Toward bidirectional photoswitchable colored

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Cite this: Chem. Commun., 2018, 54, 9356 Received 2nd July 2018, Accepted 26th July 2018

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Toward bidirectional photoswitchable colored photochromic molecules with visible light stability† Tian-Guang Zhan,‡ Huan-Huan Yin,‡ Si-Tai Zheng, Wei-Cheng Lin, Nan-Li Shen, Jiecheng Cui, Li-Chun Kong, Li-Juan Liu and Kang-Da Zhang *

DOI: 10.1039/c8cc05294h rsc.li/chemcomm

Photochromic [2]rotaxanes with bidirectional photoswitchability were fabricated, whose colored states exhibit remarkable visiblelight and thermal stabilities as revealed by systematically spectroscopic investigations.

Color-switchable photochromic molecules1 such as azobenzenes,1b diarylethenes,1c spiropyrans1e and donor–acceptor Stenhouse adducts1h have drawn tremendous attention due to their unique advantages in fabricating marvellous photoresponsive systems including optical displays and data storage,2 supramolecular assemblies3 and biological nanosystems.4 In general, the desired properties could be obtained either by chemically modifying the conjugated structures of known photochromophores1g or by synthesizing new conjugated photochromic molecules.5 However, for most reported organic photochromic systems, the changes in their absorption spectra in the visible region are directly caused by the conjugated structural variation of the photochromophores, whose colored isomers usually fade once exposed to visible light. Due to the ubiquitous existence of visible light in natural and artificial environments, undesirable colorfading or information loss tends to occur when these visible light sensitive colored photochromic compounds are used in optical displays and data storage. Some efforts have been devoted to the development of photochromic systems with visible light stable colored states. For instance, biimidazole6 and Rhodamine B7 derivatives and some organic–inorganic hybrid systems8 were reported to be stable in their colored forms while displayed under visible light. Unfortunately, these systems are not bidirectional photo-switchable as the fading processes of their colored states are non-optically driven, which make them lack spatiotemporal and remote controllability during data erasing processes. Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Science, Zhejiang Normal University, 688 Yingbin, Road, Jinhua 321004, China. E-mail: [email protected] † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c8cc05294h ‡ These authors contributed equally to this work.

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Moreover, their thermally unstable states oftentimes decline on an observable timescale, which is not conducive to the long-term display and storage of information. Definitely, color-switchable photochromic systems combining reversible bidirectional photoisomerization with excellent visible light and thermal stability should be more promising candidates in fabricating functional optical materials. To this end, the colored states of such systems must in principle be visible light inactive but photoactive, which turn out to be very challenging to achieve in currently available photochromic systems. Therefore, it is desirable to develop new photochromic strategies to address these challenges. The marriage of photochromism and mechanically interlocked molecules (MIMs), such as rotaxanes,9 has been proved to be a reasonable method in the fabrication of intelligent photochromic nanosystems as molecular devices for information storage or signal output.10 Recently, we demonstrated a new approach to construct photochromic bistable [2]rotaxanes with dual absorption spectral variation properties by optically-driven shuttling of macrocycles on the axles.11 By taking into consideration the unique advantages of this innovative [2]rotaxane-based photochromic mechanism, here we report the manufacture and in-depth studies of novel [2]rotaxane-based bidirectional photoswitchable P-type photochromic molecules, whose colored states exhibit remarkable stability toward visible light. As shown in Fig. 1, visible light insensitive photochromic bistable [2]rotaxanes could be fabricated by incorporating P-type photochromic stiff stilbene (SB) units3a,12 and 1,5-dioxynaphthalene (DNP) (for R1) or biphenyl (BP) (for R2) units into dumbbell components as recognition sites, and then interlocked with CBPQT4+ macrocycles (Scheme 1). The shuttling of the CBPQT4+ rings on the axles can be triggered by reversible bidirectional photoisomerization of the colorless SB units upon alternating UV light irradiation, resulting in the content variation of E-SB@CBPQT4+ and DNP (or BP)@CBPQT4+ inclusion complexes. Therefore, distinctive color changes of these [2]rotaxanes caused by the variation of the charge-transfer (CT) absorption bands could be observed. Furthermore, benefiting from this

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Fig. 1 Schematic representation of UV-light triggered shuttling in the photochromic bistable [2]rotaxanes of R1 and R2.

new photochromic mechanism and the high thermal stability of the T-type SB units, the colored states of such photochromic [2]rotaxanes could be kept for a long time at room temperature, which facilitates their applications in the long-term display and storage of information. The synthesis of R1 and R2 is outlined and described in detail in the ESI† (Schemes S1 and S2). By employing the threading-followed-by-stoppering approach, R14PF6 and R24PF6 could be prepared in the yields of 23% and 17%, respectively, and further converted to R14Cl and R24Cl through ion exchange mechanisms. Owing to the fact that the shuttling rates of the CBPQT4+ rings on the axles between the E-SB and DNP (or BP) units are close to the nuclear magnetic time scale at room temperature, the 1H NMR spectra of R14PF6 (Fig. S1a, ESI†) and R24PF6 (Fig. S3a, ESI†) in CD3CN exhibit hardly resolved broad signals of their aromatic protons. However, clearly identifiable 1H NMR signals for DNP@CBPQT4+ in R14PF6 (Fig. S1b and S2, ESI†) and BP@CBPQT4+ in R24PF6 (Fig. S3b, ESI†), could be observed as the temperature decreased to 233 K, indicating the existence of equilibrium

Scheme 1

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balance for isomer I and isomer II in the solutions of R1E and R2E (Scheme 1). Interestingly, it was found that the ratios of isomer I and isomer II in the solutions of R14PF6 or R24PF6 are solventdependent. The CT absorption bands in the visible region of R14PF6 (Fig. S4a, ESI†) and R24PF6 (Fig. S4b, ESI†) were varied in different solvents; in particular, the most significant enhancement and bathochromic shift of the CT bands were observed in methanol. In order to resolve the CT bands, the UV/vis absorption spectra for the mixtures of the bis-glycolchain substituted naphthalene derivative (G1)13 (Fig. S5a, ESI†) and the biphenyl derivative (G2) (Scheme S3, ESI†) (Fig. S6a, ESI†) with the CBPQT4+ rings were recorded individually. The absorption bands that appeared at around 525 nm (Fig. S5b, ESI†) and 475 nm (Fig. S6b, ESI†) could be assigned to the CT bands generated from the DNP@CBPQT4+ and BP@CBPQT4+ inclusion complexes, respectively. It was found that the lmax of the CT bands of DNP@CBPQT4+ and BP@CBPQT4+ were not obviously solvent-dependent. Moreover, both G1 C CBPQT4+ and G2 C CBPQT4+ hardly exhibit absorptions in the region of l 4 700 nm, which could be determined as the CT bands of the E-SB@CBPQT4+ complexes in R14PF6 and R24PF6. This was further confirmed from the absorption spectrum recorded for the mixture of a bis-glycol-chain-substituted E-SB derivative (3) (Scheme S1, ESI†) and CBPQT4+ (Fig. S7, ESI†). These results clearly suggest that solvent has a great impact on the isomer distributions of R1E and R2E, and the [2]rotaxanes with higher isomer I ratios could be obtained in methanol.14 Therefore, the photochromic behaviours of these [2]rotaxanes were systematically investigated in methanol by using R14Cl and R24Cl as targets considering their solubility.

Chemical structures and UV-light-trigged photo-switching processes of bistable [2]rotaxanes R1 and R2.


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Fig. 3 UV/vis absorption spectra recorded for (a) R14Cl (1.0 mM) and (b) R24Cl (1.0 mM) in MeOH at 20 1C. Black dashed line: before UV irradiation; purple line (3a) and yellow brown line (3b): after UV irradiation (l = 340 nm, 1.1 mW cm 2) for 1 h; blue line (3a) and grey line (3b): the UV-irradiated samples after irradiation by another UV light (l = 375 nm, 5.2 mW cm 2) for 1 h. The inset pictures show that the solution color changes at different PSSs.

Fig. 2 Partial 1H NMR spectra (600 MHz, CD3OD, 2.0 mM, 298 K) of solutions (a) R14Cl and (b) R24Cl, before (bottom brown line) and after (top cyan-blue line) irradiation by UV light of l = 340 nm (1.1 mW cm 2) for 1.5 h.

Firstly, the variable-temperature 1H NMR spectra of R14Cl (Fig. S8 and S9, ESI†) and R24Cl (Fig. S10 and S11, ESI†) in CD3OD were recorded as well, which revealed the dynamic equilibrium of isomer I and isomer II at room temperature. However, as displayed in Fig. 2a, when the solution of R14Cl in CD3OD (brown line) was irradiated with UV light of l = 340 nm for 1.5 h to reach the photostationary state (PSSZ), the appearance of aromatic proton signals of the Z-SB units (az/dz and bz/ez), as well as the CBPQT4+ rings (az, bz, gz and dz) and the DNP units (2z/6z, 3z/7z and 4z/8z) in the inclusion complex of DNP@CBPQT4+ from R1Z4Cl could be observed (cyan-blue line).11 Analogously, under the same UV irradiation conditions, the solution of the PSSZ mixtures of R24Cl (cyan-blue line in Fig. 2b) also exhibits newly appeared aromatic proton signals not only for the Z-SB units but also for the CBPQT4+ rings (az, bz, gz and dz) and the BP units (2z/7z and 3z/6z) in the BP@CBPQT4+ complex from R2Z4Cl.15 These observations strongly support that the UV-lightinduced E-to-Z photoisomerization of the SB units has successfully triggered the movement of the CBPQT4+ rings from the Z-SB units to the DNP or BP units. By integrating the proton signals of gz, the contents of R1Z4Cl and R2Z4Cl in the solutions of their PSSZ mixtures could be calculated to be about 45% and 48%, respectively. The reversed photoisomerization of the Z-SB units to their E-isomers could be induced by irradiating the solutions of their PSSZ mixtures with another UV light source of l = 375 nm for one hour, which was confirmed by the dramatically decreased proton signals of the Z-isomers in the resulting PSSE mixtures (Fig. S12c and S13c, ESI†). In the same way, the contents of R1E4Cl and R2E4Cl of their PSSE mixtures could be determined to be about 92% and 91%, respectively. UV/vis spectroscopic experiments were further carried out to help us gain more insight into the photochromism and dual absorption spectral variation properties of these [2]rotaxanes. The first spectral changes originate from the UV-light-induced

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photoisomerization of the SB units whose absorption spectral properties (Fig. S14, ESI†) are consistent with the results reported in the literature.3a The second spectral variations are shown in the visible region, owing to the changes in the charge-transfer interactions between the CBPQT4+ rings and the recognition sites during the reversible E/Z photoisomerization processes of the SB units. Before irradiation, the solution of R14Cl in MeOH exhibits an absorption band at 580 nm (dashed line in Fig. 3a), referring to the combined CT absorptions of E-SB@CBPQT4+ in R1E4Cl isomer I and DNP@CBPQT4+ in R1E4Cl isomer II. When the R14Cl solution was irradiated by UV light of l = 340 nm to reach PSSZ (Fig. S15a, ESI†), this CT band declined and blue-shifted obviously, which can be attributed to the movement of the CBPQT4+ rings from the E-SB units to the DNP units triggered by the E-to-Z photoisomerization of the SB units. The resulting solution of the PSSZ mixtures of R14Cl displays a purple color (inset picture in Fig. 3a) and a CT absorption band at 550 nm (purple line in Fig. 3a) owing to the increased content of R1Z4Cl in the solution.11 However, when the purple solution of the PSSZ mixtures of R14Cl was irradiated with another UV light source (l = 375 nm) to achieve PSSE, the CT band at 550 nm was red-shifted close to 580 nm again and enhanced significantly (blue line in Fig. 3a); meanwhile, the solution color reverted to blue (inset picture in Fig. 3a). This can be attributed to the reversed Z to E photoisomerization of the SB units which drives the CBPQT4+ rings back to the E-SB units. Similar results were obtained for R24Cl. The non UV-irradiated spectrum of R24Cl in methanol exhibits a broad CT band in the visible region (dashed line in Fig. 3b), referring to the combined absorptions of the E-SB@CBPQT4+ and BP@CBPQT4+ inclusion complexes in the two isomers of R2E4Cl. This CT band declined substantially after the solution of R24Cl was irradiated by UV light of l = 340 nm for one hour to reach PSSZ (yellow brown line in Fig. 3b and Fig. S15b, ESI†), and the solution color turned from grey to yellow brown concurrently (inset picture in Fig. 3b). This change is caused by the photoisomerization of E-SB to Z-SB, which pushes the movement of the CBPQT4+ ring from the Z-SB unit to the BP unit, leading to an increase in the amount of the BP@CBPQT4+ complex in the solution of R24Cl. Upon irradiation with another UV light source (l = 375 nm), the solution of R24Cl could be converted from PSSZ to PSSE, during which the


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CBPQT4+ ring was moving back to the E-SB unit triggered by the Z to E photoisomerization of the SB unit. This gives rise to the enhancement of the CT band again (grey line in Fig. 3b) and the recovery of the solution color (inset picture in Fig. 3b) due to an increase in the amount of E-SB@CBPQT4+ in the PSSE mixture of R24Cl. By employing the UV/vis spectra of G1 C CBPQT4+, 3 C CBPQT4+, R14Cl and R24Cl, the ratios of isomer I and II at the thermodynamically stable states of R14Cl (61/39) and R24Cl (64/36) in MeOH could be calculated (see Section 8 in the ESI†), based on which their isomer distributions at the photostationary states could be further determined. For R14Cl at PSSZ, the ratio of R1E isomer I/R1E isomer II/R1Z could be determined to be about 34%/21%/45%, which turned to be about 56%/36%/8% after reaching PSSE. The PSSZ of R24Cl contains R2E isomer I (33%), R2E isomer II (19%) and R2Z (48%), whereas its PSSE consists of R2E isomer I (58%), R2E isomer II (33%) and R2Z (9%), respectively. Although multicomponent distributions are observed for the PSSs of R14Cl and R24Cl owing to the incomplete trans–cis photoisomerization of the SB units, they have exhibited noticeable changes for both absorption spectra and solution colors. The visible light stabilities of the coloured states for the solutions of the PSSZ mixtures of R14Cl and R24Cl were further studied using time-resolved UV/vis spectroscopic experiments. The time lapse absorption spectra showed that no significant changes in the CT bands could be traced when the UV light (340 nm) irradiated solutions of R14Cl (Fig. S18, ESI†) and R24Cl (Fig. S19, ESI†) were exposed to visible light (l 4 400 nm) as long as 9 days. These observations strongly suggest that the coloured states of R14Cl and R24Cl are provided with remarkable stabilities towards visible light and good thermal stability under ambient conditions. The reason is that the bidirectional photoisomerization of the SB units is solely induced via UV light, and more importantly, the colorswitching behaviors of these [2]rotaxanes originate from the changes in their CT absorption bands rather than the conjugated structural changes of their photochromophores. In summary, we have prepared two color-switchable [2]rotaxanebased photochromic molecules in which P-type photochromic stiff stilbene units were incorporated into the axles as recognition sites. Thanks to the solely UV-light-induced E/Z photoisomerization of the SB units, these unique photochromic molecules are bidirectional photoswitchable, and their colored states exhibit high visible light and thermal stabilities. These features make them promising candidates for information display and storage applications. More broadly, this work well illustrates what might be an adopted approach to rationally design more elaborate photochromic systems for fabricating functional optical materials. The National Natural Science Foundation of China (21602205) and Zhejiang Normal University are acknowledged for their financial support to this research.


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Conflicts of interest There are no conflicts to declare.

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