Acid-Responsive Conductive Nanofiber of Tetrabenzoporphyrin Made

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Acid-Responsive Conductive Nanofiber of Tetrabenzoporphyrin Made by Solution Processing Yonggang Zhen, Kento Inoue, Zongrui Wang, Tetsuro Kusamoto, Koji Nakabayashi, Shin-ichi Ohkoshi, Wenping Hu, Yunlong guo, Koji Harano, and Eiichi Nakamura J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10575 • Publication Date (Web): 05 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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Journal of the American Chemical Society

Acid-Responsive Conductive Nanofiber of Tetrabenzoporphyrin Made by Solution Processing Yonggang Zhen,*,†,‡ Kento Inoue,‡ Zongrui Wang,† Tetsuro Kusamoto,‡ Koji Nakabayashi,‡ Shin-ichi Ohkoshi,‡ Wenping Hu,† ,ǁ Yunlong Guo,*,†,‡ Koji Harano,*,‡ and Eiichi Nakamura*,‡ †

Beijing National Laboratory for Molecular Science, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡ Department of Chemistry, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan ǁ Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin University & Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China Supporting Information Placeholder ABSTRACT: While cofacial one-dimensional (1-D) π-stacking of a planar aromatic molecule is ideal for the construction of conduction systems, such molecules including tetrabenzoporphyrin (BP) prefer to form edge-to-face stacking through CH–π interactions. We report here that the BP molecules spontaneously form a 1-D cofacial stack in chloroform containing 1% trifluoroacetic acid (TFA), and that a bundle of the formed nanofiber shows acidresponsive 1-D conductivity as high as 1904 S m–1. A small fraction (2.7%) of BP in the fiber exists in a cation radical state, and 1.5 equiv of TFA is located in an intercolumnar void. Dedoping and redoping of TFA with trimethylamine vapor results in 1300– 2700 times decrease and increase, respectively, of the conductivity and also the amount of the radical cation. The conductivity of the fiber also shows a correlation with the pKa of acid dopants.

Supramolecular stacks of many molecules, 1 , 2 , 3 and singlemolecule molecular wires 4 , 5 , 6 are two of the typical moleculebased strategies to achieve one-dimensional (1-D) electrical conduction, where cofacial columnar π-stacking is the most commonly exploited in the first category.7,8,9,10 The molecules stacked in this strategy have a common structural feature—an aromatic core decorated on its periphery with flexible side chains, because herringbone packing is a preferred mode of stacking for aromatics11 including benzene and tetrabenzoporphyrin (BP). 12 Thus, the side chains are necessary for prevention of edge-to-face herringbone packing caused by CH–π interaction, for nanophase segregation, and for solubilization of the molecules. The side chains however tend to reduce the density of π-electron, to sterically disturb πstacking,13,14 and hence to reduce the conductivity. The reported conductivities of small molecule based organic nanofibers are generally very low (Table S1) except for a fiber grown between two electrodes by light and electric field triggering (>5000 S m– 1 8 ). Herein, we report solution-processed preparation of conductive organic nanofibers without recourse to the side chain strategy (Figure 1a), where BP is dissolved in trifluoroacetic acid (TFA) to produce spontaneously first a cation radical (BP+•, Figure 1c) and then precipitates of BP–TFA nanofibers of approximately 40 nm diameter and their bundles (Figures 2a, b). A µm-sized single bundle of fiber shows conductivity of 1904 S m–1 as measured along the fiber axis.

Figure 1. Spontaneous formation of nanofiber of tetrabenzoporphyrin that shows acid-responsive conductivity. (a) Preparation of BP–TFA fiber. (b) A plausible stacking structure and molecular structures. (c) A plausible pathway for fiber formation. See text and Supporting Information for the determination of the 37:1 ratio. Figure 1b illustrates a schematic columnar structure modeled after a single crystal of 2BP•BP+••I3–.15 The BP–TFA fiber has a nominal composition of nBP•(BP+••TFA-)•mTFA (n = 37 ± 2, m =

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54 ± 3) as judged by elemental analysis and electron spin resonance (ESR). A π-stacking distance of 3.27 Å and an intercolumnar distance of 14.54 Å were determined by grazing incidence Xray diffraction (GIXRD) and selected area electron diffraction (SAED) analyses. Interestingly, we can switch the conductivity on and off with a ratio of 1300–2700 through TFA doping and dedoping (Figure 1b)—a rare example of 1-D organic molecular wires whose electronic property responds to acid and base treatment with retention of their structural integrity. The switching accompanies the change of radical concentration in the fiber. Various volatile carboxylic acids act as a dopant, and the conductivity correlates well with the pKa value of the acid. Insolubility of BP16 in organic solvents requires the use of vacuum deposition or of soluble precursors for its practical applications.17,18 We found that BP dissolves homogeneously (2.9×10-2 M) at 25°C as a green solution in chloroform containing 1% TFA as a diprotonated form (BPH22+, Figure 1b; 1H NMR, Figure S1, S2), from which spontaneously formed dark-colored web-like µm-diameter bundles of fibers (BP–TFA; 64% yield) of approximately 40 nm as analyzed by scanning electron microscopy (SEM) (Figure 2a,b). ESR monitoring of the solution indicated that the BP+• radical cation formed immediately after addition of BP to TFA and persisted throughout the fiber formation (Figure S3). Figure 1c is suggested as a fiber formation path involving electron transfer between BP and BPH22+, 19 modeled after chemistry of tetrathiafulvalene.20,21 C2F5COOH, C3F7COOH, and C4F9COOH also formed similar nanofibers, but hydrochloric acid, formic acid, or trifluoromethanesulfonic acid hardly dissolved BP and did not form the fibers, suggesting that the perfluorinated carboxylic acids act as a solubilizing agent as well as a necessary component of supramolecular assembly of BP columns. Tetraphenylporphyrin in TFA/chloroform forms neither radicals nor precipitates under the same conditions. Upon polarized optical microscopic analysis (Figure S4), the BP–TFA fiber bundles displayed a crystalline texture due to bundled nanofibers. Information on the molecular packing in the nanofibers was obtained by GIXRD and SAED analyses. A sum of out-of-plane and in-plane XRD signals of the BF-TFA fibers on an ITO-glass substrate (Figure 2c, blue and red lines) matches well with a simulated XRD pattern based on a single crystal struc• ture of needle-crystalline 2BP•BP+ •I3– (Figure 2c black line) in +• which BP and BP molecules cofacially π-stack along the needle axis.15 This matching of the data between BP—TFA and 2BP•BP+••I3– led us to propose the model structure of the BP– TFA column shown in Figure 1b. The XRD data for the BP-TFA • fibers and the 2BP•BP+ •I3–needles much differ from the data for herringbone-stacked neutral BP (Figure 2c, magenta line).12 The out-of-plane XRD data of BP-TFA (blue line) shows a strong peak at 6.08° (d = 14.54 Å; intercolumnar distance), while the in-plane XRD data (red) revealed a prominent peak at 27.27° (d = 3.27 Å; π-stacking distance of the cofacial BP stack), which was not seen in the out-of-plane data. The latter distance is much shorter than the π-stacking distance in the neutral BP crystal (3.359 Å) and slightly longer than the one in the 2BP•BP+••I3– crystal (3.232 Å).15 For the nanofibers precipitated from C2F5COOH, C3F7COOH, and C4F9COOH, the π-stacking distance remained unchanged (3.27–3.29 Å; in-plane XRD). The intercolumnar distance, on the other hand, jumped from 14.54 Å (TFA; 14.87 Å for C2F5COOH) to 17.89 Å (C3F7COOH; 18.30 Å for C4F9COOH) (Figure S5), supporting the structure suggested in Figure 1b in that the acid molecules fill in the intercolumnar space.

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Figure 2. Characterization of BP–TFA nanofibers. (a, b) SEM images of BP–TFA nanofibers on silicon substrate; (c) GIXRD patterns of BP–TFA nanofibers drop casted as a thin film on ITOglass for in-plane (red) and out-of-plane (blue) diffraction measurements, as well as simulated XRD based on a single crystal of 2BP•BP+••I3– (black) and neutral BP (magenta). (d) SAED pattern of a bundle of BP–TFA nanofibers. The inset is a transmission electron microscope image of the same bundle. (e) ESR spectra of BP-TFA fibers (red), after TEA treatment (BP-TFA-TEA, black) and after second TFA treatment (BP-TFA-TEA-TFA, blue). (f) Vis-NIR absorption spectra of the BP–TFA fibers (red) and thinfilm BP (blue; reported previously).28 The SAED pattern in Figure 2d shows d-spacing distances of 3.30 ± 0.02 Å and 14.73 ± 0.02 Å along the longitudinal and transverse axes of the bundled nanofibers, respectively, in agreement with GIXRD data. Various parts of a bundle showed the same SAED pattern, indicating homogeneity of the fiber at the molecular level. Surface analysis by X-ray photoelectron spectroscopy (XPS, Figure S6) indicated the presence of a small amount of BPH22+ on the fiber surface, an intermediate of the radical cation formation also detected by 1H NMR in the reaction mixture (Figure 1c). The BP–TFA fiber contains a persistent radical species with a g value of 2.0025 (Figure 2e, red line)—a value similar to that of a free electron (2.0023). The peak line width (∆H) of 0.59 G is much narrower than those of isolated porphyrin radical cations,22 conducting polyaniline, 23 and TCNQ-based donor–acceptor systems,24 suggesting that the signal comes from an electron delocalized in the 1-D stacked column. The BP:BP+• ratio was found to be 100:2.7 using 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as a reference. From these data and the elemental analysis (nBP•mTFA; n = 1, m = 1.45), we assign a composition of nBP•(BP+••TFA–)•mTFA where n = 37 ± 2, m = 54 ± 3. The fiber is stable in air, and the radical persisted for at least two months under air at room temperature.


lower than the values of ca. 105 S m–1 reported for chemically or electrochemically oxidized 0 – D porphyrin crystals,7 probably because of intercolumnar boundaries and "Y" bifurcation defects constituting localized barriers for charge carrier transportation along the column.1 The temperature-dependent resistivity measured by a fourprobe method in the range 12 to 300 K revealed a semiconductorlike behavior with an activation energy of 20 meV (Figure 3c). Further, we observed the conductivity (σ) followed ln[σ(T)] ∝ T– 1/2 dependence within this temperature range (Figure 3d), probably indicative of 1-D hopping behavior in this BP•BP+• (= 100:2.7) system.

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Figure 3. The conductivity of a single bundle of BP–TFA nanofibers. (a) SEM image of a device using a single bundle of the nanofibers; (b) I–V curve of the device at room temperature; (c) Plot of resistivity (ρ) as a function of temperature (T) by a fourprobe method; (d) Plot of log(σ) as a function of T–1/2 by a fourprobe method. The conductivity of the TFA-doped BP fiber can be reversibly switched through dedoping and redoping (Figure 4a). Removal of TFA from the BP–TFA fiber (from 145 mol% to BP to 0.91 mol% determined by fluorine elemental analysis, Table S2) by exposure of a fiber sample to TEA vapor, followed by methanol washing, reduces the conductivity by 2700 times from 1904 S m–1 to 0.7 S m–1 (1300–2700 times depending on runs), which coincides with the decrease of the ESR signal to 3% of the original (BP:BP+• = from 100:2.7 to 100:0.081) (Figure 2e, black and red). Such a low concentration of BP+• would hardly make a continuous array of polarons along the column axis (cf. Figure 1b), and hence shut off the hopping mechanism. The BP+• concentration recovered to 46%

of the original value upon redoping with TFA vapor (Figure 2e, blue). The dedoping/redoping cycle can be repeated several times at the expense of maximum conductivity reduced to nearly half at each cycle (Figure 4a). Prolonged redoping did not increase the conductivity any further, and we ascribe the incomplete recovery to defect formation because of a chemical reaction between TEA • with BP+ . The dedoping and redoping did not change either the π-stack or the intercolumnar distances (Figure 1b and Figure S7), while SEM images suggest degradation to a certain extent (Figure S8). This structural stability of the fiber indicates that the BP columnar stack once formed is stable without TFA (cf. Figure 1b), though that TFA is necessary for the column formation. Interestingly, the conductivity shows a strong correlation with the acidity of the doped organic acid. When we redoped the TEAtreated fibers with the vapor of volatile carboxylic acids other than TFA, conductivity was recovered and showed dependence on the pKa value of the acid (Figure 4b).

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Visible-near infrared (Vis-NIR) spectroscopy provided further evidence of electron delocalization through the BP columnar stack--a very broad peak in the NIR region of 1500–2500 nm (Figure 2f), which we assign to a charge-resonance band because of electron delocalization among tightly π-stacked BP molecules.25,26 The significantly blue-shifted (against neutral BP) Soret band peaked at 396 nm and the Q band peaked at 594 nm are also characteristic. The BP–TFA fiber has a 1-D delocalized electron and shows high conductivity. Utilizing an organic ribbon mask technique,27 we fabricated a two-terminal device for a single bundle of BP– TFA nanofibers (Figure 3a) and found a maximum value of electrical conductivity to be 1904 S m–1 at room temperature (Figure 3b). The conductivity measured for eight devices ranged between 1056 and 1904 S m–1 (1546 ± 324 S m–1). This conductivity is

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Figure 4. Dedoping/doping of the BP–TFA fibers. (a) Switching of conductivity normalized to the starting data through five TEAdedoping/TFA-redoping cycles. (b) Correlation between relative conductivity and pKa values of the volatile acids used for redoping. In conclusion, we found that conductive nanofibers made of BP–TFA π-stacks show high conductivity, up to 1904 S m–1, which then can be switched off >1000 times upon removal of TFA, and on by TFA redoping. This observation illustrates the function of space-filling guest molecules (TFA and other acid molecules) to tailor the electrical characteristics of supramolecular assemblies without changing the host packing structure. The ease of solution-processed self-assembly promise in situ incorporation of the fibers into molecular electronics by a bottom-up and controlled approach. We note that the present nanofiber formation has relevance to the formation of flat and uniform nm-thick p-layers of BP on highly acidic PEDOT/PSS (poly (3,4ethylenedioxythiophene) doped with poly(4-styrenesulfonate)), which contributed much to the performance of an organic solar cell using BP as a donor. 28,29

ASSOCIATED CONTENT Supporting Information Experimental details and supplementary figures. The Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *[email protected] *[email protected] *[email protected] *[email protected]

Notes The authors declare no competing financial interests.


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ACKNOWLEDGMENTS We are grateful to Dr. Reizo Kato, Satoshi Okada, Junya Yamada and Ye Zou for experimental assistance. We also acknowledge financial support (MEXT, Japan; KAKENHI, No. 15H05754 to E.N. and No. 17H05355 to K.H.; from the Youth Innovation Promotion Association of the Chinese Academy of Sciences to Y.Z. and Y.G.; Strategic Priority Research Program, No. XDB12000000), and Shanghai Synchrotron Radiation Facility for the use of the beamline BL14B1 and technical assistance.

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Acid-Responsive Conductive Nanofiber of Tetrabenzoporphyrin Made

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