Chiara Botta

Toward White Light Emission through Efficient Two-Step Energy Transfer in Hybrid Nanofibers

Nanosized zeolite L crystals containing about 550 strongly luminescent acceptor molecules have been modified by grafting a conjugated oligomer on their external surface. The 25 nm sized crystals have consequently been embedded in polymeric nanofibers obtained by electrospinning. The fluorescent molecule grafted on the external surface allows addressing the guests in the zeolite nanochannels through an efficient two-step energy transfer from the polymer nanofiber. The so obtained hybrid nanofibers exhibit intense emissions from the three fundamental colors using a single excitation wavelength. The molecule grafted on the external surface of the nanocrystal also induces a higher compatibility of the hybrid organic/inorganic nanomaterials in the conjugated polymer and therefore high concentrations of zeolites embedded in the nanofibers are obtained. Playing on this concentration, the emission of the nanofiber can be tuned and eventually be used for fabricating white-light emitting nanofibers. This hybrid nanomaterial opens new perspectives for low-cost nano organic light emitting diodes fabrication with considerable impact on the lighting and display technologies.

Highly Emissive Nanostructured Thin Films of Organic Host–Guests for Energy Conversion

All-organic nanostructured host-guest systems, based on dyes inserted in the nanochannels of perhydrotriphenylene (PHTP) and deoxycholic acid (DCA), show enhanced fluorescence properties with quantum yields even higher than those of the dyes in solution, thanks to the high concentration of emissive molecules with controlled spatial and geometrical organization that prevents aggregation quenching. Both host molecules crystallize, growing with the long axis oriented along the direction of the nanochannels where the linear-chain dyes are inserted, to yield crystals emitting well-polarized light. For the DCA-based host-guests, homogeneous thin films suitable for several applications are obtained. Colour emission in such films can be tuned by co-inclusion of two or three dyes due to resonant energy-transfer processes. We show that films obtained by low-cost techniques, such as solution casting and spin-coating, convert UV light into visible light with an efficiency much higher than that of the standard polymeric blends.

Cyclometallated platinum(II) complexes of 1,3-di(2-pyridyl)benzenes for solution-processable WOLEDs exploiting monomer and excimer phosphorescence

Two cyclometallated platinum(II) complexes, N^C^N-5-fluoro-1,3-di(2-pyridyl)benzene platinum(II) chloride, FPtCl, and N^C^N-5-methyl-1,3-di(2-pyridyl)benzene platinum(II) isothiocyanate, MePtNCS, have been synthesized and characterized. Both complexes are highly efficient phosphorescent green emitters which can also display excimer emission in the red region. They have been studied as triplet emitters in solution-processed, multilayer organic light-emitting diodes (OLEDs), together with the known complex of 5-methyl-1,3-di(2-pyridyl)benzene, MePtCl, for comparison. The trend in efficiencies of the OLEDs prepared correlates with the charge-trapping properties of the complexes. The most efficiently emitting complex, FPtCl, was used as the dopant in a solution-processed white OLED, employing monomer and excimer emission.

Precise surface patterning of silk fibroin films by breath figures

Silk produced by the silkworm Bombyx mori has gained a second youth as a sustainable material for high-technology applications. Here, we report a simple and cost-effective fabrication technique which allows the preparation of free-standing micropatterned pure protein films under ambient processing conditions. In this view, we take advantage of the auto organization of polymers in microporous films - the so-called breath figure formation - to create a specific morphology, which is then transferred to silk. The method includes, after the construction of a microcavity array, two to four soft-lithographic steps, depending on the desired surface patterning. Arrays of concave or convex microlenses, as well as packed micrometric bumps ordered in a hexagonal fashion, are possible obtainable architectures. Despite the straightforwardness of the procedures proposed, our method is able to produce, with high precision, transparent and stable silk fibroin films shaped with an ordered superficial pattern, which are attractive for the development of new biocompatible photonic devices.

Electroluminescent poly(fluorene-co-thiophene-S,S-dioxide): synthesis, characterisation and structure–property relationships

A new copolymer, constituted by the regular alternation of a fluorene derivative with a 1,1-dioxothiophene moiety, has been synthesised via Suzuki coupling. The molecular, thermal, structural, and photophysical characterisations have been performed. The material showed two dimensional order, enhanced by annealing; molecular modelling calculations afforded a consistent structural model accounting for the optical data. In comparison with both polyfluorenes and 1,1-dioxothiophene oligomers the unusual photoluminescence quantum yields observed in solution and in the solid state can be related to a significant contribution of internal conversion to the deactivation process in solution, and the formation of intra-chain hydrogen-bonds, due to a peculiar chain conformation, in the solid state. LED devices based on this new copolymer showed the highest efficiency compared with similar copolymers containing both fluorene and 1,1-dioxothiophene functionalities.

Perfluorinated polymer with unexpectedly efficient deep blue electroluminescence for full-colour OLED displays and light therapy applications

The fluorinated phenylene unit is used as a building block for an all-conjugated fluorene-based alternating copolymer (PFO–TFP). The synthesis, carried out by an optimized Suzuki cross-coupling, produces an extremely high purity compound with a series of intriguing properties. Firstly, a blue-shift of the absorption and emission spectra with respect to the parent polymer is achieved. Secondly, the photoluminescence (PL) quantum yields (QYs) of the solution, as high as 68%, are surprisingly retained in the solid-state. Finally, an improved colour stability of the film with respect to unsubstituted polymers, investigated by means of cyclic voltammetry and exposure to ultraviolet radiation, is observed. The PFO–TFP tested as an undoped active layer in a polymer light-emitting diode (PLED) shows a highly desirable pure deep blue 405 nm electroluminescence, CIE = (0.17; 0.06) and a remarkable 5.03% external quantum efficiency, thus fulfilling the requirements for both full-colour displays and biomedical applications. To date, fluorine atom incorporation has been successfully employed in the synthesis of high-performing polymers for organic field-effect transistors and photovoltaics. We give the first unambiguous proof of its efficacy in achieving pure deep blue electroluminescence in a highly efficient and stable PLED, which is competitive with the best reported conjugated oligomer-based devices.

A persulfurated benzene molecule exhibits outstanding phosphorescence in rigid environments: from computational study to organic nanocrystals and OLED applications

A molecule consisting of a hexathio-benzene core and peripheral tolyl substituents exhibits outstanding phosphorescence properties in some environments favoring rigidity, in the solid state and in a solid matrix at 77 K, while no luminescence is recorded in solution at room temperature. This peculiar behavior is attributed to the restriction of bond rotation and conformational mobility of the tolylthio substituents, which slows down the non-radiative deactivation processes of the phosphorescent excited state. The solid material (powder) shows a very high phosphorescence quantum yield (80%), which, to the best of our knowledge, is the highest reported value for organic molecules. The photophysical investigation was accompanied by a detailed computational study disclosing the role of CH–π interactions. This molecule has been exploited toward luminescent organic nanocrystals and light-emitting diodes with a fully solution processable technology.

Optical Properties and Photoexcitations in Regularly Alternating Conjugated Copolymers

We report in this short paper a preliminary account on the design, preparation, optical characterization and photoexcitation of a class of novel conjugated polymers consisting of regularly alternating thiophene and benzene sequences, which Might represent Models for one-dimensional superlattices. Optical absorption and emission spectra suggest that the barriers produced by the phenylene Moieties do not interrupt the conjugation but can produce localization effects. Resonant Raman spectra indicate that a selective response of the thiophene Moiety occurs upon excitation with visible light. Moreover these Measurements confirm the presence of a confinement on the thiophene blocks, while the biphenil Moiety does not appear to feel the energy barrier and behaves as the corresponding polymer poly (paraphenylene) (PPP). CW photomodulation spectra show that the formation of long-lived bipolarons takes place only if the thienylenic Moiety is long enough to accomodate the lattice distortion.

Synthesis and crystal structure and optical properties of fluorenic-core oligomers

Fluorenic core oligomers, displaying interesting photoluminescence properties, have been synthesized by an organometallic route. The crystal and molecular structure of a fluorene derivative and three homologous oligomers have been studied. The spiro-derivative of dibromofluorene shows a very strong interaction between the H and Br atoms. The thienyl-terminated oligomer with a spiro-substituent adopts a particular herringbone arrangement with overlap of only the end-thienyl residues, conformationally disordered. The crystal of the all-phenyl derivative consists of the packing of discrete molecules cofacially arranged. The molecule with linear alkyl chains on the 9-position of the fluorene core and thienyl rings as end-groups exhibits polymorphism and its crystal structure, tetragonal, is very peculiar because of the loose packing allowing a good separation among adjacent thienyl rings. The absorption and emission properties of this series were investigated and the electroluminescence of single-layer devices was characterized. The photoluminescence properties of the molecules are strongly dependent on their structural organization, displaying a red-shift of the emission from the amorphous phase, to the crystalline structure, to the aggregated form. A comparison of the packing of these oligomers and oligothienylenes fully accounts for the optical properties of the reported molecules.

Energy transfer in nanostructured oligothiophene inclusion compounds

Resonant energy transfers from terthiophene (T3) to quinquethiophene (T5) oligomers embedded in the nanochannels of perhydrotriphenylene (PHTP) crystals are studied by cw and femtosecond spectroscopy. The stringent geometry imposed by the host results in a very peculiar supramolecular organization of the guest molecules consisting of parallel linear arrays of chromophores. This provides a unique opportunity to make a quantitative analysis of the energy transfer phenomena and in particular to distinguish between heterotransfers and homotransfers. The experimental data fitted by a suitable theoretical analysis indicate that homotransfers become important in the long time dynamics and that the heterotransfer rate is satisfactorily described by the analytical solution obtained by assuming a continuous intermolecular spacing.