By utilizing Dy­(hfac)3(H2O)2 to react with enantiomerically pure tridentate N,N,N-pincer ligands, namely (−)/(+)-2,6-bis­(4′,5′-pinene-2′-pyridyl)­pyridine (L R and L S ), respectively, homochiral DyIII enantiomeric pairs formulated as Dy­(hfac)3L R /Dy­(hfac)3L S (R -1/S -1) (hfac– = hexafluoroacetylacetonate) were achieved and structurally characterized. Meanwhile, their magnetic, photoluminescent (PL), and chiroptical properties were probed. The PL test results indicate that the precursor Dy­(hfac)3(H2O)2 only shows very weak emission, while R -1 exhibits characteristic DyIII f–f transition emission bands at room temperature. Furthermore, the nonlinear optical responses of Dy­(hfac)3(H2O)2, L R /L S , and R -1/S -1 were investigated in detail based on crystalline samples. The results reveal that L R and L S present the coexistence of second- and third-harmonic generation (SHG and THG) responses with more intense signals for SHG responses; and Dy­(hfac)3(H2O)2 merely displays weak THG responses, while R -1 and S -1 also only exhibit THG responses. However, the THG intensities of R -1 and S -1 are more than six times larger than that of Dy­(hfac)3(H2O)2 under the identical measurement conditions. These results demonstrate that introducing homochiral N,N,N-pincer ligands to replace two H2O molecules of Dy­(hfac)3(H2O)2 results in significant improvements of both PL performances and THG responses of resultant R -1/S -1 enantiomers. R -1 and S -1 integrate PL, THG, and chiral optical activity in one molecule, suggesting their multifunctional merits. In particular, a convenient method is introduced to simultaneously test THG and SHG responses of molecular materials based on crystalline samples in this work.

The third-order nonlinear optical (NLO) materials with stimuli-responsive properties have received extraordinary attention due to their controllable photophysical properties. In this work, two attractive metal complexes thirdorder NLO switches, which are far superior to congeneric optical switches in terms of their performance conversion, versatility, and fast response, were successfully designed and synthesized. The test of their third-order NLO properties proves that the metal complexes exhibit reverse saturable absorption and self-defocusing refraction. After light irradiation, the third-order NLO behavior turns quickly into selffocusing refraction. The relation between the molecular structures and the third-order NLO properties was investigated via 1 H nuclear magnetic resonance and ultravioletvisible absorption. The results show that the metal ions have a significant influence on the NLO behavior and reveal the origin of third-order NLO properties via Z-scan determinations, pump-probe technology, and density functional theory calculations. These metal complexes can be used as third-order NLO switches with excellent fatigue resistance and broaden the application range of third-order NLO materials with adjustable performances.

The twisted intramolecular charge transfer (TICT) process endows the luminogen-containing rotatable electron donor−π–acceptor (D−π–A) structure with different electronically excited state energy levels, resulting in multiple photophysical properties. In this work, a new propeller-like molecule of 5,6-di­(4-N,N-(dimethylamino)­phenyl)­pyrido­[2,3-b]­pyrazine (APPP) with a rotatable D−π–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 (AIEgen) with third-order NLO properties and, more importantly, provides a visualized prototype to understand the origination of third-order NLO properties in TICT systems.

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