Effects of Encapsulating Tube and Encapsulated Molecular Chain Length on the Second-Order Nonlinear Optical Responses of Carbon Nanotubes Filled with Head-To-Tail Polar Molecules

Carbon nanotubes (CNTs) filled with dye molecules have been reported to yield extremely strong second-order nonlinear optical (NLO) responses. To get a clear understanding about the origin of such enhancement of NLO properties of CNTs filled with head-to-tail dipolar molecules, the current study considers three typical dipolar molecules (i.e., 1-amino-4-nitro-1,3-butadiene, para-nitroaniline, and 4-amino-4′-nitro-trans-stilbene) as guests filled inside a host tube CNT(7,5) or CNT(9,7) with their electronic structures and second-order NLO properties being investigated. The present work found that the key to the enhancement of NLO properties of complexes partially comes from the geometric deformation of the outer nanotube caused by the interaction between the tube and the inner dipolar molecules, which leads to the conspicuous polarized charge distribution of the outer nanotube. Further analysis of the transition nature shows that the long-range intramolecular charge-transfer-based electron excitations on the distorted nanotube and the intermolecular charge-transfer-based electron excitations between the nanotube and dipolar molecules make the complexes have strong second-order NLO responses. The evolution of second-order NLO properties of the molecular chain with chain length found intermolecular charge-transfer-based electron excitations occurring in the molecular chain with a certain length.