Two-photon excited luminescence and second-harmonic generation in quinacridone microstructures

Nonlinear optical microscopy was employed to investigate the properties of microstructures of quinacridone, a hydrogen-bonded pigment, derivative of quinacridine. Recently, quinacridone hierarchical microstructures (μ-QNC) having different nature-inspired morphologies were reported, e.g. in the shape of hedgehogs formed by self-assembled nano-needles. We find that such microcrystals have structure-dependent linear optical properties and exhibit strong two-photon excited fluorescence when irradiated with a near-infrared femtosecond laser. μ-QNCs exhibit different nonlinear optical properties for different crystal structures, including the emergence of symmetry-forbidden second-harmonic generation.