Unravelling Solid-State Microstructure from Conjugated Oligomers to Polymers

Abstract

Conjugated polymers exhibit complex solid-state microstructures, including assembly behavior, molecular packing, and thin-film morphology, which critically influence the optoelectronic performance. However, deciphering these features is challenging due to their broad molecular weight distributions and multiscale structural complexity. Herein, a series of isoindigo-based oligomers with backbone lengths ranging from 49 to 141 Å and side-chain lengths of 18 and 28 Å was developed. Using these conjugated oligomers with precise molecular weight, it became possible to trace the evolution of solid-state microstructure from discrete oligomers to polymers with reduced impact from broad molecular weight distributions. At short chain lengths, solid-state packing remains locally ordered due to the combined influence of backbone and side-chain interactions. With increasing chain length, packing gradually evolves into lamellar phase and more continuous film morphologies dominated by backbone π–π interactions, which favours charge transport. At a given backbone length, strengthening side-chain interactions alters this packing pathway and drives the system toward side-chain-crystallized assemblies, accompanied by increased backbone torsion and reduced effective conjugation, which in turn limits charge transport in thin films. These insights provide a coherent physical picture linking molecular chain length, solid-state microstructure evolution, and charge-transport behavior in conjugated materials.