# Thermally tunable naphthalene diimide solvates enable selective sensing, reversible photochromism, and anti-counterfeiting applications

## TL;DR

This paper introduces two naphthalene diimide solvates that can sense mercury ions, change color reversibly under light, and have potential for anti-counterfeiting uses.

## Contribution

The study presents a new strategy for creating multifunctional NDI-based materials with combined sensing and photochromic properties.

## Key findings

- NDI-1 and NDI-2 show distinct solvatochromic behavior in DMF solution.
- The compounds selectively detect Hg2+ ions with clear optical changes.
- They exhibit reversible photochromism under UV light and sunlight.

## Abstract

Recent studies have highlighted the potential of naphthalene diimide (NDI)-based molecules for their stimuli-responsive and optical properties. In this work, we report two naphthalene diimide (NDI)-based solvates, designated as NDI-1 and NDI-2, were successfully synthesized from 4-aminopyridine and 1,4,5,8-naphthalene tetracarboxylic dianhydride (NDA) and comprehensively characterized using FT-IR, thermogravimetric analysis (TGA), UV-Vis, fluorescence spectroscopy, powder X-ray diffraction (PXRD), 1H NMR, single-crystal X-ray diffraction, and Hirshfeld surface analysis. Despite being solvates of each other, NDI-1 and NDI-2 exhibit distinct solvatochromic properties in DMF solution, as demonstrated by their unique UV-Vis absorption and fluorescence responses. Notably, the compounds display exceptional selectivity toward Hg2+ ions in DMF, outperforming a range of competing metal ions and producing characteristic optical changes. Furthermore, both solvates undergo reversible photochromic transformations upon exposure to UV light (365 nm), sunlight, and tungsten light, with the fastest switching under UV irradiation. These light-induced color changes gradually revert after removal of the stimulus, and similar reversible behavior is retained in polymer-embedded films (PVDF@NDI-1). Overall, this study provides a valuable strategy for designing multifunctional NDI-based materials, addressing the limited availability of conjugated systems that simultaneously exhibit multiple responsive properties and offering promising prospects for secure data storage and anti-counterfeiting technologies.