Steven E. Wheeler, PhD , Texas A&M University
The prevailing model of substituent effects in π-stacking interactions rely on the polarization of the aryl π-system by the substituents. In this intuitive electrostatic model, electron-withdrawing substituents (CN, NO2, etc.) enhance π-stacking interactions by withdrawing electron density from the aromatic π-cloud. Electron donating substituents (CH3, NH2, etc.) are said to hinder π-stacking interactions through the opposite mechanism. Among the consequences of this model are the following: (1) in the case of π-stacking interactions between aromatic rings in which each ring is substituted, there will be a coupling between the effects of the two substituents; and (2) the presence of heteroatoms in one of the rings will alter the effects of substituents on π-stacking interactions.
We have introduced an alternative model of substituent effects in π-stacking interactions based on high-accuracy ab initio and density functional theory (DFT) computations. In this model, substituent effects do not depend on the polarization of the aryl π-system, but instead result from direct, local interactions between the substituents and the proximal vertex of the other ring. Qualitatively, this model is based on the interaction of the local dipole moment associated with the substituent and the nearby C-H dipoles on the other ring. Importantly, we have shown that the effect of a given substituent is independent of the presence of other substituents or heteroatoms on either ring as long as they are not in the local environment of the substituent. This provides a simple means of understanding substituent effects on π-stacking interactions of large, complex aromatic systems in which there are multiple substituents and heteroatoms.
Additional work concerning the nature of π-stacking interactions involved quantifying the role of aromaticity. This has implications for π-stacking interactions involving heteroaromatic systems because they will have varying degrees of aromaticity depending on the nature and placement of the heteroatoms. In contrast to prevailing assumptions, we showed through high-accuracy computations on model systems that aromaticity is not only unnecessary for π-stacking interactions, but can actually hinder these interactions.
We have recently completed work regarding the torsional potentials of oligothiophenes and the impact of neighboring chains on these torsional potentials. Because torsional motions of conjugated oligomers can significantly affect charge transport properties, understanding the factors that impact the energy required for torsional motion will be important in the design of next-generation organic electronic materials. The aim of our work is to shed light on the rigidity of oligothiophenes in the solid state, and help bridge the gap between previous studies of isolated oligothiophene chains and their physical properties in more realistic environments. Two publications related to this work will be submitted shortly. In the initial work, we have performed high-accuracy ab initio computations of torsional potentials of bithiophene and π-stacked bithiophene dimers. These reference potentials were then used to benchmark several DFT functionals and basis sets to identify functional/basis set combinations that yield accurate torsional and interaction potentials at a reasonable computational cost. Also included in this study is an extensive investigation of the effect of substituents on torsional potentials of bithiophene, to understand how substituents alter the physical properties of oligothiophenes.
In the other work, we utilized DFT to study the torsional potentials of a series of oligothiophenes (nT, n = 2-10), to examine how the torsional potentials change with increasing chain length. For sexithiophene (6T), we also predicted torsional potentials for stacked chains in several prototypical configurations to understand how a single neighboring chain impacts predicted torsional potentials. This was followed by a combined quantum mechanical/molecular mechanical (QM/MM) study of a single 6T chain in the environment of the 6T crystal structure. It was shown that neighboring chains have a significant impact on computed torsional potentials, and we suggest that studies of torsional potentials of conjugated heterocyclic oligomers should account for the effect of interchain interactions to provide an accurate portrayal of these systems.