Excellent photo actuation in crystal-polymer composite by transfer of mechanical energy

Abstract

Transfer of strong photomechanical effect from dynamic crystals to hybrid polymer composite is an evolving field of research and holds high potential for actuation and photo-switch development. An organic counter anion (dinitrobenzoate) stabilized nitropentaamminecobalt(III) complex (Co-dnb-1) showing fast photomechanical motions under UV light (365 nm) was synthesized. Detailed experimental and theoretical studies attribute the photomechanical motion to the formation of metastable nitrito-coordinated species (Co-dnb-2) after irradiation. The energy difference between nitro-nitrito complexes was estimated to be 3.54 eV which is close to irradiation energy (3.4 eV). However, in a slow process, by absorbing energy of similar to 0.4 eV/cation through external perturbations such as temperature, Co-dnb-2 recovers to the initial configuration. Three different crystal-polymer composite films (PCF-1-3) were fabricated using photoactive crystals with polyvinylalcohol (PVA), polyvinyldifluride(PVDF) and agarose polymers, respectively by solution casting method. The composite films show quick and excellent photo-actuation though, the extent varies with the polymers used. The actuation of the composite films starts immediately (<2-3 s) and can be deflected up to 90 degrees within a few seconds of the onset of UV light. PCF-3 shows reversible photo-actuation measured up to multiple cycles. Dynamical transfer of photomechanical energy from crystal to composite films leads to successful assembly of a prototype photoswitch. The presented work provides a new dimension towards the development of smart functional composite materials as photo-actuators.