External electric field modulated second-order nonlinear optical response and visible transparency in hexalithiobenzene.

External electric fields (EEFs) offer a unique opportunity to tune certain activity of molecules by orienting the alignment of the electric field along the specific axis. The second-order NLO response of hexalithiobenzene (C6Li6) is very poor due to its first mean hyperpolarizability of 0.5 a.u. Therefore, we have analyzed the effect of EEFs on the structural, electronic properties, and NLO response of C6Li6 using a density functional approach. We notice that the structure of the C6Li6 molecule remains planar, with the slight change in C-C and C-Li bond lengths, but their stability is increased under the effect of EEFs. By applying EEFs, the conductivity or reactivity of C6Li6 is increased as their HOMO-LUMO energy gap is decreased. Furthermore, C6Li6 attains a finite dipole moment in the presence of EEF, which increases linearly as the EEF increases. More interestingly, the first static hyperpolarizability of C6Li6 is significantly enhanced, becoming as high as 3.4 × 104 a.u. for EEF = 50 × 10-4 a.u. This suggests the EEF as an effective way to enhance the second-order NLO responses, leading to the design of potential NLO materials. Nevertheless, the visible transparency of C6Li6 with and without EEF may suggest its possible applications in optical devices.