Roberta Ragni

Fluorinated organic materials for electronic and optoelectronic applications: the role of the fluorine atom

In this article we highlight, by means of selected examples drawn from work performed in our or other laboratories, the features of some classes of fluorinated conjugated materials and their use in electronic devices such as electroluminescent diodes or field effect transistors. A variety of fluorinated conjugated systems, either molecular or polymeric, such as poly(phenylenevinylene)s, poly(phenyleneethynylene)s, polythiophenes, polyphenylenes, are dealt with. Attention is also focused on a different class of electroluminescent compounds, represented by the cyclometalated iridium complexes with various forms (mer and fac). In particular, fluorine atoms lower both the HOMO and LUMO energy levels. Consequently, the electron injection is made easier, the materials display a greater resistance against the degradative oxidation processes and organic n-type or ambipolar semiconducting materials may result. Moreover, the C-H...F interactions play an important role in the solid state supramolecular organization, originating a typical pi-stack arrangement which enhances the charge carrier mobility.

Blue emitting iridium complexes : synthesis, photophysics and phosphorescent devices

Homoleptic Ir(Fnppy)3 and heteroleptic (Fnppy)2Ir(acac) complexes (n = 3: F3ppy = 2-(3′,4′,6′-trifluorophenyl)pyridine; n = 4: F4ppy = 2-(3′,4′,5′,6′-tetrafluorophenyl)pyridine; acac = acetylacetonate) have been synthesized and their spectroscopic properties investigated. The homoleptic complexes exist as two stereoisomers, facial (fac) and meridional (mer), that have been isolated and fully characterized. Their electrochemical and photophysical properties have been studied both in solution and in the solid state and electroluminescent devices have been fabricated. The emissive layers in devices have been obtained mixing the iridium complexes with a PVK [poly(9-vinylcarbazole)] host matrix, in the presence of the electron carrier Bu-PBD [2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole]. The application of a voltage (5.0–6.5 V) between the electrodes of devices leads to electro-generated blue luminescence which has similar energy to the solution emissions. Interestingly, the stability of the devices made with the homoleptic fluorinated iridium complexes strongly depends on the stereochemistry of these phosphors and high (up to 5.5%) external quantum efficiencies for the fac complexes are measured.

Enhancing the Light Harvesting Capability of a Photosynthetic Reaction Center by a Tailored Molecular Fluorophore

Light machine: The simplest photosynthetic protein able to convert sunlight into other energy forms is covalently functionalized with a tailored organic dye to obtain a fully functional hybrid complex that outperforms the natural system in light harvesting and conversion ability.

Excited-state switching by per-fluorination of para-oligophenylenes.

Fluorination has become a versatile route to tune the electronic and optical properties of organic conjugated materials. Herein we report a new phenomenon, excited-state switching by per-fluorination of para-oligophenylenes, placing a low intensity 1(1)B(2) state below the 1(1)B(1) state, giving rise to large Stokes shifts. The switching is attributed to the specific impact of fluorine on the delocalized and localized frontier orbitals as elucidated by quantum-chemical calculations. The sterical demands of the fluorine atom additionally diminish efficient conjugation along the chain, leading to hypsochromic shifts with respect to the unsubstituted counterparts and to a weak chain length dependence of the absorption and unstructured emission spectra and enhanced internal conversion.

Influence of electronic and steric effects of substituted ligands coordinated to Ir(III) complexes on the solution processed OLED properties

Orange and green phosphorescent heteroleptic iridium complexes 1 and 2 [(1): iridium(III)bis[2-(5′-benzylsulfonyl)phenylpyridinato-N,C2′](2,4-decanedionate) and (2): iridium(III)bis[2-(5′-benzylsulfonyl-3′,6′-difluoro)phenylpyridinato-N,C2′](2,4-decanedionate)] are used as dopant emitters in solution processed organic light-emitting diodes. These emitters bear one sterically hindered 2,4-decanedionate and two phenylpyridine (ppy) ligands functionalized with electron withdrawing benzylsulfonyl groups and fluorine atoms. The steric and electronic effects of such substituents on the photophysical properties and performance of devices of 1 and 2 are investigated by making comparison with the green emitting reference phosphor Ir(ppy)2(acac) [iridium(III)bis(2-phenylpyridinato-N,C2′)-acetylacetonate]. In particular, the functionalization of ppy ligands with the benzylsulfonyl group enhances the photoluminescence quantum yield (Φ) and red shifts the emission of complex 1 with respect to Ir(ppy)2(acac). Further substitution of the same ligands with two fluorine atoms in 2 restores the green emission of the reference complex, increasing its Φ. Hence, the combination of the two kinds of substituents represents a suitable functionalization pattern to increase the photoluminescence efficiency of 2vs. the unfunctionalized Ir(ppy)2(acac). Interestingly, a fully solution processed phosphorescent light-emitting device (PHOLED) made using complex 2 as an emitter and with properly tailored architecture, which includes an electron transporting layer of a PEG substituted polyfluorene (PEG: poly(ethylene glycol)), exhibits high external emission quantum efficiency (EQE up to 12%) and a high luminous efficiency (LE up to 24.2 cd A−1). We attribute such a high performance to the bulky effect of both benzylsulfonyl groups in ppy and of the alkyl chain in the β-diketonate ligand, as well as to the enhanced electron mobility induced by fluorine atoms. This device outperforms the control diode based on Ir(ppy)2(acac) (EQE 5.5% and LE 17 cd A−1) and approaches the best efficiencies achieved so far for green Ir(III) complex based PHOLEDs.

''Garnishing'' the photosynthetic bacterial reaction center for bioelectronics

The photosynthetic reaction center is an extraordinarily efficient natural photoconverter, which can be ideally used in combination with conducting or semiconducting interfaces to produce electrical signals in response to absorption of photons. The actual applicability of this protein in bioelectronic devices critically depends on the finding of (a) suitable deposition methods enabling controlled addressing and precise orientation of the protein on electrode interfaces and (b) chemical manipulation protocols able to tune and enhance protein light absorption in specific or broader spectral regions. Literature reports several examples of approaches to fulfill these requirements, which have faced in different ways the fundamental issues of assembling the biological component and non-natural systems, such as electrode surfaces and artificial light harvesting components. Here we present a short overview of the main methods reported to accomplish both the objectives by properly “garnishing” the photosynthetic reaction center (RC) via chemical modifications.

Single white light emitting hybrid nanoarchitectures based on functionalized quantum dots

Colloidal white emitting nanostructures were successfully fabricated by covalently binding a blue emitting oligofluorene at the surface of silica beads, that incorporate orange luminescent colloidal CdSe@ZnS quantum dots (QDs). White light was achieved by carefully tuning the size of the QDs to complementarily match the emission color of the blue fluorophore and taking into account the delicate balance between the emission of the QDs in the core of the silica beads and the amount of the organic dye bound to the silica surface. The proposed approach is highly versatile as it can be extended to the fabrication of a variety of luminescent hybrid nano-objects, playing with the complementarity of the emission color of the inorganic and organic fluorophores at the nanoscale.

Synthesis of Conjugated Polymers by Coupling between Arenediazonium Tetrafluoroborates and Vinyl Silanes

A new and general methodology for the synthesis of conjugated polymers presenting a variety of structural features is described. The methodology is based upon the palladium-catalyzed coupling reaction of a bis(1-trimethylsilylethenyl)benzene with arenebisdiazonium tetrafluoroborates. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2002)

Synthesis of Functionalized Aryleneethynylene Oligomers and Polymers for Organic Electronics by Pd-Catalyzed Coupling Reactions

In this article we discuss synthetic routes to organic conjugated oligomers and polymers bearing triple C-C bonds that have been recently developed in our laboratories, based on Pd-catalyzed Csp-Csp 2 coupling reactions. Experimental protocols have been tuned to face synthetic challenges such as the presence in the main conjugated backbone of multifunctional substituents or uncommon conju- gated units. The relevance of the obtained products in terms of properties and applications is also shortly mentioned.

Data from two different culture conditions of Thalassiosira weissflogii diatom and from cleaning procedures for obtaining monodisperse nanostructured biosilica

Diatoms microalgae produce biosilica nanoporous rigid outershells called frustules that exhibit an intricate nanostructured pore pattern. In this paper two specific Thalassiosira weissflogii culture conditions and size control procedures during the diatoms growth are described. Data from white field and fluorescence microscopy, evaluation of cell densities and cell parameters (k value and R value) according to cell culture conditions are listed. Different cleaning procedures for obtaining bare frustules are described. In addition, FTIR and spectrofluorimetric analyses of cleaned biosilica are shown. The data are related to the research article “Chemically Modified Diatoms Biosilica for Bone Cell Growth with Combined Drug-Delivery and Antioxidant Properties” [1].