Photoconductivity in Metal–Organic Framework (MOF) Thin Films

Article abstract of DOI:10.1002/anie.201904475

Photoconductivity is a characteristic property of semi-conductors. Herein, we present a photo-conducting crystalline metal–organic framework (MOF) thin film with an on–off photocurrent ratio of two orders of magnitude. These oriented, surface-mounted MOF

Full text of DOI:10.1002/anie.201904475

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Accepted Article
Title: Photoconductivity in Metal-Organic Framework Thin Films
Authors: Xiaojing Liu, Mariana Kozlowska, Timur Okkali, Danny
Wagner, Tomohiro Higashino, Gerald Brenner-Weiß, Stefan
M. Marschner, Zhihua Fu, Qiang Zhang, Hiroshi Imahori,
Stefan Bräse, Wolfgang Wenzel, Christof Wöll, and Lars
Heinke
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201904475
Angew. Chem. 10.1002/ange.201904475
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Angewandte Chemie International Edition
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COMMUNICATION
Photoconductivity in Metal-Organic Framework Thin Films
[
a]
[b]
[a]
[c]
[d]
Xiaojing Liu, Mariana Kozlowska, Timur Okkali, Danny Wagner, Tomohiro Higashino, Gerald
[a]
[c]
[a]
[a]
[d]
Brenner-Weiß, Stefan M. Marschner, Zhihua Fu, Qiang Zhang, Hiroshi Imahori, Stefan
Bräse,[c,e] Wolfgang Wenzel, Christof Wöll, and Lars Heinke
[b]
[a]
[a],*
Abstract: Photoconductivity is a characteristic property of semi-
conductors, enabling various applications such as photoresponsive
transistors and light-sensing. Here, we present a photo-conducting
crystalline metal-organic framework, MOF, thin film with an on-off
photocurrent ratio of two orders of magnitude. These oriented,
surface-mounted MOF thin films, SURMOFs, contain porphyrin in the
framework backbone and C60 guests, loaded in the pores using a
layer-by-layer process. By comparison with results obtained for
reference MOF structures and based on density-functional theory
calculations, we conclude that donor-acceptor interactions between
the porphyrin of the host MOF and the C60 guests give rise to a rapid
charge separation. Subsequently, holes and electrons are transported
separation upon illumination.[10] An interesting band structure of
C
60 embedded in the regular MOF pores was theoretically
predicted.[ On the other hand, MOFs with porphyrin linker
molecules allow semi-conductor light-harvesting applications in
photovoltaic[12] devices and for photocatalysis.[13] These
applications take advantage of the fact that porphyrins are
excellent electron donors with delocalized п-systems.[14] In the
visible region, porphyrins exhibit sharp and intensive absorption
bands, ideal for application in light harvesting.[15] To this end,
11]
porphyrins are often integrated as active components in various
opto-electronic devices.[12,
16]
Furthermore, the porphyrin
properties can be tailored by various methods of organic
chemistry, e.g. by adding electron rich or electron poor groups,
allowing to tune photon absorption and other photophysical
through separate channels formed by porphyrin and by
60
C ,
respectively. The ability to tune the properties and energy levels of the
active molecules, porphyrin and fullerene, combined with the
controlled organization of donor-acceptor pairs in this regular
framework offers a huge potential to increase the photoconduction on-
off ratio in future work.
properties of the material. Combining porphyrin with fullerene
results in electron donor-acceptor pairs.[
17]
The light-induced
electron transfer in such dyads was investigated in solution;[
and it was demonstrated that materials of such molecules are
suitable for applications like organic solar cells.[18]
17]
Recently, remarkable photoconductance in MOFs incorporating
percolated titania nanoparticles has been reported.[19] Irradiation
Metal-organic frameworks, MOFs, are crystalline, nanoporous
hybrid materials composed of metal nodes connected by organic
linker molecules.[1] In recent years, in addition to applications in
gas loading and separation,[2] the electrical and electronic
properties of MOFs have started to attract substantial attention.[3]
In this context, taking advantage of the enormous MOF chemical
space, with the number of characterized members of this
material’s class approaching 100.000,[4] various MOF applications
with UV light of 266 nm wavelength (4.66 eV) resulted in a
pronounced increase of electron mobility as detected by terahertz
spectroscopy. Direct measurements of the photoinduced changes
in the electrical conductivity, however, were not presented, likely
due to problems in providing good electrical contacts to MOF
powders under light irradiation. Photoconductivity in MOFs with
functional organic moieties has not yet been reported, although
the virtually unlimited possibilities to tune the properties of such
materials are extremely attractive.
[
5]
have been investigated, ranging from electrocatalysis over field
effect transistor[ to energy-storage . It was found that the
6]
[7]
conductivity of the MOF material can be significantly increased by
loading with molecules like ferrocene,[3c] TCNQ,[3b] or
fullerene.[8]
C
60
C
60 fullerenes are attractive charge acceptors, since
the delocalized п-systems give rise to a large electron affinity and
strong stability.[9] In addition, they show efficient charge
[
a]
X. Liu, T. Okkali, Dr. G. Brenner-Weiß, Dr. Z. Fu, Q. Zhang, Prof. C.
Wöll, Dr. L. Heinke
Institute of Functional Interfaces (IFG)
Karlsruhe Institute of Technology (KIT)
76344 Eggenstein-Leopoldshafen (Germany)
E-mail: lars.heinke@kit.edu
[
b]
Dr. M. Kozlowska, Prof. W. Wenzel
Institute of Nanotechnology (INT)
KIT, 76344 Eggenstein-Leopoldshafen (Germany)
D. Wagner, S. M. Marschner, Prof. S. Bräse
Institute of Organic Chemistry (IOC)
KIT, 76131 Karlsruhe (Germany)
[
[
c]
d]
Dr. T. Higashino, Prof. H. Imahori
Department of Molecular Engineering, Graduate School of
Engineering, Kyoto University, Kyoto 615-8510 (Japan)
E-mail: imahori@scl.kyoto-u.ac.jp
[
e]
Prof. S. Bräse
Institute of Toxicology and Genetics (ITG),
KIT, 76344 Eggenstein-Leopoldshafen (Germany)
Supporting information for this article is given via a link at the end of
the document
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For the sample preparation, Zn(TPP) and
C60@Zn(TPP)
SURMOF thin films as well as the phenyl-based Cu(BPDC)
SURMOF films were synthesized directly on the functionalized
electrode in a step-by-step fashion by alternately spin-coating the
ethanolic metal acetate solution and the ethanolic linker molecule
solution on the substrate, Figure
1 (BPDC = biphenyl-
dicarboxylate). The C60 molecules were loaded in the MOF pores
during the synthesis by spin-coating the C60 solution on the
substrate after each linker step, see Figure 1.
The crystallinity of Zn(TPP) and Cu(BPDC) SURMOFs with and
without embedded C60 was monitored by X-ray diffraction (XRD),
Figures 2a, S1 and S6. The experimental XRD data match well
with the calculated diffractogram of the target structure. The
absence of the (110), (001), (101) and (111) diffraction peaks
reveals a high degree of orientation of the SURMOFs, i.e. the
films are grown predominantly along the (100) orientation (see
Figure S1). The XRD data recorded for C60@Zn(TPP) and
Zn(TPP) also reveal that embedding C60 did not affect the
crystallinity of Zn(TPP), i.e. diffraction peak positions did not
change and also the peak width was the same as before the
loading. Importantly, however, the form factors showed
substantial changes. The ratio of the (100) to (200) peak
intensities increases from 3.33 for Zn(TPP) to 6.25 for
C60@Zn(TPP). This change in form factor is a consequence of the
change in electron density, which increased substantially within
the pores by the loading with C60. Note that an exclusive
decoration of the outer MOF surface can be excluded from this
form factor change; such a change in relative diffraction peak
Figure 1. a) Sketch of the layer-by-layer SURMOF synthesis. The components
of C60@Zn(TPP) are shown. b) The structure of C60@Zn(TPP). O atoms are
pictured red, Zn dark grey, N blue, H white. For clarity, C in MOF scaffold is
shown in cyan, C of fullerene is grey.
intensities is only possible by affecting virtually every MOF
pore.[21]
UV-vis absorption spectra of C60@Zn(TPP) provide further
evidence of the successful loading with C60, Figure 2b. The
absorption bands at 436 nm, 561 nm and 600 nm belong to the
porphyrin chromophore, whereas the band at 330 nm originates
from the C60. From the relative intensity of the UV-vis absorption
bands, the ratio of C60 to porphyrin in each unit cell could be
determined and was found to amount to 0.92, indicating that
almost every unit cell of the porphyrin MOF contains a C60
molecule. For details, see Figure S2. This value was verified by
HPLC-mass spectrometry of the dissolved SURMOF sample,
which showed an average loading of 0.90 C60 molecules per unit
cell, Figure S3.
For directly measuring the photocurrent, as well as for device
applications, e.g. in light sensors, the MOF material must be
provided in the form of thin films. Thus, we have used surface-
mounted MOF thin films, SURMOFs,[20] grown on suitably
functionalized substrates, providing interdigitated bottom
contacts. This approach allows the measurement of electrical
properties of empty and loaded MOFs in a straightforward and
reproducible fashion. Here, several different MOF structures were
investigated: While SURMOFs fabricated from phenyl-based
linkers did not respond to light illumination, even after loading with
fullerenes, SURMOFs fabricated from different porphyrinic linkers
revealed superb photoconducting properties, likely a result of the
interesting photophysical properties of these compounds.[12]
While the electrical resistivity of these porphyrinic MOF thin films
is found to be rather high, the conductivity increased
tremendously when embedding C60 molecules in the nanopores
of these highly oriented SURMOFs, yielding C60@Zn(TPP) (TPP
=
5,15-bis-(3,4,5-trimethoxyphenyl)-
10,20-
bis-
(4-
carboxyphenyl) porphyrinato zinc(II), see Figure 1). Remarkably,
these MOF thin films showed pronounced photoconduction
features: the electrical conductivity increased by 2 orders of
magnitude when irradiated with visible light of 455 nm (2.72 eV).
Detailed theoretical investigations using density functional theory
Figure 2. X-ray diffractograms (a) and UV-vis absorption spectra (b) of Zn(TPP)
and C60@Zn(TPP) SURMOFs. The UV-vis spectrum of C60 solution in toluene
is also shown. The inset shows a magnification from 300 nm to 370 nm.
(
DFT) and time-dependent DFT calculations revealed the origin
The infrared spectra of Zn(TPP) and C60@Zn(TPP) are displayed
in Figure S4. The bands at ~1590 cm-1 and ~1410 cm-1 of both
MOFs are assigned to the carboxylate asymmetric and symmetric
of the MOF photoconductivity. First, light is absorbed by the
porphyrin moieties. Subsequently, the large electron affinity of the
C60 results in rapid charge separation.
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stretching modes. The band at 1425 cm-1 observed for
C60@Zn(TPP) appears after embedding of C60.
In order to further characterize the as-synthesized MOF films,
atomic force microscopy (AFM) was carried out to investigate the
morphology and film thickness. The images in Figure S5c and
S5d indicate that the Zn(TPP) and
C60@Zn(TPP) are
homogenous films with thicknesses of approximately 50 nm.
Such MOF films are sufficiently thin, allowing the entire
illumination of the sample, as also seen by the UV-vis spectrum
(
see Figure.2b) with a maximum absorbance of 0.6 absorption
units.
The electrical conductivities of the SURMOF samples deposited
on substrates with interdigitated gold electrodes were determined
by 2-probe DC conduction measurements. Cu(BPDC)-SURMOF-
2, a phenyl-based MOF structure, showed no significant increase
upon irradiation with various wavelengths of the visible spectra,
S7a. Cu(BPDC) with empty pores (thickness is 270 nm, see S5a)
shows a very low conductivity of approximately 2 × 10-13 S m-1
(
applied voltage was 2 V). The conductivity of the MOF thin film
increased by approximately 4 orders of magnitude upon loading
with C60, see S7b. However, the conductivity is still only slightly
affected by light irradiation, e.g. the current increases by less than
1
0 % when irradiated with light of 455 nm wavelength.
Figure 3. Photoconduction in C60@Zn(TPP). a) The DC current at a voltage of
2 V is measured while the sample is irradiated with light of 640 nm, 530 nm,
455 nm, 400 nm and 365 nm wavelength. The current without light irradiation is
0.11 nA. The photoconduction action spectrum is shown in Figure S11. b) The
A rather different photoresponse was observed for the porphyrinic
SURMOFs; although the lattice constants and pore sizes of
Zn(TPP)-SURMOF-2 and Cu(BPDC)-SURMOF-2 are very
similar, see the XRD data reproduced in Figures S1 and S6.
current-voltage-curve of the sample in the dark (black spheres) and under
irradiation with 455 nm (blue spheres). The log-plot of the data is present in
Figure S8. c) The photocurrent at different intensities of the 455 nm light
irradiation.
Zn(TPP) SURMOFs, without
C
60 embedment, shows
a
-11
-1
conductivity of approximately 1.5 × 10 S m (see S7c), which is
low but clearly higher than that of empty Cu(BPDC). In
pronounced contrast to the MOF thin films built with the phenyl-
based linkers, the irradiation with light result in a substantial
increase of the conductivity of the porphyrinic SURMOF.
Embedding fullerene in the porphyrin SURMOFs increases the
electrical conductivity. When applying 2 V to the C60@Zn(TPP)
sample, the current in the dark is approximately 0.11 nA,
corresponding to a conductivity of 1.5 × 10-9 S m . Irradiation with
light substantially increases the current, see Figure 3. The
observed change in photoconductivity strongly depends on
photon wavelength. The largest increase is obtained at 455 nm,
i.e. by exciting the porphyrin Soret band. There, a value of
approximately 9 nA is reached, corresponding to a conductivity
The current-voltage curve, Figure 3b, shows that, upon
illumination, the conductivity of C @Zn(TPP) increases over the
6
0
entire voltage range between -5 V and +5 V. In the dark, the
current increases with voltage more than linearly, roughly
exponentially. This is an indication that more conducting paths
become available with increasing voltage. Upon irradiation with
light, a different scenario is observed. For light of 455 nm, the
photocurrent is proportional to the voltage, revealing almost ideal
ohmic conduction behavior with a conductivity of 1.3 × 10-7 S m .
A linear correlation between photocurrent and the intensity of
incident light is observed (Figure 3c and Figure S9),
demonstrating that two-photon processes appear to be absent.
Repeated and long-time irradiation of the sample show the high
stability of the photoconduction phenomenon in the C @Zn(TPP)
-1
-1
increase upon illumination by 2 orders of magnitude.
6
0
This observation, and the comparison with the previous reference
experiments, indicate that the observed photoconductivity in
SURMOF, Figure S10. Noteworthy, the light irradiation increases
the conduction of this sample by approximately four orders of
magnitude.
C60@Zn(TPP) SURMOFs must be related to a cooperative effect
of the porphyrin moieties and the C60 guests.
In order to elucidate the mechanism of charge transport in
C60@Zn(TPP), a state-of-the-art quantum-chemical analysis has
been carried out. To this end, we calculated the electronic
coupling elements between the MOF linkers using a previously
developed fully ab initio Quantum Patch method[22] based on
molecular orbitals, estimated from density functional theory
(
DFT). For these calculations, the C60@Zn(TPP) structure,
optimized using periodic approach, as described in the
Supporting Information, was used. Typical for MOFs, as a result
of the charge localization and large distance between the
molecular units in the MOF, the electronic coupling elements in
the crystalline framework are generally small.[23] We find that the
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electronic coupling of the Zn(TPP) HOMO orbitals (6.20 meV) is
one order of magnitude larger than of other intermolecular pairs
in the MOF (Table S2). This suggests that hole transport is
provided within stacks of porphyrin linkers, which are localized
along the z axis, Figure S13.
interface in C60@Zn(TPP), Figure S13, enables intermolecular
charge separation (Figure 4c), where the recombination of the
generated electron-hole pair and the rate of electron back-transfer
decrease. This increases the number of mobile charge carriers
after the photoexcitation in
C60@Zn(TPP). Therefore, the
The electronic coupling of the porphyrin LUMO orbitals (0.27
meV) is approximately three times lower than for the LUMO
orbitals in the Zn(TPP)-C60 molecular pair (1meV), see Table S2,
indicating higher propensity for direct electron transfer (ET)
between porphyrin and C60.
photoconductance of C60@Zn(TPP) results from the combination
and mutual orientation of both, electron-donor porphyrin and
electron-acceptor C60: The Soret band of the electron-donor
porphyrin is optically excited by blue (455 nm) light and the
electron-acceptor C60 significantly improves the photoactivated
electron transfer from porphyrin due to the effective long-range
charge separation and the reduction of the charge recombination,
as known for C60.[27] The structure of C60@Zn(TPP) with high
density of donor-acceptor interfaces, together with the spatially
continuous network of interpenetrating donor and acceptor
domains, provides a mechanism not only for exciton formation
upon photoexcitation, but also an efficient charge separation and
transport of the generated charge carriers through the MOF
material, resulting in enhanced photoconduction properties of
Additionally, due to the donor-acceptor interactions in Zn(TPP)-
C60 and the strong electronegative character of C60 with the low
energy levels (Figure 4), there is a high rate for light-induced
electron transfer to C60, which permits carriers to move towards
the respective electrode through the C60 channels. Here, the
electronic coupling element of the LUMO orbitals is 0.33 meV, but
is prone to increase significantly when the fullerene intermolecular
distance decreases. As a result, the photo-induced charge
carriers in C60@Zn(TPP) flow in separated domains: holes within
porphyrin linkers and electrons within fullerene channels. A
similar phenomenon was reported for hexa-zirconium(IV) MOF
C60@Zn(TPP).
Among the advantages of using combinations of functional
molecules in the regular order of a MOF is that both active
components, porphyrin and fullerene, can be modified without
modifying the crystal structure. This is demonstrated by
60.[8] The same tendency was predicted
loaded with
C
theoretically[24] and proven experimentally[25] for porphyrin-
fullerene organic films.
constructing
containing fullerene guests (C60-COOH), built from a different
porphyrin linker (DAP [10,20-bis(4-carboxyphenyl)-5,15-
a
different SURMOF,
C60-COOH@Zn(DAP)
=
diazaporphyrinato]zinc(II), see S14). For this different MOF thin
film, the photoconductance properties are similar (S18). The
current increase upon blue light irradiation also amounts to about
2
C
orders of magnitude, however, the absolute current values in
60-COOH@Zn(DAP) are smaller than in 60@Zn(TPP).
C
Remarkably, as a result of the different electronic structures and
absorption spectra of the DAP porphyrin, the photoconduction
response to light of different wavelengths is slightly different. For
example, while the ratio of the photocurrent during irradiation with
455 nm compared to irradiation with 400 nm is 2.2 for
C60@Zn(TPP), it is 2.6 in C60-COOH@Zn(DAP).
In comparison to other thin film possessing photoactive porphyrin,
e.g. films of porphyrin-functionalized gold nanoparticles or
porphyrin-decorated on graphene sheets,[28] the C60@Zn(TPP)-
SURMOF has a significantly larger on-off photocurrent ratio. The
superior photoconduction properties of the SURMOF are
presumably caused by the regular, crystalline order, allowing a
high charge carrier mobility.[12] Similarly large photocurrent ratios
as in C @Zn(TPP) were achieved with ordered molecular
Figure 4: a) Visualized HOMO and LUMO orbitals of the Zn(TPP) linker (left)
and C60 (right) with the corresponding orbital energies. While the energies of the
Zn(TPP)-C60 complex are shown in black, the energies of the isolated Zn(TPP)
6
0
assemblies of various porphyrin derivatives in the form of
and
C60 are red. The labeling of orbitals, i.e. HOMO-7 and LUMO+3,
crystalline nanorods and nanowires,[
18a,
29]
however, the
corresponds to orbitals in the Zn(TPP)-C60 complex. b) and c) Electron density
difference upon the singlet-singlet excitation of isolated porphyrin (b) and
porphyrin in Zn(TPP)-C60 complex (c). The electron accepting and donating
regions are labelled in red and blue, respectively. The electron transition from
TPP to C60 is clearly visible. Electronic properties were calculated using B3LYP
functional with def2-SV(P) basis set and Grimme D3 dispersion correction using
crystalline assembly of such materials in the form of thin films with
controlled thickness has not yet been demonstrated. The
crystalline assembly also allows for the precise structure
determination, enabling a thorough theoretical analysis of the
[
26]
Turbomole 7.1 (see SI).
charge transport with
a
reliable identification of basic
mechanisms. In addition, the oriented SURMOF structure results
in efficient charge transfer in the direction of the closely packed
Combination of Zn(TPP)-SURMOF with C60 results in more
efficient photoactivated ET and exciton separation. This is
depicted in Figure 4b-c, which shows the electron density
difference after the porphyrin photoexcitation. The donor-acceptor
C60 and porphyrin molecules, which are forming charge transfer
channels parallel to the surface, connecting the electrodes. These
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Angewandte Chemie International Edition
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COMMUNICATION
channels resemble ideal nanostructured donor-acceptor hole and
electron transporting highways for photocurrent generation.
In conclusion, crystalline, oriented MOF thin films with porphyrinic
("Deutsche Forschungsgemeinschaft"). L.H. acknowledges
funding by the Volkswagen foundation and the DFG (HE7036/5),
and C.W. by the DFG through SPP COORNET. This work was
also supported by the JSPS (KAKENHI Grant Numbers
JP18K14198 (T.H.) and JP18H03898 (H.I.)). The authors thank
Frank Kirschhöfer and Michael Nusser (IFG, KIT) for the help with
the mass spectrometry.
linkers and
C60 embedded in the pores were prepared.
Photoconduction behavior under irradiation with blue light,
exciting the porphyrin Soret band, was found to increase the
electrical conductivity of the SURMOFs by 2 orders of magnitude.
The photoconductance is a result of both, the photosensitive
porphyrin acting as the electron donor and the C60 acting as the
electron acceptor, in addition to the designed MOF structure,
which enables effective electronic coupling within the donor and
acceptor phases. Due to the efficient exciton separation and
transport of the generated electron-hole pairs within the spatially
continuous network of donor and acceptor domains, hole and
electron transport is provided through the close-packed Zn(TPP)
MOF linkers and C60 channels, respectively.
Based on the virtually unlimited possibilities to tune the properties
by appropriate molecular functionalizations of C60 as an electron
acceptor and porphyrin as an electron donor as well as to tune
the absorption properties and absorption wavelength, the MOF
photoconductivity properties can be varied and adopted.
Keywords: photoconduction • metal-organic frameworks •
porphyrin • C60 fullerene • density functional theory
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COMMUNICATION
A photo-conducting crystalline metal-
organic framework, MOF, thin film with
high on-off photocurrent ratio is
Xiaojing Liu, Mariana Kozlowska, Timur
Okkali, Danny Wagner, Tomohiro
Higashino, Gerald Brenner-Weiß, Stefan
Marschner, Zhihua Fu, Qiang Zhang,
Hiroshi Imahori, Stefan Bräse, Wolfgang
Wenzel, Christof Wöll and Lars Heinke
presented. The donor-acceptor
interactions between the porphyrin in
the framework backbone of these
oriented MOF thin films and C60
guests in the pores give rise to a rapid
charge separation. Subsequently,
holes and electrons are transported
through separate channels formed by
porphyrin and by C60, respectively.
Page No. - Page No.
Photoconductivity in Metal-Organic
Framework Thin Films
7
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