Self-assembly of pyrazine-containing tetrachloroacenes

In this paper, we report self-assembly of tetrachloroacenes containing pyrazine moieties. The title compounds, phenazine and bisphenazine substituted with four chlorine atoms for increased electron deficiency and alkyloxy side groups for solubility, demonstrated excellent gelation ability in select organic solvents. The assembled structure of these two series of compounds exhibited a morphological difference. Tetrachlorophenazine containing hexadecyloxy side groups induced rigid microbelts, while more extensive entanglement of thinner, more flexible fibers was observed from tetrachlorobisphenazine compounds, characterized by scanning electron microscopy. Tetrachlorophenazine and tetrachlorobisphenazine gels showed quite different emission behavior compared to their solution state. A strong, red-shifted emission compared to that of its diluted solution state was observed from the gel of tetrachlorophenazine. We have ascertained this is a result of J-aggregate formation. From the crystal structure of a model compound, it was found that tetrachlorophenazine cores adopt π-π stacking with a short stacking distance of 3.38 Å, enabling significant intermolecular π-orbital overlap. In addition, the π-cores were displaced longitudinally, indicative of J-aggregate formation. Surprisingly, the gel of tetrachlorobisphenazine showed fluorescence comparable to that of its dilute solution, suggesting that such a close packing of the π-cores may not be possible due to the bulky tert-butyl substituents.