Aggregation/Viscosity-Induced Emission and Third-Order Nonlinear Optical Signal Inversion in a TICT System

The twisted intramolecular charge transfer (TICT) process endows the luminogen-containing rotatable electron donor-pi-acceptor (D-pi-A) structure with different electronically excited state energy levels, resulting in multiple photophysical properties. In this work, a new propellerlike molecule of 5,6-di(4-N,N-(dimethylamino)phenyl)pyrido[2,3-b]pyrazine (APPP) with a rotatable D-pi -A structure was facilely prepared, which exhibited a typical TICT character. Interestingly, aggregation/viscosity-induced emission and third-order nonlinear optical (NLO) signal inversion from reverse saturation absorption (RSA) to saturation absorption (SA) in the aggregated state or in solutions with high viscosity were observed in this system. Mechanism studies showed that these properties were originated from the restricted intramolecular rotation (RIR) process, which not only cut off the nonradiative transition channel of electronically excited states but also influenced the electronically excited state transition rate from the charge transfer (CT) state to the TICT state. According to these results, a temperature-triggered third-order NLO optical switch was successfully designed. This work affords a new type of aggregation-induced emission luminogen (AlEgen) with third-order NLO properties and, more importantly, provides a visualized prototype to understand the origination of third-order NLO properties in TICT systems.