LUMINESCENT COMPOUNDS FOR APPLICATIONS IN PHOTONICS

Luminescent compounds have seen a growing interest for the possibility to employ them for several applications, from energy conversion to the biomedical field, but also sensors, light-emitting devices, and many others. The variety of luminescent compounds is huge, and this offers a wide flexibility in terms of chemical and photophysical properties, that enables the possibility to use them for all these applications. This Ph.D. thesis deals with the synthesis and characterization of luminescent compounds for potential applications in photonics, which can be defined as a branch of optics related with generation, modulation, detection, and processing of photons. It is divided in three parts, each concerning an independent project. In the first part, made in collaboration with Prof. Gianmarco Griffini (Politecnico di Milano), the synthesis and characterization of two luminescent diol dyes, D-BT and A-PDI, have been described, and both dyes were characterized by intense absorption bands and high luminescence quantum yields. These dyes have been accurately chosen to realize luminescent solar concentrators (LSCs), considering two main aspects. Firstly, the extension of the device absorption spectrum when used together, second, an efficient energy transfer between them to improve the overall device performance. Furthermore, the presence of two hydroxyl groups was chosen for the subsequent incorporation of the dyes in polymeric structures. This was aimed to minimize the mobility of the dye in the polymeric matrix, therefore reducing the tendency to form aggregates, and to improve the dye dispersion. A series of nine luminescent polyurethanes containing one dye or both, plus three blends of polymers containing different dyes has been synthesized and characterized in thin-film configuration. Except for the shift of the emission bands, that was associated with the different chemical environment than in solution. Polymers containing two dyes were characterized by broader absorption spectra than those functionalized with only one luminophore, and energy transfer processes took place very effectively, with measured efficiencies up to 90 %. Device characterization displayed very interesting results, starting from the elevate internal photon efficiencies, up to 48 %. The LSC device based on polymer PU-DA15, containing both dyes, in particular, was characterized by an excellent external photon efficiency of 3.97 % and by a moderate device efficiency of 0.64 %. However, what characterizes this type of device is its remarkable transparency, in fact, its average visible-light transmittance was approximately 90 %. Also a high light utilization efficiency of 58 was calculated for the same polymer. The second part, made in collaboration with Dr. Henri Doucet during a stay of three months at Université de Rennes (France), reports the synthesis of fourteen new phenyl-functionalized, difluorinated NˆCˆN ligands through direct arylation via C-H activation reactions, never attempted on this kind of substrate. The synthesized ligands were then coordinated with Pt(II) cations to obtain the corresponding Pt(II) complexes. Photophysical characterization showed intense absorption bands in the near UV region, with a tail in the visible, responsible for the pale-yellow colour of the derivatives. However, a strong absorption in the visible was measured when the ligands were functionalized with triphenylamines. Luminescent quantum yields were usually high (up to 90 %) and emission lifetimes long, in the range of µs. Pyridine functionalization had a strong effect on the photophysical properties, depending on the electron-donating character of the substituents and on their steric hindrance. The third, and last, part of this doctoral thesis, was made during a stay in Ikeda (Japan) in collaboration with Dr. Kenji Kamada (National Institute of Advanced Industrial Science and Technology, AIST). The two-photon absorption properties of three samples were determined and the experimental data were compared with simulated ones. Compound T1, in spite of being an octupolar structure, was characterized by a moderate cross-section of 190 GM at 587 nm. However, this is only a preliminary result, and T1 was prepared to set and optimize the synthetic procedures to obtain a class of molecules characterized by more appropriate functional groups suited to increase the TPA cross-section and to red-shift the spectrum. Also Pt(II) complexes having NˆCˆN ligands, compounds Pt1-2, were investigated: despite the modest TPA cross-section of Pt1, a very good result was obtained for Pt2, with a cross-section of 251 GM at 805 nm. Theoretical calculations confirmed that the presence of triphenylamines on the side pyridine rings is associated with an intense charge transfer from them to the molecular core that, together with the efficient conjugation, allowed a reasonably intense TPA.