Wavelength dependent nonlinear optical response of tetraphenylethene aggregation-induced emission luminogens

Since the discovery of the striking aggregation-induced emission (AIE) phenomenon, tremendous efforts have been devoted to the exploration of brilliant organic solid-state light emitting materials and their underlying mechanistic theories. While AIE is well established in linear optics, the nonlinear optical (NLO) properties of the AIE luminogens (AIEgens) remain relatively unexplored. Here, we present a detailed study of the NLO properties of crystalline prototypical AIEgens based on tetraphenylethene (TPE). The results disclose a highly efficient NLO response of TPE crystals which is strongly dependent on the wavelength of the incident near infrared (NIR) light. TPE exhibits mainly two-photon excited fluorescence (TPF) upon excitation at 700 nm < lambda < 800 nm, while it exhibits three-photon excited fluorescence (3PF) if pumped at 800 nm < lambda < 840 nm. For pump wavelengths longer than 840 nm, mainly second harmonic generation (SHG) is observed. The introduction of halogen atoms into the TPE backbone results in comparable NLO hyperpolarizabilities, but allows for future derivations and functionalizations. The exploration of the nonlinear optical properties of AIEgens might open a new avenue to a next generation of multifunctional organic and composite photonic materials and devices.