Optimization of the crosslinker and fluorophore chain length in liquid crystal material mechanoluminiscence

Abstract

Smart materials, capable of dynamically altering their physical properties in response to external orientation-dependent stimuli, have emerged as highly relevant candidates for advanced technological applications. Within this framework, liquid single crystal elastomers (LSCEs) represent a unique class of responsive networks that combine the cooperative order of liquid crystals with the macromolecular elasticity of polymer networks. This research focuses on the molecular design, processing-dependent configuration and macroscale mechano-optical characterization of functionalized liquid single-crystal elastomers synthesized as responsive mechanofluorescent strain sensors. Chromophores based on functionalized carbazole units with varying alkyl spacer lengths and attachment geometries were integrated as minor emissive tags into a major liquid-crystalline host matrix. The study systematically compares the influence of two film-deposition techniques on the baseline network arrangement and the resulting solid-state luminescence: high-speed centrifugal spin-coating and stationary mould casting. Photophysical evaluations in dilute solution initially revealed that oxygen-linked carbazole monomers, functioning in an end-on layout, possess enhanced quantum efficiencies and a bathochromic shift compared to nitrogen-linked side-on analogues due to the electron-donating nature of the alkoxy chains attached to the carbazole core. Upon polymerization into solid-state films, the bulk matrix introduces a severe inner-filter effect and significant light scattering. To secure high-resolution optical data, photoexcitation conditions were strategically optimized to the absolute absorption maximum of the fluorophore. Under uniaxial tensile strain, the mechanical deformation is directly transduced by the architectural layout of the probe; the linear end-on derivatives align parallel to the host mesogens, where lateral film contraction drives them into tight face-to-face proximity, favouring static π-π stacking and a deep, rapid quenching. On the other hand, the perpendicular side-on pendant architectures undergo mechanical disruption into high local disorder, yielding a more progressive and continuous variation in solid-state emissions. Furthermore, the macromolecular configuration and baseline aggregate constraints were found to be strictly dictated by the fluid dynamics of film processing rather than the specific design of the minor fluorophore fraction. Traditional stationary mould casting traps the network into thick, entangled domains that promote random chromophore aggregation, leading to high baseline self-quenching and the emergence of a prominent intermolecular excited-state complex (exciplex) emission. On the contrary, the high shear rates experienced during centrifugal spin-coating mechanically counteract native aggregation, forcing the network into an ultra-thin planar conformation that maximizes initial spatial separation to preserve clean monomer fluorescence. Volumetric equilibrium swelling experiments in toluene confirmed that while the thermodynamic expansion limits are strictly governed by the bulk matrix and the uniform density of the cross-linking agent, the processing determines network anisotropy. Centrifugal deposition yields highly oriented configurations capable of substantial asymmetric swelling, whereas mould casting restricts solvent diffusion due to isotropic cross-linking constraints. Crucially, the swelling-deswelling cycle acts as an essential purification step that efficiently extracts unreacted monomers from the elastomer. Upon complete drying, the processing history is permanently locked into the network baseline density, where spin-coated frameworks sustain sufficient molecular spacing to prevent self-quenching. Ultimately, this work demonstrates that engineering highly sensitive, tailorable mechanofluorescent LSCE sensors demands a coordinated synergy between the precise spatial layout of the molecular probe and the fluid-dynamic control of the processing method.