Irreversible Solvatochromic Zn-Nanopaper Based on Zn(II) Terpyridine Assembly and Oxidized Nanofibrillated Cellulose

A new irreversible solvatochromic Zn-nanopaper has been produced through the coordination-driven assembly of Zn­(II)-terpyridine complex (Zn-tpy) on the surface of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibril (tCNF). The Zn-tpy as a photoactive center exhibits a changed emission color from greenish-blue to yellow after coordination with the carboxylate anion on the surface of tCNF. Theoretic calculations support that the longer wavelength emission is the result of a metal–ligand charge transfer. When exposed to solvents and then dried, the coordination bond between the Zn-tpy and tCNF experienced a dynamic, reversible process, where the lowest-energy excited state emitted by the Zn-tpy was “inverted”, which is a typical phenomenon of irreversible solvatochromism. The shifts of the emission colors of the Zn-nanopaper appeared result from its exposure to specific solvents and occurred in a matter of minutes. After solvent exposure, it was found that the emission colors of the nanopaper are not recovered to its original state. The different emissive Zn-nanopapers are easily prepared by post-processing using a solvatochromic process. This highly transparent Zn-nanopaper with post-processable emission offers unprecedented potential applications in the areas of memory devices, fluorescent switches, and organic light-emitting diodes (OLEDs).