In-situ photomechanical bending in a photosalient Zn-based coordination polymer probed by photocrystallography

Photomechanical bending or mechanical flexibility in single crystals is an interesting landscape for innovative technological applications, including smart medical devices, molecular machines, artificial muscles, microrobots, and flexible electronic actuators. However, metal-organic crystals with multiple dynamic effects such as bending (in-situ), jumping, fracturing, and splitting in the absence of mechanical energy or temperature is interesting and relatively unexplored. The development of these materials presents significant challenges, requiring a thorough grasp of the underlying mechanisms for practical applications. Herein, we developed a Zn based 1D coordination polymer (CP) crystal {[Zn(DCTP)(4-nvp)₂]·(CH₃OH)}_n (1) {H₂DCTP = 2,5-dichloroterephthalic acid; 4-nvp = 4-(1-naphthylvinyl)pyridine} which undergoes [2 + 2] cycloaddition under both UV irradiation and sunlight to generate a partially dimerized product of a two-dimensional coordination polymer (2D CP) [Zn(DCTP)(rctt-4-pncb)]_n (i₂₀1). During UV irradiation, these single crystals exhibit photomechanical effects like jumping, bending, cracking, and swelling to relieve anisotropic strain from the light. Surprisingly, bent-shaped single crystals (1b) identical in structure to 1 were also obtained in-situ without any external stimuli, simply by keeping reaction mixture for an extended period. A time-resolved photocrystallographic study fully described the photoinduced structural transformation. Nanoindentation measurements complemented a DFT study of mechanical property trends for irradiated and bent Zn-based photosalient crystals.