A DFT study on the two-dimensional second-order nonlinear optical (NLO) response of terpyridine-substituted hexamolybdates: Physical insight on 2D inorganic-organic hybrid functional materials

DFT calculations were carried out in order to study the second-order nonlinear optical (NLO) response of a series of proposed 2D polyoxometalate- based terpyridine-substituted compounds. These compounds can be formulated as [Mo₆O₁₇{N₄C₂₅H₁₆(X) ₂}{N₄C₂₅H₁₆(X)₂}] ^2- (X = H, F, Cl, Br, I, CF₃, or CN), which has a wedge Λ-shaped acceptor - π-conjugated bridge - donor - π-conjugated bridge - acceptor (A-π-D-π-A) configuration. The calculations showed that these compounds possess significantly large molecular second-order polarizabilities that range from approximately 1000×10^-30 to 4300×10^-30 esu. The combination of trifluoromethyl (CF ₃) and cyanide (CN) groups at the end of the terpyridine ligand strengthens the bridge conjugation, which is useful for the enhancement of the NLO response. In addition, the greatest contributions to the β_vec values are dervied from the charge transfer (CT) from the Mo≡N bond and the organoimido ligand to the terpyridine-substituted segments. This report demonstrates that various combinations of the acceptor(s) remarkably affect the second-order NLO response. The electronic transitions to the crucial excited states indicated that the y polarized transition contributed to the off-diagonal second-order polarizabiliy tensor (β_zyy) and that the z polarized transition accounted for the diagonal second-order polarizabiliy tensor (β_zzz). Thus, itsteered towards in-plane nonlinear anisotropy (u = β_zyy/β_zzz) along with a good 2D second-order NLO response. These compounds can be used as good 2D second-order NLO materials from the point of view of their large β values.