Co-optimizing photodeformation and stiffness in azobenzene polyimides for enhanced photoinduced stress and multimodal sensing.

Abstract

Photomechanical-to-electrical conversion offers an alternative pathway for optical sensing, but reliable and highly linear signals require large and reproducible photoinduced stress. Achieving such mechanical output remains challenging because efficient photodeformation and a high elastic modulus are often difficult to reconcile through molecular design. Herein, the backbone architecture of azobenzene polyimides (azoPIs) was systematically engineered by varying the dianhydride structure while maintaining identical azobenzene content and flexible-segment fraction. Among the resulting polymers, the 6FDA-based azoPI (azoPI-6FDA) exhibits the most favorable combination of mechanical stiffness and photodeformation (E = 3.27 GPa and a bending angle of 75° under UV irradiation) and possesses a high photoinduced stress of 2.0 MPa. AzoPI-6FDA was subsequently integrated with PVDF-TrFE to construct flexible photomechanical-to-electrical sensing devices in both laminated and coaxial fiber configurations. In the laminated device, photoinduced stress generated by azoPI-6FDA was transferred to PVDF-TrFE and converted into a piezoelectric voltage of up to 9.0 V. Both the photoinduced stress and electrical output exhibited nearly linear dependences on UV intensity, with correlation coefficients approaching 0.99, enabling quantitative optical sensing. The coaxial electrospun membrane combined stable electrical output with an elongation at break exceeding 200% and maintained reliable sensing performance under bending. Moreover, UV irradiation generated predominantly unipolar voltage signals, whereas mechanical loading and unloading produced characteristic bipolar piezoelectric signals, allowing direct discrimination between optical and mechanical stimuli without additional signal processing. This work connects molecular backbone design with photoinduced stress generation and device-level signal transduction, providing a robust materials strategy for flexible, self-powered multimodal sensing.