Dispersion-corrected density functional
theory (DFT) calculations
were employed to investigate stable structures of triiodobenzenes
(BzI3) inside an armchair (m,m) carbon nanotube (m = 7–9), denoted by n × BzI3@(m,m), where n is the number of guests (n = 1–4). Three BzI3 isomers were considered: 1,3,5-,
1,2,4-, and 1,2,3-BzI3. DFT calculations found that nanotube
confinement has an impact on n × BzI3@(m,m) structures, featuring guest
orientation, positioning, and alignment. Then, we estimated a threshold
value of the diameter of a tube containing BzI3 monomer
in a certain orientation without repulsive host–guest interactions
(D
threshold). To represent the degree
of nanotube confinement in 1 × BzI3@(m,m), we obtained ΔD
small values by subtracting the smallest limit of D
threshold of the guest from the host–tube diameter (D). Negative ΔD
small values
indicate strong confinement, and positive values less (much larger)
than 1.0 Å indicate medium (weak) confinement. The strong and
medium confinement in n × BzI3@(m,m) form a linear alignment of guests
in parallel orientation with the smallest D
threshold value. Reflecting from the different degrees of nanotube confinement,
various types of intermolecular interactions operate. DFT calculations
found that I–I intermolecular interactions are key in enhancing
the second-order nonlinear optical properties of n × BzI3@(m,m).
As a result, n × 1,2,4-BzI3@(8,8)
with medium nanotube confinement has the most significant hyperpolarizability.