Metal-Free Facile Synthesis of Multisubstituted 1-Aminoisoquinoline Derivatives with Dual-State Emissions.

Although isoquinoline is a good traditional fluorescent structural unit, most of its derivatives emit fluorescence in solution and a few of them can emit solid-state fluorescence as well. Herein, a series of multisubstituted 1-aminoisoquinoline derivatives were synthesized by a simple reaction of a readily available 4H-pyran derivative and secondary amines. The reaction had advantages of metal-free, mild conditions, simple operation, and good yields, which was realized by a ring-opening and sequential ring-closing mechanism. These 1-aminoisoquinoline derivatives were found to exhibit interesting dual-state emissions. In the solution, they emitted strong blue fluorescence at about 458 nm. In the solid state, they emitted solid-state blue fluorescence at 444-468 nm with high fluorescence quantum yields of 40.3-98.1%. Crystal structural analyses indicated that solid-state emissions of these compounds originated from twisted molecular conformations and the resultant loose stacking arrangements. Furthermore, their solid-state fluorescence wavelengths were demonstrated to depend on molecular conformations rather than stacking arrangements. The discovery of these 1-aminoisoquinolines with multiple reaction sites provides new possibilities for the development of solid-state fluorescent materials based on the traditional isoquinoline skeleton.