Advances in soft materials are reshaping the design of next-generation devices, particularly where flexibility, adaptability, and high performance intersect. Functional molecular liquids (FMLs) stand out in this domain. Defined as solvent-free liquids at room temperature with properties derived from a functional molecular core, they combine fluidity and free deformation with the ability to dissolve other molecules or nanomaterials. Unlike solids, which suffer from defects and orientation constraints, these liquids maintain stable performance under mechanical stress, offering unique optoelectronic and chemical capabilities.
The molecular design of FMLs typically involves attaching flexible chains—such as alkyl, poly(ethylene oxide), or siloxane—to rigid π-conjugated units. This weakens aggregation forces without compromising the core’s function. This approach differs from ionic liquids, which rely on tuning ionic interactions for liquidity. FMLs’ low polarity makes them suitable for applications where conductivity is undesirable, such as certain semiconductors and electret devices. Photostability is another advantage; dye-containing FMLs have shown roughly tenfold improvements over unmodified dyes.
Stimuli responsiveness is a defining feature. Electrical stimuli can induce electroluminescence, as demonstrated by OLEDs using neat liquid carbazole hosts doped with emitters. Microfluidic OLEDs exploit liquid flexibility for precise light-emitting structures. Electrocatalytic isomerization in arylazopyrazole liquids offers efficient thermal fuel cycling, while electrochromic behavior in alkylated double-decker lutetium phthalocyanine allows rapid color changes in thin liquid films, coupled with switchable spin-active properties.
Chemical gas stimuli can trigger vapochromism, as seen in N-heteroacene liquids exposed to HCl vapor, which alters both emission color and molecular alignment through protonation. Chiral derivatives can form helical assemblies under gas exposure, enabling circularly polarized luminescence. Host–guest chemistry also plays a role: pillar[6]arene liquids solidify upon cyclohexane uptake, reversing with heat. Porous liquids—created by integrating rigid cages or MOFs with flexible chains—retain permanent pores for gas adsorption, enabling CO₂ capture and catalysis in fluid systems.
Chemical additives can restructure FMLs at the nanoscale. Alkylated fullerenes form micelles or lamellar assemblies depending on additive type and ratio, affecting photoconductivity. Donor–acceptor complexation in π-conjugated liquids can shift fluorescence colors, while selective guest interactions enable phase transitions and enhanced optoelectronic properties.
Photo and thermal stimuli open further possibilities. Azobenzene-modified liquids can reversibly switch between solid and liquid phases under light, useful for adhesives or optical energy storage. In thermal fuel applications, UV-induced cis–trans isomerization stores heat, released on demand with visible light. Combining azobenzene triggers with phase-change materials allows heat storage below normal crystallization points. Temperature-dependent fluorescence modulation has been achieved in liquefied anthracene systems via energy transfer to dopants.
Mechanical forces can induce phase transitions in metastable supercooled FMLs, changing optical properties. Scratching or shear stress can crystallize supercooled liquids, altering fluorescence intensity and color. Mechanically induced fluorescence color changes have been demonstrated in viscoelastic π-conjugated polymer blends, where phase separation and energy transfer dynamics produce reversible optical shifts. Liquid electrets—such as branched-alkyl porphyrins—store electrostatic charge and generate voltage pulses under pressure, enabling stretchable mechanoelectric generators. Blends of glassy and liquid π-conjugated polymers can be tuned for modulus and stretchability, producing flexible devices that respond to vibration or impact.
Designing FMLs requires balancing viscosity, molecular size, and functional performance. Bulky yet flexible chains prevent aggregation, while careful core–side chain integration tailors stimulus response. Lowering viscosity without losing function is a key challenge, with mixing strategies from dye glasses offering potential solutions. Applications range from microfluidic optoelectronics and soft robotics to gas-processing catalysts and adaptive sensors, underscoring FMLs’ promise in fields where mechanical compliance and advanced functionality must coexist.