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Tsubasa Naka, [Takeshi Ueki](https://orcid.org/0000-0001-9317-6280)

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[Light-switchable LCST windows in an ionic liquid solution of azobenzene copolymers](https://mdr.nims.go.jp/datasets/19cbf7e8-b53b-4a95-9933-0be961c27e36)

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Polymer Journal (2026) 58:1337–1344https://doi.org/10.1038/s41428-026-01195-0RAPID COMMUNICATIONLight-switchable LCST windows in an ionic liquid solution ofazobenzene copolymersTsubasa Naka1,2 ● Takeshi Ueki 1,2Received: 26 February 2026 / Revised: 2 April 2026 / Accepted: 7 April 2026 / Published online: 13 May 2026© The Author(s) 2026. This article is published with open accessAbstractPhoto-induced tuning of lower critical solution temperature (LCST) behavior is an attractive route for reversible phasecontrol, yet the achievable cloud-point shift is often modest in conventional solvent systems. Here, we demonstrate acomposition-amplified, light-tunable LCST in ionic liquid polymer solutions using random copolymers of4-phenylazophenyl methacrylate and phenethyl methacrylate (P(AzoMA-r-PhEtMA)) dissolved in 1-ethyl-3-methylimidazolium trifluoromethylsulfonylimide ([C2mim][TFSI]), a medium in which the azobenzene-containingmonomer is readily compatible. Turbidity measurements reveal opposite cloud-point shifts under UV (cis-type) andvisible-light (trans-type) conditions, enabling a bistable temperature window that widens monotonically with azobenzenecomposition. At 37 °C, the solution is reversibly switched between one-phase and two-phase states by light alone andmaintains stable cycling for more than ~40,500 s. These results provide a practical molecular design guideline for targetingambient-to-physiological light-controlled phase separation in nonvolatile ionic liquid media.Light offers a uniquely gentle, contactless, and reversiblehandle to modulate polymer‒solvent interactions, enabling“on-demand” shifts of phase boundaries in solution. Inthermoresponsive polymers, the photochemical switchingof embedded chromophores, such as azobenzenes, caneffectively tune the cloud point (Tc) by altering the solvationcontrast between two molecular states [1–15]. However, theexperimentally observed shift is often not a simple reflec-tion of this intrinsic contrast. Instead, it is constrained byphotochemical and physical factors, including the achiev-able photostationary state composition, thermal back-iso-merization, and optical geometry (e.g., limited penetrationdepth and path-length effects). As a result, even when theunderlying solvation difference between the two states issubstantial, the macroscopic phase behavior may show onlya modest response unless these constraints are explicitlyaccounted for. Nonvolatile solvents such as ionic liquids(ILs) provide an attractive platform for examining andexploiting phototunable phase behavior because they sup-press concentration drift during measurements and offer adesignable solvation landscape [16–19]. Moreover, manyphotoresponsive polymer/IL systems that have been repor-ted to date exhibit phase transitions at elevated tempera-tures, where thermal back isomerization and opticalattenuation can severely limit the effective cis-rich popu-lation in the bulk [20–23]. This creates a practical andconceptual need for a design strategy that decreases thebaseline transition temperature to ambient-to-physiologicalconditions, thereby allowing the intrinsic cis-/trans-solva-tion contrast to be expressed under standard optical pathlengths and realistic operating conditions.Traditionally, photo-induced phase transition tempera-ture modulation has been widely demonstrated forazobenzene-containing thermoresponsive polymers in aqu-eous media, most notably poly(N-isopropylacrylamide)(PNIPAAm)-based copolymers and related backbones[24–27]. These studies established that changing the cis-/trans-population can shift the phase boundary; however,they also revealed that the observable temperature shift isoften limited to a narrow design window; for example, in* Takeshi UekiUEKI.Takeshi@nims.go.jp1 Research Center for Macromolecules and Biomaterials, NationalInstitute for Materials Science, Tsukuba, Ibaraki, Japan2 Graduate School of Life Science, Hokkaido University,Sapporo, Hokkaido, JapanSupplementary information The online version containssupplementary material available at https://doi.org/10.1038/s41428-026-01195-0.1234567890();,:1234567890();,:http://crossmark.crossref.org/dialog/?doi=10.1038/s41428-026-01195-0&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41428-026-01195-0&domain=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1038/s41428-026-01195-0&domain=pdfhttp://orcid.org/0000-0001-9317-6280http://orcid.org/0000-0001-9317-6280http://orcid.org/0000-0001-9317-6280http://orcid.org/0000-0001-9317-6280http://orcid.org/0000-0001-9317-6280mailto:UEKI.Takeshi@nims.go.jphttps://doi.org/10.1038/s41428-026-01195-0https://doi.org/10.1038/s41428-026-01195-0P(NIPAAm-co-N-(4-phenylazophenyl)acrylamide)(P(NIPAAm-co-AzoAm), the UV photo stationary statereaches only =60% cis-type, and the Tc is maximized onlyat low azobenzene contents (~2–3 mol%), while higherloadings can suppress solubility and diminish the apparentphoto effect [28]. Employing a more hydrophilic backbonecan accommodate a larger effective polarity swing; forinstance, poly(N,N-dimethylacrylamide-co-4-phyenylazo-phenyl acrylate) was reported to exhibit a Tc difference ofup to ~20 °C between the cis- and trans-types, which isconsistent with a higher attainable cis fraction in water [29].Even when a substantial intrinsic solvation contrast exists,the apparent response can be diminished at elevated tem-peratures because thermal back-isomerization progressivelyreduces the cis-rich population during the timescale ofmeasurements. In addition, for strongly absorbing chro-mophores such as azobenzenes, optical attenuation andfinite penetration depth can generate spatially inhomoge-neous isomer distributions, further reducing the effective cisfraction in the bulk and complicating quantitative inter-pretation [20, 23]. Collectively, the results of prior workindicate that a robust photo-induced Tc shift simultaneouslysatisfies three practical constraints: (i) a sufficiently highattainable cis fraction in the photostationary state, (ii) suf-ficiently slow thermal back-isomerization on the measure-ment timescale, and (iii) sufficient optical penetration toavoid strong spatial gradients in isomer composition.Herein, we address these limitations by combining twocomplementary design elements in a nonvolatile IL med-ium. First, we employed poly(phenethyl methacrylate)(PPhEtMA), a polymethacrylate that undergoes LCST-typephase separation at ~40 °C in an IL, 1-ethyl-3-methylimidazolium trifluoromethylsulfonylimide ([C2mim][TFSI]) [30–34], to establish a low-baseline LCST platformthat shifts the phase transition window toward ambient-to-physiological temperatures, thus reducing the impact ofthermal back-isomerization and enabling more reliableirradiation of the bulk without relying on specialized opticalgeometries. Second, we selected a photoswitchable co-monomer, 4-phenylazophenyl methacrylate (AzoMA),whose monomeric form is readily soluble in [C2mim][TFSI], suggesting strong photoswitch, solvent compat-ibility and solvation accessibility, conditions expected toamplify the effective cis-/trans-polarity contrast in solutionrather than sequestering the chromophore into poorly sol-vated microdomains. Using a systematic composition seriesof P(AzoMA-r-PhEtMA), we quantitatively mapped Tc-trans and Tc-cis in [C2mim][TFSI], which demonstratemonotonic expansion of the bistable temperature windowΔTc with the azobenzene fraction and show reversible light-driven switching between one-phase and two-phase states at37 °C (Fig. 1). Together, these results establish a practicaldesign principle for translating molecular photo-isomerization into a robust, quantifiable macroscopicphase response in IL polymer solutions.Figure 2 highlights a composition-tunable, light-programmable LCST-type phase separation of a series ofP(AzoMA-r-PhEtMA) in [C2mim][TFSI]. The preparation,characterization (Supplementary Figs. S1–S12, Supple-mentary Table S1), photochromism (SupplementaryFigs. S13, 14), and thermal relaxation (SupplementaryFigs. S15, 16, and Supplementary Tables S2, 3) ofP(AzoMA-r-PhEtMA) and their protocol are summarized inthe Supporting Information. Notably, the phase transitionwindow lies near ambient-to-physiological temperatures,reflecting the intrinsically low LCST baseline of thePPhEtMA platform in [C2mim][TFSI] (SupplementaryFig. S17). To quantify the photostationary-state composi-tion under the present conditions, we performed additional¹H NMR measurements of AzoMA in [C2mim][TFSI] afterUV and visible-light irradiation (Supplementary Fig. S14).Analysis of the photoisomerization-split signals revealedthat the cis fraction reached 87% under UV irradiation,whereas it was 14% under visible-light irradiation. ThesePoly(trans-AzoMA-PhEtMA) Poly(cis-AzoMA-PhEtMA)Visible light(436nm)UV light(365nm)@37ºCInsoluble SolubleOONNOOm nOONNOOm ntrans-Azobenzene0.5 Dcis-Azobenzene3.1 DFig. 1 Conceptual illustration of light-programmable solubilityswitching of P(AzoMA-r-PhEtMA) in the ionic liquid [C2mim][TFSI]at 37 °C. Visible-light irradiation (436 nm) enriches the trans-state,rendering the polymer insoluble and inducing phase separation,whereas UV irradiation (365 nm) enriches the cis-state, restoringsolubility and yielding a homogeneous solution. The photographsshow representative solution states under the two irradiation conditions1338 T. Naka, T. Uekiresults confirm that a sufficiently cis-rich state was attainedin the present IL medium. Upon UV irradiation, the Tc shiftsto higher temperatures (P(cis-AzoMA-r-PhEtMA)),whereas under visible light, it shifts to lower temperatures(P(trans-AzoMA-r-PhEtMA)) (Fig. 2, SupplementaryFigs. S18–21), demonstrating reversible optical control ofthe solubility of the IL. This shift direction is consistentwith polarity matching. Solvatochromic polarity scalesgenerally place [C2mim][TFSI] in a moderately high-polarity regime, comparable to that of highly polarorganic solvents [35]. In fact, the ET(30) of [C2mim][TFSI]is 52.6 kcal/mol [36], indicating that the polarity of the IL iscomparable to that of typical polar molecular solvents suchas dimethyl sulfoxide (DMSO) (45.1 kcal/mol) and ethanol(51.9 kcal/mol [37]). We therefore interpret the differentphase behaviors of the trans- and cis-rich states not simplyin terms of polarity alone but more broadly in terms ofdifferences in solvation compatibility in the IL medium.Moreover, the dipole moment of azobenzene substantiallyincreases upon trans-to-cis isomerization (from ~0.5 D to~3.1 D) [38], rendering the cis-type polymer more compa-tible with this solvation environment. A possible molecularorigin of this behavior is a change in the local solvationenvironment in the IL. Previous studies on aromaticmethacrylate derivatives in imidazolium-based ILs havesuggested preferential cation–π interactions between theimidazolium cation and aromatic side groups, leading to acharacteristic solvation structure [39, 40]. By analogy,Fig. 2 Temperature-dependenttransmittance profiles ofP(AzoMA-r-PhEtMA) solutionsin [C2mim][TFSI] showinglight-tunable LCST-type phaseseparation. Panelsa–f correspond to copolymerswith AzoMA contents of 0.41,0.77, 1.61, 2.54, 4.51, and14.3 mol%, respectively. Theblue circles and red squaresdenote the transmittancemeasured for the trans-type(visible light irradiation) and cis-type (UV light irradiation)states, respectively. Cloud pointswere determined from thetemperature at 50%transmittance. The polymerconcentration= 1 wt%, opticalpath length= 1 cm, monitoringwavelength= 700 nm, andheating rate= 0.5 °C min−1Light-switchable LCST windows in an ionic liquid solution of azobenzene copolymers 1339similar preferential solvation is plausible for the PhEtMAunits in [C2mim][TFSI]. Photoisomerization of the azo-benzene unit is therefore expected to perturb the balance ofthese polymer‒IL interactions and alter the relative stabilityof the dissolved state, resulting in different phase-transitiontemperatures for the trans-rich and cis-rich states. At thehighest azobenzene loading examined (P(AzoMA-r-PhEtMA) [AzoMA]= 14.3 mol%, Fig. 2f), the trans-typepolymer becomes sufficiently solvatophobic such that Tc-trans shifts below the lower limit of our measurementwindow; thus, only Tc-cis (≈56 °C) could be observedwithin the accessible temperature range.This composition-dependent divergence of Tc-trans andTc-cis is quantified in Fig. 3, which shows that the resultingbistable temperature window, ΔTc (= Tc-cis – Tc-trans), canbe continuously expanded by increasing the AzoMA frac-tion. Over the investigated range ([AzoMA]= 0.41 ~ 14.3mol%), Tc-trans decreases monotonically, whereas Tc-cisincreases monotonically, leading to a systematic wideningof ΔTc up to 24.9 °C at [AzoMA] = 4.51 mol% within thepresent temperature range. This monotonic widening isparticularly notable because in many aqueous azobenzene-polymer systems, the observable Tc separation is confined toa narrow composition window and can become non-monotonic at higher chromophore loadings, reflecting acompetitive hydrophilic–hydrophobic balance and limitedisomerization contrast under experimental conditions. In thepresent IL system, by contrast, Tc-trans and Tc-cis remainwell separated and continuously tunable within the studiedrange, allowing ΔTc to be treated as a genuine designvariable. To assess whether this bistable window is sensitiveto polymer loading, we further examined the concentrationdependence of Tc for P(AzoMA-r-PhEtMA) with [AzoMA]= 0.77, 2.91, and 4.51 mol% (Supplementary Fig. S23).Within the concentration range examined, the Tc-cisdecreased slightly with increasing polymer concentrationfor all three copolymers. This trend is consistent with thegeneral behavior of LCST-type phase transitions, in whichincreasing the polymer volume fraction shifts the phaseboundary to lower temperatures [41]. Notably, the two Tcsshift in a largely parallel manner over the investigatedrange, so ΔTc remains nearly constant. This concentration-insensitive ΔTc indicates that the cis-/trans-solvation con-trast is preserved and that the photoprogrammable tem-perature window is robust against moderate variations in thepolymer concentration.This behavior can be rationalized within the practicalconstraint framework highlighted in the introduction: themacroscopic Tc shift reflects not only the intrinsic cis-/trans-solvation contrast but also (i) the attainable cis fraction at thephoto stationary state, (ii) the timescale of thermal back-isomerization relative to the measurement, and (iii) opticalattenuation/penetration that can introduce spatial gradients inisomer composition. In aqueous media, early demonstrationsalready emphasized that the effective cis fraction can besubstantially less than unity even under UV irradiation [29]and that thermal relaxation at elevated temperatures can leadto the underestimation of Tc-cis. A similar limitation becomeseven more consequential in high-temperature LCST platformsemploying strongly absorbing chromophores, where opticalattenuation and penetration effects can produce a cis-richregion only near the illuminated surface, whereas the bulkrelaxes toward the trans-type, resulting in a reduced apparentTc-cis-/trans- and, in extreme cases, a near collapse of thedifference [20, 23]. The present “low-baseline LCST” plat-form mitigates these constraints by operating in an ambient-to-physiological temperature window: the lower transitionmaintains a meaningful cis-enriched population throughoutthe sample. In this regime, the intrinsic polarity contrast ofazobenzene is more faithfully translated into opposite shifts inTc, thus yielding a robust, composition-amplified ΔTc understandard optical conditions.Fig. 3 Composition dependence of the Tcs and bistable temperaturewindow for P(AzoMA-r-PhEtMA) in [C2mim][TFSI]. a Tc for trans-(blue circles; visible light irradiation) and cis- (red square; UV irra-diation) states as a function of AzoMA composition. The greentriangle denotes the corresponding values for the PPhEtMA homo-polymer ([AzoMA] = 0 mol%). b Bistable temperature window,ΔTc= Tc-cis – Tc-trans, plotted against AzoMA composition, showingmonotonic expansion with respect to the azobenzene fraction1340 T. Naka, T. UekiBeyond these measurement-level constraints, the mag-nitude of ΔTc is expected to depend on whether azobenzeneunits can effectively participate in the solvent environmentrather than be sequestered into strongly aggregated micro-domains. Zhao and coworkers demonstrated in mixed-solvent aqueous systems (water/dioxane) that a large photo-induced Tc shift emerges only when the local solvationenvironment allows azobenzene units to remain sufficientlysolvated and responsive, as evidenced by NMR analyses ofsegment mobility and domain partitioning [6]. Importantly,in our case, the photoswitch unit itself appears highlycompatible with the IL medium: the AzoMA monomer isreadily soluble in [C2mim][TFSI] at room temperature(Supplementary Fig. S23), suggesting that azobenzene unitsremain accessible to solvation in this environment. Thisdiffers from the azobenzene acrylamide/acrylate derivativesoften used in aqueous LCST systems [25, 26, 28, 42–45],which can exhibit very limited water solubility and maytherefore be prone to reduced solvation access. While dif-ferences in the cis-/trans-solubility of the monomer couldnot be directly quantified because of strong opticalabsorption at high concentrations, the high baseline solu-bility of AzoMA in the IL provides a plausible molecularbasis for the large ΔTc achieved here even at low chromo-phore contents. Consistently, the relatively large ΔTcobserved here, even at low AzoMA contents, is consistentwith the efficient solvation of AzoMA units in the ILenvironment, enabling the cis-/trans-polarity change todirectly modulate the effective polymer‒solvent interaction.Sequence-level arrangement can, in principle, furthermodulate this response: end-group azobenzene designsoften display near-additive, composition-proportional shiftsbecause chromophore proximity and cooperative aggrega-tion are minimized [2, 46], whereas side chain azobenzenecopolymers can exhibit stronger nonlinearity when chro-mophore adjacency becomes significant. In our earlier ILupper critical solution temperature (UCST) system [47], weproposed that increased azobenzene adjacency associatedwith somewhat blocky sequence statistics could promotecooperative chromophore interactions and amplify thetransition temperature. In the present P(AzoMA-r-PhEtMA)series, sequence distribution analysis indicated a randomarrangement within the studied composition range(r1= 1.27, r2= 2.36 as [M1]= PhEtMA, [M2]=AzoMA;Supplementary Fig. S12), suggesting that strong coopera-tive chromophore clustering is not required to account forthe systematic widening of ΔTc. Taken together, theseconsiderations support the view that the low-baseline LCSTdesign, combined within an IL solvation environment thateffectively “activates” the azobenzene polarity contrast,provides large and tunable bistable temperature windows.The reversible, light-triggered solubility switching ofP(AzoMA-r-PhEtMA) [AzoMA]= 2.91 mol% in [C2mim][TFSI] at a fixed, physiologically relevant temperature(37 °C) is shown in Fig. 4, taking advantage of the bistablewindow between Tc-trans= 30.7 °C and Tc-cis= 45.8 °C.The solution was first converted to a homogeneous, trans-parent, cis-type polymer by UV irradiation at 37 °C. Sub-sequent visible-light irradiation rapidly decreased thetransmittance and drove complete phase separation within~100 s, indicating that the trans-type polymer renders thepolymer less compatible with the IL solvation environmentat this temperature. The transmittance switching is repro-ducible over 9 cycles spanning ~40,500 s without noticeableloss of contrast, supporting stable operation in a nonvolatileIL medium where concentration drift is negligible. Whenthe turbid, phase-separated sample was irradiated with UVlight again, the transmittance recovery was markedlyslower: after an induction period of ~300 s, the solutiongradually clarified and returned to transparency over ~300 s.This kinetic asymmetry is readily rationalized by opticalattenuation in the turbid state, as the polymer-rich domainsFig. 4 Reversible, light-triggered solubility switching ofP(AzoMA2.91-r-PhEtMA) in [C2mim][TFSI] at 37 °C. The transmit-tance at 700 nm was monitored over time while the solution wasalternately irradiated with UV light and visible light. Red squares andblue circles denote the transmittance values during UV and visibleirradiation, respectively. Reversible transmittance switching is main-tained over nine cycles spanning ~675 minLight-switchable LCST windows in an ionic liquid solution of azobenzene copolymers 1341scatter incident UV light more strongly than the transparentstate does, reducing the effective photon flux available fortrans-to-cis conversion in the bulk and thus slowing redis-solution. In contrast, the cis-to-trans switching initiatedfrom a transparent solution proceeds efficiently becauselight penetration is high and isomerization can occur moreuniformly throughout the sample. Importantly, these dataprovide a direct “operation mode” demonstration: a non-volatile IL solution can be switched reversibly between one-phase and two-phase states by light alone at constant tem-perature. The switching timescales are expected to be fur-ther tuned through optical intensity, wavelength, and loadedAzoMA, offering a practical route to programmable,ambient-to-physiological photocontrolled phase behavior inIL media. Taken together, the present results suggest asimple design rule for realizing large, reliable photo inducedTc shifts in nonvolatile ILs: (i) choose a host polymer with asufficiently low baseline LCST to suppress the impact ofthermal back-isomerization on the measurement/operationtimescale, (ii) ensure that the photoswitch unit is well sol-vated (or at least not strongly segregated) in the targetsolvent environment so that cis-/trans-polarity changestranslate into a bulk interaction change, and (iii) operatewithin an optical regime where irradiation can access thebulk (especially for strongly absorbing chromophores).In conclusion, we established a practical design frame-work for robust phototunable LCST phase separation in anonvolatile IL medium by combining a low-baseline ther-moresponsive platform with strong photo switch–solventcompatibility. Using a systematic composition series ofP(AzoMA-r-PhEtMA) in [C2mim][TFSI], we achieved acontinuous expansion of the ΔTc and demonstrated rever-sible, repeatable switching between one-phase and two-phase states at 37 °C over extended operation times. Beyondsimply lowering the LCST, our results highlight that largeand quantifiable photo-induced shifts require that azo-benzene units remain solvation accessible so that the changein the cis-/trans-polarity can be efficiently translated into achange in macroscopic solubility. Together with opticalpenetration considerations, these findings identify action-able design levers, baseline transition temperature, photoswitch solvation accessibility, and irradiation/geometryconditions that govern both the magnitude and kinetics ofswitching. Notably, reversible photoswitching is achieved at37 °C because this temperature is directly relevant to phy-siological and cell culture conditions [17, 48, 49]. Thisfeature makes the present polymer-IL system a potentiallyuseful platform for future photoresponsive soft materialsthat operate in biologically relevant environments. We alsoanticipate that this framework will enable programmable,nonvolatile soft materials in IL media, including photo-addressable ion-gel platforms and dynamic phase-separatedsystems for responsive transport and actuation.Acknowledgements This work was financially supported by JSPSKAKENHI grants (23H02030 and 23K26726 to TU).Compliance with ethical standardsConflict of interest The author declares no competing interests.Publisher’s note Springer Nature remains neutral with regard tojurisdictional claims in published maps and institutional affiliations.Open Access This article is licensed under a Creative CommonsAttribution 4.0 International License, which permits use, sharing,adaptation, distribution and reproduction in any medium or format, aslong as you give appropriate credit to the original author(s) and thesource, provide a link to the Creative Commons licence, and indicate ifchanges were made. 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Uekihttps://doi.org/10.1021/ma2014244https://doi.org/10.1002/adma.202310105https://doi.org/10.1002/marc.202500909 Light-switchable LCST windows in an ionic liquid solution of azobenzene copolymers Abstract Compliance with ethical standards ACKNOWLEDGMENTS References