Sensitizer-free photon up conversion in (HQ)ZnCl and HQCl crystals: systems involving resonant energy transfer and triplet-triplet annihilation.

This work deals with normal luminescence and up-conversion luminescence involving charge transfer and triplet-triplet annihilation in the lead free hybrid materials (HQ)2[ZnCl4] and HQCl salt; HQ is the hydroxyquinolate cation (HQ+ = C9H8NO+). The crystal structures were determined by X-ray diffraction and the optical properties were investigated by optical absorption and photoluminescence measurements and electronic band structure calculations. Under UV excitation, the normal luminescence is associated with π-π* transitions within the organic cation and involves energy and charge transfer between the inorganic ion and organic cation. Moreover, photoluminescence measurements under various excitation wavelengths performed on the hybrid (HQ)2[ZnCl4] and the salt HQCl have shown efficient up conversion of light from the near infrared (855 nm) to the visible region at 471 nm and 490 nm respectively. This behavior is described as sensitizer-free up-conversion luminescence based on the triplet-triplet annihilation process (TTA-UCL). These compounds are believed to be the first sensitizer-free TTA up-converting materials found in the organic metal halide family. Compared with the conventional TTA-UC solid systems based on precious and heavy metal organic complexes, the title compounds exhibit efficient photon up-conversion. Furthermore, they have extended the NIR conversion photons to a wide spectral interval of about 300 nm centred around 850 nm.