Modulating the Electronic Properties of Multimeric Thiophene Oligomers by Utilizing Carbon Nanotube Confinement

We investigated the arrangement of methyl-terminated terthiophenes inside a nanotube by using density functional theory (DFT) including dispersion corrections. After DFT calculations were conducted, a variety of arrangements of the inner terthiophene chains was found, depending on host-tube diameters and the number of chains. Because of the various inner thiophene arrangements, the terthiophene chains interact differently. The interactions in a smaller nanotube are stronger than those within a larger nanotube, indicating the importance of nanotube confinements to the interchain couplings. The interchain interactions split the orbitals of the multimeric terthiophene chains, which are built from single-chain frontier orbitals, broadening their energy levels. Therefore, nanotube confinements are key factors in determining the energy levels of the frontier orbitals of contained multimeric terthiophenes. As a result, their electronic transitions are affected by the encapsulation in a restricted nanotube space. According to time-dependent DFT calculations, a specific electronic transition occurs from a HOMO-built orbital to a LUMO-built orbital. The broadening of the orbital energies by the aggregation of terthiophene chains in a nanotube leads to a widened range of excitation energies (Ex) in their electronic transitions relative to the single-chain. With respect to the strongest transition of multimeric terthiophenes, the excitation energy is enhanced by confinement to a nanotube. The Ex enhancement within a smaller nanotube is more significant than that within a larger nanotube because of the stronger interchain interactions in a smaller nanotube. Therefore, it is proposed from the DFT calculations that nanotube confinements can modulate electronic and absorption properties of multimeric terthiophene chains by changing the interchain interactions.