Modulating the charge transfer of D-S-A molecules: structures and NLO properties

Very recently, the investigation of an Li atom doped effect on the "through-space" electronic interaction (S) of a donor-S-acceptor (D-S-A, 1) shows that the Li-doping effect can modulate the first hyperpolarizability of 1 ( Dyes Pigm. 2014 , 106 , 7 - 13 ). Can we further enhance the first hyperpolarizability (βtot) of 1 by modulating the charge transfer of D-S-A molecules? The present work indicates that the βtot value can be successfully modulated by replacing the sp(2)-hybridized CH═CH moiety connected with substituted para-cyclophane (PCP). On the other hand, the NO2 contributes more than NH2 to the βtot value. The results of time-dependent density functional theory (TD-DFT) provide a good explanation for the variation in the βtot value. Interestingly, the βtot value of 3 (4.09 × 10(3) au) is larger than 1.52 × 10(3) au of 4, while the difference between the dipole moments (Δμ) of the ground state and the crucial excited state of 3 (2.93 D) is smaller than that of 4 (7.79 D). Further, the charge-transfer excitation length (D(CT)) of 3 (1.41 Å) is smaller than that of 4 (2.89 Å). Therefore, D(CT) is the major factor in determining the Δμ value.