Complete stereochemical control to unlock monosign circularly polarised luminescence with superior circularly polarised brightness for chameleon security inks

Abstract

A novel arylalkynylpyridine-sensitised nine coordinate quasi-C₃ symmetric all carboxylate donor europium(III) complex (EuL) possessing exceptionally high circularly polarised brightness in both ΔJ = 1 and ΔJ = 2 transitions was prepared and tested in spin-coated solid-state PMMA thin films. The authentication of the circularly polarised luminescence (CPL) layer was successfully performed using CPL photography (CPLP) and enantioselective differential chiral contrast (EDCC) imaging for both transitions simultaneously using appropriate band pass filters. The effect of reflective properties of different thin film substrate materials on the recorded chiral contrast was quantified using the newly introduced CPLP dissymmetry factor (g_CPLP) which compared to the average dissymmetry factor values obtained using a photo elastic modulator (PEM) based CPL spectrometer. Circularly polarised brightness (CPB) of ΔJ = 2 (590 mol⁻¹ dm³ cm⁻¹ at 607 nm) was the highest ever reported and that of ΔJ = 1 (307 mol⁻¹ dm³ cm⁻¹ at 596 nm) was third best across other CPL-active materials with reported CPB. This makes EuL the best candidate for next-generation CPL-active multi-tier ‘chameleon security inks’.