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

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 ¹H 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.