Chameleon Security Inks

A novel arylalkynylpyridine-sensitised nine-coordinate quasi-C3 symmetric all carboxylate
donor europium(III) complex EuL1 possessing circularly polarised luminescence (CPL) with
exceptionally high CPL brightness (CPB) in both ΔJ = 1 and ΔJ = 2 transitions was prepared
and tested in solid-state polymethylmethacrylate (PMMA) spin-coated thin films. The structural
design was based on the previously reported parent EuL0 which contained phenylphosphinate
donor groups instead of their carboxylate equivalents in EuL1. This structural modification
resulted in 58% brightness increase (34650 mol-1 dm3 cm-1) and eliminated the dependence
of the emission properties on the applied solvent. The use of arylalkynylpyridine facilitated
sensitisation via a low-energy internal charge transfer state, resulting in optimal excitation with
commercially available 365 nm LEDs. The Laporte forbidden 5D0 →7FJ transitions gave rise to
~1 millisecond emission lifetime, allowing time-gated methods when used in combination with
short-lived (nanoseconds) organic fluorophores. EuL1 produced the highest ever reported
branching factor-corrected CPB (CPBi) values of 984 mol-1 dm3 cm-1 and 590 mol-1 dm3 cm-1
for the ΔJ = 2 (at 607 nm) using the branching factor for the whole and single-sign only regions
respectively. The CPBi of 307 mol-1 dm3 cm-1 for the ΔJ = 1 (at 596 nm) was the third best
across all europium(III) complexes and the highest reported for isostructural materials.
The authentication of the CPL security layer of EuL1 in PMMA thin films was performed using
CPL photography (CPLP) and enantioselective differential chiral contrast (EDCC) imaging for
both ΔJ = 1 and ΔJ = 2 transitions simultaneously using band pass optical filters. The effect
of film substrate light reflections on the recorded chiral contrast was studied and quantified
using the newly introduced CPLP dissymmetry factor (gCPLP), which was found to be equivalent
to the average dissymmetry factor obtained using a standard CPL spectrometer. The
developed reproducible method of thin film preparation via spin-coating resulted in identical
gCPLP values for the consecutively spin-coated films, suggesting high reproducibility of the CPL
security feature in combination with reliability of the authentication method.
A set of novel EuL1-based materials compatible with the complex-stage functionalisation via
‘click-chemistry’ cycloaddition and nucleophilic substitution reactions was developed for the
applications where a covalently linkable material with strong CPL-activity is desired. A proof
of-concept study demonstrated EuL3 functionalisation with propargyl glucose units which
improved the live-cell uptake for a better bioimaging performance. All novel materials were
photophysically characterised and explored in terms of their CPL properties which revealed
that structural modifications away from the europium(III) coordination site did not affect the
CPL intensity, glum or spectral shape, even when the total complex symmetry was lowered in
EuL4 with unsymmetric macrocyclic ligand containing structurally different chromophores.