NEW NCN-PLATINUM(II) COMPLEXES WITH LUMINESCENT PROPERTIES
During the doctoral period, my research has been focused on the synthesis and characterization of novel NCN-based platinum(II) complexes, obtaining more than twenty completely new compounds. These complexes have a terdentate chelating ligand with the general scaffold of a 1,3-di(2-pyridyl)benzene and were designed following different strategies: i) Complexes PtCl1-PtCl7 have unsubstituted pyridines and different groups in position 5 of the benzene ring, i.e. a mesityl, a triphenylamine, a pyrenyl, a phenyl-carbazole, a carbazole, a 2-thienyl and a methyl. The synthesis of the NCN ligands started from some dibromo intermediates, obtained in different ways: from a Suzuki-Miyaura coupling of 1,3,5-tribromobenzene with the suitable aryl-boronic acid in the case of L1-L4; from 1,3-dibromo-5-fluorobenzene reacting with carbazole for L5; from a Stille coupling of tribromobenzene and 2-SnBu3-thiophene for L6. Once obtained the dibromo intermediates, the Stille coupling of them (and of 1,3-dibromotoluene for L7) with 2-SnBu3-pyridine provided the terdentate ligands. Finally, the complexation step with K2PtCl4 in glacial AcOH leads to complexes PtCl1-PtCl7. ii) The second group of new complexes is composed of fifteen derivatives of the NCN-PtCl compounds PtCl1-PtCl2 and PtCl6-PtCl7, by replacement of the chloride ligand with six anionic species. These ancillary ligands are two aromatic thiolates, a simple azide, a phthalimide, a thioacetate and an isothiocyanate. The substitution reactions were carried out in acetone or MeOH, at room temperature and under Argon atmosphere, using a large excess of the new ligand. iii) Finally, PtCl8-PtCl13 still have the same -Cl ancillary ligand and substituents on the benzene ring (mesityl, 4-NPh2-phenyl, 2-thienyl, methyl) of some previously reported compounds, but new groups (i.e. 4-hexyl-2-thienyl, 4-NPh2-phenyl) have been added to the pyridines, to test the effect of an expanded aromatic system. These complexes are a development of some compounds of the first group, not having substituents on the pyridines. The synthesis started from the two different 4-aryl-2-chloropyridines (having a hexyl-thienyl or a triphenylamino moiety in para position), whose substituents were introduced by Suzuki-Miyaura coupling. Then, they reacted in a further Suzuki-Miyaura coupling with boronic diesters obtained from the already discussed dibromo intermediates. Finally, the typical complexation reaction was carried out in the presence of K2PtCl4. For all complexes reported before, the UV-Vis absorption spectra in CH2Cl2 were registered at different concentrations, in order to calculate the molar extinction coefficients (ε). Moreover, most of the complexes were studied from the luminescence point of view, using dearated dichloromethane solutions; in that way, the excitation and emission spectra, together with the absolute Quantum Yields and the Lifetimes, were obtained. The Freeze-Pump-Thaw procedure, used to remove oxygen from the solution, was necessary since the long-living triplet states of the complexes are very efficiently quenched by the fundamental triplet state of O2. Concerning the Quantum Yields, very high values were achieved in the case of complexes Pt1 (90%), Pt3 (87%), Pt8 (83%), Pt9 (89%) and Pt10 (79%), so the substitution of the -Cl is very often useful to improve the photophysical properties. Moreover, a remarkable increase of the QY is brought about also by the expansion of the aromatic system via new moieties on the pyridines. In fact, the values pass from 62% (PtCl1) to 96% (PtCl8) and 89% (PtCl12), and from 54% (PtCl6) to 100% (PtCl10). Complex Pt1, being very interesting because of its luminescence properties, was used for the preparation of three OLEDs having different amounts (8%, 25% and 100%) of NCN-Pt complex in the emissive layer. Since this family of complexes undergoes aggregation when the concentration reaches high values, and consequently a red-shifted emission is observed, the three devices emitted light with different color on the basis of the aggregation level. In this way the emission could be tuned, passing from green (8% Pt1) to yellow (25%) to red (100%). Up to now, all complexes have been studied from the UV-Vis absorption point of view, calculating the molar extinction coefficient for all of them; moreover, complexes Pt1-Pt3, Pt5, Pt7-Pt10, Pt12-Pt14, PtCl8, PtCl10, PtCl12 and PtCl13 have been fully characterized for their photophysical properties, measuring the excitation and emission spectra, the absolute Quantum Yields and the lifetimes in dichloromethane solutions. For what concerns the luminescence, some key points have been confirmed: i) Considering the emission of the monomeric species having unsubstituted pyridines, the emission measured from diluted solutions only depends on the substituent on the benzene ring, not being particularly influenced (besides a negligible shift of few nanometers) by the change of the -Cl ligand. ii) Regarding the absolute Quantum Yields, the change of the chloride is generally helpful to improve the QY values, in many cases causing an important increase up to 90%; this influence is not observed for the lifetimes, which remain in the ranges typical of the parent Pt-Cl compounds. iii) The most relevant effects are surely visible for those complexes presenting an expanded aromatic system on the pyridines, since for all of them a very important growth in the QY has been achieved (in the case of PtCl10 reaching unity), together with a noticeable red-shift of the emission. iv) For all complexes, an emission band at lower energies (with maxima in the range 650-750 nm) appears when the concentration is increased; this is due to formation of aggregates and can be seen also from pure layers of compounds in solid-state analysis; nevertheless, the concurrent formation of excimers, participating in the luminescent properties, cannot be excluded. In the case of Pt1, the luminescence has been assessed also for the powders, both at Room and Low Temperature; furthermore, the X-ray crystal structure of the compound has been obtained. Compound Pt5, bearing a thioacetate on the Pt center, has been studied non only as powders but also as blend in PMMA matrix at two different concentrations. Together with Pt9, the aforementioned Pt1 complex has been employed to produce some OLED devices, showing different emission colors on the basis of the amount of compound in the Emitting Layer. These tests can open the way for further strategies of color tuning, designing the device with the proper concentration of complex in order to modulate the emission. The next thing to do will be surely the study of the remaining complexes, to achieve a complete and clear overview of the photophysical properties of these series of compounds. Moreover, will also be important the testing of the already characterized compounds, both by preparing other OLED devices based on these complexes (to explore the different colors which can be achieved through them) and by applying the synthesized compounds in the biological field, as sensitizers in the bio-imaging of cells and tissues and as active dyes for photodynamic therapies. To conclude, another important step for the near future can be the synthesis of new members of the NCN-Pt(II) family, in particular following various strategies: i) the substitution of the ancillary -Cl in complexes such as PtCl8, PtCl10, PtCl12 and PtCl13, to observe the variation in the luminescence arising by the replacement with thiolates or azides, which is in general positive for what concerns the QY values; ii) the use of new thiolates or, in general, of new anionic species to replace the chloride ligand; iii) the introduction of new moieties on the pyridines, to confirm the mentioned helpful changes in the properties of such complexes, and to better understand the effect exerted by electron-donor or withdrawing groups, in particular on the emission color.
Settore CHIM/03 - Chimica Generale e Inorganica
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