Key factors in determining the arrangement of π-conjugated oligomers inside carbon nanotubes

Density functional theory (DFT) calculations with dispersion corrections elucidated the key factors for determining the arrangement of π-conjugated oligomers inside a carbon nanotube. The current study considered methyl-terminated terfurans as guests inside a host tube; the results were compared with those obtained in previous studies for methyl-terminated terthiophenes inside a nanotube. DFT calculations found that the most important factor in determining the guest arrangement is the host-guest interactions arising from long-range CH-π and π-π interactions. In particular, the host-guest interactions play a crucial role in the arrangement of π-conjugated oligomers inside a larger-diameter tube: π-conjugated oligomers sit near the inner tube wall to maximize attractive host-guest interactions. Within a smaller-diameter tube, host-guest interactions, as well as guest-guest (interchain) interactions, are responsible for the guest arrangement. Then, stronger host-guest (weaker guest-guest) interactions are a major (minor) factor. The stronger host-guest interactions are sufficient to deform the inner π-conjugated oligomers. Due to the tube encapsulation, inner terfurans lose their planarity, leading to a weakening of the interchain interactions. In contrast, tube encapsulation induces terthiophenes to assume nearly planar structures, enhancing their interchain interactions. As a result, the magnitude of the interchain interactions is dependent on the type of inner oligomer. Reflecting the different interchain interactions, multimeric terfurans inside a nanotube exhibit molecular packings that are different from those of the corresponding terthiophenes. Considering the importance of the arrangement of inner π-conjugated oligomers to their electronic properties, nanotube containers can thus tune the properties of inner oligomers.