Solid-state phosphorescence-to-fluorescence switching in a cyclometalated Ir(iii) complex containing an acid-labile chromophoric ancillary ligand: Implication for multimodal security printing

In this study, we have demonstrated the reconstruction of encrypted information by employing photoluminescence spectra and lifetimes of a phosphorescent Ir(III) complex (IrHBT). IrHBT was constructed on the basis of a heteroleptic structure comprising a fluorescent N^O ancillary ligand. From the viewpoint of information security, the transformation of the Ir(III) complex between phosphorescent and fluorescent states can be encoded with chemical/photoirradiation methods. Thin polymer films (poly(methylmethacrylate), PMMA) doped with IrHBT display long-lived emission typical of phosphorescence (λ_max = 586 nm, τ_obs = 2.90 μs). Meanwhile, exposure to HCl vapor switches the emission to fluorescence (λ _max = 514 nm, τ_obs = 1.53 ns) with drastic changes in both the photoluminescence color and lifetime. Security printing on paper impregnated with IrHBT or on a PMMA film containing IrHBT and photoacid generator (triphenylsulfonium triflate) enables the bimodal readout of photoluminescence color and lifetime.