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.