Photophysics and Spin‐Physics Studies on Persistent Upconversion Luminescence from Nonlinearly Polarizable Ferroelectric‐Like Lattice Prepared by Orderly Packing Donor–Acceptor Structures under Multiphoton Excitation

Abstract The ultralong‐lived upconversion luminescence with the lifetime of 0.48 s in a broad spectral range (530–650 nm) is observed in CD49 (9‐(3‐(5‐bromopyridin‐3‐yl)prop‐2‐yn‐1‐yl)‐9H‐carbazole) crystal designed with donor–acceptor (carbazole–pyridine) structures under infrared excitation, simultaneously accompanied with second harmonic generation (SHG). This phenomenon indicates orderly packing donor–acceptor structures form a nonlinearly polarizable ferroelectric‐like lattice with ultralong‐lived light‐emitting states, leading to much prolonged nonlinear optical behaviors. The persistent upconversion luminescence together with SHG is largely reduced when lowering crystallinity. This implies that nonlinearly polarizable ferroelectric‐like lattice provides the necessary condition to generate persistent upconversion luminescence. Evidently, persistent upconversion luminescence becomes completely lacking when only using ultralong‐lived light‐emitting states without nonlinearly polarizable ferroelectric‐like lattice, exampled by 4‐(dimethylamino)benzonitrile dispersed in polyvinyl alcohol matrix. Magneto‐photoluminescence shows that persistent upconversion luminescence is essentially a super‐delayed fluorescence from crystalline intermolecular charge‐transfer excitons formed in the nonlinearly polarizable ferroelectric‐like lattice. Magnetodielectrics indicate crystalline intermolecular charge‐transfer excitons are coupled with nonlinearly polarizable ferroelectric‐like lattice, leading to prolonged nonlinear optical behaviors shown as persistent upconversion luminescence through super delayed fluorescence. Therefore, crystalline intermolecular charge‐transfer excitons formed in nonlinearly polarizable ferroelectric‐like lattice provide an interesting platform to generate prolonged nonlinear optical behaviors toward developing persistent upconversion luminescence under multiphoton excitation.