Ultraviolet up-conversion luminescent tetrahalozincates(II) with triphenylphosphonium cation: Synthesis, optical properties, and DFT calculations

Luminescent [HL][ZnX] [X = Cl, Br] tetrahalozincate(II) compounds with triphenylphosphonium cation (HL = PhPH) were isolated using hydrohalic acids. This study reports the synthesis, crystal structures, and photophysical properties of novel tetrahalozincate(II) compounds. The [HL][ZnCl] and [HL][ZnBr] crystallize in the 2/ monoclinic and rhombic space groups, respectively. The compounds exhibit a strong ultraviolet (UV–B, λmax ≈ 290 nm) fluorescence and blue-green and green phosphorescence in solid state, with a high quantum efficiency (up to 78 %). The luminescence mechanism was investigated, revealing that the UV-B and green emissions are ascribed to the {ZnX}-{PhPH} exciton-phonon emission, and the blue-green emission arises from the transition within the organic moiety. The [HL][ZnCl] compound shows an unexpected up-conversion luminescence at 305 nm under 405 nm excitation, presumably caused by stepwise two-photon absorption accompanied by proton-coupled electron transfer. The up-conversion luminescence make this compounds a promising candidate for simple and cost-effective materials for visible-to-ultraviolet converter devices.