Factors Impacting Dihedral Angle Rotation and Classification in π-Conjugated Systems

The conformational variability of π-conjugated molecules or polymers, defined by the capacity for rotation among the connected ring moieties that comprise the structure, directly impacts properties of organic semiconductors ranging from material processability to the electronic, redox, optical, and mechanical characteristics. Thus, the shapes of the potential energy surfaces and corresponding energy barriers of inter-ring rotations as a function of the system chemistry serve as critical parameters for both molecular/polymer and material design. Here, we systematically examine the effects of various ring chemistries, ortho-positioned substituents, and oligomer length on the rotation of two rings in the center of the system. Two primary factors, as one might expect, dictate the potential energy surface (PES) for rotation: the degree of π-electron delocalization across the dihedral bond and the noncovalent interactions among ortho-substituted atoms and/or groups. Each factor can stabilize or destabilize fully planar conformations. Notably, the oligomer chain length has a little-to-no discernible impact on the rotation energetics in systems with more than six rings. We identify four distinct shapes for PES of rotation that relate to chemical composition, which enable us to develop classification models to classify a system's dihedral potential energy surface from a two-dimensional structure. We then combine these classification models and previously reported analytical methods to quickly predict polymer chain dimensions from a monomer structure. The insights derived here are expected to help direct and accelerate the in silico design of new molecular and polymer classes with the desired optoelectronic properties and ease of processing.