title: Single Molecule Study of Force-Induced Rotation of Carbon-Carbon Double Bonds in Polymers.
abstract: Carbon-carbon double bonds (C═C) are ubiquitous in natural and synthetic polymers. In bulk studies, due to limited ways to control applied force, they are thought to be mechanically inert and not to contribute to the extensibility of polymers. Here, we report a single molecule force spectroscopy study on a polymer containing C═C bonds using atomic force microscope. Surprisingly, we found that it is possible to directly observe the cis-to-trans isomerization of C═C bonds at the time scale of ∼1 ms at room temperature by applying a tensile force ∼1.7 nN. The reaction proceeds through a diradical intermediate state, as confirmed by both a free radical quenching experiment and quantum chemical modeling. The force-free activation length to convert the cis C═C bonds to the transition state is ∼0.5 Å, indicating that the reaction rate is accelerated by ∼109times at the transition force. On the basis of the density functional theory optimized structure, we propose that because the pulling direction is not parallel to C═C double bonds in the polymer, stretching the polymer not only provides tension to lower the transition barrier but also provides torsion to facilitate the rotation of cis C═C bonds. This explains the apparently low transition force for such thermally "forbidden" reactions and offers an additional explanation of the "lever-arm effect" of polymer backbones on the activation force for many mechanophores. This work demonstrates the importance of precisely controlling the force direction at the nanoscale to the force-activated reactions and may have many implications on the design of stress-responsive materials.
authors.affiliation: Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University , Nanjing 210093, P.R. China.
authors.forename: Wenmao
authors.initials: W
authors.lastname: Huang
authors.affiliation: Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University , Nanjing 210093, P.R. China.|Department of Pharmaceutical Analysis, Key Laboratory of Drug Quality Control and Pharmacovigilance, Ministry of Education, China Pharmaceutical University , Nanjing 210009, P.R. China.
authors.forename: Zhenshu
authors.initials: Z
authors.lastname: Zhu
authors.affiliation: Key Laboratory of Mesoscopic Chemistry of MOE, Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University , Nanjing 210023, P. R. China.
authors.forename: Jing
authors.initials: J
authors.lastname: Wen
authors.affiliation: Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University , Nanjing 210093, P.R. China.
authors.forename: Xin
authors.initials: X
authors.lastname: Wang
authors.affiliation: Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University , Nanjing 210093, P.R. China.
authors.forename: Meng
authors.initials: M
authors.lastname: Qin
authors.affiliation: Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University , Nanjing 210093, P.R. China.
authors.forename: Yi
authors.identifier: 0000-0003-1493-7868
authors.initials: Y
authors.lastname: Cao
authors.affiliation: Key Laboratory of Mesoscopic Chemistry of MOE, Collaborative Innovation Center of Chemistry for Life Sciences, School of Chemistry and Chemical Engineering, Nanjing University , Nanjing 210023, P. R. China.
authors.forename: Haibo
authors.identifier: 0000-0001-7915-3429
authors.initials: H
authors.lastname: Ma
authors.affiliation: Collaborative Innovation Center of Advanced Microstructures, National Laboratory of Solid State Microstructure and Department of Physics, Nanjing University , Nanjing 210093, P.R. China.
authors.forename: Wei
authors.initials: W
authors.lastname: Wang
keywords: atomic force microscope; carbon−carbon double bonds; cis-to-trans isomerization; force-induced rotation; mechanochemistry