Light-Responsive Fluorescent Security Ink Based on Calixpyridinium–Protoporphyrin IX Disodium Salt Supramolecular Nanoassemblies

Abstract

In this work, the coordination effects of protoporphyrin IX disodium salt (PpIX) with CuII, FeII, and ZnII were systematically studied, and the host–guest interactions of calixpyridinium with PpIX and the three metalloporphyrins were further explored. The bonding ratio between calixpyridinium and PpIX was different from that between calixpyridinium and the other three metalloporphyrins. The reduction of the electronegativity of PpIX caused by the coordination of metal cations may be the reason for the change of the bonding ratio. The reduction of the electronegativity of PpIX caused by the coordination of metal cations also leads to a weaker host–guest interaction with calixpyridinium. The supramolecular nanoassemblies prepared by the host–guest interactions of calixpyridinium with PpIX and the three metalloporphyrins under alkaline conditions could emit a bright fluorescence due to the pH-dependent luminescence characteristics of calixpyridinium and PpIX. Among calixpyridinium and the three metal ions, calixpyridinium alone had the best effect on promoting the photoactivity of PpIX. Its fluorescence gradually disappeared after being irradiated by an incandescent lamp not only in aqueous solution but also on a filter paper. More interestingly, the supramolecular nanoassemblies formed by calixpyridinium and the photoreaction product of PpIX had a reversible temperature responsiveness, which could lead to a reversible fluorescence on–off control. As a result, this material was successfully applied as an intelligent light-responsive fluorescent security ink.