A thiazolo[5,4-: D] thiazole-bridged porphyrin organic framework as a promising nonlinear optical material

Article abstract of DOI:10.1039/c9cc05415d

Porphyrin-based porous organic frameworks are an important group of materials gaining interest due to their structural diversity and distinct opto-electronic properties. However, these materials are seldom explored for nonlinear optical (NLO) applications

Full text of DOI:10.1039/c9cc05415d

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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
towards bifunctional catalysts of tunable basicity
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Thiazolo[5,4-d]thiazole-Bridged Porphyrin Organic Framework as
Promising Nonlinear Optical Material
Received 00th July 2019,
Accepted 00th xxxx 2019
Mahalaxmi Samal,a Sreeramulu Valligatla,b Nabil A. Saad,c M. Veeramohan Rao,d D. Narayana Rao,c
Rojalin Sahu*,a and Bishnu P. Biswal*,e,f
DOI: 10.1039/x0xx00000x
www.rsc.org/
Porphyrin-based porous organic frameworks are an important group of applications like NLO.3,4 There are few reports on linear
materials gaining interest due to their structural diversity and distinct conjugated porphyrin oligomers, which have shown excellent
opto-electronic properties. However, these materials are seldom explored NLO response. However, arduous multistep synthesis appears
for nonlinear optical (NLO) applications. In this work, we investigate a to be the bottleneck toward its development.5
thiazolo[5,4-d]thiazole-bridged porous, porphyrin framework (Por-TzTz-
Conjugated, porous organic frameworks comprising of
POF) with promising NLO properties. The planar TzTz moiety coupled with porphyrin units (POFs)6 are of high interest due to their
integrated porphyrin units enables efficient π-conjugation and charge scalable protocols, in addition to excellent chemical stability,
distribution in Por-TzTz-POF resulting in a high nonlinear absorption diverse functionality, and unique opto-electronic properties.
coefficient (β = 1100 cm/GW) with figure of merit (FoM), σ1/σ0 = 5571, in Despite this, few have reported successful introduction of
contrast to analogous molecules and material counterparts e.g. metal- heterocyclic units into porous networks.7 Introduction of fused
organic frameworks (MOFs; β = ~0.3-0.5 cm/GW), molecular porphyrins (β (bi)heterocyclic units e.g. thiazolo[5,4-d]thiazole (TzTz) into a
= ~100-400 cm/GW), graphene (β = 900 cm/GW), and covalent organic porous polymeric network is interesting because of its
frameworks (Por-COF-HH; β = 1040 cm/GW and FoM = 3534).
semiconductor characteristics associated to the electron
deficient nature, high oxidative stability, and a rigid planar
structure.8 Notably, TzTz moieties feature an excellent
photoabsorbing ability9 and strong electron acceptor
characteristics; when linked with porphyrin units, they are
expected to be highly useful in photo(electro)catalysis and
opto-electronics applications.10 Until recently, none of the
POFs or COFs composed of porphyrin units have been explored
as a NLO material. We recently reported three regioregular
crystalline Por-COFs11 with outstanding NLO activity, and that
inspired us to further investigate and understand analogous
porous materials for advanced NLO applications.
Herein, we present a new conjugated, porous TzTz-linked
porphyrin organic framework (Por-TzTz-POF) by single step
condensation of 5,10,15,20-tetrakis(4-formylphenyl)-21H,23H-
porphyrin (TFPP) and dithiooximide (DTO) (Figure 1). The Por-
TzTz-POF is porous with a Brunauer–Emmett–Teller (BET)
surface area of 740 m2g-1, which is higher than most of the
TzTz-linked frameworks reported so far in the literature.12 We
also examined the NLO activity of this Por-TzTz-POF by
nanosecond laser open aperture Z-scan technique at 532 nm
excitation.13 Interestingly, Por-TzTz-POF shows optical
switching behavior from saturable absorption (SA) to reverse
saturable absorption (RSA) with increased input laser intensity,
and a promising nonlinear absorption coefficient value (β =
~1100 cm/GW; σ1/σ0 (FoM) = 5571) compared to our
previously reported metal-free imine-linked porphyrin-COF
In the realm of nonlinear optics (NLO), molecular porphyrins
have unique prominence due to their outstanding opto-
electronic properties, large and fast NLO responses, possibility
of engineering with metals, and excellent thermo-chemical
stabilities, making them suitable for applications in
telecommunication, data storage, sensors, and display
technologies.1 However, there are still enough opportunities to
fine-tune the optical nonlinearities of singular molecular
porphyrins, which can be achieved by de novo design of
integrated porphyrin units with extended π-conjugations and
periodic incorporation of opto-electronically active
heterocyclic units.2 The large π-electron delocalization in
heterocycles would offer efficient light absorbability and thus
red shifting of their absorption bands along with in-plane facile
charge transport, which is crucial for opto-electronic
aDepartment of Chemistry, School of Applied Sciences, Kalinga Institute of Industrial
Technology (KIIT), Deemed to be University, Bhubaneswar-24, Odisha, India.
E-mail: rsahufch@kiit.ac.in; Tel: (+91)-9778127994; Fax: (+91)-674 2725113.
bInstitute for Integrative Nanosciences, IFW- Dresden, 01069, Dresden, Germany.
cSchool of Physics, University of Hyderabad, Hyderabad-500046, India.
dDepartment of Physics, Pondicherry University, Pondicherry - 606015, India.
eFaculty of Chemistry and Food Chemistry, Center for Advancing Electronics
Dresden, Technische Universitat Dresden, 01062, Dresden, Germany.
fPresent Address: Max-Planck-Institute for Solid State Research, Heisenbergstraße
1, 70569 Stuttgart, Germany. E-mail: b.biswal@fkf.mpg.de.
†Electronic Supplementary Information (ESI) available: [Detailed experimental and
characterizations. See DOI: 10.1039/x0xx00000x
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Figure 1. Schematic representations of the synthesis of Por-TzTz-POF and Por-C=N-COF (previously reported as Por-COF-HH),11 b) and c) Experimental (red) PXRD profiles of Por-
TzTz-POF and Por-C=N-COF (black pattern is the simulated one derived from the eclipsed model) respectively with molecular models (C: grey; N: blue; S: yellow and H: white).
(Por-COF-HH),11 β = 1040 cm/GW and σ1/σ0 (FoM) = 3534. This as well as higher chemical shifts for the carbons of the
result endows the prevalence of bi(heterocycle)-bridged (bi)heterocyclic TzTz ring,  = ~170 ppm (S-C=N) and 152 ppm
porphyrin frameworks for optical switching NLO applications.
The synthesis of Por-TzTz-POF was achieved using the
(C=C).
The thermal stability of Por-TzTz-POF was verified using
standard solvothermal protocol as described in Section S2, thermogravimetric analysis (TGA) under N2 atmosphere. Mass
Supporting Information. To carve out the structure-activity loss shows that about 7% of residual solvent molecules were
relationship in the porphyrin frameworks, and for comparison, trapped inside the Por-TzTz-POF framework, which were
we have synthesized
a reported crystalline, Schiff-base removed upon gradual heating to ~150 ºC. Overall, the
analogous, Por-C=N-COF by condensation of TFPP and framework was stable up to ~470 ºC. After decomposition, a
5,10,15,20-tetrakis(4-aminophenyl)-21H,23H-porphyrin (TAPP) weight loss of ~50% for Por-TzTz-POF was observed until the
(Figure 1a). The synthesized porphyrin frameworks were end of the scan (Figure S4). N2 adsorption/desorption analysis
examined via powder X-ray diffraction (PXRD) to investigate of Por-TzTz-POF displays a typical reversible type-II isotherm
their crystallinity and phase purity. From PXRD, it is confirmed with a rapid uptake at low relative pressures giving rise to the
that Por-TzTz-POF is amorphous in nature with a broad BET surface area of 740 m2 g-1 (Figure 2b). This value is
reflections centred at 18º 2θ (Figure 1b). However, the Por- comparatively higher than most of the TzTz-linked POFs
C=N-COF is crystalline with the most intense first peak at a low reported in the literature such as 604 m2g-1 (TzTz-POP-3), 491
2θ angle of ∼4.7° corresponding to (100) reflections, along m2g-1 (TzTz-POP-5), 299 m2g-1 (TzTz-POP-1) and 488 m2g-1
with other minor peaks (Figure 1c). To gain further insight into (TzTz-POP-2).12 The pore size distribution (PSD) was derived
the functional groups and the local bonding modes in the Por- from the N2 adsorption data for Por-TzTz-POF using nonlocal
TzTz-POF, we recorded the Fourier Transform Infrared (FT-IR) density functional theory (NLDFT), which suggests narrow
spectrum, from which, the newly formed heterocyclic C=C micropores of this polymeric frameworks with major pore
bond (1736 cm-1) and C=N bond (1594 cm-1) can be identified. diameter of 1.3 nm. This is slightly smaller than the pore
The near disappearance of C=O stretching vibration of the diameter value obtained (1.6 nm) from ideal structure model
carbonyl group (expected range 1693 cm-1) confirms the (Figure S5). The external morphologies of the Por-TzTz-POF
condensation of the formyl group of TFPP and DTO to form the was visualized by scanning electron microscopy (SEM), which
(bi)heterocyclic TzTz linkages. Further, the FT-IR spectrum revealed that small particles (~30-50 nm), with cubic
shows minor bands correspond to the —C=N and —C-S bonds morphology, for large agglomerations (Figure S6a,b). Similar
of the TzTz ring (Figure S3, Supporting Information). The morphology was also observed in transmission electron
framework structure and composition of Por-TzTz-POF was microscopy (TEM) images (Figure 2d and Figure S6c,d). It is
confirmed by 13C cross-polarization magic-angle spinning (CP- worth mentioning that the particle size and shape of any
MAS) NMR spectroscopy (Figure 2a). All distinct signals can be organic porous materials synthesized via solvothermal method
assigned to the respective carbon atoms of the porphyrin are hard to predict and they drastically vary due to a range of
framework. The spectrum of Por-TzTz-POF shows the presence interactions such as π-π stacking, van der Waal’s interaction
of aromatic groups (chemical shift region of ca. 110-150 ppm) etc.
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Figure 2. a) 13C cross-polarization magic-angle spinning (CP-MAS) solid state NMR spectra of Por-TzTz-POF; b) N2 physisorption isotherms collected at 77 K; c) UV-vis and
photoluminescence spectrum (measured in the integration sphere in dispersion state in 2-propanol); d) TEM images of Por-TzTz-POF (scale bar = 50 nm); and e) Time-correlated
single-photon counting experiment for Por-TzTz-POF measured in 2-propanol. Sample was excited with a λex = 380 nm laser and emission was measured at λem = 655 nm.
The optoelectronic properties such as UV-vis absorption RSA behaviour as depicted in Figure 3. However, in case of Por-
and photoluminescence (PL) measurements of Por-TzTz-POF C=N-COF, at the input intensity of 0.25 GW/cm2, an optical
were accessed using the center mount position of an switching behavior switch from RSA to SA is observed and that
integrating sphere in the dispersion state (2-propanol). The attributes to the dominant excited state absorption at lower
UV-vis spectral studies display all four Q-band components intensities (far-focus) and saturation of excited states at higher
[500-700 nm: Qy(1,0), Qy(0,0), Qx (1,0), Qx (0,0)] for Por-TzTz- intensity (near-focus). To further support this hypothesis, the
POF (Figure 2c). The Soret B-band (B(0,0)) is associated to a known theoretical model was applied with five level rate
single component of 418 nm (dominant major peak) similar to equations (Section S5).14 It has been found that the
the reported Por-C=N-COF11. However, an additional shoulder experimental results are in good agreement with the
peak appears at 442 nm for Por-TzTz-POF in contrast to Por- theoretical fits at different input intensities (Figure 3). From
C=N-COF, which corresponds to the TzTz group. We observed the fitting results, the nonlinear absorption coefficient (β),
an emission at 655 nm in the PL analysis (Figure 2c), which is ground state absorption cross section (σo), first excited state
identical to the reported Por-C=N-COF compound. The (σ1) and second excited state absorption cross sections (σ2)
fluorescence decays can be fitted with (bi)exponential have been determined for both Por-TzTz-POF and Por-C=N-
functions (Figure 2e) and the amplitude weighted average COF. A slightly higher β value is observed for Por-TzTz-POF (β =
lifetime for Por-TzTz-POF is 7.5 ns [τ1 = 0.41 ns (12.2%) and τ2 = 1100 cm/GW) compared to Por-C=N-COF (β = 1040 cm/GW)
8.46 ns (87.8%)], which is lower than the Por-C=N-COF (τavg
=
(Table S1), and the value is much higher compared to many
9.17 ns).
other reported materials such as metalated porphyrins (β =
In the light of exciting opto-electronic features of Por-TzTz- 132-366 cm/GW), MOFs (β = 0.28-0.46 cm/GW) and graphene
POF, the NLO response was examined by the open aperture Z- (β = 900 cm/GW).11,14,15 We assume that the higher NLO
scan technique at the excitation wavelength of 532 nm from response of Por-TzTz-POF could be attributed to a combined
frequency doubled Nd: YAG laser with varying input effect of structural planarity of the TzTz-phenyl core (quasi-
intensities. A purely intensity dependent, nonlinear absorption planar: dihedral angle 6.9º), effective inter-layer stacking,
mechanism was observed from the Z-scan profiles at different efficient π-conjugation-electron transport and framework
input intensities (Figure 3a). For comparison, the NLO response disorderliness (loss of symmetry) compared to the imine
of Por-C=N-COF11 is also presented in Figure 3b. Interestingly, (C=N)-phenyl core (dihedral angle 62.8º) in the case of Por-
it has been found that both the porphyrin frameworks exhibit C=N-COF (Figure S9).12 Moreover, the obtained figure of merit
NLO switching behavior as the nonlinear absorption swap from (FoM = σ1/σo; 5571) for Por-TzTz-POF is also higher than that of
saturable absorption (SA) to reverse saturable absorption our recently reported porphyrin based COFs; Por-COF-HH
(RSA) with increasing input intensity. Figure 3 denotes the (FoM = 3534), Por-COF-ZnCu (FoM = 3565) and Por-COF-ZnNi
open aperture Z-scan curves for a) Por-TzTz-POF and b) Por- (FoM = 3762), tabulated in Table S2.11 Overall, these results
C=N-COF at the peak intensities of 0.04, 0.06 and 0.25 signify a structure-activity relationship in porphyrin based
GW/cm2. At the lower intensity of 0.04 GW/cm2, both porous organic materials that can be useful in optical switching
materials showed SA behavior. As the input intensity applications.16,17
increases, the excited state absorption leads to characteristic
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Figure 3. Open aperture Z-scan traces for a) Por-TzTz-POF and b) Por-C=N-COF11 at the
excitation of 532 nm, ns laser at different input intensities. The solid lines represent the
theoretical fitting, triangles, spheres and hexagons are the experimental data.
In summary, we have synthesized a conjugated TzTz-linked
porphyrin organic framework employing single-step direct
condensation of A4 porphyrin tetra-aldehyde and DTO.
Moreover, to prove the concept and to draw the benefits from
infinite conjugated porphyrin network, the NLO response of
Por-TzTz-POF have been investigated using Z-scan technique.
Next, the NLO activity of Por-TzTz-POF was compared with a
reported analogous crystalline Schiff base Por-COF (Por-C=N-
COF). Although a rare optical switching behavior from SA to
RSA with input intensity was observed for both materials, the
amorphous Por-TzTz-POF is found to display higher nonlinear
absorption coefficient values (β = ~1100 cm/GW and FoM =
5571). Through this work, we tried to establish a structure-
activity relationship in porphyrin containing organic
frameworks. However, there is still a huge opportunity to
further explore these materials at the molecular level, to help
develop them for next generation NLO applications.
9
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Conflicts of interest
The authors declare no competing financial interest.
Acknowledgements
B.P.B. acknowledges the Alexander von Humboldt foundation
for a research fellowship. We are thankful to Prof. Xinliang
Feng for the research support and helpful discussion. We
thank Prof. Ulrich Starke and Dr. Kathrin Müller for XPS
measurement.
Notes and references
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2
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TABLE OF CONTENT GRAPHIC
We report a porous Thiazolo[5,4-d]thiazole-bridged porphyrin organic framework, Por-TzTz-POF, with promising nonlinear optical
(NLO) activity.