Fluorescent molecular photoswitch based on basket-shaped porphyrins

Article abstract of DOI:10.1016/j.tetlet.2020.152104

Here we present two novel basket-shaped porphyrins bridged by photochromic diarylethene derivatives, which show significant photoswitchable fluorescent properties. Their synthesis, structures, photochromism and fluorescence characteristics were reported in detail. They are desirable for reversible light-driven fluorescent molecular switches.

Full text of DOI:10.1016/j.tetlet.2020.152104

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Fluorescent Molecular Photoswitch Based on Basket-shaped Porphyrins
Liying Yan, Chaochao Xiong, Piaopiao Jin, Qianfu Luo
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S0040-4039(20)30555-4
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TETL 152104
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Tetrahedron Letters
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Please cite this article as: Yan, L., Xiong, C., Jin, P., Luo, Q., Fluorescent Molecular Photoswitch Based on
Basket-shaped Porphyrins, Tetrahedron Letters (2020), doi: https://doi.org/10.1016/j.tetlet.2020.152104
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1
Tetrahedron Letters
journal homepage: www.elsevier.com
Fluorescent Molecular Photoswitch Based on Basket-shaped Porphyrins
Liying Yan, Chaochao Xiong, Piaopiao Jin and Qianfu Luo*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry and Molecular Engineering, East China University of Science &
Technology, Shanghai 200237, China.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Here we present two novel basket-shaped porphyrins bridged by photochromic diarylethene
derivatives, which show significant photoswitchable fluorescent properties. Their synthesis,
structures, photochromism and fluorescence characteristics were reported in detail. They are
desirable for reversible light-driven fluorescent molecular switches.
Available online
2020 Elsevier Ltd. All rights reserved.
keywords:
Photochromism
Porphyrin
Diarylethylene
Fluorescence
1. Introduction
Light-driven fluorescent molecular switches have currently attracted great interest in stimuli-responsive materials and molecular
opto-electronics [1-3]. Photochromic diarylethenes have their inherent advantages as photoswitchable molecules due to their high
photosensitivity, excellent fatigue resistance and outstanding thermal stability [4-7]. However, most diarylethene molecules have very
weak fluorescence with short emission wavelengths or even no fluorescence, which limited their applications in the longer wave range.
Therefore, the complementary designs of diarylethenes and various fluorescent molecules have been developed. Among them, the
connection of diarylethenes and porphyrins has attracted considerable attentions. It is well-known that porphyrins are one of the most
representative macrocyclic compounds with excellent light stability, thermal stability and photoelectric properties due to their highly
aromaticity [8-12]. They have been widely applied in light-induced electron transfer systems, molecular energy storage devices and
photoelectric conversion system based on the strong near-infrared fluorescence, large molar elimination coefficient and long lifetime of
excited singlet states [13-20]. In recent years, our group has carried out a series of studies on the preparation and characteristics of
functional porphyrins based on photochromic diarylethenes [21-23]. There are two main methods to combine diarylethenes with
porphyrin groups. On the one hand, porphyrins can be covalently bonded to diarylethylene molecules to form various linear or planar
functional molecules [24], on the other hand, diarylethylenes containing coordination groups can be used as ligands to coordinate with
the central metal ion of porphyrins to form coordination compounds, which can be further assembled into supramolecular compounds
[25]. The two approaches have many promising applications, but the problem of π-π stacking caused by the rigid planar structure of
porphyrins and the relatively weak coordination ability between ligands and metal ions need to be solved. Therefore, we further
developed binding porphyrins-dithienylethene system [26], in which diarylethylene was axially bonded to the porphyrin ring. This
design made the system performing excellent photochemical double-throw switches. However, once ions such as zinc ions were added,
it would no longer have photochromic properties, which limited its application as a fluorescent switch in the presence of ions. For this
reason, we have improved the molecular design in this paper by incorporating pyridine groups at the meso positions of the porphyrin
and two novel basket-shaped porphyrins strapped by diarylethene derivatives (DTE-1o and Zn-DTE-1o) for fluorescent molecular
photoswitches were designed and constructed (Scheme 1). Their structures have been confirmed by nuclear magnetic resonance
spectroscopy (1H NMR and 13C NMR), high resolution mass spectrometry and single crystal X-ray diffraction. The light-triggered
reversible fluorescence emission in the near infrared and photochromism of the diarylethene-porphyrin system was investigated in
detail.

2
Tetrahedron Letters
S
S
N
N
N
N
M
N
N
M= 2H,Zn
Scheme 1. The structure of basket-shaped porphyrins.
2. Results and discussion
2.1. Synthesis and Structure Characterization
The synthetic procedures of basket-shaped porphyrins are shown in Scheme 2. The preparation and characterization of intermediates,
diarylethylene and other related compounds (compound 1-5), were reported in the supporting information in detail. With them in hand,
the chlorinated diarylethylene 1 reacted with n-BuLi to give the corresponding borate 2, and further reacted with bromobiphenyl
aldehyde 3 via classical Suzuki reaction to obtain diarylethylene dialdehyde 4. DTE-1o was obtained from the cyclization of pyridyl
dipyrrolylmethane 5 and dialdehydes derivative 4 at 0 ℃. Compound Zn-DTE-1o was prepared conveniently by the dropwise addition
of zinc acetate solution in methanol.
N
5
S
S
S
S
N
NH
NH
1. TFA, DCM, 20min, 0o
C
NH
N
N
2. DDQ, 1h
CHO
OHC
HN
4
N
Br
DTE-1o
3
CHO
(H3CO)2B
B(OCH3)
2
S
S
2
S
S
N
N
N
N
Zn
N
Cl
Cl
S
S
1
N
Zn-DTE-1o
Scheme 2. The synthetic procedures for the basket-shaped porphyrins DTE-1o and its zinc complex.
The 1H NMR spectra of DTE-1o and Zn-DTE-1o were given in Figure 1. Compared with the chemical shifts of the protons in the
diarylethene 4 (SI), the chemical shifts of H protons in the diarylethene bridged over the porphyrin are shifted to the upfield region
because of the shielding effect of porphyrin ring. Especially there is a striking singlet at δ = -2.89 ppm in the upfield region of DTE-1o
(the top of Figure 1), ascribed to the two hydrogen atoms of N-H in pyrroles. However, this singlet disappeared when DTE-1o
complexed with Zn2+ (the lower part of Figure 1), which indicated that the two N-H in pyrroles were replaced by zinc and the Zn-
DTE-1o complex formed as expected. Moreover, the rigidity and planarity of Zn-DTE-1o increased after the addition of Zn2+, which
led to the increase in the symmetry and showed a simple splitting in 1H NMR spectra.
Figure 1. Comparison of the 1H NMR spectra of compound DTE-1o and Zn-DTE-1o.

3
The configuration of the target compound DTE-1o was also confirmed clearly by the single crystal X-ray diffraction data. The
porphyrin was connected to diarylethene via two benzene rings, and the other two meso positions of porphyrin were attached with
pyridine groups, which makes the spatial configuration of the molecule like a basket (Figure 2). DTE-1o crystallized in a triclinic
crystal system. The spatial distance of the two methylated activated carbon-carbon bond was 4.2 Å, and the dihedral angle between the
two thiophene ring was found to be α = 76.8o. According to the literature [27], only when the distance between the two photoactive
carbon atoms is not greater than 4.2 Å, diarylethenes can undergo photochromism. It means that DTE-1o could take the process of
photochromism judging from the structure.
Figure 2. Single crystal structure of compound DTE-1o.
2.2. Photochromic performances of the basket-shaped porphyrins
UV-vis absorption spectrum of DTE-1o (1.0 × 10-5 M) in dichloromethane solution was shown in the top of Figure 3. It shows the
typical absorption bands of porphyrin: a strong Soret band and four weak Q bands. When irradiated with ultraviolet light (254 nm), it
can be seen that the absorption peaks alternately fall and rise from the ultraviolet region to the near-infrared region. The reason is that
the π electron delocalization of the diarylethene derivative has expanded after photoirradiation, and its π-π* transition energy level has
decreased. Especially the Soret band at around 423 nm dramatically decreased and red-shifted, while a new peak at around 450
appeared. After several minutes of irradiation with UV light, the absorption spectra did not change any more and the system reached a
photostationary state accompanying a series of isosbestic points emerging, which indicated that the two isomers (ring-closed and ring-
open forms) of DTE-1o reached equilibrium during photochromism [28].
When the solution of zinc acetate in methanol was added dropwise to the solution of DTE-1o in dichloromethane, the
metalloporphyrin Zn-DTE-1o was obtained with a relatively high yield (82%). The UV-vis absorption spectrum of Zn-DTE-1o in
dichloromethane solution (1.0 × 10-5 M) was shown in Figure 3. Although the rigidity of the molecule is increased after binding Zn2+,
Zn-DTE-1o still exhibits obvious photochromic properties, which is different from the reported metalloporphyrin-diarylethenes without
pyridyl groups [26]. It shows that the introduction of the pyridine groups at the porphyrin ring is beneficial to promote the
photochromic performance of the metalloporphyrin-diarylethene system. It may be because the N atom on pyridine can interact with the
Zn atom in the center of the metal porphyrin [29]. As can be seen from Figure 3, there is a characteristic strong Soret band and two
weak Q bands at around 443 nm and 500-700 nm, respectively. After irradiation with ultraviolet light, the Soret band gradually
decreased, and the near infrared absorption band in the range of 700-900 nm increased slowly. Compared with DTE-1o, the number of
Q bands of Zn-DTE-1o reduced, and the Soret band red-shifted obviously. The reason was that when the N atoms of the porphyrin
coordinated with Zn2+, it made the rigidity, symmetry and the degree of conjugation of the porphyrin all increased [30], and the
molecular orbital energy levels were closer.
The fatigue resistance property of DTE-1o and Zn-DTE-1o were measured upon alternating irradiation with UV/Vis light. The
results show their fatigue resistance is a bit low in the presence of air. The absorbance declined gradually after 5 cycles (Figure S5).

4
Tetrahedron Letters
3.0
2.5
2.0
1.5
1.0
0.5
0.0
DTE-1o
300
400
500
600
700
800
Wavelength / nm
0.6
0.4
0.2
0.0
Zn-DTE-1o
300
400
500
600
700
800
900
Wavelength
Figure 3. Changes in the absorption spectra of DTE-1o (top) and Zn-DTE-1o (bottom) in DCM (1.0×10-5 M) upon alternating irradiation with
UV/vis light.
2.5. Reversible light-driven fluorescent switches
The fluorescence performances of the two target compounds in DCM solution (1.0×10-5 M) were investigated at room temperature.
Compound DTE-1o emitted violent red fluorescence under 365 nm light, which was the characteristic fluorescence of porphyrins. As
shown in Figure 4 (top), there are two strong fluorescence emission peaks at 669 nm and 727 nm could be observed when DTE-1o was
excited by 455 nm light. Its fluorescence quantum efficiency is 19.3% using rhodamine as a reference (Table S1). Interestingly, with the
extension of the UV irradiation, the fluorescence intensity decreased gradually, and the fluorescence was almost completely quenched
when the photostationary state reached. However, the fluorescence intensity restored slowly when irradiated with visible light (> 510
nm). It indicates that irradiation with different wavelengths could control the fluorescence intensity effectively. This characteristic has
potential application in molecular-level stimuli-responsive materials and non-destructive readout system for photon storage.
600
DTE-1o
Vis UV
500
400
300
200
100
0
Vis UV
650
700
750
800
Wavelength / nm

5
800
600
400
200
0
UV Vis
Zn-DTE-1o
UV Vis
400
500
600
700
Wavelength
Figure 4. Changes in the emission spectra of DTE-1o (top) and Zn-DTE-1o (bottom) through the alternate photoirradiation with UV and
visible light in DCM (1.0×10-5 M, λex= 455 nm).
The fluorescent behavior of Zn-DTE-1o is similar to that of DTE-1o. The strong red fluorescence of Zn-DTE-1o could be distinctly
observed under 365 nm light, and its fluorescence quantum yield is 56.9% (Table S1), which is much higher than that of DTE-1o
because of the complexation of Zn. When compound Zn-DTE-1o was excited by 455 nm light, two strong fluorescence emission peaks
were tested at 471 nm and 636 nm, respectively (the bottom of Figure 4). What's more, its fluorescence intensity decreased gradually
under the irradiation of UV light, and returned to the initial state when irradiated with visible light, respectively. However, without the
pyridine groups, no matter how irradiated, its fluorescence will not reversibly change because of the absence of the photochromic
properties.
3. Conclusion
In this paper, photochromic diarylethylene was designed to stretch across the two meso positions of macrocyclic porphyrin. The
pyridines group were introduced at the other two meso positions of the porphyrin ring to construct two light-driven fluorescent
molecular switches. The synthesis, photochromism and fluorescent performances were investigated. The reversible changes in the UV-
vis absorption and fluorescence emission spectra of the target molecules before and after photoirradiation were studied thoroughly. This
new design and synthesis of basket-shaped porphyrin strapped by photochromic diarylethene can provide a new choice for molecular-
level light-controlled fluorescent switches and non-destructive readout materials.
Appendix A. Supplementary data
Supplementary data to this article can be found online
References
1. M. Irie, T. Fukaminato, T. Sasaki, N. Tamai, T. Kawai, Nature. 2002, 420, 759–760.
2. J. T. Li, H. K. Bisoyi, S. Y. Lin, J. B. Guo, Q. Li, Angew. Chem. Int. Ed. 2019, 58, 16052–16056
3. J. J. Yu, L. Y. Zhao, Z. T. Shi, Q. Zhang, G. London, W. J. Liang, C. Gao, M. M. Li, X. M. Cao, H. Tian, B. L. Feringa, D. H. Qu, J. Org. Chem. 2019,
84, 5790−5802.
4. A. X. Ding, Q. Tang, Y. G. Gao, Y. D. Shi, A. Uzair, Z. L. Lu, ACS Appl. Mater. Interfaces. 2016, 8, 14367–14378.
5. H. C. Xi, Z. P. Zhang, W. W. Zhang, M. Q. Li, C. Lian, Q. F. Luo, H. Tian, W. H. Zhu, J. Am. Chem. Soc. 2019, 141, 18467–18474.
6. S. H. Um, H. J. Kim, D. Kim, J. E. Kwon, J. W. Lee, D. Hwang, S. K. Kim, S. Y. Park, Dyes and Pigments. 2018, 158, 36–41.
7. Y. S. Cai, Z. Q. Guo, J. M. Chen, W. L. Li, L. B. Zhong, Y. Gao, L. Jiang, L. F. Chi, H. Tian, W. H. Zhu, J. Am. Chem. Soc. 2016, 138, 2219−2224.
8. M. Herder, M. Utecht, N. Manicke, L. Grubert, M. Pätzel, P. Saalfrank, S. Hecht, Chem. Sci. 2013, 4, 1028–1040.
9. G. Mező, L. Herényi, J. Habdas, Z. Majer, B. Mysliwa-Kurdziel, K. Tóth, G. Csík, Biophys. Chem. 2011, 155, 36–44.
10. F. Ricchelli, Photochem. Photobiol. B: Biol. 1995, 29, 109–118.
11. N. Amiri, S. Nouir, M. Hajji, T. Roisnel, T. Guerfel, G. Simonneaux, H. Nasri, J. Saudi Chem. Soc. 2019, 23, 781–794.
12. G. Lu, G. H. Yu, X. Kong, Y. X. Chen, D. X. Yin, W. X. Lu, Q. Y. Liu, Inorg. Chem. Commun. 2019, 110, 107591–107595.
13. L. L. Hou, X. Y. Zhang, T. C. Pijper, W. R. Browne, B. L. Feringa, J. Am. Chem. Soc. 2014, 136, 910–913.
14. P. Zwick, K. J. Weiland, J. Malinčík, D. Stefani, D. Häussinger, H. S. J. van der Zant, D. Dulic, M. Mayor, J. Org. Chem. 2020, 85, 118–128.
15. A. M. Amado, J. H. Uliana, T. Z. Pavan, I. Borissevitch, Chem. Phys. Lett. 2020, 738, 136875–36878.
16. J. M. Yuen, J. R. Diers, E. J. Alexy, A. Roy, A, K. Mandal, H. S. Kang, D. M. Niedzwiedzki, C. Kirmaier, J. S. Lindsey, D. F. Bocian, D. Holten, J.
Phys. Chem. A. 2018, 122, 7181–7201.
17. Z. Yao, H. Li, Y. Y. Fan, X. Liang, X. H. Xu, J. F. Li, Dyes and Pigments. 2020, 173, 107961–107965.
18. N. G. Singh, K. D. Borah, S. Majumder, J. Bhuyan, J. Mol. Struct. 2020, 1200, 127116–127123.
19. M. A. Conger, A. R. Cornetta, M. D. Liptak, Inorg. Chem. 2019, 58, 15455–15465.
20. K. W. Zeng, Y. Y. Lu, W. Q. Tang, S. L. Zhao, Q. Y. Liu, W. H. Zhu, H. Tian, Y. S. Xie, Chem. Sci. 2019, 10, 2186–2192.
21. Q. F. Luo, B. Z. Chen, M. Z. Wang, H. Tian, Adv. Funct. Mater. 2003, 13, 233–239.
22. Q. F. Luo, S. H. Cheng, H. Tian, Tetra. Lett. 2004, 45, 7737–7740.
23. Q. F. Luo, Y. Liu, H. Tian, Struct. Bond. 2010, 135, 89–104.
24. J. Andrasson, S. D. Straight, T. A. Moore, A. L. Moore, D. Gust, Chem. Eur. J. 2009, 15, 3936–3939.

6
Tetrahedron Letters
25. J. R. Nilsson, M. C. O’Sullivan, S. Li, H. L. Anderson, J. Andréasson, Chem. Commun. 2015, 51, 847–850.
26. P. P. Jin, M. Q. Liu, F. Cao, Q. F. Luo, Dyes and Pigments. 2016, 132, 151–158.
27. S. Z. Pu, C. B. Fan, W. J. Miao, G. Liu, Tetrahedron. 2008, 64, 9464–9470.
28. X. C. Li, H. Tian, Tetra. Lett. 2005, 46, 5409–5412.
29. E. Deiters, V. Bulach, M. W. Hosseini, Chem. Commun. 2005, 31, 3906–3908.
30. J. A. Shelnutt, X. Z. Song, J. G. Ma, S. L. Jia, W. Jentzen, C. J. Medforth, J. Chem. Soc. Rev. 1998, 27, 31–42.
Highlights
Two basket-shaped porphyrins strapped by diarylethene was constructed
Their reversible photochromic properties were studied.
The photoswitchable fluorescent characteristics have been investigated.