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.