Nonlinear Optical Switching in Regioregular Porphyrin Covalent Organic Frameworks

Article abstract of DOI:10.1002/anie.201814412

Covalent organic frameworks (COFs) have garnered immense scientific interest among porous materials because of their structural tunability and diverse properties. However, the response of such materials toward laser-induced nonlinear optical (NLO) applications is hardly understood and demands prompt attention. Three novel regioregular porphyrin (Por)-based porous COFs—Por-COF-HH and its dual metalated congeners Por-COF-ZnCu and Por-COF-ZnNi—have been prepared and present excellent NLO properties. Notably, intensity-dependent NLO switching behavior was observed for these Por-COFs, which is highly desirable for optical switching and optical limiting devices. Moreover, the efficient π-conjugation and charge-transfer transition in ZnCu-Por-COF enabled a high nonlinear absorption coefficient (β=4470 cm/GW) and figure of merit (FOM=σ₁/σ_o, 3565) value compared to other state-of-the-art materials, including molecular porphyrins (β≈100–400 cm/GW), metal–organic frameworks (MOFs; β≈0.3–0.5 cm/GW), and graphene (β=900 cm/GW).

Full text of DOI:10.1002/anie.201814412

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
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Accepted Article
Title: Nonlinear Optical Switching in Regioregular Porphyrin Covalent
Organic Frameworks
Authors: Bishnu P. Biswal, Sreeramulu Valligatla, Nabil A. Saad,
M. Bala Murali Krishna Mariserla, Mingchao Wang, Naisa
Chandrasekhar, Daniel Becker, Matthew Addicoat, Irena
Senkovska, Reinhard Berger, D. Narayana Rao, Stefan
Kaskel, Xinliang Feng, and Tanmay Banerjee
This manuscript has been accepted after peer review and appears as an
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814412
Angew. Chem. 10.1002/ange.201814412
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Nonlinear Optical Switching in Regioregular Porphyrin Covalent
Organic Frameworks**
Bishnu P. Biswal,*Sreeramulu Valligatla, Mingchao Wang, Tanmay Banerjee, Nabil A. Saad, M. Bala
Murali Krishna Mariserla, Naisa Chandrasekhar, Daniel Becker, Matthew Addicoat, Irena Senkovska,
Reinhard Berger, D. Narayana Rao, Stefan Kaskel, Xinliang Feng*
Abstract: Covalent organic frameworks (COFs) have aroused
immense scientific interest as an exhilarating class of porous
materials due to their structure tunability and diverse properties.
However, understanding of their response towards laser induced
nonlinear optical (NLO) applications is in its infancy and demands
prompt attention. Herein, we report three novel regioregular porphyrin
based porous COFs, Por-COF-HH and its dual metalated congeners
storage, display technologies, sensors and in bio-medical
devices.[ In this regard, molecular porphyrins have been widely
studied due to their versatile optical and electrochemical
properties, large and fast NLO responses, possibility of
incorporating a wide range of metals and their good thermo-
chemical stabilities for optical limiting and optical switching
applications.[2] Nonetheless, to enhance and tune optical
nonlinearities of singular porphyrin moieties, bathochromic
shifting of their absorption bands by large π-electron
delocalization are crucial, which can be possible through de novo
design of integrated porphyrin units with extended π-
conjugations.[3][4] Along this line, we propose, porphyrin-linked
covalent organic framework (COF)[ as model system, wherein
enhancement and switching of the NLO response can be studied
by manipulation of the framework. Among the recognized
crystalline materials, COFs belongs to most recent class, known
for being mechanically robust and offering high accessible
surface area. By virtue of their freedom to be structurally and
electronically tuned, so far COFs have engrossed particular
1]
(
Por-COF-ZnCu and Por-COF-ZnNi) with excellent NLO properties.
Notably, intensity dependent NLO switching behavior was observed
for these Por-COFs, which is highly desirable for optical switching and
optical limiting devices. Moreover, the efficient π-conjugation and
charge transfer transition in ZnCu-Por-COF enable a high nonlinear
absorption coefficient (β=4470 cm/GW) and figure of merit (FOM =
5]
σ
1
/σo, 3565) values compared to other state-of-art materials including
molecular porphyrins (β=~100-400 cm/GW), metal-organic
frameworks (MOFs; β=~0.3-0.5 cm/GW) and graphene (β=900
cm/GW).
Molecules/materials with inherent nonlinear optical (NLO)
properties have profound importance in telecommunication, data
interest in research areas like adsorption/storage,[6] chemical
sensors,[7] electronics[8] and catalysis.[9] Hitherto, despite having
potential, none of the COFs composed of porphyrin units have
been explored as NLO materials. For this purpose, a regioregular
ordering of multiple metal centres in a crystalline, 2D porphyrin
[*]
Dr. B. P. Biswal, M. Wang, Dr. N. Chandrasekhar, D. Becker, Dr. R.
Berger and Prof. Dr. X. Feng
Faculty of Chemistry and Food Chemistry, Center for Advancing
Electronics Dresden, Technische Universität Dresden, 01062,
Dresden, Germany.
framework, prefereably without a post-synthetic modification is
desirable, which has been a difficult task[10] and prospect to have
large demand for applications in optoelectronics and
photo/electrocatalysis.
In this work, we demonstrated a conjugated imine linked
porphyrin-homopolymeric COF (Por-COF-HH) by Schiff base
A4B4 condensation of 5,10,15,20-tetrakis(4-formylphenyl)-
Email: xinliang.feng@tu-dresden.de
bishnu_prasad.biswal@tu-dresden.de
Dr. S. Valligatla
Institute for Integrative Nanosciences, IFW- Dresden, 01069,
Dresden, Germany.
21H,23H-porphyrin
(TFPP)
and
5,10,15,20-tetrakis(4-
Dr. T. Banerjee
Max Planck Institute for Solid State Research, Heisenbergstraße 1,
aminophenyl)-21H,23H-porphyrin (TAPP) (Figure 1). Adopting the
same pathway, we also synthesized regioregular multi-
metalloporphyrin COFs with two different metal centres such as
Zn, Cu (Por-COF-ZnCu) and Zn, Ni (Por-COF-ZnNi) respectively
imbedded within a solitary framework. These porphyrin COFs
70569 Stuttgart, Germany.
N. A. Saad and Prof. Dr. D. N. Rao
School of Physics, University of Hyderabad, Hyderabad-500046,
India.
Dr. M. B. Murali Krishna Mariserla
School of Physical Sciences, Central University of Karnataka,
Kadaganchi – 585367, India.
(
Por-COFs) are highly crystalline and porous with a Brunauer–
2
-1
Emmett–Teller (BET) surface area up to ~1300 m g , which are
well comparable with the porphyrin COFs reported in the
literature.[5] Further, we have evaluated the NLO properties of
Dr. M. Addicoat
School of Science and Technology, Nottingham Trent University
Clifton Lane, NG11 8NS Nottingham (UK).
these Por-COFs by the open aperture Z-scan technique at 532
nm excitation.[
1c,11,13]
Interestingly, on one hand, these Por-COFs
Dr. I. Senkovska and Prof. Dr. S. Kaskel
Chair of Inorganic Chemistry, Technische Universität Dresden,
shows optical switching behaviour from saturable absorption (SA)
to reverse saturable absorption (RSA) along increase in the input
laser intensity and on the other hand metallated Por-COFs
display high nonlinear absorption coefficient values (β = ~4500
cm/GW), which could play potential role in photonics and optical
limiting devices.
01062, Dresden, Germany.
[**]
This project has received funding from the European Union’s
Horizon 2020 research and innovation programme under grant
agreement No 696656 (Graphene Flagship) and was supported
financially by the Center for Advancing Electronics Dresden (cfaed),
the European Social Fund, and the Federal State of Saxony (ESF-
Project “GRAPHD”, TU Dresden). We thank Ms. Viola Duppel for
SEM and TEM images, Ms. Kerstin Gottschling for TGA, Ms. Marie-
Luise Schreiber for elemental analysis, Prof. Ulrich Starke and Dr.
Kathrin Müller for XPS data, Dr. Xiaodong Zhuang and Dr. Renhao
Dong for helpful discussions.
The Por-COF-HH was synthesized using an optimized
solvothermal
protocol;
in
brief,
5,10,15,20-tetrakis(4-
aminophenyl)-porphyrin (TAPP) and 5,10,15,20-tetrakis(4-
formylphenyl)-porphyrin (TFPP) are reacted in the presence of
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the corresponding author.
1
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Figure 1. a) Schematic representations of the synthesis of Por-COFs (Por-COF-HH, Por-COF-ZnCu and Por-COF-ZnNi; b) Experimental (red) compared with
Powder refined (blue) PXRD profiles of Por-COF-HH with eclipsed (AA) arrangement; difference plot is given in black and observed reflections in green); c) The
packing models of corresponding Por-COFs indicating the pore aperture and π-π stacking distances (Zn: orange; Cu: green and Ni: purple).
aqueous acetic acid (6M) using o-dichlorobenzene and n-butanol
1:1 by vol.) as the solvent combination in a Pyrex tube at 120 ºC
(Figures S11-13) possibly correspond to the uncondensed
functional groups at the edges/termini of the COF crystallites. The
13C cross-polarization magic-angle spinning (CP-MAS) NMR
spectroscopy of Por-COFs confirms the chemical structure of the
backbone by showing the characteristic chemical shift of the —
C=N carbon at ~160 ppm, along with characteristic chemical
shifts attributed to the porphyrin units. The full assignment is
presented in Figure 2a and Figure S14. Transmission electron
microscopy (TEM) images revealed significant crystallinity of the
COF samples with well-defined, square shaped crystallites with
average sizes between 50-100 nm (Figure 2b-d). Tetragonal
pores of ~1.5 nm were clearly visible which is close to the
(
for 3 days. The same protocol was employed to synthesize Por-
COF-ZnCu and Por-COF-ZnNi using A4 and B4, metallo-
porphyrins as detailed in Section S2, Supporting Information. The
synthesized Por-COFs were first examined via powder X-ray
diffraction (PXRD) to investigate the crystallinity and phase purity.
As revealed from PXRD, all the Por-COFs (Por-COF-HH, Por-
COF-ZnCu and Por-COF-ZnNi) show good crystallinity with the
first and most intense peak at
a low 2θ angle of ∼4.7°
corresponding to the (100) reflection planes, along with other
minor peaks (Figure 1b). Based on the initial experimental PXRD
patterns, 2D COF models were built and optimized using the Self-
Consistent Charge Density Functional Tight-Binding (SCC-DFTB)
method.[12a] The experimental PXRD patterns match fairly well
with the simulated ones derived from the eclipsed (AA) stacking
models (Figure 1b and Figure S6-8). The centre-to-centre
distance (pore aperture) is ~1.4 nm and the π–π stacking
distances between the individual layers were calculated as ∼4 Å
for all three Por-COFs (Figure 1c). The latice parameters of Por-
COF-HH were calculated to be a = 27.32 Å, b = 26.60 Å, c = 7.96
Å, α = 88.51º, β = 89.83º and γ = 89.76º using the Pawley
refinement on Reflex module of BIOVIA Materials Studio 2017
calculated pore size. SAED analysis shows
a prominent
diffraction ring at ~1.8 nm corresponding to the (100) reflection for
all three Por-COFs. A similar morphological feature was also
observed in scanning electron microscopy (SEM) images,
wherein cubic shape crystals of Por-COFs can be clearly
visualized (Figure S16). Interestingly, for Por-COF-HH and Por-
COF-ZnNi the crystallite sizes are close to 100 nm, while for Por-
COF-ZnCu, the crystallite sizes are ~50 nm. Moreover, Energy
Dispersive X-Ray Analysis (EDX) confirms the presence of two
different metals within the single framework of Por-COF-ZnCu
and Por-COF-ZnNi (Figure S17-S19). We have also verified the
individual metal content in the Por-COF-ZnNi and –ZnCu
frameworks using induced coupled plasma (ICP) analysis
(Section S2). X-ray photoelectron spectral (XPS) analysis
validates our proposed Por-COF-ZnNi and -ZnCu structures
wherein metal atoms have been successfully integrated in the
COF backbone (Figure S21). Thermogravimetric analysis (TGA)
(
Figure 1b). The refinement details for Por-COF-ZnCu and Por-
COF-ZnNi are presented as Figures S9 and S10.
The formation of the Por-COFs by the Schiff-base
condensation reaction was evidenced by the almost complete
-
1
disappearance of the –N-H stretching bands (3100–3400 cm ) of
-
1
TAPP, Zn-TAPP; and the -C=O stretching bands (~1695 cm ) of
TFPP, Cu-TFPP and Ni-TFPP; and appearance of new
a
2
was performed for all three Por-COFs under both N and air, to
study the thermal stability and to estimate roughly the total metal
content (Figure S22 and S23).
-
1
characteristic -C=N stretching band (~1625 cm ) in the Fourier-
transform infrared (FT-IR) spectrum of Por-COFs (Figures S11-
S13, ESI). The residual -N-H and -C=O stretching signals
2
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Figure 2. a) Representive 13C cross-polarization magic-angle spinning (CP-MAS) solid state NMR spectra of Por-COF-HH (*corresponds to peripheral –CHO
groups); b, c and d) TEM images of Por-COF-HH, Por-COF-ZnNi and Por-COF-ZnCu respectively (The main scale bars correspond to 60 nm in all images and
the scale bar in the insets correspond to 1/0.37 nm); e) N physisorption isotherms at 77K; Photophysical properties of Por-COFs: f) UV-Vis spectra (measured in
2
the integration sphere in dispersion state in 2-propanol) and g) Photoluminescence (PL) spectral profiles.
Figure 3. Open aperture Z-scan traces for Por-COFs: a) Por-COF-HH; b) Por-COF-ZnCu; and c) Por-COF-ZnNi at the excitation of 532 nm, ns laser at the input
intensity of 0.25 GW/cm2 (represented by circles) and 0.01 GW/cm2 (represented by hexagons). The solid lines represent the theoretical fitting, spheres and
hexagons are the experimental data
Permanent porosity of these Por-COFs was evaluated by
employing N adsorption-desorption experiments at 77K. All
three Por-COFs show similar isotherm shape with a steep
increase in adsorption amount below 0.1 bar, characteristic for
the filling of the micropores (Figures 2e). The surface areas of
2
calculated from N adsorption data using Quenched Solid State
2
Density Functional Theory (QSDFT) model and cylindrical-slit
adsorption kernel for carbon show the maxima centered
between 1.0 and 1.3 nm. It corresponds well to the values
obtained by Poreblazer (1.2 nm for pore limiting diameter, 1.3
nm for maximum pore size).
2
the Por-COFs derived from the N adsorption data using the
BET model are 1316 m2g-1 for Por-COF-HH, 1027 m2g-1 for Por-
COF-ZnCu and 551 m2g-1 for Por-COF-ZnNi, respectively. The
acsessible geometrical surface areas calculated by Poreblazer
To understand the optoelectronic properties of the
synthesized Por-COFs, we performed UV-Vis absorption in the
centre mount position of an integrating sphere in the dispersion
state (2-propanol) and photoluminescence measurements. The
photo-physical properties of Por-COF-HH drastically differ
compared to the corresponding metalated counterparts, Por-
COF-ZnCu and Por-COF-ZnNi (Figure 2f and 2g). The UV-Vis
spectral studies display an obvious shift in all four Q-band
components [500-700 nm: Qy(1,0), Qy(0,0), Qx (1,0), Qx (0,0)]
software[
12b]
amount to 1512 m g for Por-COF-HH, to 1457 m g
2
-1
2 -
1
for Por-COF-ZnCu and to 1479 m2g-1 for Por-COF-ZnNi. The
significantly lower surface area value measured for the Por-
COF-ZnNi is possibly due to its moderate crystallinity in
comparison to others. The pore size distribution plots of Por-
COF-HH, Por-COF-ZnNi and Por-COF-ZnCu (Figure S24)
3
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of Por-COF-HH, in terms of their positions and intensity ratios
compared to its metalated counterparts. Whereas, in the case of
Soret B-band (B(0,0)) of Por-COF-HH associated to two
components [401 (shoulder) and 418 nm (dominant major peak)]
and the corresponding metalated ones (Por-COF-ZnCu and Por-
COF-ZnNi) exhibit only one peak (417 and 419 nm respectively)
cm/GW), MOFs (β=0.28-0.46 cm/GW), graphene (β=900
cm/GW), etc. (Table S2).[11,16-20] Noteworthy, the observed
switching behaviour from SA to RSA is highly interesting to be
considered for optical switching applications.[21] Moreover, the
switch over from RSA to SA behaviour can be applicable in
optical limiting devices based on RSA behaviour at higher
intensities and mode locked lasers as it shows SA behaviour at
low input intensities.
In summary, three regioregular, crystalline COFs
composed of entirely porphyrin units employing Schiff base
condensation of A4B4 porphyrin aldehydes and amines have
been synthesized. By de novo design, we could systematically
embed two different metals (ZnCu and ZnNi) within a crystalline
organic framework. Moreover, to prove the concept and to draw
the benefits from infinite conjugated porphyrin network, the NLO
properties of these Por-COFs were investigated under 532 nm
excitation wavelength using Z-scan technique. Interestingly, rare
optical switching behavior from SA to RSA with input intensity
was observed for these Por-COFs, while the metalated
counterparts (Por-COF-ZnNi and –ZnCu) display high nonlinear
absorption coefficient values (β = ~4500 cm/GW), surpassing
many state-of-art materials. In brief, we established a structure-
property relationship in A4B4 COF. However, there is much
opportunity to be explored at the molecular level that will
undoubtedly promote COFs as next generation nonlinear optical
switching and optical limiting materials.
(
Figure 2f). Moreover, the broadening of the Soret B-band was
noticed in the metalated Por-COFs compared to the metal-free
Por-COF. These minor changes can be attributed to the
metalation of the porphyrin network and associated structural
changes.[ In addition, we also observed photoluminescence
13]
(
PL) quenching for both Por-COF-ZnCu and Por-COF-ZnNi,
whereas the metal-free Por-COF-HH shows reasonable
emission at 650 nm (Figure 2g). The estimated excited-state
lifetimes are τavg = 9.17 ns, 1.77 ns and 1.73 ns for Por-COF-HH,
COF-ZnCu and Por-COF-ZnNi respectively measured in the
dispersion state (2-propanol) employing time-correlated single-
photon counting (TCSPC) method (Figure S25).
The NLO properties of Por-COFs were investigated by the
open aperture Z-scan technique at the excitation wavelength of
532 nm from frequency doubled Nd: YAG laser with different
input intensities. The Z-scan measurements with intensities
ranging from 0.01 to 0.25 GW/cm2 reveal that nonlinear
absorption mechanism is purely intensity dependant.
Interestingly, all the Por-COF samples exhibited NLO switching
behaviour as the nonlinear absorption swap from saturable
absorption (SA) to reverse saturable absorption (RSA) upon
increasing input intensity. Figure 3 represents the open aperture
Z-scan curves for a) HH Por-COF-HH, b) Por-COF- ZnCu, and
c) Por-COF-ZnNi at the peak intensities of 0.01 and 0.25
Keywords: covalent organic frameworks • microporous
materials • porphyrin • nonlinear optics • regioregular
2
2
GW/cm . Notably, at the lower intensity of 0.01 GW/cm all Por-
COFs show SA behavior. Remarkably, as the input intensity
increases, in Por-COF-HH, the nonlinear absorption behavior
switches from RSA to SA and then again back to RSA. This
RSA-SA-RSA switching behavior can be attributed to the
dominant excited state absorption at lower intensities (far from
focus) and saturation of excited states at higher intensity (near
the focus). Whereas, in the case of Por-COF-ZnCu and Por-
COF-ZnNi, excited state absorption leads to characteristic RSA
behavior. To explain these results, we employed theoretical
modelling based on five level rate equations as explained in the
literature for metal-porphyrins (Section S5).[14] In Figure 3,
redlines are the best theoretical fits to the experimental data at
different input intensities. From the fit, we deduced the nonlinear
absorption coefficient (β), ground state absorption cross section
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(
σ
o
1
), first excited state (σ ) and second excited state absorption
cross sections (σ ) for Por-COF-HH, Por-COF-ZnCu and Por-
2
[
3]
a) Y.-L. Liu, Z.-B. Liu, J.-G. Tian, Y. Zhu, J.-Y. Zheng, Optics Commun.,
COF-ZnNi (Table S1, Supporting Information). Higher β value is
observed for metallated porphyrin frameworks (Por-COF-ZnCu
and Por-COF-ZnNi) than that of Por-COF-HH (Table S1), which
is expected and attributed to the planarity of the metal-porphyrin
cores, effective inter-layer stacking, efficient π-conjugation and
successful charge transfer transition between the metal ion’s d-
orbital and the porphyrin π-orbital.[14,15] The progressive
increment of β value is in the following order: Por-COF-ZnCu >
Por-COF-ZnNi > Por-COF-HH. The detailed intensity dependent
open aperture z-scan data for all the three samples are
presented in the Supporting Information (Figure S26-28, Section
S5). Moreover, the obtained NLO parameters, β and the figure
2
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of merit (FOM = σ /σ ) values are as high as 4470 cm/GW for
[
a) X. Feng, L. Liu, Y. Honsho,A. Saeki, S. Seki, S. Irle, Y. Dong, A.
Por-COF-ZnCu and 3762 cm/GW, for Por-COF-ZnNi,
respectively, which are found to be higher than those of other
reported materials such as metalated porphyrins (β= 132-366
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4
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This article is protected by copyright. All rights reserved.

Angewandte Chemie International Edition
10.1002/anie.201814412
COMMUNICATION
COMMUNICATION
In this work, we demonstrated three
regioregular porphyrin based porous
COFs, Por-COF-HH and its dual
metalated congeners (Por-COF-ZnCu
and Por-COF-ZnNi) with excellent
NLO properties. Remarkably, the NLO
switching and high parameter values
were obtained for these Por-COFs,
which is highly demanding for optical
switching and optical limiting devices.
Bishnu P. Biswal,* Sreeramulu
Valligatla, Mingchao Wang, Tanmay
Banerjee, Nabil Momen, M. Bala Murali
Krishna Mariserla, Naisa
Chandrasekhar, Daniel Becker, Matthew
Addicoat, Irena Senkovska, Reinhard
Berger, D. Narayana Rao, Xinliang
Feng*
Page No. – Page No.
Nonlinear
Optical Switching
in
Regioregular
Porphyrin Covalent
Organic Frameworks
6
This article is protected by copyright. All rights reserved.