Bis-TTF-Ge derivatives: promising linear and nonlinear optical properties, a theoretical investigation

Abstract

In this work, based on bis(tetrathiafulvalenyldithio)-germane (bis-TTF-Ge), 37 compounds (denoted T0–T36) are designed by the introduction of donor and acceptor groups at different substituent positions. The ground state electronic structures, reactivity indices, electronic transition, charge transfer properties (charge, distance and dipole moment), hyper-Rayleigh scattering and depolarization ratios (static and dynamic) of these compounds are fully investigated using DFT and TD-DFT calculations. The quantum calculations were performed at the CAM-B3LYP/6-311g(d,p) level and using the sum-over-states (SOS) approach in both static and dynamic cases. The chemical hardness (η) of the two substituted bis-TTF-Ge derivatives at the b position has a smaller value than those with the same substituent at positions d, e, a and c. In both static and dynamic regimes the investigations show that the bis-TTF-Ge derivatives substituted with donor or acceptor groups have larger first hyperpolarizability than T0. Our work predicts that the introduction of the substituent group at the b position can increase the hyperpolarizability values more than those at the other positions, and the NO₂, NO and COCN acceptor groups lead to the largest values. For example, the dynamic β^λ=1064_HRS value of T25 is about 145 times larger than that of T0 and about 11 times that in the static regime. Interestingly, the β^λ=1064_HRS of bis-TTF-Ge substituted with the NO₂ group increases with the number of NO₂ in the TTF fragment. The large nonlinear optical (NLO) origin of bis-TTF-Ge substituted with NO₂, NO and COCN is attributed to charge transfer from the TTF to the acceptor group at the second TTF fragment (overlap S_r index = 0). The studied bis-TTF-Ge substituted with NO₂ compounds exhibits the possibility of being an excellent second-order NLO material.