Photodimerization-Triggered Photopolymerization of Triene Coordination Polymers Enables Macroscopic Photomechanical Movements

Controlling the packing of olefinic molecules in crystals is essential for triggering solid-state [2 + 2] photocycloaddition reactions and the synthesis of photocontrolled smart materials. Herein, we report the stepwise photodimerization-triggered photopolymerization of two triene coordination polymers (CPs), {[Zn(2-BBA)₂(tpeb)]·0.5CH₃CN}_<i>n</i> (<b>1</b>, 2-HBBA = 2-bromobenzoic acid, tpeb = 1,3,5-tri-4-pyridyl-1,2-ethenylbenzene) and {[Zn(3-BBA)₂(tpeb)]·CH₃CN)}_<i>n</i> (<b>2</b>, 3-HBBA = 3-bromobenzoic acid). Upon irradiation with 420 nm light, each pair of closely packed and parallel olefinic bonds in <b>1</b> undergoes a [2 + 2] cycloaddition reaction, which connects two adjacent <i>Z</i>-shaped chains into a ladder-like coordination chain [Zn(2-BBA)₂(bpbdpvpcb)_0.5]_<i>n</i> (<b>1a</b>, bpbdpvpcb = 1,3-bis(4-pyridyl)-2,4-bis(3,5-di(2-(4-pyridyl)vinyl)phenyl]cyclobutene) through single-crystal to single-crystal (SCSC) transformation. After photodimerization from <b>1</b> to <b>1a</b> has occurred, the olefinic bonds that were initially distant are brought in close enough proximity to meet the requirements for a subsequent [2 + 2] cycloaddition reaction. Upon further light irradiation, the neighboring bpbdpvpcb ligands in <b>1a</b> experience a SCSC photopolymerization based on [2 + 2] photocycloaddition and transform into poly-3b,4,5,5a,8b,9,10a-octahydro-4,5,9,10-tetrapyridyl-2,7-di(2-(4-pyridyl)vinyl)dicyclobuta[<i>e</i>,<i>l</i>]-pyren (poly-otpdpvdcbp). <b>2</b> showed similar structural changes under UV light illumination. Under light exposure, single crystals of <b>1</b> and <b>2</b> with different morphologies exhibit bending, cracking, and jumping photomechanical motions. The composite film (<b>1-PVA</b>) engineered by dispersing crystalline particles of <b>1</b> in poly(vinyl alcohol) (PVA) displays interesting light-wavelength-dependent photomechanical motions and can perform photodriven swimming on a liquid surface. This work provides a useful and promising approach to enable photodimerization of those photoinactive olefin pairs embedded in CPs and opens a new route to synthesize organic polymers by using olefinic CP platforms.