Luminescence of lanthanide ions: an optical tool against counterfeiting

Abstract

Security labels such as barcodes or QR codes have been used as anti-counterfeiting tools in passports, drivers' licenses, banknotes, and medicine packs. For such a proposal, luminescent materials invisible to naked eyes but revealed by near-UV radiation have been applied, mainly those based on lanthanide (Ln3+) complexes such as [Eu(tta)3phen] (tta = 2-tenoyltrifluoroacetonate; fen = 1,10- phenanthroline) due to their bright luminescence, narrow emission bands featuring high color purity, strong absorption within the near-UV spectral region and thermal and chemical stability. Yet, the dissemination of knowledge regarding this measurement ends up making it possible for security agents to be included or circumvented during the counterfeiting process. To overcome this issue, circularly polarized luminescence (CPL) appears as an alternative tool since replicating CPL, without advanced knowledge and methods of chiral chemistry, hinders the process of counterfeiting. CPL arises from chiral ligands excited with UV radiation for instance, while it can be easily detected by using photomultipliers adapted with manual polarizers or even adapted 3D glasses. Therefore, this proposal aims to investigate new chiral molecules and their use as ligands in Ln3+ complexes for application as anti-counterfeiting inks. The proposed chiral ligands are bis-oxazolines pyridines and bis-oxazolines pyrazines which will be combined with ²- diketone ligands known as efficient Ln3+ antennas to synthesize heteroleptic complexes combining large emission quantum yield and CPL. The chiral ligands may be used for UCNP surface modification leading to circularly polarized upconversion naoparticles (CP-UCNP) and also interacting them to lanthanide molecular cluster aggregates (MCA). (AU)