White-light-emitting lanthanide and lanthanide-iridium doped supramolecular gels: modular luminescence and stimuli-responsive behaviour†
Light-emitting materials are of significant interest owing to their potential applications in electroluminescent devices/displays and sensors. Herein, light-emitting metallogel systems are developed using self-assembled Ln(III)-complexes (Ln(III) = Eu(III) and Tb(III)) of 4′-p-fluorophenyl-2,2′:6′,2′′-terpyridine (L-F) and 4′-p-chlorophenyl-2,2′:6′,2′′-terpyridine (L-Cl) ligands. The luminescence of these metallogels could be tuned over a wide spectrum. Near-white light emission (CIE coordinates: (0.31, 0.36) and (0.34, 0.36)) was achieved by controlling the [L-X] (X = F, Cl)/[Eu(III)]/[Tb(III)] stoichiometry. Adopting an alternate strategy, near white light emission was also achieved using metallogels derived from Eu·L-F-Ir (CIE coordinates: 0.28, 0.35) and Eu·L-Cl-Ir (CIE coordinates: 0.31, 0.35). For this, a blue emissive [IrIII(F2ppy)2(biimid)]PF6 complex was used along with Eu(III)-terpyridine gel systems (red emissive component). Eu·L-F, Tb·L-F and Eu·L-F-Ir (1 wt%) could also be incorporated into a poly(methyl methacrylate) (PMMA) polymer matrix for developing transparent red, green and white luminescent PMMA films with excellent UV-shielding properties. Being colourless to the naked eye and luminescent on exposure to UV-irradiation, such metallogels have the potential to be used as an invisible security ink. Moreover, the dynamic nature of Ln–N(terpyridine) coordination bonds was utilized for demonstrating the chemo/vapor-responsive behaviour of the lanthanide-based gel systems, which could offer a suitable pathway for future engineering of stimuli-responsive gel materials.