Design and Synthesis of Photofluorochromic Coordination Polymers Featuring Dual-Mode Output for Dynamic Anticounterfeiting

Photofluorochromic compounds exhibit dual-mode output characteristics of photochromism and fluorescence, enabling high-security anticounterfeiting. However, constructing such materials is difficult due to the competition between the photoinduced electron transfer (PIET) process and fluorescence. Herein, we propose that prolonging the conjugation length of the ligand to form a charge transfer excited state may be an efficient method for constructing photofluorochromic materials. Accordingly, we designed and synthesized a ligand featuring an extended conjugation structure (denoted as (HL)Cl = 1-((4′-carboxy-[1,1′-biphenyl]-4-yl)methyl)-[4,4′-bipyridin]-1-ium) chloride. Through a thermal reaction with metal ions and a second ligand, two coordination polymers [Cd(L)(HL1)]·DMF·2H2O (1, H3L1 = 1,3,5-benzenetricarboxylic acid) and [Zn2(L)(L2)(H2O)2]·2H2O (2, H4L2 = 1,2,4,5-benzenetetracarboxylic acid) were successfully synthesized. Both compounds exhibit excellent photochromic and luminescent properties. Their photochromic processes show excellent reversibility and can be repeated for more than five cycles. Both compounds exhibited excellent luminescence after photochromism, which was attributed to promoted charge transfer, resulting from the extended conjugation length of the ligand. These dual-mode signal output materials demonstrate significant potential for anticounterfeiting applications. Based on their dual-mode behavior, this study demonstrates various anticounterfeiting technologies, including banknote anticounterfeiting. The dynamic photochromic process combined with the luminescent authentication significantly enhances anticounterfeiting security, confirming the great feasibility of these dual-mode output materials for high-level security applications. This work is expected to provide new insights into the development of photofluorochromic coordination polymers.