Angle-Resolved Iridescent Day-Long Afterglow in Flexible Polymeric Exciplex Films

Abstract

The creation of polymer-based persistent luminescence materials exhibiting tunable afterglow colors hold significant promise for anti-counterfeiting, flexible displays, and data storage. Nevertheless, realizing day-long organic persistent luminescence featuring angle-resolved afterglow color remains challenging. Herein, a day-long organic afterglow (DOA) system is developed by doping 9-(naphthalen-2-yl)-9′-phenyl-9H,9′H-3,3′-bicarbazole into polyethylene terephthalate. This system demonstrates a remarkably long afterglow (>24 h ambient, >100 h at 4 °C), ranking among the longest durations reported for polymeric afterglow materials. The DOA originates from radiative recombination of charge-separated radicals, which reform the emissive exciplex state. Importantly, this recombination is modulated by the donor/acceptor energy gap, and the cation radicals are stabilized by charge delocalization over the donor components. Furthermore, by integrating this long-persistent luminescent system into designed polymeric multilayer interference structures, iridescent afterglow color with distinct viewing-angle dependence is achieved. Based on the exciplex's broad emission, the angle-resolved DOA color exhibits wide spectral tunability, covering red, green, and blue hues, making it suitable for complex security features. Additionally, the system demonstrates exceptional mechanical flexibility and plastic- deformation-dependent optical properties, thereby expanding the potential of color-tunable organic afterglow materials for advanced applications in authentication technologies and optical security features.