 700 nm are shown in Figure 6(b)). This indicates that electron injection efficiency from SiPc to PCBM is increased up to 60 min, probably because the energy potential of SiPc is decreased. This is supported by UV-vis absorption spectra measurements that is red-shift of Q bands. However, in longer immersion time such as 120 min, it has a limitation of long electron transfer. From AFM results, height of bamboo shoots is stopped in 60 min, and thick films of SiPc acts as an ohmic component in the photocurrent devices. Based on these results, the electron-transfer mechanism upon light irradiation is described in Figure 7. When the light in the region of 400–600 nm is irradiated onto the ITO/SiPc/PCBM, the excitation of PCBM occurs. After that, the SiPc injects an electron in HOMO to the hole of the PCBM; ITO electrode injects an electron to the oxidized SiPc. An oxygen included in an electrolyte solution accepts the electron of the reduced PCBM. Thus, in the region of 400–600 nm, PCBM acts as a photocurrent sensitizer. On the other hand, when the light in the region of 600–800 nm is irradiated onto the ITO/SiPc/PCBM, the excitation of SiPc occurs. In this case, ITO electrode injects an electron to the hole of the SiPc; PCBM accepts the electron from the reduced SiPc. After that, an oxygen included in an electrolyte solution accepts the electron from the reduced PCBM. Figure S8 shows on-off responses of photocurrent

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generation before and after Ar bubbling. Thus, in the region of 600–800 nm, SiPc acts as a photocurrent sensitizer and PCBM acts as an electron acceptor. Conclusion. We have conveniently synthesized a soluble silicon phthalocyanine derivative. The phthalocyanine shows epitaxial growth on a substrate in a solution. The key to successful formation of a large number of nanowiers composed of phthalocyanine molecules is the employment of a very simple molecule and a smart chemical reaction. In the previous case of ttbSiPc(OH)2, the maximum absorbance based on the Q band is less than 0.1. On the other hand, in the present case of SiPc(OBu)2, the growth reached the limit at the absorbance of ca. 0.45 (at 712 nm). Note that the flat molecules can be easily connected on the substrate. Furthermore, we have clarified that phthalocyanine nanowiers act as an effective photo-electron transfer material. This convenient and reproducible method of preparing nanowiers will be useful to construct high-performance organic solar cells and FET devices. And we have also clarified that longer SiPc wire has a limitation for photocurrent generation. ACKNOWLEDGMENTS This work was supported by JSPS KAKENHI Grant Number JP16K05715.

ASSOCIATED CONTENT Supporting Information TG, CVs, MM2 calculation, AFM, SEM, electronic spectrum, and photocurrent. This material is available free of charge via the Internet at http://pubs.acs.org.

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FIGURES

Figure 1. Chemical structures of poor and highly soluble phthalocyanine used in this study (pictures show the compounds in CHCl3), and the concept for the construction of SiPc wires via an axial substitution reaction on a surface.

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0.4

a.u

0.5

Abs.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.3 300

0.2

500 700 900 Wavelength / nm

0.1 0 300 400 500 600 700 800 Wavelength / nm

Figure 2. UV-vis absorption spectra of glass/SiPc by the addition of SiCl4 in CHCl3 for various immersion times at room temperature (10 min, 30 min, 1h, 3h, 6h, 9h, 12h, 15h, and 18h from down to top). Inset shows UV-vis absorption spectrum of SiPc(OBu)2 in a chloroform solution

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Figure 3. AFM images of glass/SiPc after immersion for 10 min (top view (a) and 3D view (b)) and 60 min (top view (c) and 3D view (d)).

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Figure 4. AFM images of ITO/SiPc/PCBM (top view (a) and 3D view (b)).

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off

on

off

on

100

Photocurrent / nA

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-200 -500 -800 -1,100 0

5

10

15

20

Time / s

Figure 5. On–off responses of photocurrent upon light irradiation of the ITO/SiPc/PCBM at 710 nm in 0.1 M Na2SO4 aqueous solution at -0.3 V vs. Ag/AgCl.

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(b)

(a)

φ at peak top / %

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Current efficiency/ φ

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5

0

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0 400

500

600

700

800

Wavelength / nm

0

50

100 150 200 250

Immersion Time / min

Figure 6. Plots of photocurrent generation efficiency vs. wavelength of ITO/SiPc(10min)/PCBM (green),

ITO/SiPc(30min)/PCBM

(black),

ITO/SiPc(60min)/PCBM

(red),

and

ITO/SiPc(120min)/PCBM (blue) in 0.1 M Na2SO4 aq. solution at -0.3 V vs. Ag/AgCl (a). Plots of peak photocurrent generation efficiency based on Q bands vs. various immersion periods of time in SiPc of ITO/SiPc/PCBM (b).

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E / V vs. Fc / Fc+

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Excitation of SiPc Excitation of PCBM S2 -1.95 -1.44 -1.19

S1

-1.06 O2 / O2-

-0.3 V ITO +0.65

S0 SiPc

Direction of photocurrent PCBM

+1.76

Figure 7. Plausible cathodic photocurrent mechanism in ITO/PcSi/PCBM.

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Corresponding Author *Tel.: +81-23-628-4856. E-mail: [email protected] (K. K.)

Notes The authors declare no competing financial interest.

ABBREVIATIONS ITO, indium tin oxide Pc, phthalocyanine PCBM, [6,6]-Phenyl C61 butyric acid methyl ester AFM, atomic force microscope CVs, cyclic voltammograms SEM, scanning electron microscope

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