Characterisation of a dipolar chromophore with third-harmonic generation applications in the near-IR

E-2-Tricyanovinyl-3-n-hexyl-5-[4-{bis(4-n-butylphenyl)amino} -2-methoxystyryl]-thiophene, 1, has previously been used to demonstrate applications relying on frequency tripling of 1.55 μm light. Here we report the synthesis and chemical characterisation of 1, along with quantum-chemical calculations and additional experimental investigations of its third-order nonlinear properties that give more insight into its frequency tripling properties. Although 1 can be processed into amorphous films, crystals can also be grown by slow evaporation of solutions; the crystal structure determined by X-ray diffraction shows evidence of significant contributions from zwitterionic resonance forms to the ground-state structure, and reveals centrosymmetric packing exhibiting π-π and C-H⋯NC interactions. Both solutions and films of 1 exhibit near-infrared two-photon absorption into the low-lying one-photon-allowed state with a peak two-photon cross-section of ca. 290 GM (measured using the white-light continuum method with a pump wavelength of 1800 nm) at a transition energy equivalent to degenerate two-photon absorption at ca. 1360 nm; two related chromophores are also found to show comparable near-IR two-photon cross-sections. Closed-aperture Z-scan measurements and quantum-chemical calculations indicate that the nonlinear refractive index and third-harmonic generation properties of 1 are strongly dependent on frequency in the telecommunications range, due the aforementioned two-photon resonance.