Watching two conjugated polymer chains breaking each other when colliding in solution

Abstract

While collision theory successfully describes the kinetics of chemical reactions, very little is known about the processes at the molecular level, especially if the reacting molecules are large. In this study, using single-molecule spectroscopy, we visually observed that collision between two conjugated polymer (CP) molecules in solution leads to simultaneous rupture of both chains. In addition to opening up the possibility of monitoring chemical processes in solution at the single-molecule level, these results demonstrate that mechanical bending of two stiff conjugated backbones against each other (the effect of leverage) by Brownian motion can weaken the chemical bond and markedly accelerate photochemical oxygen-induced chain scission by at least 20 times. The catalytic effect of the chain bending is also enhanced by a prolonged interaction between the chains owing to their entanglement. These findings are important for the solution processing of CPs in their application in organic electronics, for understanding the degradation mechanisms in CPs and for the development of new catalysts based on mechanical interactions with target molecules. Mechanical forces that result from collisions between molecules play a central role in chemical reactions. The finer details of their involvement remain poorly understood, however, especially for larger molecules possessing an enormous number of degrees of freedom. Now, a team led by Ivan Scheblykin from Lund University, Sweden, has developed a fluorescence microscopy approach that enables a direct visualization of collisions between two individual polymer molecules in solution. The researchers found that the randomly moving molecules, which exhibit a rigid, double-bonded carbon backbone, broke into smaller fragments upon impact. This disintegration stems from molecular entanglement and bending stress where the stiff polymer chains touch each other. The team's single-molecule chemistry approach provides a deeper knowledge of reaction mechanisms at the molecular level and opens the door to new synthetic routes in materials chemistry. By applying single-molecule imaging technique in free solution, we visually observed polymer chains break apart when they collide with each other just because of their Brownian motion. The oxidation scission reaction is catalyzed by the leverage effect, analogous to breaking a stiff wooden stick over the knee. Our surprising results suggest that new catalysts could be designed with the idea of stiff molecules working as ‘knives’ and ‘leverages’ breaking chemical bonds. We believe that our work opens up the possibility of monitoring chemical processes in solution at the single-molecule level.