Diarylethene-powered photo-switching between precipitated nanostrips and dispersed supramolecular polymers

Abstract

UV and visible light irradiation reversibly switches diarylethene-based supramolecular assemblies between precipitated, ordered nanostrip aggregates and uniformly dispersed supramolecular fibers in a nonpolar solvent by modulating conformational freedom.

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Controlling the aggregation behavior of supramolecular polymers (SPs) is essential for translating molecular design into practical materials. Molecular design must address not only the primary one-dimensional architecture of supramolecular polymers but also their propensity for higher-order organization, i.e., bundling or lateral association, which ultimately governs solubility, processability, and bulk material properties.^1–8 In this context, supramolecular polymer polymorphism offers powerful opportunities: a single monomer can access multiple one-dimensional morphologies and their distinct higher-order aggregation states.^9–14 Yet, reversible interconversion between such one-dimensional morphologies remains challenging because each assembled state is typically stabilized by a different balance of noncovalent interactions and entropic penalties and is frequently kinetically separated by substantial barriers.

In this regard, photochromic molecules offer an attractive strategy to regulate the structural organization of supramolecular polymers, as light can reversibly modulate the monomer conformation that governs their mode of packing and resulting morphology.^15–18 However, most photoresponsive SP systems exhibit photocontrol over the degree of polymerization or drastic morphology changes by exploiting the different behaviors of the two photoisomers.^19–21 In contrast, inducing light-driven reorganization of the packing mode—together with changes in their higher-ordered aggregation state—without complete disassembly remains highly challenging.^22–24 We envisioned that a photoswitch capable of modulating conformational flexibility, rather than undergoing a drastic geometric transformation, could provide a subtle yet effective means to achieve this level of structural control.

Diarylethenes (DAEs) offer precisely this capability: the open-ring isomer (DAE_o) is conformationally flexible, whereas the closed-ring isomer (DAE_c) is rigid and more π-conjugated.^25,26 These DAE properties were previously incorporated in SP systems, exhibiting distinct differences in the SP morphologies between the two isomers.^27–37 Yet, photo-controlling the higher-ordered SP aggregation by leveraging the capability of DAEs has not been explored. Here, we show that a side-chain-functionalized DAE monomer (1, Fig. 1a) enables fully reversible switching between a precipitated, ordered aggregate (nanostrip) and a uniformly dispersed supramolecular polymer (nanofiber) simply by alternating UV and visible light irradiation (Fig. 1b). To gain mechanistic insight, we directly visualize the photoinduced reorganization at the solid–liquid interface by high-speed atomic force microscopy (AFM), revealing anisotropic photoinduced dissolution of ordered aggregates followed by reassembly into uniformly dispersed supramolecular polymers.

Fig. 1 (a) Molecular structures of 1 in the open-ring (1o) and closed-ring (1c) forms. (b) Schematic illustration of the photoswitchable self-assembly behavior of 1, showing reversible interconversion between 1o and 1c under UV/Vis light irradiation and their respective assembly into Agg_1o (insoluble nanostrip) and SP_1c (soluble nanofiber).

Photoswitchable monomer 1 was synthesized according to Scheme S1 (supplementary information, SI) and characterized by ¹H and ¹³C NMR spectroscopy and electrospray ionization mass spectrometry. ¹H NMR and UV/Vis absorption spectroscopy confirmed quantitative ring-opening to the open-ring isomer (1o) upon visible-light irradiation (λ = 620–645 nm). A hot solution of 1o (c = 100 µM) in n-octane/CHCl₃ (95 : 5, v/v) was prepared by heating to 100 °C, followed by cooling to room temperature (r.t.), upon which the absorption band at 312 nm intensified and slightly blue-shifted to 310 nm (orange line in Fig. S1a, SI). Immediately after cooling, ill-defined aggregates were observed by AFM (Fig. S1b, SI). Upon aging the above solution of 1o in the dark, the absorption intensity decreased while the band maximum red-shifted to 320 nm, accompanied by the formation of macroscopic precipitates (yellow line in Fig. S1a, SI). UV/Vis analysis of the supernatant after centrifugation showed that ca. 90% of the initially dissolved 1o had been removed from solution as precipitates (Fig. S1d, SI). Increasing the initial concentration of 1o accelerated the decrease in transmittance at 400 nm, indicating concentration-dependent precipitation behavior, consistent with precipitation from a supersaturated state generated upon cooling (Fig. S1c, SI). These results suggest that cooling initially produces metastable ill-defined aggregates, which subsequently undergo slow spectral change and macroscopic precipitation under the present conditions.^38–40

Scanning electron microscopy (SEM) of the precipitates revealed an entangled fibrous microstructure (Fig. 2a and Fig. S2, SI). Hereafter, we refer to these precipitated aggregates of 1o obtained by the cooling protocol as Agg_1o. Powder X-ray diffraction (XRD) of Agg_1o displayed multiple reflections, most of which can be assigned to lamellar ordering with a d-spacing of 3.44 nm (Fig. 2b). Consistent with this, upon spin-coating a dispersion of Agg_1o onto a highly oriented pyrolytic graphite (HOPG) substrate, AFM imaging of the Agg_1o surface showed nanostrips with 8.31 nm periodicity and a thickness of 3.45 nm, observed as a single or multi-layer (Fig. 2c–e and Fig. S3, SI). Thus, despite its conformational flexibility, 1o forms multi-layered nanostrips, which further associate to form macroscopic aggregates.

Fig. 2 (a) SEM image of the precipitated Agg_1o. (b) Powder XRD pattern of the precipitated Agg_1o (c = 0.8 mM). (c) AFM image of Agg_1o and (d) a magnified view of the region in (c) enclosed by a white rectangle. (e) Cross-sectional analysis along an orange arrow in (d). (f) Proposed packing model of Agg_1o.

FT-IR spectroscopy of an Agg_1o film on a KBr plate showed an N–H vibrational band at 3238 cm⁻¹ and a C=O stretching band at 1632 cm⁻¹, attributable to the amide groups. Compared with the monomeric state in CHCl₃ solution (ν_N–H = 3454 cm⁻¹ and ν_C=O = 1659 cm⁻¹), both bands were markedly shifted to lower wavenumbers, indicating the formation of strong intermolecular hydrogen bonding in Agg_1o (Fig. S4a, SI). Upon UV-light irradiation (λ = 290 nm) of the Agg_1o-coated KBr plate, the ring-closure reaction of the DAE_o moieties proceeded to give a ratio of 1o/1c = 40 : 60 (determined by ¹H NMR), and the two bands shifted slightly to 3233 cm⁻¹ and 1630 cm⁻¹, respectively. These results suggest that the DAE_o moieties in Agg_1o are preorganized in the photoreactive antiparallel conformation^41–43 and that ring closure to DAE_c occurs while largely preserving the hydrogen-bonding motif. Based on the above results and molecular modeling, we propose the packing model for Agg_1o shown in Fig. 2f. In this model, 1o first forms a linear hydrogen-bonded chain through intermolecular N–H⋯O=C amide interactions. Along this chain, neighboring DAE_o units are further stabilized by π–π stacking interactions, which rigidify the assembly into a ribbon-like nanostrip. The methylene C–H stretching bands shifted slightly to lower wavenumbers upon aggregation (Fig. S4b, SI), indicating increased local conformational ordering of the alkyl chains.^44,45 The ordered supramolecular strands can further associate laterally, and the resulting two-dimensional sheets hierarchically stack, potentially assisted by short contacts involving C–F groups (e.g., possible F⋯F close contacts), to form the layered structures. Because the present packing model cannot fully explain the observed 8.3 nm periodicity, further structural investigation, including attempts at single-crystal growth, is currently underway.

When a stirred dispersion of Agg_1o in n-octane/CHCl₃ (95 : 5, v/v) was irradiated at r.t. with UV light (λ = 290 nm, 3.4 mW cm⁻²), a new broad absorption band centred at 593 nm grew, consistent with the ring-closure reaction to the DAE_c form (blue line in Fig. 3a). Concomitantly, the initial light scattering from the precipitate diminished, indicating photoinduced dissolution (Fig. 3b). The growth of the 592 nm band plateaued after irradiation for 22 min, suggesting that the photostationary state under UV irradiation (PSS_UV) had been reached (Fig. S6a, SI). The 1o/1c ratio at PSS_UV was 6 : 94 as determined by ¹H NMR, comparable to that measured in the monomeric state (Fig. S5, SI). AFM analysis of the resulting homogeneous solution, spin-coated onto an HOPG substrate, revealed flexible supramolecular polymers (SP_1c) with a width of 6.8 nm and a height of 1.2 nm (Fig. 3e–g). In contrast to Agg_1o, SP_1c remained dispersed for at least 14 days (Fig. S6a, SI).

Fig. 3 (a) UV/vis absorption spectral changes of 1o (c = 100 µM) in a 95 : 5 n-octane/CHCl₃ mixture before (red line) and after (blue line) UV-light irradiation (λ = 290 nm, 3.4 mW cm⁻²) to reach a photostationary state (PSS_UV). (b) UV-induced dissolution process until reaching the PSS_UV. (c) SAXS profiles of SP_1c obtained either by UV-light irradiation to Agg_1o (pale blue) or by cooling 1c (blue) (c = 300 µM). Black curves represent fitting with a “core–shell cylinder” model. (d) Schematic illustration of SP_1c. (e) AFM image of SP_1c and (f) a magnified image of bundled fibers in (e) enclosed by a white rectangle. (g) AFM cross-sectional analysis of bundled SP_1c along a blue arrow in (f). (h)–(k) Clipped HS-AFM images of fragmented Agg_1o upon UV-light irradiation (λ = 365 nm, 4.8 mW cm⁻²) at the interface between a HOPG substrate and n-octane (see Video S1). (l) Length-change plots of Agg_1o along blue and pale blue double-headed arrows in (h) before and after UV-light irradiation.

To examine the self-assembly behavior of 1c independent of the irradiation protocol, we studied its temperature-dependent aggregation. Upon heating the solution of SP_1c to 100 °C at a rate of 1.0 °C min⁻¹, the absorption bands around 400 nm and 600 nm shifted to shorter wavelengths, indicating weak interactions between DAE_c moieties (Fig. S6b, SI). Monitoring the absorbance at 415 nm revealed a non-sigmoidal dissociation curve, characteristic of a nucleation–elongation (cooperative) supramolecular polymerization mechanism.⁴⁶ Upon subsequent cooling, no thermal hysteresis was observed (Fig. S7a, SI). In accordance with this, AFM imaging of the cooled PSS_UV solution showed the same SP_1c morphology as that obtained by UV-light irradiation of Agg_1o, suggesting that, unlike 1o, 1c undergoes an equilibrium-controlled assembly process and the organization of SP_1c is independent of the self-assembly pathway (Fig. S7b–d, SI).

The morphology of SP_1c was further supported by in situ small-angle X-ray scattering (SAXS). The scattering profile was consistent with a long and cylindrical object and could be fitted with a core–shell cylinder model (r_core = 1.02 ± 0.01 nm, d_shell = 0.62 ± 0.1 nm, l ≥ 100 nm) (Fig. 3c). The core radius nearly matches the size of the DAE_c moiety including the ester units on both sides (r = 0.96 nm) (Fig. S8, SI), supporting its face-to-face stacking arrangement (Fig. 3d). The smaller apparent outer diameter (2(r_core + d_shell) ≈ 3.3 nm) compared with the AFM cross-section (6.8 nm) likely reflects the similarity of electron density at the alkyl spacers and in the solvent. FT-IR measurements of SP_1c in solution showed N–H and C=O stretching bands at 3295 and 1632 cm⁻¹, respectively (Fig. S6c, SI). Compared with Agg_1o, the N–H band exhibited a smaller shift from the monomeric state in CHCl₃, suggesting weaker hydrogen-bonding interactions in SP_1c. In the C–H stretching region, the FT-IR spectrum of SP_1c showed only minor differences from that of Agg_1o, suggesting that the local conformational ordering of the alkyl chains is comparable in the two assemblies (Fig. S6d, SI). We infer that the closed DAE_c unit, together with the protruding methyl group, introduces packing frustration between the face-to-face DAE_c stacking and the optimal geometry required for amide hydrogen bonding. Consequently, the monomers in SP_1c stack with reduced axial registry and weaker cooperative hydrogen bonding, leading to a lower degree of ordering along the supramolecular main chain and suppressing the development of distinct lateral organization.

The Agg_1o → SP_1c conversion under UV-light irradiation was directly observed using cell-equipped, tip-scan high-speed AFM (HS-AFM).⁴⁷ Because large aggregates hindered stable tip-scanning, Agg_1o was first fragmented by sonication (Fig. S9, SI). In the absence of UV light, the fragments remained stable at the solid–liquid interface between an HOPG substrate and n-octane and could be imaged continuously even under tapping forces (Fig. 3h and i, Movie S1). When UV light (λ = 365 nm, 4.8 mW cm⁻²) was switched on after 60 s, we immediately observed anisotropic dissolution from the strip termini along the long axis at a rate of 1.0 nm s^-1 (Fig. 3h–j, Movie S1 and Movie S2–S5). The dissolution proceeded linearly with time and was independent of the initial strip length (Fig. 3l), suggesting that the ring-closure reaction of DAE_o moieties at the exposed cross-section is the rate-determining step of the nanostrip dissolution.

Notably, dissolution always initiated from the termini rather than from within the nanostrip interior. Upon continued UV-light irradiation, the formation of SP_1c could be observed independently on the substrate (Fig. 3k and Movie S1). The continuous release of 1c from the dissolving termini would increase its local concentration near the interface, potentially reaching the critical concentration required for SP_1c nucleation. The critical total concentration for SP_1c formation in bulk solution was estimated to be much lower than c ≈ 0.39 µM in a solution experiment (Fig. S10, SI). Although interfacial conditions may differ from those in the bulk solution, the low critical concentration suggests that nucleation of 1c can be readily induced upon its release. We could also image SP_1c directly by HS-AFM when a solution of SP_1c was deposited on an HOPG substrate (Movie S6). Together, these observations indicate that the Agg_1o → SP_1c conversion proceeds via an off-pathway route that involves dissolution into monomers (or small oligomeric species), followed by reassembly into SP_1c.

Visible-light-induced ring opening of the DAE_c moieties in SP_1c regenerated Agg_1o. When a solution of SP_1c was irradiated with visible light (620–645 nm, 69 mW cm⁻²), the DAE_c absorption bands bleached quantitatively within 11 min (Fig. S11a, SI). Similar to the cooling protocol for 1o, the solution transiently entered a supersaturated state containing ill-defined aggregates and subsequently yielded precipitates within a day (Fig. S12, SI). AFM and FT-IR measurements of the resulting precipitates confirmed the re-formation of Agg_1o (Fig. S11b, SI).

In conclusion, we have shown that aggregation of one-dimensional DAE supramolecular assemblies can be reversibly controlled by light through the interplay between the conformational flexibility of the DAE core and amide hydrogen bonding. The flexible ring-open isomer forms densely packed aggregates that promote macroscopic precipitation and insolubility, whereas the rigid ring-closed isomer assembles into soluble flexible nanofibers, thereby suppressing lateral organization. We anticipate that exploiting the switchable conformational freedom of DAEs will further expand the design space of reconfigurable supramolecular soft materials.

S. Y., K. M. and T. S. designed the project. K. M. and H. M. performed all the experimental work. K. M. and S. Y. prepared the overall manuscript, including figures. All authors, including T. U., C. G., T. K., S. D. and H. H., contributed by revising and/or commenting on the manuscript. S. Y. supervised the overall research.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data that support the findings of this work have been included in the main text and supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cc01370h.

Acknowledgements

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI grant no. JP23H04873, JP23H04873 and JP24H01717, JP26H02288 through a Grant-in-Aid for Transformative Research Areas “Materials Science of Meso-Hierarchy”. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2024G542). The authors are grateful to Dr Hideaki Takagi and Dr Rie Haruki for the SAXS measurements.

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