Luminescent 2D materials for smart inks

Abstract

The increment of documents or currency counterfeiting is being a problem for society, making it necessary to find a solution. One possible method is the development of smart inks, which are luminescent under certain light wavelength. To do it, metal-organic frameworks (MOFs) with structure [Ln(CH3COO)(PhCOO)2] of Ln = Tb, Eu and Sm are used. Homometallic and heterometallic compounds are synthetized with Ln:Ln’ proportions of approximately 0,5:0,5, 0,25:0,75 and 0,75:0,25. All of them are luminescent under UV light: Tb is green, Eu red and Sm violet. Due to the great properties of 2D materials, such as graphene, exfoliations of homometallic MOFs are carried out to achieve single-layer compounds dispersed in i-propanol. Both for MOFs and for exfoliations, a luminescence study has been done to see where the electronic transitions responsible of their colour appear. Although f-f transitions are prohibited, due to the antenna effect of the ligand, the expected electronic transitions of Lanthanides can be observed in all compounds. Furthermore, Tb in heterometallic compounds acts as sensitizer, making the emission of the partner Ln’ more intense. After the confirmation of exfoliations luminescence, they are stamped. In a preliminary study, it is observed that through the fluorimeter the corresponding electronic transition appears, but it is not possible to see its luminescence at naked eye under UV light.