Determination of Side‐Chain Absolute Configuration Reveals a Stereochemical Strategy for Controlling Conjugated Backbone Conformations

ABSTRACT The dihedral angle of π‐conjugated backbones critically influences the properties of organic semiconductors, yet whether and how side‐chain stereochemistry controls backbone torsion at the molecular level remains poorly understood. Here, we establish a direct relationship between the absolute configuration of 2‐ethylhexyl (EH) side chains and the preferred dihedral angles of CPDT–A–CPDT molecules (CPDT = cyclopenta[2,1‐ b :3,4‐ b ’]dithiophene; A = quinoxaline‐based acceptors). The absolute configurations of mono‐formylated CPDT (CPDT‐CHO) stereoisomers were determined by combining density functional theory calculations with experimental 1 H NMR analysis, enabling stereochemical assignment of the corresponding CPDT–A–CPDT derivatives. A clear correlation between the chemical shift of the thiophene β proton adjacent to the acceptor unit (H d ) and the CPDT–A dihedral angle established H d as a spectroscopic probe of local backbone conformation. Analysis of stereochemically defined stereoisomers led to a steric‐interaction model in which EH side‐chain stereochemistry modulates the CPDT–A dihedral angle through differences in local steric interactions. The characteristic ninefold H d splitting was assigned to distinct stereochemical environments and was reproduced in derivatives containing different quinoxaline acceptors, demonstrating the generality of this relationship. These findings provide a molecular design principle for controlling conjugated backbone conformations through side‐chain stereochemistry.