Evolution of Chain Dynamics and Oxidation States with Increasing Chain Length for a Donor-Acceptor-Conjugated Oligomer Series

While it is known that the chain length strongly affects the properties of pi-conjugated polymers, the effects of chain length on the molecular structure, chain conformation, and oxidation state properties in donor-acceptor-type conjugated structures remain unclear. This limits our understanding of how the polymer molecular weight impacts material properties. Here, a discrete and monodisperse oligomer series (n = 3, 5, 7, 9, 15, and 21) and polymers (nPB), composed of the donor 3,4-propylenedioxythiophene (ProDOT), the acceptor benzothiadiazole (BTD), and methylthio end-capping groups, are synthesized by C-H-activated cross-coupling. The molecular structure, molecular weight, and dispersity of each oligomer/polymer are thoroughly characterized by nuclear magnetic resonance spectroscopy, mass spectrometry, and gel permeation chromatography. This series reveals a rod-to-coil transition at n = 15 and coil formation at polymer length scales of similar to 28 units via solution small-angle neutron scattering characterization. The oxidation states are deciphered via cyclic voltammetry, differential pulse voltammetry, spectroelectrochemistry, and density functional theory calculations. Oligomers 3-9 undergo successive one-electron oxidation steps, while 15 and higher undergo multielectron oxidations per step in CH2Cl2-TBAPF(6) at a Pt ultramicroelectrode. The electronic transition of each oxidation state (1+, 2+, 3+, etc.) is tracked by absorption spectroscopy, revealing a bipolaron to di-polaron transition at n = 7 at which shorter oligomers prefer bipolaron formation and longer oligomers prefer di-polaron formation in their dication states. Furthermore, oxidized 15 has a lower electronic transition energy compared to its polymer homologue, as shown by spectroelectrochemistry, revealing a synergy between the chain length and the oxidation state properties. This study proves that the convergence limit between small molecule and polymer behavior occurs at approximately 15 units and highlights the property transitions that occur as a function of chain length for a donor-acceptor class of conjugated organic materials.

Automated summary

This study investigates the effects of chain length on the properties of donor-acceptor conjugated organic materials by synthesizing oligomers and polymers with varying chain lengths. It reveals a convergence limit between small molecule and polymer behavior at around 15 units, highlighting the synergy between chain length and oxidation state properties. The study also demonstrates the importance of chain length in influencing material properties and property transitions as a function of chain length.