Large static first and second hyperpolarizabilities dominated by excess electron transition for radical ion pair salts M2˙+TCNQ˙− (M = Li, Na, K)

The interesting radical ion pair salts M2˙+TCNQ˙− (M = Li, Na, K) are a particular class of charge transfer complexes with excess electron. The ground states of these complexes are triplet. The C2v symmetry geometrical structures of the M2˙+TCNQ˙− (M = Li, Na, K) with all-real frequencies are obtained at the density functional theory (DFT) B3LYP/6-31+G(d) level. All calculations of electric properties in this paper have been carried out at the restricted open-shell second order Møller–Plesset perturbation theory (ROMP2) level. Owing to existing excess electron (from the polarized alkali metal atoms) these charge transfer complexes exhibit large nonlinear optical (NLO) responses dominated by excess electron transitions. For these radical ion pair salts M2˙+TCNQ˙−, the static first hyperpolarizabilities (β0) are large. The order of β0 values is 19 203 (M = Li) < 24 140 (M = Na) < 29 065 a.u. (M = K). Specially, the second hyperpolarizability (γ0) of the complexes with excess electron is obtained for the first time. These static second hyperpolarizabilities are also large. The order of γ0 values is 2 213 006 (M = Li) < 3 136 754 (M = Na) < 7 905 623 a.u. (M = K). Among the three structures, K2˙+TCNQ˙− has the largest γ0 value to be 7.9 × 106 a.u. (3982 × 10−36 esu), which is about 9 times larger than that of the intramolecular charge transfer complex σ-arylvinylidene trans-[Ru(4-CCHC6H4CCC6H4NO2)Cl(dppm)2]PF6 [Hurst et al., Organometallics, 2001, 20, 4664]. The present investigation provides a new kind of candidates for the high-performance NLO materials.