Synthesis, characterization and photophysical properties of first-row transition metal complexes

According to Horizon 2020, Europe's photonics industry is strong (e.g. in laser-based manufacturing, medical photonics, sensing, lighting) and has the possibility to exploit new emerging market opportunities. Advanced lighting technology includes the use of Solid State Lighting (SSL) based on organic light-emitting diodes (OLEDs). The advantages involve higher quality lighting in terms of energy efficiency, quality (robustness, longer lifetime, colour tunability), and therefore cost reduction and energy saving. Lighting represents around 19% of electricity consumption worldwide and the replacement of old lighting technologies such as incandescent bulbs with SSL systems will allow to save up to 70% of energy. With all of this in mind, it is evident that the development of light technology based on SSL is of paramount importance. Nowadays most of the luminescent complexes applied in SSL are based on lanthanides and d-block metals of the second and third transition series. Platinum(II) and iridium(III) are commonly used as emitters for OLEDs due to their long-lived triplet states. However, recent studies have demonstrated that it is possible to obtain highly luminescent complexes based on first-row transition elements, such as chromium(III), iron(III), manganese(II), copper(I) and zinc(II). These elements have the advantage to be less expensive and toxic, as well as more abundant. The main focus of the following PhD thesis is the synthesis and characterization of luminescent manganese(II), copper(I) and zinc(II) complexes. As concerns the first one, the metal-centred emission related to the 4T1(4G) → 6A1(6S) transition is strongly dependent upon the coordination sphere: tetrahedral complexes are normally green emitters, while octahedral species emit in the red range. The presence of rigid structures and light harvesting fragments in the ligand skeleton allows to enhance the luminescent features as well as the UV-light absorption. Several [O=P]-donor ligands were considered for the preparation of both tetrahedral and octahedral derivatives, sometimes affording dual emissions in the corresponding manganese(II) complexes due to concurrent metal- and ligand-centred transitions. As regards copper(I) complexes, derivatives containing polydentate N-donors based on indazole and benzotriazole were synthesized and their emission properties were attributed to metal-to-ligand charge transfer mechanisms involving triplet emitting states. Small changes in the ligand skeleton determined appreciable variations in the photophysical properties, as testified by the complexes obtained using 2,1,3-benzothiadiazole as N-donor. The [O=P]-donors previously used for manganese(II) revealed to be suitable ligands also for the preparation of luminescent zinc(II) halide complexes thanks to the enhancement of ligands fluorescence due to coordination. In selected cases intersystem crossing was observed, causing intense green phosphorescence or dual emission.

Abstract According to Horizon 2020, Europe's photonics industry is strong (e.g. in laser-based manufacturing, medical photonics, sensing, lighting) and has the possibility to exploit new emerging market opportunities. Advanced lighting technology includes the use of Solid State Lighting (SSL) based on organic light-emitting diodes (OLEDs). The advantages involve higher quality lighting in terms of energy efficiency, quality (robustness, longer lifetime, colour tunability), and therefore cost reduction and energy saving. Lighting represents around 19% of electricity consumption worldwide and the replacement of old lighting technologies such as incandescent bulbs with SSL systems will allow to save up to 70% of energy. With all of this in mind, it is evident that the development of light technology based on SSL is of paramount importance. Nowadays most of the luminescent complexes applied in SSL are based on lanthanides and d-block metals of the second and third transition series. Platinum(II) and iridium(III) are commonly used as emitters for OLEDs due to their long-lived triplet states. However, recent studies have demonstrated that it is possible to obtain highly luminescent complexes based on first-row transition elements, such as chromium(III), iron(III), manganese(II), copper(I) and zinc(II). These elements have the advantage to be less expensive and toxic, as well as more abundant. The main focus of the following PhD thesis is the synthesis and characterization of luminescent manganese(II), copper(I) and zinc(II) complexes. As concerns the first one, the metal-centred emission related to the 4T1(4G) → 6A1(6S) transition is strongly dependent upon the coordination sphere: tetrahedral complexes are normally green emitters, while octahedral species emit in the red range. The presence of rigid structures and light harvesting fragments in the ligand skeleton allows to enhance the luminescent features as well as the UV-light absorption. Several [O=P]-donor ligands were considered for the preparation of both tetrahedral and octahedral derivatives, sometimes affording dual emissions in the corresponding manganese(II) complexes due to concurrent metal- and ligand-centred transitions. As regards copper(I) complexes, derivatives containing polydentate N-donors based on indazole and benzotriazole were synthesized and their emission properties were attributed to metal-to-ligand charge transfer mechanisms involving triplet emitting states. Small changes in the ligand skeleton determined appreciable variations in the photophysical properties, as testified by the complexes obtained using 2,1,3-benzothiadiazole as N-donor. The [O=P]-donors previously used for manganese(II) revealed to be suitable ligands also for the preparation of luminescent zinc(II) halide complexes thanks to the enhancement of ligands fluorescence due to coordination. In selected cases intersystem crossing was observed, causing intense green phosphorescence or dual emission.