A systematic study of the effects of thionation in naphthalene dimide derivatives to tune their nonlinear optical properties.

In the present study, the number and position of sulfur atoms on naphthalene diimide (NDI) is systematically investigated to tune its nonlinear optical (NLO) response properties. Our DFT calculations for third-order polarizability (γ) show that the thionation significantly influences the nonlinear optical property of NDI as it is seen among its several designed derivatives (NDI-1 to NDI-10). The smallest and the largest γ amplitudes are 503.49 × 10 and 1299.5 × 10 esu for NDI-1 (having tetraone group) and NDI-10 (having tetrathione group), respectively. The increase in γ amplitude for NDI-10 is 796 × 10 esu, which is ∼150% from the γ amplitude of NDI-1. A comparison of the γ amplitudes of our designed derivatives are made with para-nitroaniline i.e. a prototype NLO molecule. The γ amplitude of pNA is found to be 42.64 × 10 esu at the same B3LYP/DZVP2 level of theory. Using two-level model, the origin of larger γ amplitudes is traced in lower transition energy of NDI-10. Furthermore, the calculation of vertical ionization potentials (VIPs) shows that the thionation does not affect the stability of designed derivatives, where a slight difference of 0.06 eV is seen between the VIPs of NDI-1 (6.63 eV) and NDI-10 (6.57 eV). Thus, a systematic comparison of the third-order polarizability and other electro-optical properties of our designed derivatives shows that our derivative systems possess good potential for their practical realization in the field of optical and NLO materials.