Optical-switchable energy transfer controlled by multiple-responsive turn-on fluorescence via metal–ligand and host–guest interactions in diarylethene-based [2]pseudo-rotaxane polymers

Abstract

Multi-responsive and optically-active diarylethene-based [2]pseudo-rotaxane polymers Zn²⁺-TC₂/DS(O & C) were synthesized and prepared via metal–ligand and host–guest interactions, where Zn²⁺ ion was coordinated with two ligands (TC) containing a macrocyclic dibenzo-24-crown-8 (DB24C8) linked to a terpyridyl terminus to form a symmetrical host (Zn²⁺-TC₂) and diarylethene (DAE) bearing two terminal secondary ammonium salts to form a symmetrical guest (DS). Upon irradiation of UV-light (λ = 355 nm), the prominent photo-induced electron transfer (PET) off process to promote optical-switchable Förster resonance energy transfer (FRET) process from the emissive metal-coordination Zn²⁺-TC₂ host to the fluorescent DS(C) guest (in the closed form of emissive DAE unit) was explored for the multiple-responsive turn-on ratiometric fluorescence in supramolecular polymer Zn²⁺-TC₂/DS(C). Remarkably, endowed with the novel features of reversible chemical/chelation-stimuli responsive dis-assembly/re-assembly of non-covalent interactions in response to pH/chelation, the polymer can be used for the sensing of pH and cyclen. Accordingly, the “molecular wire and stimuli-responsive effects” of diarylethene-based [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(C) reveal significant supramolecular interactions to elucidate the turn-on fluorescence via the controllable FRET-ON process from Zn²⁺-TC₂ host donor to PET-OFF DS(C) guest acceptor, which pave a promising route to elaborate optical-switchable supramolecular platforms for the future energy transfer applications.

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Introduction

Supramolecular self-assemblies have attracted significant interest in developing innovative concepts for the design multiple-stimuli-responsive smart and adaptive materials, and have also shown great potential in numerous research areas.^1–5 The repeating units of supramolecular polymers are associated with reversible or dynamic non-covalent interactions of complementary receptor units,^6–8 so the intrinsic dynamic properties of non-covalent interactions are usually responsive to a wide range of external stimuli, such as pH value,⁹ light,¹⁰ stress,¹¹ temperature¹² and chemical reaction.^13–15 For instance, the host–guest interactions of a dibenzo-24-crown-8 (DB24C8) host and dibenzyl ammonium salt (DBAs) guest via the supportive hydrogen-bonding of (N⁺–H⋯O) and (C–H⋯O) are highly responsive towards the interactions with metal ions, pH value and temperature.^16–18 The unique luminescence features of the metal–ligand-coordinated terpyridyl (tpy)-Zn²⁺ complex are well-utilized for the construction of photo-luminescent supramolecular coordination compounds and polymers.^19–24 Notably, the host–guest and metal–ligand interactions are independent of each other and thus both of them could be facilitated to construct novel stimuli-responsive supramolecular materials.

Recently, photochromic fluorescent diarylethenes (DAEs) have received considerable attention due to their remarkable potential including optoelectronics.^25–27 The reversible photo-isomerization of DAE materials can be interconverted between two states with different spectroscopic properties by irradiation with UV and visible light. To achieve a greater fluorescence contrast between the two states of DAE units, various fluorescent fluorophores have been attached to the diarylethene core.²⁸ However, the fluorescence properties of some photochromic DAE derivatives were turned off due to the Förster resonance energy transfer (FRET) from the excited fluorophore to the photo-cyclized DAE core upon the irradiation of UV light.^29,30 Despite the development of photochromic fluorescent DAEs, only a few turn-on fluorescent DAEs were reported to show high fluorescence in their photo-cyclized state upon UV irradiation.^31–34 Nevertheless, very minor efforts have been explored in the insights of energy/electron modulation events by incorporating the photo-cyclized turn-on fluorescent DAEs with other fluorophores in supramolecular polymers via non-covalent interactions. Moreover, the photo-controllable supramolecular polymers are considered to be better candidates over small molecules for preparing light-responsive self-healing materials, where the assembly and disassembly of non-covalent connections within polymer backbones could be regulated via photo-switching the configuration of the photochromic units.^35–39 So far, there have been only a few reports related to the coupled photochromic activities of non-emissive DAE units (in the closed form) with tpy ligands enabled by covalent linkages^40–43 rather than host–guest approaches. Recently, a symmetrical non-emissive DAE guest (in the closed form) connected with two AIE-active pillar[5]arene hosts in a photo-controllable pseudo[3]rotaxane system was observed due to the reversible FRET behavior controlled by the photo-isomerization of the DAE unit.⁴⁴ To the best of our knowledge, there is only one report on coupled photo-switchable FRET activities from an asymmetrical non-emissive DAE guest (in the closed form) complexed with an asymmetrical Eu³⁺-tpy host in a monomeric pseudo[2]rotaxane system rather than both symmetrical emissive DAE guests (in the closed form) complexed with the symmetrical Zn²⁺-tpy host in our supramolecular polymer approach.⁴⁵ Swager et al. also reported that the “molecular wire and stimuli-responsive effects” of supramolecular polymers have contributed significantly to magnifying incredible sensory signals and sensitivities to enhance their interactions towards specific analysts as compared to analogous monomeric systems.⁴⁶ Therefore, the multiple-responsive supramolecular polymer designs by the integration of the Zn²⁺-bis-terpyridyl host with the photochromic emissive DAE guest (in the closed form) would be more interesting for exploring their associated energy transfer properties in this study.

Herein, our strategy involves the integration of multiple-responsive host–guest, metal–ligand and photoactive interactions for the construction of the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O & C) in Chart 1 to investigate novel optical-switchable energy transfer phenomena. This multiple-responsive supramolecular polymer consists of optical-switchable turn-on fluorescence symmetrical diarylethene (DAE)-based guest DS bearing two terminal secondary ammonium salts and a Zn²⁺-TC₂ host, which contains two macrocyclic dibenzo-24-crown-8 (DB24C8) termini linked to terpyridyl (tpy) ligands (TC₂) coordinated with the Zn²⁺ metal ion. Notably, no such efficient control of energy transfer by optical-switchable and pH/chelation stimuli-response in supramolecular polymer has been reported to date, where the FRET process from the fluorescent metal-coordinated Zn²⁺-TC₂ host to the fluorescent DS(C) guest (i.e., the photo-cyclized form of the emissive DAE unit) in a supramolecular polymer has been explored with ratiometric PL emissions. In this study, the first controllable energy transfer via the multiple-responsive capabilities of host–guest, metal–ligand and photoactive interactions in the supramolecular polymer Zn²⁺-TC₂/DS(C) are investigated by means of acid–base and another competitive chelating ligand (i.e., cyclen).

Chart 1 Schematic pictorial representations of (a) chemical structures of the metal-coordinated Zn²⁺-TC₂ host, optically-switchable DS(O & C) guest (i.e., open to closed form) before and after UV-irradiation, respectively. (b) The formation of a [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O & C) and the novel features of the reversible optical/chemical/chelation-stimuli responsive interactions in response to light/pH/chelation.

Results and discussion

The formation of the coordination compound (Zn²⁺-TC₂) via metal–ligand interactions

The capability of the terpyridyl (tpy) ligand (TC) coordinated with Zn²⁺ metal ions, i.e., Zn(OTf)₂, to form the coordination compound (Zn²⁺-TC₂) was investigated as shown in Fig. 1. The corresponding UV-Vis and PL spectral changes of ligand TC containing dibenzo-24-crown-8 (DB24C8) linked to the terpyridyl terminus (1 mM in CHCl₃/CH₃CN (1/1, v/v)) as coordinated with Zn²⁺ are demonstrated in Fig. 1(b) and (c), respectively. Upon the addition of Zn(OTf)₂ (0 to 50 μL or 0.5 equiv.), the absorption bands of ligand TC at 242 and 279 nm were slightly red- and blue-shifted to 247 and 275 nm, respectively, and two new absorption bands with vibronic progressions at 311 and 322 nm were generated with clear isosbestic points at 269 and 301 nm (Fig. 1(b)). The related PL spectral changes of ligand TC upon the addition of Zn(OTf)₂ are illustrated in Fig. 1(c). Upon the addition of Zn(OTf)₂, the emission band (at 403 nm) of ligand TC was red-shifted to a more prominent band at 417 nm, along with a less pronounced new emission band at 338 nm (with a quantum yield of 18.40% for coordination compound Zn²⁺-TC₂ in CHCl₃/CH₃CN (1/1, v/v) medium, using quinine hemisulfate standard). According to Fig. S1 in the ESI,† the metal–ligand complex formation between Zn²⁺ and ligand TC was verified by Job's plot to be a molar ratio of 1 : 2 from the free ligand form to the coordination compound Zn²⁺-TC₂.

Fig. 1 (a) Pictorial demonstration of the metal–ligand interaction of TC and Zn²⁺. (b) UV-Vis spectral changes of ligand TC (1 mM) upon the addition of Zn(OTf)₂ (50 μL or 0.5 equiv. in CHCl₃/CH₃CN (1/1, v/v)). (c) PL spectral changes of ligand TC (1 mM) upon the addition of Zn(OTf)₂ (50 μL or 1 equiv. in CHCl₃/CH₃CN (1/1, v/v)). Insets: Photoimages of TC in the absence and presence of Zn²⁺ in CHCl₃/CH₃CN (1/1, v/v) (λ_ex = 300 nm for PL exp.).

¹H-NMR titrations between ligand TC and Zn(OTf)₂

To further explore insights into the formation of coordination compound Zn²⁺-TC₂ with a molar ratio of the metal–ligand complex of 1 : 2, the coordination process was further investigated by ¹H-NMR spectroscopy (400 MHz) in CDCl₃/CD₃CN (1/1, v/v) as shown in Fig. 2. Upon the addition of Zn²⁺ (0 to 0.5 equiv.), the chemical shifts of the pyridyl protons H^a and H^d in TC were shifted upfield to 0.96 and 0.120 ppm, while the downfield shifts of 0.186 and 0.23 ppm were observed for protons H^c and H^e, mainly ascribed to the shielding effect of the neighboring terpyridyl unit after complexation as illustrated in Fig. 2.

Fig. 2 ¹H-NMR (400 MHz) titration during the formation of Zn²⁺-TC₂ in CDCl₃/CD₃CN (1/1, v/v) with 1 equiv. of ligand TC complexed with increasing equivalents of Zn²⁺: (a) 0 equiv.; (b) 0.1 equiv.; (c) 0.2 equiv.; (d) 0.3 equiv.; (e) 0.4 equiv. (f) 0.5 equiv.; (g) 0.6 equiv.; (h) 0.7 equiv.; (i) 0.8 equiv.; (j) 0.9 equiv.; (k) 1.0 equiv. of Zn(OTf)₂versus the terpyridyl unit (i.e., ligand TC).

The chemical shifts of the aromatic protons of crown ether DB24C8 (i.e., H^Ar) were shifted downfield along with a downfield shift of 0.3 ppm for the benzyl protons (H^f). According to ¹H-NMR results, all of these chemical shifts are the characteristic properties of the efficient conversion from the free ligand form to the coordination compound Zn²⁺-TC₂ with a metal–ligand complex molar ratio of 1 : 2. All these pieces of evidence are consistent with previously reported results,^47,48 indicating the metal–ligand interactions between Zn²⁺ and terpyridine-based ligands. However, once the Zn²⁺ concentrations were oversupplied (i.e., 1 equiv. with an equal molar ratio of terpyridyl ligand TC), a third set of proton signals were ripened and gradually strengthened along with the deinsertion of the pre-formed Zn²⁺-TC₂ (1 : 2) proton signals as shown in Fig. 2, signifying the formation of a 1 : 1 metal–ligand complex (Zn²⁺-TC) with the additional coordination sites saturated by solvent or triflate counterions.^49,50

Optical switchable behaviour of guest DS

The optical switchable property of the diarylethene (DAE)-based secondary ammonium salt guest DS was measured in CHCl₃/CH₃CN (1/1, v/v) and demonstrated in Fig. 3(a) and Fig. S2 (ESI†). Upon UV-irradiation, the open-form of DAE in DS was optically switched to its closed-form, i.e., from the open-form of DS, i.e., DS(O) (O = open-form) to the closed-form of DS, i.e., DS(C) (C = closed-form). The UV/Vis spectral changes of DS during UV-irradiation are summarized in Fig. 3.

Fig. 3 Optical-switchable DS(O & C) guest in CHCl₃/CH₃CN (1/1, v/v) before and after UV-irradiation (λ = 355 nm). (a) Pictorial diagram of DS(O & C) (the open to closed form) before and after UV-irradiation. (b) UV-Vis spectral changes of DS (from the open to closed form). Insets: Photoimages of DS(O) and DS(C) under UV-irradiation at 0 and 24 min, respectively. (c) PL spectral changes and photoimages (insets) of DS guest in CHCl₃/CH₃CN (1/1, v/v) before and after UV-irradiation (at λ = 355 nm for 24 min) (λ_ex = 300 and 425 nm for PL exp.).

The UV-Vis spectrum of DS(O) exhibited absorption bands at 236 nm (n–π* transition of DAE) and 280 nm as shown in Fig. 3(b). After UV-irradiation (λ = 355 nm) for 0–24 min, a broad and intense absorption band at 425 nm was observed and saturated at 15 min due to the generation of DS(C) with the photo-cyclized form of the DAE unit as shown in Fig. 3(b). Even with a longer period of UV-irradiation (i.e., λ = 355 nm up to 24 min), the intensity of the absorption band at 425 nm was not altered and was thus saturated over 15 min of UV-irradiation for DS(C). On the contrary, DS(C) was reversible towards its original open-form DS(O) upon irradiation with visible light at λ ≥ 400 nm as illustrated in Fig. S2 (ESI†), where the de-cyclization process was completed within 25 min.

The PL spectral changes from DS(O) to DS(C) during UV-irradiation (i.e., λ = 355 nm up to 24 min) are illustrated in Fig. 3(c). Notably, during the photo-cyclization of the DAE unit, a less intense emission band at 545 nm was observed (λ_ex = 300 nm for PL exp. with a quantum yield of 0.76% for DS(C) in CHCl₃/CH₃CN (1/1, v/v) medium by using quinine hemisulfate standard). This was due to the prohibition of the photo-induced electron transfer (PET) mechanism from the protonated ammonium group to the photo-cyclized form of DAE unit in DS(C) as shown in Fig. 3(c), which was also reported in the literature where the annihilation of PET (i.e., the PET-OFF process) was induced by the charge introduction to the fluorophore.⁵¹ However, the intensity of the emission band at 545 nm was 3-fold enhanced if DS(C) was excited at the newly photo-cyclized absorption band at λ_ex = 425 nm, where the emission maxima of photo-cyclized DS(C) could be observed only at a critical concentration of ∼1 mM. We also explored the structural changes of the DAE unit in DS (from the open to the closed form) via¹H-NMR spectroscopy as illustrated in Fig. S3 (ESI†). From the ¹H-NMR spectra of DS(O) and DS(C) in Fig. S3 (ESI†), the chemical shift of the methyl proton (–CH₃ group) on the thiophene ring of DAE in DS(O) is located at 1.92 ppm and the methyl proton (–CH₃ group) is slightly shifted to up-field at 1.91 ppm for photo-cyclized DAE in DS(C). Moreover, after photo-cyclization, the methylene protons (i.e., –CH₂ group, H⁴ and H⁵ protons) were shifted to up-field from 4.15 and 4.27 ppm to 4.14 and 4.25 ppm, respectively, which further confirmed the structural changes before and after UV-irradiation. Thus, the integrated areas of the methyl proton (–CH₃ group) indicated 100% conversion of DS(O) to DS(C).

The formation of [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O) via host–guest interactions

It is noteworthy that several research groups have explored the host–guest interactions between secondary ammonium salts and dibenzo-24-crown-8 (DB24C8) via the supportive hydrogen-bonding of [N⁺–H⋯O] and [C–H⋯O].^16–18 Since we have investigated the formation of Zn²⁺-TC₂ (1 : 2) by the complexation of Zn²⁺ with two equivalents of ligand TC containing DB24C8 linked to the terpyridyl terminus and the photo-switching behavior of the DAE unit from the open to the closed form in the guest DS bearing two terminal secondary ammonium salts in Fig. 1 and 3, respectively, a novel [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O)via host–guest interaction is introduced in this study as illustrated in Fig. 4(a). We performed a series of important experiments to discover the host–guest interactions due to the impact of the optical-switchable properties of the DAE unit in the supramolecular polymer Zn²⁺-TC₂/DS upon UV-irradiation and thus explored the associated properties of energy transfer from the metal–ligand complex Zn²⁺-TC₂ to DS(C) by means of photo-cyclization events in the supramolecular polymer Zn²⁺-TC₂/DS(C) as demonstrated in Fig. 5(a). For the formation of the supramolecular polymer Zn²⁺-TC₂/DS(O), the corresponding PL spectral changes of metal–ligand complex Zn²⁺-TC₂ that occurred upon the addition of DS(O) (1 mM or 1 equiv.) in CHCl₃/CH₃CN (1/1, v/v) are shown in Fig. 4(b). The emission bands of the metal–ligand complex Zn²⁺-TC₂ at 338 and 417 nm were slightly increased, which was due to the host–guest complex formation of the supramolecular polymer Zn²⁺-TC₂/DS(O) (1 : 1) and was saturated up to 100 μL of DS(O) in Fig. 4(b). According to Fig. 4(c), the host–guest complex formation between the Zn²⁺-TC₂ host and DS(O) guest was verified by Job's plot with a molar ratio of 1 : 1 to generate the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O).

Fig. 4 (a) Schematic demonstration of the supramolecular formation via the host–guest and metal–ligand interaction. (b) PL spectral changes of the metal–ligand complex Zn²⁺-TC₂ (1 : 1) upon the addition of DS(O) (1 mM or 1 equiv.) in CHCl₃/CH₃CN (1/1, v/v). Insets: Photoimages of Zn²⁺-TC₂ in the absence and presence of DS(O) in CHCl₃/CH₃CN (1/1, v/v). (c) Job's plot of the Zn²⁺-TC₂/DS(O) complex upon the addition of DS(O) (1 mM or 1 equiv.) in CHCl₃/CH₃CN (1/1, v/v) (λ_ex = 300 nm for PL exp.).

Fig. 5 (a) Schematic demonstration of the optical-switchable behavior from the open to closed form of the DAE unit in the supramolecular polymer Zn²⁺-TC₂/DS(O & C) (1 : 1) under UV and Vis light irradiation. (b) PL spectral changes of supramolecular polymer Zn²⁺-TC₂/DS(C) after UV-irradiation (CHCl₃/CH₃CN (1/1, v/v). Insets: Photoimages of Zn²⁺-TC₂/DS(O) and Zn²⁺-TC₂/DS(C) under UV-irradiation at 0 and 25 min, respectively. (c) Reversible PL behavior at λ = 545 nm of Zn²⁺-TC₂/DS(O to C) under UV and Vis light irradiation (λ_ex = 300 nm for PL exp.).

Optical-switchable behaviour of the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O & C) under UV & vis-irradiation

It is well known that the photo-cyclized form of the DAE unit acts as an energy acceptor unit with a combination of other fluorophores.^28–30 Therefore, we are more interested in the impact of the optical-switchable property of the DAE unit in the supramolecular polymer Zn²⁺-TC₂/DS(C) after UV-irradiation and exploring the related properties of energy transfer from the metal–ligand complex Zn²⁺-TC₂ to DS(C) by means of photo-cyclization events as demonstrated in Fig. 5(a). After UV-irradiation (λ = 355 nm), the corresponding PL spectral changes of supramolecular polymer Zn²⁺-TC₂/DS(C) (1 : 1 mM in CHCl₃/CH₃CN 1/1, v/v) were saturated after 25 min, as illustrated in Fig. 5(b). In contrast to Zn²⁺-TC₂/DS(O) before the photo-cyclization of the DAE unit, upon exposure to UV, the supramolecular polymer Zn²⁺-TC₂/DS(C) showed a new major emission band of the DS(C) guest at 545 nm (with a quantum yield of 12.57% for DS(C) in CHCl₃/CH₃CN (1/1, v/v) medium by using quinine hemisulfate standard) and the emission band of the Zn²⁺-TC₂ host at 417 nm decreased (with a quantum yield of 7.60% for Zn²⁺-TC₂ in CHCl₃/CH₃CN (1/1, v/v) medium by using quinine hemisulfate standard) in Fig. 5(b) due to the FRET process from the energy donor Zn²⁺-TC₂ host to the energy acceptor DS(C) guest (i.e., the FRET efficiency of supramolecular polymer Zn²⁺-TC₂/DS(C), E_FRET = 55%). As revealed in the inset of Fig. 5(b), the dark blue emission of the open form of the supramolecular polymer Zn²⁺-TC₂/DS(O) changed to cyan (i.e., greenish-blue) for the closed form of Zn²⁺-TC₂/DS(C).

Notably, the emission intensity of the photo-cyclized DAE unit at 545 nm was increased by about 10 times in the supramolecular polymer Zn²⁺-TC₂/DS(C) (see Fig. 5) as compared to the emission intensity of the DS(C) guest (at 545 nm) alone (see Fig. 3(c)) due to the occurrence of the FRET event from the Zn²⁺-TC₂ host to the DS(C) guest in addition to the prohibition of the PET phenomena from the ammonium group (i.e., coordinated with the macrocyclic host which acts as a strong donor) to the photo-cyclized DAE unit simultaneously in the supramolecular polymer Zn²⁺-TC₂/DS(C). The “molecular wire and stimuli-responsive effects” of the supramolecular polymer also play a key role in enhancing the FRET behavior from the Zn²⁺-TC₂ host to the DS(C) guest in this study.⁴⁶ Therefore, it is significant that the exploration of the FRET behavior from the metal-coordinated Zn²⁺-TC₂ host to the closed form of the DS(C) guest in our supramolecular polymer with two different PL emission peaks is much better clarified in contrast to the reported FRET in the monomeric system with a single PL emission peak.⁴⁵ Moreover, the occurrence of the FRET event in the supramolecular polymer Zn²⁺-TC₂/DS(C) is fully supported by the significant overlap of the emission (at 417 nm) in the Zn²⁺-TC₂ host and the absorption (at 425 nm) in the DS(C) guest as shown in Fig. S4(a) (ESI†) (i.e., the integral of the overlapped area = 60%). As a result, the PL excitation spectrum of the supramolecular polymer Zn²⁺-TC₂/DS(C) in Fig. S4(b) (ESI†) displayed the absorption bands of the Zn²⁺-TC₂ host (306 and 333 nm) and the DS(C) guest (425 nm), which validated the occurrence of the FRET process in supramolecular polymer Zn²⁺-TC₂/DS(C).

The preferential excitation of a donor fluorophore and recognition of the increased emission of an interacting acceptor fluorophore was proof of concept to support the FRET process.⁵² Therefore, another excitation-dependent control experiment was executed to confirm the FRET process from the Zn²⁺-TC₂ host to the DS(C) guest in Zn²⁺-TC₂/DS(C), where the emission spectra of Zn²⁺-TC₂/DS(C) in CHCl₃/CH₃CN (1/1, v/v) solutions (1 mM) were recorded under various excitation wavelengths (from 300 to 330 nm) as shown in Fig. S5 (ESI†). It is worth noting that the optimal excitation wavelength for the occurrence of the strongest FRET process in Zn²⁺-TC₂/DS(C) is 300 nm among all excitations (305, 310, 315, 320, 325 and 330 nm) owing to the formation of the isosbestic point (at 301 nm, see Fig. 1) when Zn²⁺ ions are coordinated with ligand TC. Thus, the supramolecular polymer Zn²⁺-TC₂/DS(C) demonstrated excitation-dependent PL spectra which further confirmed the proposed FRET mechanism from Zn²⁺-TC₂ to DS(C).

The reversibility of the energy transfer from the photo-cyclized DAE unit of DS(C) for the supramolecular polymer Zn²⁺-TC₂/DS(C) to convert the de-cyclized DAE unit of DS(O) upon the irradiation of visible light at λ ≥ 400 nm without energy transfer for supramolecular polymer Zn²⁺-TC₂/DS(O) was further verified as shown in Fig. 5(c). Therefore, the efficient energy transfer from the metal–ligand complex Zn²⁺-TC₂ to the photo-cyclized DAE unit of DS(C) was only produced in the supramolecular polymer Zn²⁺-TC₂/DS(C) by optical switching of the DAE unit from DS(O) to DS(C). According to Fig. S6 in the ESI,† the emissions of the Zn²⁺-TC₂ host (at λ_max = 417 nm) in the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS before and after UV-irradiation in CHCl₃/CH₃CN (1/1, v/v) displayed the fluorescence lifetime values of τ = 2.00 ns and τ = 1.40 ns, respectively. Thus, our findings for energy transfer from Zn²⁺-TC₂ to DS(C) in Fig. 5 were further validated by the decrease in the fluorescence lifetime in the TRPL results of Fig. S6 (ESI†) for host emission in Zn²⁺-TC₂/DS(C) under UV-irradiation.

¹H-NMR titrations between the metal–ligand complex Zn²⁺-TC₂ and DS(O) guest

The host–guest interactions between secondary ammonium salts and DB24C8 via supportive (N⁺–H⋯O) and (C–H⋯O) hydrogen-bonding in less polar solvents are well reported.^16–181H-NMR measurements were carried out to prove the formation of the supramolecular polymer by mixing the DS(O) guest and metal–ligand complex Zn²⁺-TC₂ in CDCl₃/CD₃CN (1/1, v/v) solution as demonstrated in Fig. 6. By mixing the equal molar ratio (1 : 1) of both the DS(O) guest and the Zn²⁺-TC₂ host as shown in Fig. 6, both benzyl protons (H⁴, H⁵) of the DS(O) guest were shifted upfield by 0.35 and 0.15 ppm, respectively, and the alkyl ammonium protons (H⁶) split into a doublet with an upfield-shift of 0.89 ppm. Moreover, H⁷ protons of the thiophene ring in the DAE unit and aromatic protons Ar-H⁸ in the DS(O) guest were shifted downfield by 0.29 and 0.11 ppm, respectively. Likewise, the terpyridine and benzene aromatic protons (H^a, H^b, H^c, H^d, H^e and H^Ar) of the Zn²⁺-TC₂ host were broadened and slightly downfield-shifted. According to the ¹H-NMR results, the formation of the supramolecular polymer Zn²⁺-TC₂/DS(O) was confirmed by the host–guest interaction of both the DS(O) guest and Zn²⁺-TC₂ host.

Fig. 6 ¹H-NMR spectra (300 MHz, CDCl₃/CD₃CN (1/1, v/v) at 298 K for (a) the Zn²⁺-TC₂ host, (b) the supramolecular polymer Zn²⁺-TC₂/DS(O) and (c) the DS(O) guest.

The concentration-dependent ¹H-NMR measurements for studying the formation of the supramolecular polymer Zn²⁺-TC₂/DS(O) were carried out and are illustrated in Fig. S7 (ESI†) to indicate the gradual conversion from cyclic oligomers to a supramolecular polymer.⁵³ Furthermore, the convincing results in the self-assembly mode between the Zn²⁺-TC₂ host and guest DS(O & C) in supramolecular polymers were verified by two-dimensional diffusion-ordered NMR (DOSY) spectra (see Fig. S8 and S9, ESI†) and rotating-frame nuclear Overhauser effect NMR (ROESY) spectra (see Fig. S10, ESI†). Subsequently, we performed the SEM measurements by taking the morphological images of the metal-coordinated Zn²⁺-TC₂ host and supramolecular polymers Zn²⁺-TC₂/DS(O & C) to further elucidate the induced morphological changes via non-covalent interactions. As shown in Fig. S11 (ESI†), the metal-coordinated Zn²⁺-TC₂ host exhibited a uniform morphological pattern of molten grains. There were no remarkable differences in the uniform morphological patterns between supramolecular polymers Zn²⁺-TC₂/DS(O) and Zn²⁺-TC₂/DS(C), which showed some granular images that were smaller as compared to the metal-coordinated Zn²⁺-TC₂ host.

To compare the metal ion effects on the formation of supramolecular polymers, the demetalated component TC₂/DS(C) was investigated for chelation with different metal ions (i.e., 0.5 equiv.) in CHCl₃/CH₃CN (1/1, v/v), and their PL spectra are shown in Fig. S12 (ESI†). Upon the addition of various metal ions Ag⁺, K⁺, Ca⁺, Na⁺, Cd²⁺, Cu²⁺, Fe²⁺, Hg²⁺, Mn²⁺, Ni²⁺, Zn²⁺, Pb²⁺, Al³⁺ and Fe³⁺ to demetalated TC₂/DS(C) solutions in CHCl₃/CH₃CN (1/1, v/v), only Zn²⁺ and Cd²⁺ metal ions showed different extents of stimuli-responsive effects on the PL emissions of DS(C) at λ = 545 nm because of the FRET processes from emissive metal-coordination hosts Zn²⁺-TC₂ and Cd²⁺-TC₂ to the fluorescent DS(C) guest (in the closed form of the emissive DAE unit). Importantly, compared with the Cd²⁺-chelated supramolecular polymer Cd²⁺-TC₂/DS(C), the formation of the supramolecular polymer Zn²⁺-TC₂/DS(C) with Zn²⁺ ions displayed the most significant PL emission enhancement as verified by the appearance of the corresponding emission bands of the metal-coordinated Zn²⁺-TC₂ host at 338 and 417 nm, as well as the DS(C) guest at 545 nm shown in Fig. S12 (ESI†), which might be due to the distinct electronic structures of chelating Zn²⁺ and Cd²⁺ metal ions. Therefore, the excellent selectivity of the demetalated component TC₂/DS(C) toward Zn²⁺ ions over the other metal ions was verified in the pronounced PL response during the formation of supramolecular polymer Zn²⁺-TC₂/DS(C).

A study of the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O) by concentration-dependent viscometry measurements

The viscometry studies were performed to prove the formation of the supramolecular polymer Zn²⁺-TC₂/DS(O), where the polyelectrolyte effect was excluded by adding 0.05 M tetrabutylammonium hexafluorophosphate in the specific viscosity measurements in CDCl₃/CD₃CN (1/1, v/v).⁵⁴ A double logarithmic plot of the specific viscosity versus the equal concentration of the DS(O) guest and Zn²⁺-TC₂ host (1 : 1) is shown in Fig. S11 (ESI†), where the slopes of the curves were 0.0075 and 0.0253 in the low and high concentration ranges, respectively. The formation of the supramolecular polymer Zn²⁺-TC₂/DS(O) was verified in Fig. S11 (ESI†) and calculated to be 25 mM as the critical polymerization concentration (CPC) at the slope change point, which indicated a ring-chain transition from cyclic oligomers to a linear supramolecular polymer.⁵⁵

Dis-assembly/re-assembly of the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O & C) via interactions with base/acid

The impacts of the chemical-stimuli-responsive supramolecular assembly (i.e., host–guest interaction) on energy modulations from the Zn²⁺-TC₂ host to the DS(C) guest in supramolecular polymer Zn²⁺-TC₂/DS(C) were investigated. As illustrated in Fig. 7(a), the secondary ammonium salt was converted to free amine under a basic environment with trimethylamine (TEA), and it was reversed to the secondary ammonium salt under an acidic environment with trifluoroacetic acid (TFA), which induced the dis-assembly and re-assembly of the supramolecular polymer Zn²⁺-TC₂/DS(C), respectively. The corresponding UV/Vis and PL spectral changes are displayed in Fig. 7(b) and (c), respectively. As demonstrated in Fig. 7(b), upon the addition of base (TEA), the absorption bands at 311 and 322 nm of Zn²⁺-TC₂ host and at 425 nm of DS(C) guest in supramolecular polymer Zn²⁺-TC₂/DS(C) were slightly reduced and were regained via the addition of an equal amount of acid (TFA). The corresponding PL spectra were obtained upon the addition of base (TEA); the dis-assembly of supramolecular polymer Zn²⁺-TC₂/DS(C) occurred to turn off the FRET process and the recovered emissions of the monomer component of the Zn²⁺-TC₂ host appeared at 338 and 422 nm, but the emission peak of neutralized-DS(C) at 545 nm was completely quenched. As illustrated in Fig. 3(c), this result confirmed the PET-ON process⁵¹ from the neutralized ammonium group to the photo-cyclized DAE unit, owing to the removal of charge as compared with the emission of the protonated DS(C) guest at 545 nm. Accordingly, the FRET-OFF process was induced by the disassembly of the host–guest interaction of the photo-cyclized supramolecular polymer Zn²⁺-TC₂/DS(C) resulting in the full recovery of the emission intensity of the Zn²⁺-TC₂ host along with the PET-ON process of neutralized-DS(C).

Fig. 7 (a) Pictorial demonstration of supramolecular polymer Zn²⁺-TC₂/DS(C) (1 : 1) dis-assembly/re-assembly by means of interactions with base/acid in CHCl₃/CH₃CN (1/1, v/v). (b) UV/Vis spectral changes of Zn²⁺-TC₂/DS(C) upon the addition of TEA (0 to 100 μL) and TFA (0 to 100 μL) in CHCl₃/CH₃CN (1/1, v/v). (c) PL spectral changes of Zn²⁺-TC₂/DS(C) upon the addition of TEA (0 to 100 μL) and TFA (0 to 100 μL) in CHCl₃/CH₃CN (1/1, v/v). Insets: Photoimages of Zn²⁺-TC₂/DS(C) with acid/base in CHCl₃/CH₃CN (1/1, v/v). (λ_ex = 300 nm for PL exp.).

After the addition of an equal amount of acid (TFA), the reversible PL behavior and the recovery of energy transfer from host to guest were observed to prove the re-assembly of supramolecular polymer Zn²⁺-TC₂/DS(C) in Fig. 7(c). As illustrated in the inset of Fig. 7(c), the cyan emission color (i.e., greenish-blue) for supramolecular polymer Zn²⁺-TC₂/DS(C) changed to dark blue for free components of the Zn²⁺-TC₂ host and the neutralized-DS(C) guest after the dis-assembly of the supramolecular polymer. Likewise, similar experimental findings of the dis-assembly/re-assembly of supramolecular polymer Zn²⁺-TC₂/DS(O)via base/acid interactions were also observed as summarized in Fig. S12 (ESI†). According to this result, the reversible chemical-stimuli-responsive energy transfer of the dis-assembly/re-assembly of supramolecular polymer Zn²⁺-TC₂/DS(C)via the addition of base/acid was confirmed and can be used for the ratiometric sensing of pH. Therefore, the FRET process from the Zn²⁺-TC₂ host to the turn-on fluorescence of the DS(C) guest in the closed form of the emissive DAE unit through the supramolecular host–guest interactions was further enhanced by the significant “molecular wire and stimuli-responsive effects” of supramolecular polymer Zn²⁺-TC₂/DS(C) in this study.

Dis-assembly/re-assembly of [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(O & C)via interactions with chelating agent (cyclen)/Zn(OTf)₂

It is noteworthy that cyclen (1,4,7,10-tetraazacyclododecane) has a higher binding affinity towards Zn²⁺ in contrast to terpyridine.⁵⁶ Therefore, we performed experiments to investigate the chelation capability of Zn²⁺ ions via interactions with the competing ligand (cyclen) in the demetalation/re-metalation supramolecular polymer Zn²⁺-TC₂/DS(C) as depicted in Fig. 8(a). The corresponding UV/Vis and PL spectral changes in the supramolecular polymer Zn²⁺-TC₂/DS(C)via interactions with chelating agent cyclen and Zn(OTf)₂ are displayed in Fig. 8(b) and (c). Upon the addition of cyclen, the absorption bands of 311 and 322 nm of Zn²⁺-TC₂ host in supramolecular polymer Zn²⁺-TC₂/DS(C) were diminished completely and the absorption bands of 311 and 322 nm were regained via the addition of an equal amount of Zn(OTf)₂, respectively, as shown in Fig. 8(b).

Fig. 8 (a) Pictorial demonstration of supramolecular polymer Zn²⁺-TC₂/DS(C) (1 : 1) dis-assembly/re-assembly by means of interactions of cyclen/Zn²⁺ in CHCl₃/CH₃CN (1/1, v/v). (b) UV/Vis spectral changes of Zn²⁺-TC₂/DS(C) upon the addition of cyclen (0 to 100 μL) and Zn(OTf)₂ in CHCl₃/CH₃CN (1/1, v/v). (c) PL spectral changes of Zn²⁺-TC₂/DS(C) upon the addition of cyclen (0 to 100 μL) and Zn(OTf)₂ in CHCl₃/CH₃CN (1/1, v/v). Insets: Photoimages of Zn²⁺-TC₂/DS(C) in the absence and presence of cyclen (0 and 100 μL) (λ_ex = 300 nm for PL exp.).

Upon the addition of cyclen, related PL spectral changes in the emission bands at 338 and 417 nm of Zn²⁺-TC₂ host and the emission band at 545 nm of DS(C) guest in the supramolecular polymer Zn²⁺-TC₂/DS(C) were all quenched significantly, which was due to the dis-assembly of the supramolecular polymer into the demetalated component TC₂/DS(C) without energy transfer due to the lack of zinc-coordinated emission centers as shown in Fig. 8(c). On the contrary, after the addition of an equal amount of Zn(OTf)₂, the re-assembly of supramolecular polymer Zn²⁺-TC₂/DS(C) was further verified by the re-appearance of the corresponding emission bands as shown in Fig. 8(c). Similarly, the comparable experimental findings of the dis-assembly/re-assembly of supramolecular polymer Zn²⁺-TC₂/DS(O)via chelating agent cyclen and Zn(OTf)₂ were also observed as illustrated in Fig. S13 (ESI†). Hence, the reversible chelation-stimuli-responsive energy transfer for the demetalation/re-metalation of supramolecular polymer Zn²⁺-TC₂/DS(C)via the addition of cyclen and Zn(OTf)₂ was confirmed.

To investigate the temperature effect on the reversibility of non-covalent interactions in our supramolecular polymer, the PL spectral changes of the supramolecular polymer Zn²⁺-TC₂/DS(C) in CHCl₃/CH₃CN (1/1, v/v) were measured within the temperature range of 40–60 °C as shown in Fig. S16 (ESI†). When the temperature was increased above 40 °C, the fluorescence intensity of Zn²⁺-TC₂/DS(C) showed slight decreases in the emission maxima of the DS(C) guest (i.e., at 545 nm) in the supramolecular polymer Zn²⁺-TC₂/DS(C), due to the reduction of the supramolecular interactions between the metal-coordinated Zn²⁺-TC₂ host and the DS(C) guest in supramolecular polymer Zn²⁺-TC₂/DS(C) by heating. The reversible fluorescence changes with temperature variations were maintained, but there were fewer PL changes affected by temperature in contrast to TEA and chelating agents in supramolecular systems (see Fig. 7 and 8). Accordingly, normal reduced non-covalent interactions by heating and reversible thermal-responsive phenomena were observed in the supramolecular polymer Zn²⁺-TC₂/DS(C).

Conclusion

We have designed and constructed optical-switchable diarylethene-based [2]pseudo-rotaxane polymers Zn²⁺-TC₂/DS(O & C) to elucidate the optical and multi-stimuli responsive energy transfer from the metal-coordinated Zn²⁺-TC₂ host to the fluorescent DS(C) guest in the closed form of the emissive DAE unit. The confirmation of the supramolecular polymer Zn²⁺-TC₂/DS(O & C) was further validated by ¹H-NMR, UV/Vis and PL spectra along with specific viscosity measurements. Spectroscopic studies revealed that upon the UV irradiation of λ = 355 nm to cyclize the DAE unit of DS(C), the promoted FRET-ON process from the fluorescent metal-coordinated Zn²⁺-TC₂ host to the PET-OFF DS(C) guest in the closed form of the emissive DAE unit was predominantly produced in the supramolecular polymer Zn²⁺-TC₂/DS(C). The reversibility of energy transfer can be controlled by the de-cyclized DAE unit of DS(O) upon the irradiation of visible light at λ ≥ 400 nm to diminish energy transfer for supramolecular polymer Zn²⁺-TC₂/DS(O). Endowed with the promoted FRET process, our supramolecular polymer displayed ratiometric PL emission peaks that could demonstrate a prominent FRET process triggered via multi-stimuli responses. In the dis-assembly/re-assembly of the host–guest interactions of the supramolecular polymer Zn²⁺-TC₂/DS(C) in response to base/acid (TEA/TFA), the FRET-OFF process was induced by the disassembly of the host–guest interactions of photo-cyclized supramolecular polymer Zn²⁺-TC₂/DS(C), resulting in the full recovery of the emission intensity of the Zn²⁺-TC₂ host along with the PET-ON process of neutralized-DS(C). The reversible chelation-stimuli responsive energy transfer for the demetalation/re-metalation of the supramolecular polymer Zn²⁺-TC₂/DS(C)via the addition of cyclen and Zn(OTf)₂ was further confirmed by its controllable energy transfer. All these attractive findings illustrate the efficient control of energy transfer by the multiple-responsive capabilities of the host–guest, metal–ligand and photoactive interactions in the [2]pseudo-rotaxane polymer Zn²⁺-TC₂/DS(C), which were investigated for the first time in this study. This can provide ideal viewpoints for the development of multi-responsive and photo-switchable energy transfer for versatile supramolecular applications.

Experimental section

Materials and characterization

All reagents and starting materials were commercially obtained with high purities and the solvents were dried by distillation under appropriate drying agents. In general, the progress of the reactions was monitored using TLC plates and the desired compounds were purified by column chromatography on silica gel. The characterization of the precursors and target compounds was performed via¹H and ¹³C NMR spectra (recorded on a 300 MHz spectrometer and samples were dissolved in CDCl₃ and DMSO-d₆), mass spectra (HRMS, (obtained on a mass spectrometer) and elemental analysis (carried out using an Elemental Varo EL)). The chemical shifts were expressed in ppm and coupling constants (J) in Hz. UV-Vis absorption and fluorescence spectra were measured on a V-670 spectrophotometer and F-4500 fluorescence spectrophotometer, respectively. The UV-irradiation was carried out using a 500 W xenon arc lamp (Solar Electro-Optics Co., Ltd) with an intensity of 1.17 mW cm⁻² as the light source.

The synthetic routes to obtaining the target components of the novel optical-switchable supramolecular polymers via multiple-responsive host–guest and metal–ligand interactions are outlined in Scheme 1. Zn²⁺ metal ion was coordinated with two TC ligands containing a macrocycle (DB24C8) linked to a terpyridyl terminus to form the Zn²⁺-TC₂ host and diarylethene (DAE) bearing two terminal secondary ammonium salts to form the DS guest.

Scheme 1 Synthetic routes to the ligand (TC) and guest (DS).

Commercially available starting compounds (catechol and 2-[2-(2-chloroethoxy)ethoxy]ethanol) have been used to obtain the intermediates 1 and 2 by tosylation protection of hydroxyl groups. Intermediate 3 was obtained by the cyclization of intermediate 2 with 3,4-dihydroxybenzaldehyde under refluxing conditions. Intermediate 4 was obtained by using reducing agent NaBH₄. After that, ligand TC was synthesized with the tri-pyridyl intermediate under NaH reaction conditions with a desirable yield. Intermediate 7 was successfully synthesized in our previous report³⁰ and was used in this study to synthesize the target guest DSvia the corresponding condensation with benzylamine to generate the Schiff base intermediate 8, where the reduction of compound 7 by NaBH₄ produced the secondary amine 8, followed by sequential treatments with HCl and NH₄PF₆ to acquire guest DS. The synthetic routes of all intermediates and target compounds are provided in detail with their characterization data as follows.

Synthesis of compound 1

Catechol (5.5 g, 50 mmol), K₂CO₃ (20.81 g, 150 mmol) and KI (2.52 g, 12.50 mmol) were dissolved in acetonitrile (CH₃CN, 500 mL) at room temperature under a N₂ gas atmosphere. The reaction mixture was stirred at 40 °C for 1 h and followed by the addition of 2-[2-(2-chloroethoxy)ethoxy]ethanol (13 mL, 150 mmol) dropwise in the above reaction mixture and refluxed at 80 °C for 36 h. The progress of the reaction was monitored by TLC. After completion, the reaction was cooled to room temperature, filtered and evaporated to get the crude residue. The crude residue was subjected directly to column chromatography (silica gel, ethyl acetate/hexane = 3/2 and then ethyl acetate/MeOH = 9/1) to afford the red-brown oily desired precursor 1 with a yield of 13.5 g (72.1%).

¹H-NMR (300 MHz, CDCl₃): δ = 3.32 (s, 2H), 3.56–3.59 (m, 4H), 3.64–3.74 (m, 12H), 3.84–3.87 (m, 4H), 4.13–4.16 (m, 4h), 6.88 (s, 4H).; ¹³C-NMR (75 MHz, CDCl₃): δ = 61.3, 68.5, 69.5, 70.1, 70.6, 72.6, 114.3, 121.5, 148.6; HRMS (ESI): [M + Na]⁺(calcd for C₁₈H₃₀NaO₈): m/z = 397.1833; found: 397.1839; elemental analysis: calcd for C₁₈H₃₀O₈: C = 57.74; H = 8.08; O = 34.18, found: C = 57.70; H = 8.08; O = 34.22.

Synthesis of compound 2

Compound 1 (5 g, 13.35 mmol) was dissolved in THF (200 mL) and cooled to 0 °C by using an ice bath. Then, NaOH (4.31 g, 106.85 mmol) was dissolved in H₂O (100 mL) and added to the above reaction mixture with stirring for 30 min. After that, 4-toluenesulfonyl chloride (TsCl, 10.21 g, 53.42 mmol) was dissolved in THF (200 mL) and added dropwise to the above reaction mixture with vigorous stirring at 0 °C for up to 1 h. The reaction mixture was allowed to stir at room temperature overnight. The progress of the reaction was monitored by TLC. After completion, the reaction mixture was diluted with DCM (200 mL) and neutralized by 0.1 N HCl (200 mL × 2). The organic layer was separated and washed with a saturated solution of NaCl (200 mL × 2). The organic layer was dried over MgSO₄, filtered and evaporated to get the crude residue, which was purified by column chromatography (silica gel, ethyl acetate/hexane = 3/2) to give a colorless oily compound 2 with a yield of 7.05 g (i.e., 78%).

¹H-NMR (300 MHz, CDCl₃): δ = 2.41 (s, 6H), 3.44–3.47 (m, 4H), 3.51–3.58 (m, 8H), 3,68 (m, 4H), 4.03–4.11 (m, 8H), 6.86–6.98 (m, 4H), 7.32 (d, J = 8.3 Hz, 4H), 7.78 (d, J = 8.3 Hz, 4H); ¹³C NMR (75 MHz, CDCl₃): δ = 21.6, 68.6, 68.7, 69.4, 69.7, 70.6, 70.7, 114.6, 121.6, 127.9, 129.9, 132.8, 144.9, 148.8; HRMS (ESI): [M + 1]⁺(calcd for C₃₂H₄₃O₁₂S₂): m/z = 683.2190; found: 683.2190; elemental analysis for C₃₂H₄₂O₁₂S₂: calculated: C = 56.29; H = 6.20; O = 28.12; S = 9.39, found: C = 56.33; H = 6.19; O = 28.08; S = 9.40.

Synthesis of compound 3

3,4-Dihydroxybenzaldehyde (1.38 g, 9.99 mmol) and K₂CO₃ (16.30 g, 49.95 mmol) were mixed in acetonitrile (300 mL) at room temperature. The reaction mixture was heated under reflux for 1 h, followed by the addition of compound 2 (6.83 g, 9.99 mmol) in acetonitrile (100 mL) during reflux and allowed to stir for 24 h. The progress of the reaction mixture was monitored by TLC. After completion, the reaction mixture was cooled to room temperature and the solvent was removed by evaporation. The crude residue was dissolved in DCM (200 mL) and washed with 1 N HCl and saturated NaCl aqueous solution. After drying with MgSO₄, filtration and removal of solvent, the crude product was purified by column chromatography (SiO₂, ethyl acetate/MeOH = 10/1) to give a yellowish solid compound 3 with a yield of 3.7 g (i.e., 77% w/w).

¹H-NMR (300 MHz, CDCl₃): δ = 3.74(s, 8H), 3.78–3.95 (m, 8H), 4.19–4.33 (m, 8H), 6.96 (s, 4H), 7.09 (d, 1H), 7.47–7.53 (m, 2H), 9.84 (s, 1H); ¹³C-NMR (75 MHz, CDCl₃): δ = 67.4, 67.7, 68.4, 69.7, 111, 116.9, 122.9, 123.6, 127.7, 131.4, 147.4, 148.1, 153.8, 162.3, 190.7; HRMS (ESI⁺): [M + Na]⁺ (calcd for C₂₅H₃₂NaO₉): m/z = 499.1939; found: 499.1947.; Elemental Analysis: Calculated for C₂₅H₃₂O₉: C = 63.01; H = 6.77; O = 30.22, found: C = 63.05; H = 6.75; O = 30.20.

Synthesis of compound 4

Compound 3 (2 g, 4.19 mmol) was dissolved in EtOH/CH₂Cl₂ (v/v = 50/50 mL) and the solution was cooled to 0 °C. NaBH₄ (0.79 g, 12.59 mmol) was added slowly to the reaction mixture, which was allowed to warm up to room temperature and kept stirring overnight. The progress of the reaction was monitored by TLC. After completion, the reaction mixture was quenched by the addition of water (100 mL) and the product was extracted with CH₂Cl₂ (100 mL × 2). The combined organic layer was dried with MgSO₄, filtered and the solvent removed to give a white solid compound 4 with a yield of 1.81 g (i.e., 90%).

¹H-NMR (300 MHz, DMSO-d₆): δ = 3.67 (s, 8H), 3.74–3.82 (m, 8H), 4.04–4.21 (m, 8H), 4.52 (s, 2H), 6.74–6.84 (m, 7H); ¹³C-NMR (75 MHz, CDCl₃): δ = 69.4, 69.9, 71.2, 113.7, 114.1, 121.0, 121.4, 131.4, 148.4, 148.9; HRMS (ESI): [M + Na]⁺(calcd for C₂₅H₃₄NaO₉): m/z = 501.2095; found: 501.2104; elemental analysis: calcd for C₂₅H₃₄O₉: C = 62.75; H = 7.16; O = 30.09, found: C = 62.73; H = 7.17; O = 30.10.

Synthesis of compound TC

NaH (60%, 0.25 g, 6.25 mmol) was added to a stirred suspension of compound 4 (0.61 g, 1.25 mmol) in anhydrous DMF (15.0 mL) at 70 °C. After stirring for 30 min, 4′-chloro-2,2′:6′,2′′-terpyridine (0.37 g, 1.4 mmol) was added to the mixture and stirred for 48 h at 70 °C. After the reaction, the mixture was cooled to room temperature and DMF was removed by evaporation. The residue was dissolved in CHCl₃ (100 mL) and washed with saturated NaCl aqueous solution. The organic layer was dried with MgSO₄, filtered and concentrated by evaporation. The crude product TC was purified by column chromatography (neutral, aluminum oxide, DCM/hexane = 7/3 to EtOAc/DCM = 3/7) to give a white solid compound 5 with a yield of 0.75 g (i.e., 85%).

¹H-NMR (300 MHz, CDCl₃): δ = 3.82 (s, 8H), 3.91 (d, 8H) 4.12–4.18 (m, 8H), 21 (s, 2H), 6.85 (s, 5H), 6.92 (d, 2H), 7.29–7.34 (m, 2H), 7.80–7.86 (m, 2H), 8.09 (s, 2H), 8.61 (d, 2H, J = 7.9 Hz), 8.68 (d, 2H, J = 4.7 Hz).; ¹³C-NMR (75 MHz, CDCl₃): δ = 69.4, 69.5, 69.9, 71.3, 107.7, 113.5, 113.8, 114.1, 120.8, 121.4, 121.5, 123.9, 129.1, 136.9, 149.1, 156.1, 157.2, 162.4, 167.0; HRMS (ESI): [M + Na]⁺(calcd for C₄₀H₄₃N₃NaO₉): m/z = 732.2892; found: 732.2884; elemental analysis: calculated for C₄₀H₄₃N₃O₉: C = 67.69; H = 6.11; N = 5.92; O = 20.29, found: C = 67.64; H = 6.10; N = 5.94; O = 20.32.

Synthesis of intermediates 5, 6 and 7

Intermediates 5, 6 and 7 were successfully synthesized as indicated in our previous report +.³⁰

Synthesis of compound 8

Compound 7 (1 g, 3.16 mmol) and benzyl amine (0.92 g, 9.48 mmol) were dissolved in methanol (100 mL) at room temperature and allowed to stir for 24 h. After completion, the methanol was removed by evaporation and the residue was dissolved in CH₂Cl₂ (100 mL) and cooled to 0 °C using an ice bath. Then, NaBH₄ (0.54 g, 15.8 mmol) was added in portions and the suspension was stirred for 1 h at 0 °C and further reacted for another 24 h at room temperature. The progress of the reaction was monitored by TLC and the reaction mixture was quenched with water (100 mL) after completion. The organic layer was separated and washed with water (50 mL × 2), then dried with MgSO₄, filtered and concentrated by evaporation. The residue was purified by column chromatography (silica gel, ethyl acetate/hexane = 6 : 4) to give an orange-colored oily compound 8 with a yield of 1 g (i.e., 70%).

¹H-NMR (300 MHz, DMSO-d₆): δ = 1.86 (s, 6H), 1.96 (m, 2H), 2.71 (m, 4H), 3.61 (s, 4H), 3.68(s, 4H), 6.61 (s, 2H), 7.19–7.33 (m, 10H). ¹³C-NMR (75 MHz, CDCl₃): δ = 14.44, 23.12, 38.46, 47.47, 52.43, 126.64, 127.20, 128.47, 128.56, 133.79, 134.62, 1336, 138.89, 139.77. HRMS (ESI): [M + 1]⁺ (calcd for C₃₁H₃₅N₂S₉): m/z = 499.2236; found: 499.2236. Elemental analysis: calculated for C₃₁H₃₄N₂S₂: C = 74.65; H = 6.87; N = 5.62; S = 12.86, found: C = 74.66; H = 6.90; N = 5.59; S = 12.85.

Synthesis of compound DS

Compound 8 (0.5 g, 1 mmol) was dissolved in methanol (50 mL) and conc. HCL was added to adjust the pH to less than 2, then the solvent was removed by evaporation to get the crude residue. After that, the crude residue was suspended in acetone (50 mL), followed by the addition of a saturated aqueous solution of NH₄⁺PF₆⁻ until the suspension became clear. The solvent was removed by evaporation and the residue was filtered, and washed with water (200 mL). The product was dried in a vacuum oven at 100 °C to give a yellow solid guest DS with a yield of 0.75 g (i.e., 95%).

¹ H-NMR (300 MHz, CD₃CN): δ = 1.92 (s, 6H), 2.06 (m, 2H), 2.77 (m, 4H), 4.16 (s, 4H), 4.27 (s, 4H), 5.56 (s, 2H), 6.99 (s, 2H), 7.46 (s, 10H); ¹³C-NMR (75 MHz, CD₃CN): δ = 14.5, 23.7, 39.1, 46.4, 51.8, 128.2, 130.1, 130.8, 131.2, 133.5, 135.8, 137.2, 183.7; elemental analysis: calculated for C₃₁H₃₆N₂S₉: C = 47.09; H = 4.59; N = 3.54; S = 8.11, found: C = 47.08; H = 4.61; N = 3.53; O = 8.12.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial supports are provided by the Ministry of Science and Technology (MOST) in Taiwan through MOST 103-2113-M-009-018-MY3, MOST 103-2221-E-009-215-MY3, MOST 106-2113-M-009-012-MY3 and MOST 107-3017-F009-003 and Ministry of Education, Taiwan (SPROUT Project-Center for Emergent Functional Matter Science of National Chiao Tung University). In addition, thanks to Ms. Shou-Ling Huang of Ministry of Science and Technology (Instrumentation Center at National Taiwan University) for the assistance in two-dimensional diffusion-ordered (DOSY) and rotating-frame nuclear overhauser effect (ROESY) (500 MHz NMR) experiments.

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis and PL spectra, ¹H- and ¹³C-NMR data, mass data (HRMS-ESI) and elemental analysis (EA) data. See DOI: 10.1039/d0qm00605j

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