Dispersion Overwhelms Charge Transfer in Determining the Magnitude of the First Hyperpolarizability in Triindole Octupoles

Abstract

A comprehensive series of systematically functionalized C3-symmetric hexakis (para-substituted) triindoles has been studied for its linear and second-order nonlinear optical properties. The carbazole-derived triindole central core is electron rich and electron donating. The peripheric substitution pattern, resulting in a donor–donor or a donor–acceptor charge transfer, is reflected in the UV–vis absorption spectra, where a lower energy charge-transfer band is observed for the donor–acceptor pattern. On-resonance all compounds exhibit a strongly enhanced second-order nonlinear optical response, critically dependent on the particular wavelength but showing no clear correlation with the charge-transfer character imparted by the peripheral substituents. Nonetheless, extremely large values are obtained: we measured the highest value ever reported for octupolar compounds in transparent conditions on-resonance. Off-resonance significantly smaller values are found, which are very similar for all compounds and show no correlation with the charge-transfer character as well. Both observations have been unambiguously confirmed by (linear and nonlinear) spectroelectrochemistry on a donor–donor structure, effectively transforming this to an acceptor–donor structure (by oxidizing the donor triindole core to an electron-accepting triindole-based cation radical). The strong wavelength dependence of the first hyperpolarizability values around resonance is clearly shown to be overwhelmed by dispersion effects and not to be determined by the charge-transfer pattern in these octupolar materials. This finding provides insight for independent tuning of the linear absorptive properties, determined by the charge-transfer pattern, and the second-order nonlinear polarizability, not determined by this pattern but strongly dispersive.