Adjustable Photochromic Behavior of a Diarylethene-Based Bistable

Letter pubs.acs.org/OrgLett

Cite This: Org. Lett. 2018, 20, 5626−5630

Adjustable Photochromic Behavior of a Diarylethene-Based Bistable [3]Rotaxane Guoxing Liu,*,† Jing Zhu,† Yu Zhou,‡ Zhenhua Dong,† Xiufang Xu,*,‡ and Pu Mao† †

Org. Lett. 2018.20:5626-5630. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 09/21/18. For personal use only.

College of Chemistry, Chemical and Environmental Engineering, Henan University of Technology, Zhengzhou 450001, People’s Republic of China ‡ Department of Chemistry, Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), College of Chemistry, Nankai University, Tianjin, 300071, People’s Republic of China S Supporting Information *

ABSTRACT: An acid−base stimulus-responsive diarylethene-based bistable [3]rotaxane has been constructed through a threading-stoppering method, manifesting a reversible shuttling motion undergoing acid−base stimulation. In this system, two macrocycles can be driven to unfasten or restrict the photoswitchable framework by addition of the appropriate acid or base, revealing that photocyclization quantum yield of the [3]rotaxane in the “Near” state is superior to that in the “Far” state. These findings offer a new approach for directional improvement of photochromic performance and construction of molecular machine with a shuttling motion.

T

cyclization or cycloreversion, quantum yield becomes one of the hotspots of photochemical research. However, for application of optical memory storage, high cyclization quantum yields, low ring-opening quantum yields, and high absorption coefficients of the closed-ring forms (CFs) are also required. Consequently, it is especially significant to enhance cyclization quantum yield and absorption coefficients of CF. Many reports on improving photochromic property focus on altering the substituents12 or metal coordination.6,13 Recently, Zhu et al. introduced very high steric strain barriers to the rotation of benzothiophene units, and even completely blocked the interconversion between parallel conformation and antiparallel conformation to separate completely pure antiparallel conformers and suppress intramolecular charge transfer, to realize extremely high photocyclization quantum yields.14 In addition, host−guest complexation as a simple method has been encouraged to improve photochromic performance.15 Herein, an acid−base stimulus-responsive bistable [3]rotaxane has been rationally designed and constructed successfully through host−guest complexation, expressing

he study on mechanically interlocked molecules, especially rotaxanes, has been attracting increased attention, because of the superiority of designing artificial molecular machines and stimuli-responsive “smart” materials. Notably, stimuli-responsive bistable rotaxanes with two-station shuttling mode, based on the complexation of dibenzo-24crown-8 (DB24C8), have been applied to develop molecular elevators,1 controllable charge-transfer (or electron-transfer) complexes,2 and modulated energy-transfer systems.3 Among various stimulators, such as light, ions, voltage, and temperature, acid−base is a common stimulating factor, which is utilized to regulate reciprocating motion in bistable rotaxanes. On the other hand, diarylethene (DAE) undergoes photocyclization and cycloreversion reactions under irradiation of different wavelength light, and exhibits fast responsiveness, excellent thermostability, and fatigue resistance, in comparison to other molecular photoswitches.4 Therefore, it has been generally applied in controlled release,5 photoswitching singlet oxygen generation,6 photomodulating chiral helical,7 photoswitching cell imaging,8 photoprinting,9 photocontrolling electric conduction device,10 and optical memory storage materials.11 Among photochromic properties, quantum yield is an important factor in evaluating the performance of photochromic materials. Hence, to efficiently improve photo© 2018 American Chemical Society

Received: July 24, 2018 Published: August 31, 2018 5626

DOI: 10.1021/acs.orglett.8b02293 Org. Lett. 2018, 20, 5626−5630

Letter

Organic Letters

obtained [3]rotaxane 1F (two DB24C8 macrocycles “far” from DAE) with acid−base responsive function in the yield of 92%. Moreover, axles of these [3]rotaxanes 3 and 4 as reference substances were synthesized by the same reactions to affirm the structure of 1−2. The synthesis processes and characterizations (1H NMR, 13C NMR, and ESI-MS) of all new compounds above were presented in Figures S1−S18 in the Supporting Information. In addition, other evidence for the formation of 1 and 2 in this process came from the analysis of their 1H NMR spectra. As displayed in Figure S19 in the Supporting Information, an apparent downfield shift of the resonance for the methylene protons (H14′ and H15′) and upfield shifts for the benzene ring protons (H12′, H13′, H16′, and H17′) on 2, as revealed by a comparison of the spectra of 4, which showed that the crown ether units in 2 were threaded by the ammonium template. Meanwhile, benzene ring protons (Ha and Hb) of 7 displayed obvious upfield shifts. It was notable that the protons of NH2+ in the template of rotaxane 2 were detected at 7.51 ppm, because of the stabilizing effect of the hydrogen-bonding interactions of the oxygens on the crown ether 7 with the ammonium H atoms. Furthermore, similar chemical shift changes for the characteristic protons on [3]rotaxane 1F were observed, as shown in Figure S20 in the Supporting Information. It is well-known that dibenzylammonium (DBA) with DB24C8 possesses stronger binding ability than N-methyltriazolium (MTA).18 Therefore, DB24C8 was initially located at dibenzylammonium recognition (Figure 1a). However,

adjustable photochromic behaviors. In this system, two binding sites, i.e., dibenzylammonium (DBA) and N-methyltriazolium (MTA), were introduced to fasten two crown ether macrocycles near or far from the DAE skeleton and meanwhile, the rings could slide back and forth between DBA and MTA to proceed a shuttling motion under the stimulus of acid and base. Benefiting from diverse locations of macrocycles relative to DAE, the influence of host rings on DAE’s photoswitching behavior was subsequently investigated and tunable photochromic performance was promised. The chemical structure and synthetic route of [3]rotaxanes 2 and 1 are shown in Scheme 1. Condensation of benzylamine Scheme 1. Synthetic Route for Bistable [3]Rotaxane 1

Figure 1. (a) Partial 1H NMR spectra (400 MHz, 298 K, CD3CN) of (a) [3]rotaxane 1F, (b) deprotonation with addition of 8 equiv of DBU to sample a, and (c) reprotonation with addition of 16 equiv of TFA to sample b.

916 with aldehyde 8 then produced the corresponding reversible dynamic imine, which was reduced by NaBH4 in the solution of THF and MeOH to give the kinetically stable amine 10 in 88% yield. Protonation of the free amine with excess trifluoroacetic acid (TFA) and subsequent counterion exchange with saturated NH4PF6 solution afforded the dialkylammonium salt 6 in 92% yield for the two steps. DAE-modified alkyne 5 was prepared using a method reported previously.17 It is well-known that dibenzo-24-crown-8 (DB24C8) can bind with dialkylammonium of 6. Then, [3]rotaxane 2 was constructed through “click” reaction by threading-stoppering strategy from 5, 6, and 7 in 66% yield. Subsequently, methylation reaction of triazole group in 2

when 8 equiv of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) was added in the solution of [3]rotaxane 1F, deprotonation of dialkylammonium (−NH2+−) led to the conclusion that the rings were slipped to MTA. This proof was provided by the analysis of comparison of 1H NMR spectra. As shown in Figure 1a and 1b, an obvious upfield shift of the resonance for the methylene protons (H15 and H16) and a downfield shift of adjacent N-methyltriazolium protons (H5, H7, and H3) were observed, implying that macrocycles moved from DBA to MTA recognition site. Subsequently, reprotonation of the −NH− center with the addition of 16 equiv of trifluoroacetic acid (TFA) resulted in the return of the DB24C8 ring to the DBA station, as testified by the re-emergence of the original 1H 5627

DOI: 10.1021/acs.orglett.8b02293 Org. Lett. 2018, 20, 5626−5630

Letter

Organic Letters NMR spectrum (Figure 1c). Therefore, two DB24C8 might be slipped between DBA and MTA undergoing acid−base stimulus, indicating reversible constraint and relaxation of DAE framework by the two macrocycles to some extent. Benefiting from the existence of DAE framework, an investigation was performed of photophysical properties of bistable [3]rotaxane 1. As displayed in Figure S21 in the Supporting Information, an open form of the molecular switch (OF-1F) gave an absorption maximum at 275 nm (ε = 4.33 × 104 L mol−1 cm−1), while a new absorption peak appeared at 530 nm (ε = 1.28 × 104 L mol−1 cm−1) upon UV irradiation for 4 min at 254 nm, with the accompanying obvious color change from colorless to fuchsia, implying the formation of a new compound. In addition, a well-defined isosbestic point at 330 nm indicated that the open form of 1F was converted to the photocyclized product, i.e., the closed form of 1F (CF-1F). A photostationary state ratio of 78:22 closed/open form is observed by NMR spectral examination of the irradiated sample (see Figure S24 in the Supporting Information). Photoirradiation, using light at a wavelength of >450 nm, of this sample, which contained CF-1F, led to a complete recovery of the original absorption spectrum of OF-1F, indicating reversible photocyclization/reversion between the open form and the closed form with good fatigue resistance, as illustrated in Scheme 2. The photocyclization quantum yield

Figure 2. Absorbance variation curves of closed-form of 1, 1+DBU and 1+DBU+TFA over the irradiation time of 254 nm UV light in aqueous solution.

Furthermore, the absorbance of the OF (276 nm, ε = 4.55 × 104 L mol−1 cm−1) and CF (534 nm, ε = 1.51 × 104 L mol−1 cm−1) at a photostationary state in 1N were higher than 1F. Moreover, photocyclization conversion yield was enhanced appreciably from 78% to 88% with the addition of DBU (see Figure S25 in the Supporting Information). In contrast, the Φc−o of 1N was slightly degraded in comparison to 1F (see Table 1). The probable reason for the above changes might be that the antiparallel configuration was fixed by the two DB24C8 macrocycles, to some extent. Furthermore, the higher absorbance (∼254 nm), varying photostationary state ratios, or a reduced quantum yield for ring opening at 254 nm irradiation were also possible causes for the acceleration of photocyclization speed. However, when 16 equiv TFA was added into the above resulted system, original absorption spectrum, conversion yield, photocyclization speed, and quantum yield were nearly completely recovered (see Figures S23 and S26 in the Supporting Information, Figure 2, and Table 1). To eliminate the influence of the acid−base environment, some control experiments were performed. We selected the axle molecule of [3]rotaxane, i.e., compound 3 without DB24C8 as reference to perform the same illumination experiments as 1. The result showed that the ultraviolet−visible light (UV-vis) absorption spectrum, Φo−c and Φc−o remained basically unchanged, which is other than [3]rotaxane 1 (see Figures S27−S30 in the Supporting Information, as well as Table 1). This indicated that macrocycle DB24C8 played a key role in acid−base adjusted photochromic performance of [3]rotaxane 1, further validating aforementioned speculation. To further excavate the cause of the aforementioned distinction in photocyclization speed and quantum yield between OF-1F and OF-1N, the relevant computations were

Scheme 2. Regulation of Photocyclization Speed and Quantum Yield of [3]Rotaxane 1 by Acid−Base Stimulation

(Φo−c) and photocycloreversion quantum yield (Φc−o) corrected for the active conformer were determined to be 0.15 and 0.0057 for 1. The relevant optical properties are shown in Table 1. Very significantly, as shown in Scheme 2, when the two macrocycles were staying at the binding site MTA and “near” DAE framework after the addition of 8 equiv DBU (1F was changed to be 1N), the photocyclization speed was dramatically accelerated and the Φo−c was determined to be 0.3, which was enlarged to double its original state (see Figure 2, Figure S22 in the Supporting Information, and Table 1).

Table 1. Photochromic Parameters of 1 and 3 (2 × 10−5 M) in CH3CN Open-Ring Isomers

Quantum Yielda of Ring Closing [λ/nm]

Closed-Ring Isomers

compound

absorption maxima, λmax [nm]

ε [× 104 cm−1 M−1]

absorption maxima, λmax [nm]

ε [× 104 cm−1 M−1]

isosbestic point, IP [nm]

l 1+DBU 1+DBU+TFA 3 3+DBU 3+DBU+TFA

275 276 275 273 272 272

4.33 4.55 4.29 3.06 3.08 3.08

530 534 530 530 530 530

1.28 1.51 1.24 1.10 1.03 1.11

330 331 330 328 328 328

ΦO−C

ΦC−O

± ± ± ± ± ±

0.0057 ± 0.0001 0.0044 ± 0.0001 0.0054 ± 0.0001 0.0064 ± 0.0002 0.0070 ± 0.0001 0.0067 ± 0.0001

0.15 0.30 0.17 0.17 0.15 0.17

0.002 0.004 0.002 0.003 0.003 0.002

a

The determination of quantum yields is shown in Figures S31−S42 and the Remarks section in the Supporting Information. 5628

DOI: 10.1021/acs.orglett.8b02293 Org. Lett. 2018, 20, 5626−5630

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subsequently performed. As shown in Figure 3 and Table 2, in comparison with OF-1F, the C1−C6 distance of [3]rotaxane

ORCID

Guoxing Liu: 0000-0003-1609-3865 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Ph.D. Foundation of Henan University of Technology, China (No. 2017BS020), The Science and Technology Foundation of Henan Province (No. 162102210038), The Colleges and Universities Key Research Program Foundation of Henan Province (No. 17A150007), Natural Science Foundation of China (Project No. 21421001), and National Key R&D Program of China (No. 2018YFA0306002).

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Figure 3. Optimized geometries of OF-1F and OF-1N. Geometry optimization were performed using the B3LYP/6-31G(d)/SMD (acetonitrile) method with the Gaussian 16 program.

(1) (a) Zhang, Z.-J.; Han, M.; Zhang, H.-Y.; Liu, Y. Org. Lett. 2013, 15, 1698−1701. (b) Badjic, J. D.; Balzani, V.; Credi, A.; Silvi, S.; Stoddart, J. F. Science 2004, 303, 1845−1849. (c) Badjic, J. D.; Ronconi, C. M.; Stoddart, J. F.; Balzani, V.; Silvi, S.; Credi, A. J. Am. Chem. Soc. 2006, 128, 1489−1499. (2) (a) Jiang, Q.; Zhang, H.-Y.; Han, M.; Ding, Z.-J.; Liu, Y. Org. Lett. 2010, 12, 1728−1731. (b) Zhang, H.; Hu, J.; Qu, D.-H. Org. Lett. 2012, 14, 2334−2337. (c) Li, H.; Zhang, J.-N.; Zhou, W.; Zhang, H.; Zhang, Q.; Qu, D.-H.; Tian, H. Org. Lett. 2013, 15, 3070−3073. (3) Yao, J.; Li, H.; Xu, Y.-N.; Wang, Q.-C.; Qu, D.-H. Chem. - Asian J. 2014, 9, 3482−3490. (4) (a) Irie, M.; Fukaminato, T.; Matsuda, K.; Kobatake, S. Chem. Rev. 2014, 114, 12174−12277. (b) Qu, D.-H.; Wang, Q.-C.; Zhang, Q.-W.; Ma, X.; Tian, H. Chem. Rev. 2015, 115, 7543−7588. (c) Feringa, B. L.; van Delden, R. A.; Koumura, N.; Geertsema, E. M. Chem. Rev. 2000, 100, 1789−1816. (5) (a) Han, M.; Michel, R.; He, B.; Chen, Y.-S.; Stalke, D.; John, M.; Clever, G. H. Angew. Chem., Int. Ed. 2013, 52, 1319−1323. (b) Luo, F.; Fan, C. B.; Luo, M. B.; Wu, X. L.; Zhu, Y.; Pu, S. Z.; Xu, W.-Y.; Guo, G.-C. Angew. Chem., Int. Ed. 2014, 53, 9298−9301. (6) Hou, L.; Zhang, X.; Pijper, T. C.; Browne, W. R.; Feringa, B. L. J. Am. Chem. Soc. 2014, 136, 910−913. (7) Cai, Y.; Guo, Z.; Chen, J.; Li, W.; Zhong, L.; Gao, Y.; Jiang, L.; Chi, L.; Tian, H.; Zhu, W.-H. J. Am. Chem. Soc. 2016, 138, 2219− 2224. (8) (a) Zou, Y.; Yi, T.; Xiao, S.; Li, F.; Li, C.; Gao, X.; Wu, J.; Yu, M.; Huang, C. J. Am. Chem. Soc. 2008, 130, 15750−15751. (b) Piao, X.; Zou, Y.; Wu, J.; Li, C.; Yi, T. Org. Lett. 2009, 11, 3818−3821. (c) Pang, S.-C.; Hyun, H.; Lee, S.; Jang, D.; Lee, M. J.; Kang, S. H.; Ahn, K.-H. Chem. Commun. 2012, 48, 3745−3747. (9) Liu, G.; Zhang, Y.-M.; Zhang, L.; Wang, C.; Liu, Y. ACS Appl. Mater. Interfaces 2018, 10, 12135−12140. (10) Liu, Z.; Wang, H. I.; Narita, A.; Chen, Q.; Mics, Z.; Turchinovich, D.; Kläui, M.; Bonn, M.; Müllen, K. J. Am. Chem. Soc. 2017, 139, 9443−9446. (11) (a) Chan, J. C.-H.; Lam, W. H.; Yam, V. W.-W. J. Am. Chem. Soc. 2014, 136, 16994−16997. (b) Liu, G.; Zhang, Y.-M.; Xu, X.; Zhang, L.; Liu, Y. Adv. Opt. Mater. 2017, 5, 1700149. (12) (a) Irie, M.; Sakemura, K.; Okinaka, M.; Uchida, K. J. Org. Chem. 1995, 60, 8305−8309. (b) Fan, C.; Pu, S.; Liu, G.; Yang, T. J. Photochem. Photobiol., A 2008, 194, 333−343. (c) Pu, S.; Yan, L.; Wen, Z.; Liu, G.; Shen, L. J. Photochem. Photobiol., A 2008, 196, 84− 93. (d) Pu, S.; Zheng, C.; Le, Z.; Liu, G.; Fan, C. Tetrahedron 2008, 64, 2576−2585. (13) (a) Qin, B.; Yao, R.; Zhao, X.; Tian, H. Org. Biomol. Chem. 2003, 1, 2187−2191. (b) Wu, Y.; Zhu, W.; Wan, W.; Xie, Y.; Tian, H.; Li, A. D. Q. Chem. Commun. 2014, 50, 14205−14208. (14) Li, W.; Jiao, C.; Li, X.; Xie, Y.; Nakatani, K.; Tian, H.; Zhu, W. Angew. Chem., Int. Ed. 2014, 53, 4603−4607.

Table 2. Calculated Bond Lengths and Dihedral Angles compound

rC1−C6 [Å]

φ(C1, C2, C3, C4)

φ(C3, C4, C5, C6)

OF-1F OF-1N

3.72 3.67

53.68 49.92

51.86 50.91

OF-1N is shorter by 0.05 Å, and the dihedral angle between two thiophene rings is smaller nearly by 5°, because of the constraint of DB24C8 on antiparallel configuration of DAE, leading to the conclusion that the photocyclization of [3]rotaxane 1N will be easier than 1F. In addition, the dibenzo-24-crown-8 distal to the diarylethene and the more straight molecule chain in [3]rotaxane 1F may make it difficult for C1 and C6 to get close, because the resisting arm is much larger. In summary, we have developed an acid−base responsive bistable DAE-based [3]rotaxane 1 through a threading− stoppering method, where two DB24C8 macrocycles were far away from DAE. Subsequently, when a certain amount of base was added into 1, DB24C8 macrocycles moved to get close to the DAE skeleton, exhibiting faster photocyclization speed and higher cyclized quantum yield, because of the influence of DB24C8 on DAE. Afterward, the addition of excess acid drove two macrocycles to return to the original location, resulting in the re-emergence of original photochromic performance. Consequently, these findings, adjustable photoswitching behavior, and regular reciprocation provided a new strategy for directional improvement of photochromic performance and construction of a molecular machine with a shuttling motion.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b02293. Synthesis and characterization of all new compounds, other additional data (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (G. Liu). 5629

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Organic Letters (15) (a) Takeshita, M.; Kato, N.; Kawauchi, S.; Imase, T.; Watanabe, J.; Irie, M. J. Org. Chem. 1998, 63, 9306−9313. (b) Liu, G.; Zhang, Y.-M.; Wang, C.; Liu, Y. Chem. - Eur. J. 2017, 23, 14425− 14429. (16) Atilgan, S.; Ozdemir, T.; Akkaya, E. U. Org. Lett. 2010, 12, 4792−4795. (17) Lin, Y.; Yin, J.; Yuan, J.; Hu, M.; Li, Z.; Yu, G.-A.; Liu, S. H. Organometallics 2010, 29, 2808−2814. (18) (a) Coutrot, F.; Busseron, E. Chem. - Eur. J. 2008, 14, 4784− 4787. (b) Ragazzon, G.; Credi, A.; Colasson, B. Chem. - Eur. J. 2017, 23, 2149−2156.

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DOI: 10.1021/acs.orglett.8b02293 Org. Lett. 2018, 20, 5626−5630