Conformational Freedom-Enhanced Optomechanical Energy Conversion Efficiency in Bulk Azo-Polyimides

Azobenzene-doped glassy polyimides (azo-polyimides) offer some of the most efficient optomechanical power densities to date rivaling electrostrictive polymers. Despite such potential attributes, the optomechanical efficiency remains low in comparison to other smart materials. Using high-fidelity coarse-grained molecular dynamics simulations, the authors reconcile both experimental and theoretical challenges to understand the limiting factors for the optomechanical conversion in photostrictive polymers. Interestingly, the ideal optomechanical efficiency of 10–24% for a single-chain azo-imide monomer predicted here is equal to or a little higher than experimental reports, suggesting experimental design space. The time-dependent optomechanical efficiency of bulk azo-polyimide is quantified, for the first time, to be strongly correlated with the initial free volumes, a measure of polymer conformational freedom. This trend is elaborated by conformational order parameters and viscoelastic relaxation moduli. Resembling the role of porosity in azobenzene-contained metal/covalent organic frameworks to enhance the photo-switching efficiency, a larger conformational freedom enables >10 times increase in optomechanical efficiency comparing to existing experiments. This is primarily due to facilitated viscoelastic relaxation after photo-switching which alleviates residual stresses quickly and reduces heat dissipation. These findings suggest opportunities to improve the optomechanical performance through targeted strategies, such as porosity control and thermal annealing.     

Asymmetric Organocatalyzed Aza-Henry Reaction of Hydrazones: Experimental and Computational Studies

The first asymmetric catalyzed aza-Henry reaction of hydrazones is presented. In this process, quinine was used as the catalyst to synthesize different alkyl substituted β-nitrohydrazides with ee up to 77 %. This ee was improved up to 94 % by a further recrystallization and the opposite enantiomer can be obtained by using quinidine as the catalyst, opening exciting possibilities in fields in which the control of chirality is vital, such as the pharmaceutical industry. Additionally, experimental and ab initio studies were performed to understand the reaction mechanism. The experimental results revealed an unexpected secondary kinetic isotope effect (KIE) that is explained by the calculated reaction pathway, which shows that the protonation of the initial hydrazone and the C−C bond forming reaction occur during a concerted process. This concerted mechanism makes the catalysis conceptually different to traditional base-promoted Henry and aza-Henry reactions.