Single-sign CPL emitting Ln(III) complexes and their applications

Abstract

The present work concerns the synthesis and photophysical characterisation of novel europium(III) complexes that are versatile, robust, bright, linkable, and can display same-sign circularly polarised luminescence (CPL) spectra, i.e. all positive or negative transitions within a single emission manifold. Broad, intense, same-sign CPL emission manifolds are expected to aid spectroscopic detection using low-cost broad band pass filters or wider slit opening, increasing signal without opposite sign CPL bands cancelling out.

Highly conjugated arylalkynylpyridyl antennae attached to the 1,4,7-triazacyclononane macrocycle, and mixed donor moieties, i.e. both carboxylate and phosphinate donor groups to coordinate the central europium(III) ion, were incorporated in the complexes. Different peripheral linking groups, such as carboxylate anions and para-nitro groups, are embedded in the structure to allow facile late-stage conjugation of the complex. The effects of chemical modifications on CPL spectra were investigated.
A terbium(III) chiral complex was designed and tested as a green dopant for the fabrication of Circularly Polarised Organic Light Emitting Diodes (OLEDs). Despite some preliminary good results, the limited choice of available commercial host materials with an adequately broad bandgap is a problem that needs to be overcome to achieve efficient CP-OLEDs.

A europium(III) complex was found to display same-sign CPL transition for the ΔJ = 1 and ΔJ = 2 emission manifolds. Further investigation of the dependence of CPL properties (both shape and intensity) on solvent polarity highlighted that CPL emission tends to be brighter and mono-signate in low polarity solvents. Racemisation kinetics of enantiopure europium(III) complexes in different solvents was investigated. Kinetic stability is key to designing durable lanthanide(III) doped polymeric films with definite CPL properties.

Unlocking same-sign CPL emission allowed a further advancement in the field of CPL imaging. A handheld non-moving parts setup that can acquire a CPL image in a single shot, called Circularly Polarised Luminescence Photography (CPLP), was designed and validated using the europium(III) complex described above. This high-throughput instrument will integrate five layers of security readout comprising of multi-coloured, multi-spectral, opposing-helicity, combined with high spatial and temporal resolution in unclonable luminescent QR codes, which was prototyped in the present study.