DFT Investigation of Triarylamine-α-cyanoacrylic Acid Compounds: Structural, Electronic, and Nonlinear...

Using the density functional theory and finite field method, nonlinear optical properties of nine triarylamine-α-cyanocinnamic acid derivatives were investigated at the M06-2X/6-311++ G(d,p) and ωB97X-D/6-311++ G(d,p) levels of theory. Except for (E)-2-cyano-3-(4-(di([1,1'-biphenyl]-4-yl)amino)phenyl)acrylic acid (a), which had a D-π-A electronic structure, all the other eight derivatives had an A-π-D-π-A structure. The results suggest that the lowest energy transition of the nine triarylamine derivatives was the π-π* transition from the HOMO to LUMO. The absorption maxima of the derivatives in their ethanol solution were redshifted with respect to those in the gas phase. The introduction of conjugated C = C or C≡C bonds between the biphenyl unit of molecule a had a minor effect on the second-order nonlinear optical properties of the molecule. However, the introduction of C = C bond into the parent molecule improved the third-order nonlinear optical properties. The introduction of a heterocyclic ring (furan ring or thiophene ring) between the triarylamine moiety and the branched chain containing the cyanocinnamic acid group enhanced the second- and third-order nonlinear optical properties; especially, the second- and third-order polarisabilities of molecules b3 and c3, which were obtained by introducing a thiophene ring, were the highest. The second- and third-order polarisabilities of b3 were 0.13 × 10⁵ and 27.13 × 10⁵ a.u., respectively, while those of c3 were 0.14 × 10⁵ and 28.10 × 10⁵ a.u., respectively. This suggests that b3 and c3 have desirable second- and third-order nonlinear optical properties and can be used for designing efficient second- and third-order nonlinear optical materials.