In organic semiconductors, the flexibility of a molecule or polymer and the degree of π-delocalization directly impact properties ranging from the semiconductor processability to electronic, optical, and mechanical characteristics. The flexibility and π-conjugation of connected ring systems are dictated by the capacity for rotation amongst any two rings. Thus, the energetics of ring rotations as a function of the system chemistry serve as critical parameters, both for 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 curves for rotation: The degree of π-delocalization across the dihedral bond and the (exchange) repulsion among ortho-substituted atoms and/or groups. Both factors either stabilize or destabilize the planar conformations. Notably, the oligomer chain length had little-to-no discernable impact on the rotation energetics. Further, distinct patterns relating the ring chemistries and shapes of the potential energy surfaces for rotation were found, which are helpful towards the goal of developing machine-learning algorithms for molecular design. The insights derived here will help direct and speed-up the design of new molecular classes that afford semiconductors with a priori design of electronic and optical properties and ease of processing.

A systematic examination of factors impacting dihedral angle rotation to enlighten materials design

ACS Spring 2022

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