Third-order nonlinear optical properties of a series of pyrroles and dipyrroles featuring π-conjugated substituents and linkers

The third-order nonlinear optical properties of a series of novel pyrrole-containing compounds were explored. A custom nonlinear optical microscope was used to measure the third harmonic generation (THG) intensity ratio from two interfaces of an ultrasmall cuvette: a glass-solution interface incorporating the pyrrole-containing compound, and an air-glass interface. The THG intensity ratio, along with refractive index measurements of the solution at the laser wavelength (1030 nm) and the third harmonic wavelength (343 nm), were used to calculate <i>χ</i>⁽³⁾ values of each pyrrole-containing solution. The second hyperpolarisability <i>γ</i> was then extracted from concentration-dependent measurements of <i>χ</i>⁽³⁾. Trends in the magnitude of <i>γ</i> for the studied pyrrole-containing compounds became apparent, including that several formyl-substituted dipyrrole derivatives exhibited elevated <i>γ</i> values. Further, an enhancement in <i>γ</i> was observed with increasing conjugation length when formyl-substituted dipyrroles with differing internal linkers were studied. Lastly, the complexation of ruthenium by a formyl-substituted dipyrrole enhanced the magnitude of the <i>γ</i> value by ∼3 to ∼300 times, depending on the dipyrrole derivative. These trends can be used to understand the chemical features needed for the emergent development of dyes for THG microscopy.