A phosphorescent liquid-crystal elastomer combines shape change with persistent afterglow, enabling soft materials that visibly track motion, encode hidden patterns and respond to heat or targeted ultraviolet illumination.
(Nanowerk Spotlight) Phosphorescent pigments appear in places where light must persist after illumination disappears. They line emergency exits, aircraft aisles and safety indicators because they absorb photons and release them gradually. In europium- and dysprosium-doped ceramics, electrons absorb ultraviolet energy and fall into microscopic traps formed by dopant ions. When excitation ends, trapped electrons escape and emit visible photons.
These long-afterglow materials do not require batteries or circuitry, tolerate heat and humidity and maintain performance over long periods. Their limitation is structural. The crystals are rigid and they perform best when immobilized in coatings or plastic layers.
Soft, responsive materials excel at movement rather than signaling. A liquid-crystal elastomer is a polymer containing rod-shaped segments that align below a certain temperature and lose alignment above it. This molecular ordering drives reversible contraction or expansion. Soft-robotic actuators and artificial muscles use this effect because the material can bend or twist without mechanical joints. Yet the elastomer gives no visual indication of its motion or stress. If it bends in darkness, nothing reveals what occurred without sensors or external imaging.
A study in Advanced Materials (“Stimuli‐Responsive Afterglow from Luminescent Liquid Crystal Elastomers”) combines long-afterglow phosphors with a thermoplastic liquid-crystal elastomer to create a material that both moves and emits persistent light. The researchers prepared a polyurethane-based liquid crystal elastomer (PULCE) with two complementary domains. The first contains liquid-crystal segments that generate actuation. The second contains thio-urethane domains that form hydrogen bonds. These reversible bonds stabilize the network at low temperature and reorganize at elevated temperature, allowing the material to be reconfigured without destroying the polymer backbone.
a) Schematic illustration of the chemical structure of the PULCE. The liquid crystal monomers used are shown at the bottom (green background). The dynamic hydrogen bonding is highlighted on the right (gray background). b) Photographs of (i) the polydomain PULCE film, (ii) the monodomain film after uniaxial stretching, and (iii) the deformed film after heating to 60 °C, showing contraction along the alignment direction and expansion perpendicular to it (scale bar = 1 cm). c) 2D-WAXS pattern of the polydomain PULCE film, showing an isotropic diffraction ring. d) Temperature-dependent FT-IR spectra of the PULCE from 30 to 190 °C, highlighting changes in the hydrogen-bonded N─H and C═O stretching bands upon heating. e) 2D-WAXS pattern of the monodomain PULCE film, exhibiting strong diffraction anisotropy and confirming molecular alignment. f) Thermal actuation of the aligned film between 25 and 100 °C, showing reversible dimensional changes. g) Actuation stability over 20 heating-cooling cycles, demonstrating consistent and repeatable shape transformation without performance loss. Rose color denotes heating and light blue regions represent cooling periods. (Image: Reproduced from DOI:10.1002/adma.202516922, CC BY) (click on image to enlarge)
Characterization shows that the elastomer has an average molecular weight of 59 kg mol⁻¹, remains thermally stable up to 260 °C and undergoes a nematic-to-isotropic transition at about 60 °C. This transition defines actuation. Below 60 °C, chain segments align and the elastomer behaves as an ordered rubber. Above 60 °C, alignment vanishes and the material relaxes. Unaligned films scatter light and appear opaque but tolerate large deformation.
The neat elastomer sustains up to 2500 % strain before failure, and adding phosphors does not significantly change its mechanical response. When stretched to 200 % of its original length and annealed at 130 °C, polymer chains align permanently. These single-domain films contract along the alignment direction when heated above 60 °C and recover their length when cooled.
The optical component comes from two well-established long-afterglow phosphors. SrAl₂O₄:Eu²⁺,Dy³⁺ emits green light centered near 500 nm. Sr₂MgSi₂O₇:Eu²⁺,Dy³⁺ emits blue light near 470 nm. In both cases, europium ions generate light and dysprosium ions act as traps. Under ultraviolet illumination, electrons enter excited states and some fall into trap sites.
After illumination stops, trapped electrons gradually return to europium centers and emit photons. The green phosphor produces brighter and longer-lasting emission, while the blue emits less intensely and decays faster, though still over useful timescales.
To form the composite, the researchers dispersed 2 wt% phosphor and 1 wt% UV-absorber into the elastomer solution. After solvent removal and hot-pressing, the films looked identical to the undoped polymer. Microscopy confirmed uniform phosphor distribution, and thermal measurements showed that transition points and actuation thresholds were unchanged.
Brief ultraviolet exposure caused strong afterglow, and emission remained visible for >1000 s for the blue-doped composite, and the green-doped composite shows longer persistence. The decay matched the native phosphor behavior, indicating that the polymer matrix did not interfere with charge trapping.
The persistent emission allows the material to store information. Ultraviolet projections create glowing patterns that remain readable in darkness. Heating accelerates charge release and erases the pattern, after which new information can be written. A focused UV beam can inscribe detailed shapes by hand. Emission spectra and brightness remain stable over repeated write-erase cycles, demonstrating that the inorganic phosphors withstand mechanical and thermal stress within the elastomer.
The polymer’s optical scattering adds another layer of control. In its relaxed, multi-domain state, the film hides underlying afterglow. Stretching aligns domain structures and reduces thickness, making the material more transparent and revealing hidden patterns. Heating above the liquid-crystal transition has the same effect.
The UV-absorber enables a third option. A narrow UV beam produces localized photothermal heating, selectively revealing regions without altering the full sample. A single liquid-crystal elastomer therefore supports decoding through strain, bulk temperature change or targeted illumination.
Layered encoding highlights this capability. Thin green-emitting segments can be embedded at different depths inside a blue-emitting base film and hot-pressed into a unified sheet. Under ultraviolet illumination, only front-facing features appear. Embedded segments remain concealed until the film is stretched or heated. Because emission is persistent, the revealed information remains visible after the stimulus ends. Storing data in depth is difficult with single-color plastics or rigid composites.
Aligned elastomer films are transparent and retain their afterglow as they act. When heated from room temperature to 60 °C, they contract by about 30 % and re-expand when cooled. A focused UV beam acts as a light-driven actuator trigger, producing localized contraction wherever illumination falls.
Because the actuator glows, its motion is visible without cameras. Both green- and blue-emitting versions maintain actuation strain over repeated cycles and preserve emission spectra after storage in air or immersion in water.
The material also enables three-dimensional designs. Hot-welded segments form multicolor sheets without visible seams. Direct-ink-written fibers bend toward illumination and leave faint afterglow trails of their movement. Coiled fibers shorten under ultraviolet stimulation and relax as emission fades. Ring-shaped samples roll when light heats one side, and molded hemispherical structures emit uniformly.
The work published in Advanced Materials demonstrates a class of phosphorescent liquid-crystal elastomers that store light, respond to external energy and communicate their movements in real time. Combining reversible network mechanics with stable inorganic phosphors makes possible soft devices that encode information in depth, robotic components that show their actuation directly and materials that reveal prior stimulation through persistent afterglow.
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Albert P. H. J. Schenning (Eindhoven University of Technology)
, 0000-0002-3485-1984 corresponding author
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