Lanthanoid Security Ink for UV-A Machine-Readable Banknote Patterns
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Solution Overview
Problem
Existing luminescent substances used for safeguarding documents of value, such as banknotes, lack sufficient lightfastness and are limited to broad emission spectra, making intricate patterns difficult to print and unsuitable for machine reading.
Innovation Solution
A polynuclear, heteroleptic lanthanoid complex with specific NN and OO ligands, excitable in the UV-A range, emitting in the visible wavelength range with high lightfastness and intensity, particularly with green and/or red light.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If inorganic luminophores are used for luminescent printing, then luminescence emission is achieved, but the emission spectra are broad and loading must be high, making intricate patterns impossible to print
Solution Approach 1:
The patent changes the fundamental parameters of the luminescent material by transitioning from inorganic luminophores to organic luminescent substances. This parameter change results in narrowband emission spectra that enable high-resolution printing of intricate patterns while requiring significantly lower loading amounts, thus resolving the contradiction between pattern intricacy and luminophore loading.
2Illumination intensity
If europium diketonate complexes are used for UV-A excitation, then luminescence emission is achieved, but light stability and thermal stability are low
Solution Approach 1:
The patent employs composite material design by combining europium or terbium metal centers with specifically designed organic ligands (diketonate or carboxylic acid ligands with extended conjugation). This composite structure leverages the advantages of both components: the metal center provides stable luminescence emission while the engineered organic ligands provide enhanced light stability and thermal stability, resolving the contradiction between luminescence emission and light stability.
3Reliability
If europium-carboxylic acid complexes are used for UV-A excitation, then light stability is improved, but intense luminescence is only achieved with short-wave UV light in UV-C range
Solution Approach 1:
The patent modifies the molecular parameters of the carboxylic acid ligands by introducing extensions of the conjugated system. This parameter change in the ligand structure enables the complexes to absorb UV-A radiation effectively while maintaining high light stability, thus resolving the contradiction between light stability and excitation wavelength range by shifting the absorption characteristics without sacrificing stability.
4Illumination intensity
If mononuclear Tb complexes are used for green phosphorescence, then energy level matching is achieved, but back-energy transfer to ligand occurs resulting in low luminescence quantum yields
Solution Approach 1:
The patent applies segmentation by transitioning from mononuclear to polynuclear terbium complexes. This segmentation creates multiple metal centers that can cooperate to enhance energy transfer efficiency to the ligands, thereby increasing the luminescence quantum yield and reducing back-energy transfer losses, thus resolving the contradiction between green phosphorescence emission and luminescence quantum yield.
5Illumination intensity
If known Tb complexes are used for safeguarding, then luminescence emission is achieved, but they are excitable only in short-wave UV light below λ=350 nm and/or have low quantum yield or low lightfastness
Solution Approach 1:
The patent systematically changes the molecular parameters of both the metal center coordination environment and the organic ligands. By adjusting the ligand field strength, extending conjugation, and modifying the coordination geometry, the patent broadens the excitation spectrum to include UV-A range while maintaining or enhancing quantum yield and lightfastness, thus resolving the contradiction between luminescence emission and excitation spectrum adaptability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a luminescent security feature with improved lightfastness and intensity, enabling detailed patterns and machine-readable emissions, suitable for documents of value like banknotes.
Implementation Method 1
Luminescent substances that are normally invisible under daylight are excited by radiation of suitable wavelength (e.g. UV, NIR), and the presence thereof is verified by the luminescent light
Implementation Method 2
Lanthanoid complexes are therefore of particular interest because of their narrowband emission spectrum, their emission in the visible part of the spectrum, their long luminescence lifetime (in some cases up to a few milliseconds)
Data Source
AI summary
A security feature for safeguarding a document of value, includes an organometallic luminescent substance which, on excitation with radiation in the UV-A range, has emission of radiation in the visible wavelength range, especially with green and/or red light, wherein the luminescent substance is based on a polynuclear, heteroleptic lanthanoid complex of the formula (I) [Mx(NN ligand)a(OO ligand)b] (I) where x, a and b are natural numbers, where x≥2, a≥2 and b≥6; M is a lanthanoid selected from the group consisting of Eu and Th; the OO ligand is a ligand that coordinates via oxygen atoms and the NN ligand is a nitrogen ligand having a phenanthroline base skeleton.