An n-channel two-dimensional covalent organic framework

Article abstract of DOI:10.1021/ja2052396

Co-condensation of metallophthalocyanine with an electron-deficient benzothiadiazole (BTDA) block leads to the formation of a two-dimensional covalent organic framework (2D-NiPc-BTDA COF) that assumes a belt shape and consists of AA stacking of 2D polymer sheets. Integration of BTDA blocks at the edges of a tetragonal metallophthalocyanine COF causes drastic changes in the carrier-transport mode and a switch from a hole-transporting skeleton to an electron-transporting framework. 2D-NiPc-BTDA COF exhibits broad and enhanced absorbance up to 1000 nm, shows panchromatic photoconductivity, is highly sensitive to near-infrared photons, and has excellent electron mobility as high as 0.6 cm² V^-1 s^-1.

Full text of DOI:10.1021/ja2052396

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An n-Channel Two-Dimensional Covalent Organic Framework
Xuesong Ding,†,z Long Chen,†,z Yoshihito Honsho,‡ Xiao Feng,† Oraphan Saengsawang,§,^ Jingdong Guo,||
Akinori Saeki,‡ Shu Seki,*,‡ Stephan Irle,§ Shigeru Nagase,|| Vudhichai Parasuk,^ and Donglin Jiang*,†,#
†Department of Materials Molecular Science, Institute for Molecular Science, National Institutes of Natural Sciences, 5-1 Higashiyama,
Myodaiji, Okazaki 444-8787, Japan
‡Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan
§Department of Chemistry, Graduate School of Science, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8601, Japan
^Computational Chemistry Unit Cell, Department of Chemistry, Chulalongkorn University, Phyathai Road, Patumwan,
Bangkok 10330, Thailand
Department of Theoretical and Computational Molecular Science, Institute for Molecular Science,
National Institutes of Natural Sciences, 38 Nishigo-naka, Myodaiji, Okazaki 444-8585, Japan
#PRESTO, Japan Science and Technology Agency, Chiyoda-ku, Tokyo 102-0075, Japan
S Supporting Information
b
Chart 1. (A) Schematic Representation of the Synthesis of
ABSTRACT: Co-condensation of metallophthalocyanine
with an electron-deficient benzothiadiazole (BTDA) block
leads to the formation of a two-dimensional covalent organic
framework (2D-NiPc-BTDACOF) that assumes a belt shape
and consists of AA stacking of 2D polymer sheets. Integration
of BTDA blocks at the edges of a tetragonal metallophtha-
locyanine COF causes drastic changes in the carrier-transport
mode and a switch from a hole-transporting skeleton to an
electron-transporting framework. 2D-NiPc-BTDA COF ex-
hibits broad and enhanced absorbance up to 1000 nm, shows
panchromatic photoconductivity, is highly sensitive to near-
infrared photons, and has excellent electron mobility as high
2D-NiPc-BTDA COF with Metallophthalocyanine at the
Vertices and BTDA at the Edges of the Tetragonal Frame-
work, and (B) Top and Side Views of a Graphical Represen-
tation of a 2Â2 Tetragonal Grid Showing the Eclipsed
Stacking of 2D Polymer Sheets (Pc, sky blue; BTDA, violet;
Ni, green; N, blue; S, yellow; O, red; B, orange; and H, white)
as 0.6 cm2 VÀ1 sÀ1
.
ovalent organic frameworks (COFs) are a class of crystalline
materials with predesignable compositions and porous
C
structures.1À7 COFs have emerged as a new material for gas
adsorption as well as a novel platform for the design of
π-electronic skeletons. In this context, two-dimensional COFs offer
a unique way to construct π-electronic 2D polymers with eclipsed
stacking of aromatic components.3À7 Such a stacking alignment
provides a pathway for charge carrier transport.7a Previously,
p-type 2D COFs for hole transport have been reported,3 but
electron-transporting 2D COFs are unprecedented. Here we
report the synthesis of an n-channel conducting COF and high-
light its high-mobility electron-transporting property and pro-
minent photoconductivity.
in sharp contrast to the hole-transporting nature of the nickel(II)
phthalocyanine COF that bears phenyl units at the edges of the
framework (NiPc COF).3c
2D-NiPc-BTDA COF was synthesized by condensation of
(2,3,9,10,16,17,23,24-octahydroxyphthalocyaninato)nickel(II)
([OH]8PcNi) and 1,4-benzothiadiazole diboronic acid (BTDADA)
in a mixture of o-dichlorobenzene and dimethylacetamide (1/2 v/v)
under solvothermal condition in 85% isolated yield (Supporting
Information (SI)). Infrared spectroscopy revealed the formation
Our strategy is to employ electron-withdrawing blocks for the
construction of n-channel 2D COFs. We placed an electron-
deficient building block, benzothiadiazole (BTDA), at the edges
of a two-component tetragonal nickel(II) phthalocyanine COF
(Chart 1A, 2D-NiPc-BTDA COF). Integration of BTDA units
into the 2D COF caused a drastic change in the carrier-transport
mode from a hole-transporting to an electron-conducting frame-
work. Owing to the eclipsed stacking arrangement of the 2D
polymer sheets (Chart 1B), 2D-NiPc-BTDA COF exhibited
Received: June 7, 2011
Published: August 24, 2011
excellent electron mobility as high as 0.6 cm2 VÀ1 À1. This is
s
r
2011 American Chemical Society
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Figure 1. FE-SEM images at (A) low and (B) high magnifications.
(C) HR-TEM image.
of ester linkages between NiPc and BTDA in 2D-NiPc-BTDA
COF (Figure S1, Table S1). Elemental analyses showed the con-
tent of C, H, and N to be 48.6, 2.0, and 16.7%, respectively. Field-
emission scanning electron microscopy (FE-SEM, Figures 1A,B)
revealed that 2D-NiPc-BTDA COF assumed a belt shape with
lengths of several tens of micrometers and widths of ∼100 nm.
High-resolution transmission electron microscopy (HR-TEM)
allowed the direct visualization of tetragonal pores with widths of
∼2.0 nm (Figure 1C).
2D-NiPc-BTDA COFs are crystalline belts that show strong
peaks in X-ray diffraction (XRD) at 3.80, 7.68, 11.50, and 27.04ꢀ,
which are assignable to the 100, 200, 300, and 001 facets,
respectively (Figure 2A, blue curve). The XRD pattern is similar
to that of NiPc COFs.3c The tetragonal pores were simulated
with hybrid density functional theory at the B3LYP level using a
double-ζ basis set (Lanl2dz) effective core potential (Lanl2
ECP) for Ni atoms and the 3-21G(d) basis set for all other
atoms using the Gaussian 03 program,8 and single-point calcula-
tions with basis set Lanl2dz for Ni and 6-31G(d) for the other
atoms to improve energies, to predict the orientation of the
BTDA units on the edges of the pore (SI). The preferred
structure reveals that a D2h geometry—i.e., one of the two BTDA
units on opposite edges is inward and the other is outward—had
the lowest energy, 2.6 kcal molÀ1, among the possible orientation
patterns (Figure S3). Based on this structure, Pawley refinement
(Figure 2A, green curve) of the experimental XRD pattern
confirmed the assignment of the observed diffractions and gave
a good match to the observed XRD pattern, as evidenced by their
negligible difference (Figure 2A, black curve). The crystalline
structure was deduced from the XRD pattern using Reflex imple-
mented in Materials Studio version 4.4. Structure simulation
using P4 space group no. 81 with parameters α = β = γ = 90ꢀ, a =
b = 46.49 Å and c = 3.40 Å (Tables S2 and S3) gave an XRD
pattern in good agreement with the experimentally observed one
(Figure 2A, red curve). The AA stacking unit cell structures are
shown in Figure 2BÀD. In contrast, a staggered model offset by
distances of a/2 and b/2 (AB stacking, Figure 2EÀG) could not
reproduce the experimental XRD pattern (Figure 2A, orange curve).
These results indicate that the condensation reaction of [OH]8-
PcNi and BTDADA leads to the formation of 2D-NiPc-BTDA
Figure 2. (A) Experimental XRD pattern of 2D-NiPc-BTDA COFs
(blue curve), Pawley refined pattern (green curve), their difference
(black curve), and simulated XRD patterns using the eclipsed AA
stacking (red curve) and staggered AB stacking models (orange curve).
Views of the unit cell derived from the AA stacking P4 space group along
the z (B) and y axes (C) and a 16-pore structure (D). Views of the 2D
staggered structure along the z (E) and y axes (F) and a staggered stack-
ing 16-pore structure (G).
COFs that assume a belt shape and consist of AA-type stacking of
2D-NiPc-BTDA sheets.
To elucidate the layer structures and stacking energies of 2D-
NiPc-BTDA COFs, we applied the density-functional tight-
binding method including Lennard-Jones (LJ) dispersion with
consideration of periodic boundary conditions. For a 2D mono-
layer structure, the obtained optimal cell length was a = b = 46.30 Å.
All atoms form a perfectly planar layer. The optimized stacking
distances and the corresponding crystal stacking energies per
monolayer and unit cell for both AA and AB forms are summar-
ized in Table S4. The crystal stacking energy for eclipsed AA
(312.84 kcal molÀ1) is higher than that for stacked AB (178.31
kcal molÀ1), indicating a significant energetic preference for AA
stacking. AB stacking causes severe loss of π-overlap between the
layers, as can be seen by its significantly reduced LJ stacking
energy (Table S4). Furthermore, we calculated the AA stacking
modes for slightly slipped structures and found that the crystal
stacking energies are similar (Table S4). The AA stacking alignment
exhibited an optimum stacking distance of 3.4À3.5 Å (Figure S3).
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Figure 3. (A) Nitrogen sorption isotherm curves. (B) Pore width and
pore width distribution curves.
Accordingly, formation of the AA stacking structure is required
by the maximum stacking energy demand for the layered
structure.
2D-NiPc-BTDA COF has tetragonal pores with a theoretical
width of 2.2 nm. The porous structure was investigated by
nitrogen sorption isotherm measurements at 77 K. 2D-NiPc-
BTDA COF exhibited reversible isotherm curves (Figure 3A).
The BrunauerÀEmmettÀTeller surface area was evaluated to be
877 m2 gÀ1. On the basis of the nonlocal density functional
theory model, the pore width was calculated to be 2.2 nm
(Figure 3B, red circles), which was identical to the theoretical
pore width. The pore width distribution profile (Figure 3B, open
circles) revealed that the porosity originated from 2.2-nm-
wide pores.
2D-NiPc-BTDA COFs displayed an absorption band at
397 nm (Figure 4A, red curve), which was 15-nm blue-shifted
from the B band of the monomeric [MeO]8PcNi (black curve).
The blue-shifted B-band indicated that the phthalocyanine units
stacked in the form of H-aggregates, in good agreement with the
results of XRD measurements and structure simulations. On the
other hand, in the Q-band region, the absorbance was signifi-
cantly enhanced. The intensity ratio of Q-band to B-band was
estimated to be 1.72, which was 1.5-fold as high as that of
[MeO]8PcNi. The significantly enhanced absorbance over a
broad wavelength range extending up to 1000 nm is interesting
for the exploration of photofunction.
The AA stacking arrangement of 2D phthalocyanine sheets
provides a pathway for carrier transport. We investigated intrinsic
carrier mobility using a flash photolysis time-resolved microwave
conductivity (FP-TRMC) method. To discern the charge carrier,
the transient conductivity was investigated under different atmo-
spheres of O2, SF6 (electron quenchers), and Ar. Excitation
with 355-nm pulsed laser at a photon density at 1.6 Â 1015
photon cmÀ2 under Ar gave a Φ∑μ (Φ is the photocarrier
generation yield and ∑μ is the sum of the mobilities of charge
carriers) value of 5.8 Â 10À4 cm2 VÀ1 sÀ1 (Figure 4B, red curve).
The double-exponential decay curve kinetics (Figure S4, Table
S5) is related to the kinetic trace of conductivity transients of
electrons (first decay) and holes (second decay). The first decay
was considerably accelerated when measured under SF6 (blue
curve) or upon annealing under SF6 (orange and green curves)
or O2 (black curve), supporting that the fast decay component
was due to the mobile electrons. In contrast, the second decay
component was almost independent for the SF6 and O2 annealed
samples, and the pre-exponential factor of the second decay
component gave a ΦΣμ the value of only 2.9 Â 10À5 cm2 VÀ1sÀ1
for the holes.
Figure 4. (A) Electronic absorption spectra of 2D-NiPc-BTDA COFs
(red curve) and [MeO]8PcNi (black curve). (B) Transient conductiv-
ities irradiated with a 355-nm pulsed laser at the photon density of 1.6 Â
1015 under Ar (red curve), SF6 (blue curve), and after annealing under
SF6 for 1 h (orange curve) and 24 h (green curve) or under O2 for 24 h
(black curve). (C) Number of charge carriers measured by the time-of-
flight transient current integration at different bias voltages, irradiated
with a 355-nm pulsed laser of 5.0 Â 1014 photons cmÀ2. (D) IÀV curves
of 2D-NiPc-BTDA COFs in the dark (black curve) and upon irradiation
(red curve) with visible light. (E) OnÀoff switching of photocurrent of
2D-NiPc-BTDA COFs at the bias voltage of 1.0 V, by switching the light
on and off. (F) Wavelength-dependent onÀoff switching of photocur-
rent at the bias voltage of 1.0 V.
To determine the yield of photocarrier generation, the time-
of-flight transient conductivity was integrated at different bias
voltages at a photon density of 5.0 Â 1014 photons cmÀ2
(Figure 4C). The number of charge carriers estimated by extra-
polation of the bias at 0 V was 9.6 Â 1010, leading to Φ = 9.2 Â
10À4. Thus, 2D-NiPc-BTDA COFs are electron-transporting
frameworks with outstanding mobility as high as 0.6 cm2 VÀ1 sÀ1
.
The AA stacking arrangement of 2D polymer sheets endows
2D-NiPc-BTDA COFs with efficient absorbance over a wide
range of wavelengths from the visible to the near-infrared region
up to 1000 nm and excellent electron-transporting property.
Along this line, we investigated the photoconductivity of 2D-
NiPc-BTDA COFs. Upon irradiation with visible light (>400 nm),
2D-NiPc-BTDA COFs exhibited an enhanced photocurrent from
250 nA to 15 μA (Figure 4D). The photocurrent can be switched
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onÀoff many times without deterioration (Figure 4E). To probe
the wavelength-dependent response, we utilized band-path filters
for selective excitation of 2D-NiPc-BTDA COFs at different
wavelengths. Interestingly, 2D-NiPc-BTDA COFs were panchro-
matic responsive and exhibited extreme sensitivity to near-infrared
photons (Figure 4F). Photoconductive COFs with vertically
aligned π columns are interesting for the development of organic
electronics. 2D-NiPc-BTDA-COF is unique because it is an
n-channel semiconductor combining broad absorbance up to
1000 nm, panchromatic photoconductivity, and high IR sensitiv-
ity. Such an n-type semiconductor is unprecedented.
In summary, we have developed a strategy for the synthesis of
the first example of n-type semiconducting 2D COFs using
electron-withdrawing blocks. 2D-NiPc-BTDA COFs adopt AA-
type stacking arrangement and exhibit enhanced absorbance over
a wide range of wavelengths up to 1000 nm; they exhibit
panchromatic photoconductivity and are highly sensitive to near-
infrared lights. Owing to the AA-type stacking, 2D-NiPc-BTDA
COFs provide pathways for carrier transport. As a result, the
n-channel 2D-NiPc-BTDA COFs transport electrons with high
carrier mobility. These characteristics clearly originate from the
structural features of the 2D COFs and are hard to achieve with
other n-type semiconductors; thus, this constitutes a new aspect
in the design of π-electronic fucntions of COFs.
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’ ASSOCIATED CONTENT
S
Supporting Information. Complete ref 8, detailed experi-
b
mentalprocedures, FTIRspectra, and simulations. Thismaterialis
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
seki@chem.eng.u-osaka.ac.jp; jiang@ims.ac.jp
Author Contributions
zThese authors contributed equally.
’ ACKNOWLEDGMENT
We are grateful for the financial support of PRESTO, JST. O.S.
thanks the National Research University Project of CHE and the
Ratchadaphiseksomphot Endowment Fund (AM1078I) for fi-
nancial support and a postdoctoral fellowship.
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