End-substitution effect on the geometry and electronic structure of oligoheterocyclics

Abstract  The end-substitution effects on the geometric and electronic structures of oligoheterocyclics are systematically studied using
the density functional theory. It is found that the influence of the end-substitution does not depend on the heteroatom. End-substitution
plays a fine-tune effect on the geometry and the excitation state. While the influences on the conducting type (p-type or
n-type) and the inter-chain charge carrier hoping channels are much different between the electron-donating –CH
₃ and electron–accepting –CN substitutions. Both molecular electrostatic potentials and charge carrier injection rates indicate
that the –CH
₃/–CH
₃ substitution is beneficial to the p-type doping, while the –CN/–CN substitution is in favor of the n-type doping, which is
in agreement with the experimental observations. The –CH
₃ substituted packing dimers exert similar intermolecular interactions to the unsubstituted ones. The –CN substituted packing
dimers yield much stronger intermolecular interactions comparing to the –CH
₃ substituted ones. It could be anticipated that the –CN substitution would be helpful to the charge carrier hopings between
chains and thereby enhance the conductivity.