Charge transport and optical properties of the complexes of indigo wrapped over carbon nanotubes

A new molecular system comprising the non-covalently functionalized complexes of single walled (6,6) carbon nanotubes (SWCNTs) of finite length with indigo is proposed based on the dispersion-corrected density functional theory calculations. In the complexes viz. the dyad and triad, indigo is wrapped over carbon nanotubes in the ratio of 1 : 1 and 2 : 1, respectively. A comprehensive study of stabilization energy, ionization energy, electron affinity, the energy gap between the highest occupied and lowest unoccupied molecular orbitals (ΔELUMO–HOMO), and absorption spectra unravels the structure–property relationship of the complexes. The energy gap of ∼1 eV between the HOMO and the LUMO of the complexes suggests that they can be semiconductive. The energy levels of the frontier molecular orbitals of indigo and CNT suggest the possibility of the photoinduced charge transfer between them. Using the charge hopping rate based on Marcus theory, a hole mobility as high as 8.77 cm2 V−1 s−1 is obtained for the dyad. For both the dyad and triad, a higher value of hole mobility than electron mobility is observed, thereby suggesting them to be useful for p-type semiconductor devices. The time-dependent density functional theory (TD-DFT) calculations predict that the absorption of indigo–CNT complexes occurs in the visible and the near-infrared regions finding applications in organic light emitting diodes (OLEDs). Furthermore, the effects of the length and the capping of CNTs as well as the orientation of indigo over the CNTs on the charge transport properties are also discussed.