Controlling the electron-deficiency of self-assembling pyrazine-acenes: a collaborative experimental and theoretical investigation

This paper reports novel pyrazine-acenes containing electron-deficient heteroaromatic π-extenders, such as pyridine, pyrazine, and benzothiadiazole, directly fused with pyrazine. Electronic properties of these systems were characterized by UV-Vis, fluorescence spectroscopy, and cyclic voltammetry. Computational electronic property evaluation of all experimentally synthesized compounds is provided, and is coupled with electronic calculations of closely related compounds that were not synthetically feasible. Our theoretical results provide insight into the overall analysis and interpretation of the experimentally observed trends. In this study, we found a systematic decrease in the LUMO energy (ELUMO) with an increasing number of imine functions in the π-extender. Additionally, when comparing the pyrazine-acene containing pyrazine π-extender to a reference compound with CN peripheral substituents, we found that the imine function is comparable to the CN substituent in lowering ELUMO. The most dramatic ELUMO lowering was experimentally observed using dibromobenzothiadiazole as a π-extender. In all cases, the HOMO energy (EHOMO) was negligibly affected, thus we found options for electronic property control based solely on ELUMO manipulation. This is computationally validated by an examination of the molecular orbitals in which the LUMO orbital was found predominantly on the π-extender section of the molecules, while the HOMO orbital was localized away from the π-extender. Interestingly, the self-assembly of all the experimentally synthesized compounds showed excellent one-dimensional fiber formation in spite of their large π-core framework. These fibers were characterized by atomic force microscopy and UV-Vis spectroscopy.