Acid-Responsive Conductive Nanofiber of Tetrabenzoporphyrin Made by Solution Processing

Article abstract of DOI:10.1021/jacs.7b10575

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 acid-responsive 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-fold decreases and increases, respectively, in the conductivity and also the amount of the radical cation. The conductivity of the fiber also shows a correlation with the pK_a of acid dopants.

Full text of DOI:10.1021/jacs.7b10575

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Communication
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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AcidꢀResponsive Conductive Nanofiber of Tetrabenzoporphyrin Made
by Solution Processing
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†,‡
‡
†
‡
‡
Yonggang Zhen,* Kento Inoue, Zongrui Wang, Tetsuro Kusamoto, Koji Nakabayashi, Shinꢀichi
‡
† ,ǁ
,†,‡
,‡
,‡
Ohkoshi, Wenping Hu, 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, 7ꢀ3ꢀ1 Hongo, Bunkyoꢀku, Tokyo, 113ꢀ0033, Japan
ǁ
Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Science, Tianjin Uniꢀ
versity & Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China
Supporting Information Placeholder
ABSTRACT: While cofacial oneꢀdimensional (1ꢀD) πꢀstacking
of a planar aromatic molecule is ideal for the construction of conꢀ
duction systems, such molecules including tetrabenzoporphyrin
(
BP) prefer to form edgeꢀtoꢀface stacking through CH–π interacꢀ
tions. We report here that the BP molecules spontaneously form a
ꢀD cofacial stack in chloroform containing 1% trifluoroacetic
acid (TFA), and that a bundle of the formed nanofiber shows acidꢀ
1
–
1
responsive 1ꢀD conductivity as high as 1904 S m . A small fracꢀ
tion (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–
700 times decrease and increase, respectively, of the conductiviꢀ
2
ty and also the amount of the radical cation. The conductivity of
the fiber also shows a correlation with the pK of acid dopants.
a
1
, 2 , 3
Supramolecular stacks of many molecules,
molecule molecular wires
and singleꢀ
4
,5,6
are two of the typical moleculeꢀ
based strategies to achieve oneꢀdimensional (1ꢀD) electrical conꢀ
duction, where cofacial columnar πꢀstacking is the most commonꢀ
7
,8,9,10
ly exploited in the first category.
The molecules stacked in
this strategy have a common structural feature—an aromatic core
decorated on its periphery with flexible side chains, because herꢀ
11
ringbone packing is a preferred mode of stacking for aromatics
1
2
including benzene and tetrabenzoporphyrin (BP). Thus, the side
chains are necessary for prevention of edgeꢀtoꢀface 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 πꢀelectron, to sterically disturb πꢀ
13,14
stacking,
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
Figure 1. Spontaneous formation of nanofiber of tetrabenzoporꢀ
phyrin that shows acidꢀresponsive 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.
1
8
). Herein, we report solutionꢀprocessed preparation of conducꢀ
tive 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 ꢀmꢀsized single
Figure 1b illustrates a schematic columnar structure modeled
–1
+•
– 15
bundle of fiber shows conductivity of 1904 S m as measured
after a single crystal of 2BP•BP •I . The BP–TFA fiber has a
3
+•
ꢀ
along the fiber axis.
nominal composition of nBP•(BP •TFA )•mTFA (n = 37 ± 2, m =
ACS Paragon Plus Environment

Journal of the American Chemical Society
a
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4 ± 3) as judged by elemental analysis and electron spin resoꢀ
b
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nance (ESR). A πꢀstacking distance of 3.27 Å and an intercolumꢀ
nar distance of 14.54 Å were determined by grazing incidence Xꢀ
ray 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 1ꢀD organic molecular
wires whose electronic property responds to acid and base treatꢀ
ment with retention of their structural integrity. The switching
accompanies the change of radical concentration in the fiber. Varꢀ
ious volatile carboxylic acids act as a dopant, and the conductivity
30 ꢀm
100 nm
c
d
correlates well with the pK value of the acid.
a
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Insolubility of BP in organic solvents requires the use of vacꢀ
uum deposition or of soluble precursors for its practical applicaꢀ
3.30 Å
1
4.73 Å
(110)
(210)
(211)
(002)
320)
17,18
ꢀ2
tions.
We found that BP dissolves homogeneously (2.9×10
(
(
100) (200) (300)
M) at 25°C as a green solution in chloroform containing 1% TFA
2+
1
as a diprotonated form (BPH2 , Figure 1b; H NMR, Figure S1,
S2), from which spontaneously formed darkꢀcolored webꢀlike
ꢀmꢀdiameter bundles of fibers (BP–TFA; 64% yield) of approxiꢀ
mately 40 nm as analyzed by scanning electron microscopy (SEM)
5
10 15
2
20 25 30
theta (degree)
1 ꢀm
(
Figure 2a,b). ESR monitoring of the solution indicated that the
e
f
+•
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 BPH
tetrathiafulvalene.
BP-TFA
BP film
2
+ 19
,
modeled after chemistry of
2
20,21
C F COOH, C F COOH, and C F COOH also formed similar
2
5
3
7
4 9
nanofibers, but hydrochloric acid, formic acid, or trifluoroꢀ
methanesulfonic 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 supramoꢀ
lecular assembly of BP columns. Tetraphenylporphyrin in
TFA/chloroform forms neither radicals nor precipitates under the
same conditions.
1
.98 1.99 2.00 2.01 2.02 2.03
500 1000 1500 2000 2500
wavelength (nm)
g factor
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 ITOꢀ
glass for inꢀplane (red) and outꢀofꢀplane (blue) diffraction measꢀ
urements, as well as simulated XRD based on a single crystal of
Upon polarized optical microscopic analysis (Figure S4), the
BP–TFA fiber bundles displayed a crystalline texture due to bunꢀ
dled nanofibers. Information on the molecular packing in the nanꢀ
ofibers was obtained by GIXRD and SAED analyses. A sum of
outꢀofꢀplane and inꢀplane XRD signals of the BFꢀTFA fibers on
an ITOꢀglass substrate (Figure 2c, blue and red lines) matches
well with a simulated XRD pattern based on a single crystal strucꢀ
+•
–
2BP•BP •I (black) and neutral BP (magenta). (d) SAED pattern
3
of a bundle of BP–TFA nanofibers. The inset is a transmission
electron microscope image of the same bundle. (e) ESR spectra of
BPꢀTFA fibers (red), after TEA treatment (BPꢀTFAꢀTEA, black)
and after second TFA treatment (BPꢀTFAꢀTEAꢀTFA, blue). (f)
VisꢀNIR absorption spectra of the BP–TFA fibers (red) and thinꢀ
+
•
–
ture of needleꢀcrystalline 2BP•BP •I (Figure 2c black line) in
which BP and BP molecules cofacially πꢀstack along the needle
axis. This matching of the data between BP—TFA and
BP•BP •I led us to propose the model structure of the BP–
3
+
•
28
film BP (blue; reported previously).
15
The SAED pattern in Figure 2d shows dꢀspacing distances of
.30 ± 0.02 Å and 14.73 ± 0.02 Å along the longitudinal and
+•
–
2
3
3
TFA column shown in Figure 1b. The XRD data for the BPꢀTFA
transverse axes of the bundled nanofibers, respectively, in agreeꢀ
ment 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 Xꢀray photoelectron specꢀ
troscopy (XPS, Figure S6) indicated the presence of a small
on the fiber surface, an intermediate of the
radical cation formation also detected by H NMR in the reaction
mixture (Figure 1c).
+
•
–
fibers and the 2BP•BP •I needles much differ from the data for
3
1
2
herringboneꢀstacked neutral BP (Figure 2c, magenta line).
The outꢀofꢀplane XRD data of BPꢀTFA (blue line) shows a
strong peak at 6.08° (d = 14.54 Å; intercolumnar distance), while
the inꢀplane XRD data (red) revealed a prominent peak at 27.27°
2+
amount of BPH
2
1
(
d = 3.27 Å; πꢀstacking distance of the cofacial BP stack), which
was not seen in the outꢀofꢀplane data. The latter distance is much
shorter than the πꢀstacking distance in the neutral BP crystal
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,
conducting polyaniline, and TCNQꢀbased donor–acceptor sysꢀ
tems, suggesting that the signal comes from an electron delocalꢀ
ized in the 1ꢀD stacked column. The BP:BP ratio was found to
be 100:2.7 using 2,2,6,6ꢀtetramethylpiperidinꢀ1ꢀoxyl (TEMPO) as
reference. From these data and the elemental analysis
nBP•mTFA; n = 1, m = 1.45), we assign a composition of
+•
–
(
3.359 Å) and slightly longer than the one in the 2BP•BP •I
3
15
crystal (3.232 Å).
2
2
For the nanofibers precipitated from C F COOH, C F COOH,
2
5
3 7
23
and C F COOH, the πꢀstacking distance remained unchanged
4
9
24
(3.27–3.29 Å; inꢀplane XRD). The intercolumnar distance, on the
other hand, jumped from 14.54 Å (TFA; 14.87 Å for C F COOH)
+•
2
5
to 17.89 Å (C F COOH; 18.30 Å for C F COOH) (Figure S5),
3
7
4 9
a
(
supporting the structure suggested in Figure 1b in that the acid
molecules fill in the intercolumnar space.
+•
–
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.
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Visibleꢀnear infrared (VisꢀNIR) spectroscopy provided further
evidence of electron delocalization through the BP columnar
stackꢀꢀa very broad peak in the NIR region of 1500–2500 nm
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
πꢀstack 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 coꢀ
lumnar stack once formed is stable without TFA (cf. Figure 1b),
though that TFA is necessary for the column formation.
1
2
3
4
5
6
7
8
9
(
Figure 2f), which we assign to a chargeꢀresonance band because
of electron delocalization among tightly πꢀstacked BP moleꢀ
25,26
cules.
The significantly blueꢀshifted (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 1ꢀD delocalized electron and shows
27
high conductivity. Utilizing an organic ribbon mask technique,
we fabricated a twoꢀterminal device for a single bundle of BP–
TFA nanofibers (Figure 3a) and found a maximum value of elecꢀ
1
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0
1
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0
1
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0
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–
1
trical conductivity to be 1904 S m at room temperature (Figure
Interestingly, the conductivity shows a strong correlation with
the acidity of the doped organic acid. When we redoped the TEAꢀ
treated fibers with the vapor of volatile carboxylic acids other
than TFA, conductivity was recovered and showed dependence on
3
1
b). The conductivity measured for eight devices ranged between
–
1
–1
056 and 1904 S m (1546 ± 324 S m ). This conductivity is
5
–1
lower than the values of ca. 10 S m reported for chemically or
the pK value of the acid (Figure 4b).
a
7
electrochemically oxidized 0 – D porphyrin crystals, probably
a
01
00
b
1
0
CF
3
COOH
TFA
because of intercolumnar boundaries and "Y" bifurcation defects
constituting localized barriers for charge carrier transportation
1
-
1
CHF
2
COOH
10-1
0-2
0-3
10-4
1
-
2
along the column.
1
-
3
The temperatureꢀdependent resistivity measured by a fourꢀ
probe method in the range 12 to 300 K revealed a semiconductorꢀ
HCOOH
1
-4
-5
-6
TEA
like behavior with an activation energy of 20 meV (Figure 3c).
CH
3
COOH
–
Further, we observed the conductivity (σ) followed ln[σ(T)] ∝ T
0-5
1
1
/2
0
1
2
3
4
5
-1
0
1
2
3
4
5
dependence within this temperature range (Figure 3d), probaꢀ
bly indicative of 1ꢀD hopping behavior in this BP•BP (=
00:2.7) system.
cycle number
pK
a
+•
1
Figure 4. Dedoping/doping of the BP–TFA fibers. (a) Switching
of conductivity normalized to the starting data through five TEAꢀ
dedoping/TFAꢀredoping cycles. (b) Correlation between relative
a
b
30
conductivity and pK values of the volatile acids used for redopꢀ
20
a
ing.
10
0
In conclusion, we found that conductive nanofibers made of
BP–TFA πꢀstacks show high conductivity, up to 1904 S m ,
–1
-
10
-20
-30
which then can be switched off >1000 times upon removal of
TFA, and on by TFA redoping. This observation illustrates the
function of spaceꢀfilling guest molecules (TFA and other acid
molecules) to tailor the electrical characteristics of supramolecular
assemblies without changing the host packing structure. The ease
of solutionꢀprocessed selfꢀassembly promise in situ incorporation
of the fibers into molecular electronics by a bottomꢀup and conꢀ
trolled approach. We note that the present nanofiber formation has
relevance to the formation of flat and uniform nmꢀthick pꢀlayers
of BP on highly acidic PEDOT/PSS (poly (3,4ꢀ
ethylenedioxythiophene) doped with poly(4ꢀstyrenesulfonate)),
which contributed much to the performance of an organic solar
3
0 ꢀm
1
ꢀm
-1
-0.5
0
0.5
1
voltage (V)
c
1
1
04
d
0
cooling
heating
03
-2
-
4
102
-
6
8
01
1
-
1
00
-10
0.05 0.10 0.15 0.20 0.25 0.30
0
100
200
300
T (K)
T
-1/2 (K-1/2
)
28,29
cell using BP as a donor.
Figure 3. The conductivity of a single bundle of BP–TFA nanoꢀ
fibers. (a) SEM image of a device using a single bundle of the
nanofibers; (b) I–V curve of the device at room temperature; (c)
ASSOCIATED CONTENT
Supporting Information
Plot of resistivity (ρ) as a function of temperature (T) by a fourꢀ
–
1/2
Experimental details and supplementary figures. The Supportꢀ
ing Information is available free of charge on the ACS Publicaꢀ
tions website.
probe method; (d) Plot of log(σ) as a function of T
probe method.
by a fourꢀ
The conductivity of the TFAꢀdoped 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,
AUTHOR INFORMATION
Corresponding Author
*zhenyg@iccas.ac.cn
–1
reduces the conductivity by 2700 times from 1904 S m to 0.7 S
*
*
*
guoyunlong@iccas.ac.cn
harano@chem.s.uꢀtokyo.ac.jp
nakamura@chem.s.uꢀtokyo.ac.jp
–1
m
(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
Notes
The authors declare no competing financial interests.
+•
the hopping mechanism. The BP concentration recovered to 46%
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Journal of the American Chemical Society
Page 4 of 5
(
11) Gautam R. Desiraju, G. R.; Gavezzotti, A. J. Chem. Soc., Chem.
ACKNOWLEDGMENTS
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Commun., 1989, 621
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 Proꢀ
motion 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.
(12) Aramaki, S.; Mizuguchi, J., Acta Cryst. 2003, E59, o1556.
(13) Murphy, A. R.; Frëchet, J. M. J.; Chang, P.; Lee, J.; Vivek S. J. Am.
Chem. Soc. 2004, 126, 1596.
(14) Oh, J. H.; Lee, W.ꢀY.; Noe, T.; Chen, W.ꢀC.; Könemann, M.; Bao Z.
J. Am. Chem. Soc. 2011, 133, 4204.
(15) Murata, K.; Liou, K.; Thompson, J. A.; McGhee, E. M.; Rende, D. E.;
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