Excimer and Red Luminescence Due to Aggregation-Induced Emission in Naphthalene Based Zinc Phosphonate

The phosphonate Zn(PO3-CH2-C10H7) has been obtained as single crystals and polycrystalline powder by hydrothermal synthesis from the parent acid 1-naphthylmethylphosphonic acid H2PO3CH2-C10H7. This Zn phosphonate exhibits a unique layered structure (P2(1)/c), built up of single inorganic [ZnPO3C] layers stacked with organic bilayers of naphthylmethyl moieties CH2-C10H7. The inorganic [ZnPO3C] layers consist of infinite zig-zag chains of corner-sharing ZnO4 tetrahedrons running along the b direction and interconnected through PO3C tetrahedrons along c. The luminescence properties of this compound show two features never observed so far in any hybrid matrix containing naphthalene ligand: excimer emission characterized by one band fitted at 403 nm; and red luminescence, at room temperature, characterized by six bands fitted at 588, 611, 629, 667, 689, and 734 nm. Comparison with the structure and luminescence properties of the parent phosphonic acid shows that these exceptional luminescence properties originate from particularly strong - interactions, between the close-packed naphthalene rings with F2F and L-shaped conformations. The extremely fast decay of red emission ( approximate to 50 ns) shows that the latter is due to aggregation-induced emission (AIE), rather than to phosphorescence. The role of the inorganic [ZnPO3C] layers, which enhance the rigidity of the structure and restrict intramolecular motions, is also discussed to explain the appearance of the luminescence properties.