In the last few years, white-light emission from organic compounds has been the subject of increasing interest due to its potential impact on the lighting industry and backlight applications. In order to obtain white light from organic lightemitting devices (OLEDs), the simultaneous excitation of different molecular species emitting at different primary colors is required. [1,2] So far, the most exploited approach has been the fabrication of multilayer devices by consecutive evaporations or co-evaporation of different emitting compounds.[1±6]However, this technique requires complex technological processes and a large amount of wasted organic materials, resulting in relatively high fabrication costs. Spin-coating of a blend of different soluble emitters in a single layer seems to be a more cost-effective technique. [7,8] Though cheaper, this approach has the drawback that customized color combinations are not always possible due to Förster transfer from the highenergy emitting material (donor) to the low-energy one (acceptor), which induces emission only from the lower-gap compound. [9,10] An alternative approach which overcomes such a problem is to blend two blue-light-emitting organic molecules of different electron affinities, whose interaction gives rise to exciplex states. [11,12] The combination of the exciplex emission with the blue-light emission of the individual donor molecule results in the generation of white light. However, in both of these approaches the purity of the color emission is strongly dependent on the relative concentration of the different molecular species and, generally, on the applied voltage. This is a problem for lighting applications in which the source intensity (but not the color) has to be varied by changing the applied electrical power. [13] In this frame, the synthesis of a soluble compound showing white-light emission in the solid state is strongly desirable because it would enable the fabrication of a new class of devices which combine the simplicity and low cost of the single-layer spin-coated structures without the problems connected with the material concentration and bias.In this work, we demonstrate a bright single-layer white OLED realized by spin-coating a single emitting molecular material, namely 3,5-dimethyl-2,6-bis(dimesitylboryl)-dithieno[3,2-b:2¢,3¢-d]thiophene (compound 1 in Fig. 1). In 1, white electroluminescence is achieved by the superposition of the intrinsic blue±green-light emission (BGE) of the single molecule with a red-shifted emission (RSE) that occurs only in the solid state. The origin of the RSE peak is due to the formation of cross-like dimers between the molecules. This has been demonstrated by optical measurements and theoretical calculations, and also performed on similar compounds functionalized with different substitution patterns (2, 3, and 4 in Fig. 1) in order to control the self-assembling of the molecules. By virtue of the excellent properties of compound 1, namely the good electron-acceptor characteristics of the dimesitylboryl moieties...

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The photoluminescence (PL) frequencies and quantum efficiencies of dithieno [3,2-b;2′,3′-d]thiophene-4,4-dioxide (1), its 3,5-dimethyl derivative (2), and the corresponding 4-oxide (3) are reported and discussed in relation to their single-crystal X-ray structures. The peculiar packing modalities of dioxides 1 and 2, based on dimeric units originated by short S···O contacts between molecules related by an inversion center, cause an unusually high bathocromic shift of PL frequencies from solution to solid state. This effect is accompanied by a marked decrease in solid-state PL efficiencies (12% and 16%) compared to those in solution (75% and 77%). In monoxide 3 the loss of local symmetry inherent to the change SO 2 f SO deeply modifies the self-assembly and PL properties, and the PL efficiency in the solid state is close to that in solution. Ab initio calculations on the ground and excited states of compound 1 were performed and compared to those of a conformationally mobile counterpart. Oligomers containing dithienothiophene 2 as internal core were synthesized and found to be characterized by high PL efficiencies in the solid-state (up to 48%) as well as in solution (up to 85%). The current-voltage and luminance-voltage plots of an electroluminescent diode fabricated with one of these new rigid-core oligothiophene-S,Sdioxides are reported.

Among the most appealing features of oligothiophenesÐwhich are actively investigated for a broad range of electronic applications [1±7] Ðare their chemical stability and ease of functionalization, which allows the fine tuning of relevant properties. Unsubstituted oligothiophenes are p-type, hole-transporting, semiconductor materials and one of the research objectives in the field of organic-based devices is to achieve the kind of functionalization capable of giving stable derivatives displaying n-type semiconductor properties [8±10] for the preparation of electron-transporting layers. One of the possible ways to achieve this objective is functionalization with strongly electron-withdrawing groups, which can be designed to increase the electron affinity of the material.[11]We report here a new strategy towards functionalization of oligothiophenes that consists of the chemical transformation of thienyl sulfurs into the corresponding S,S-dioxides. We show that this kind of functionalizationÐwhich implies the dearomatization of the thiophene ringsÐleads to very stable oligomers with increased electron delocalization and has also a dramatic effect on their electron affinity as deduced from their electrochemical reduction potential. It is worth mentioning that the synthesis and the characterization of unsubstituted thiophene S,S-dioxide has been reported only very recently [12] and that only S,S-dioxides derived from polysubstituted thiophene are stable at room temperature.[13±15] So far, no electrochemical data on this kind of compound have yet been published. The oligothiophene S,S-dioxides were prepared either by action of m-chloroperoxybenzoic acid (m-CPBA) on the parent oligothiophenes [16] or by assembly of the appropriate building blocks via the Stille reaction, according to the improved procedure recently described by us. [17] As an example, the synthesis of a,w-bis(dimethyl(t-butyl)silyl)-2,2¢:5¢,2²:5²,2²¢-quaterthiophene-1,1-dioxide, 5, is illustrated in Scheme 1.As shown in Table 1, which includes the main optical and electrochemical data for a series of structurally correlated tetramers and pentamers, the maximum absorption wavelength of 5 (OTTT) is red shifted by 48 nm with respect to that of the parent quaterthiophene (TTTT), indicating a greater delocalization of the electronic charge. Moreover, while the oxidation potential of OTTT only increases by 0.09 V with respect to that of TTTT, the reduction potential is shifted by 0.84 V towards less negative values, resulting in a substantial increase of the electron affinity and also a decrease of the energy gap, DE el , of OTTT with respect to that of the precursor quaterthiophene.A similar trend in the variation of the oxidation and reduction potentials evaluated by cyclic voltammetryÐfol-lowing the dearomatization of one terminal ring by formation of the corresponding S,S-dioxideÐwas observed for the entire series of oligomers from the dimer to the pentamer bearing ±SiMe 2 t Bu or ±(CH 2 ) 5 CH 3 groups at the a,w positions. This trend is shown in Figu...

vices the required Q S to obtain a specific m is slightly lower than in SiO 2 -and BZT-based devices assuming that there is a reduced concentration of traps in the pentacene channel of solution-processed BST-based TFTs relative to ones comprising BZT or SiO 2 gate insulators.The use of solution-processed BST films as the high e gate insulator in pentacene IGFETs, required annealing at 400 C. Hence, despite their excellent low voltage device characteristics and performance they are not compatible with transparent plastic substrates, in contrast to the sputtered mixed metal oxide BZT insulators reported in the literature, [8] which were deposited at room temperature. However, given their performance, which is very close to that of the widely used a-Si:H TFT, pentacene IGFETs comprising sol-gel deposited BST films could be good candidates for applications involving AMLCDs or AMOLEDs on glass substrates. The low operating voltage required to produce such performance and the very low subthreshold slope of pentacene/BST-based IGFETs make them very attractive for applications. Furthermore, due to the solutionbased deposition process used, such IGFETs can potentially reduce manufacturing costs, especially if other layers of the device are also deposited from solution. This can prove very important in the future, as cost is expected to become an increasingly important factor in the flat panel display industry.In conclusion, based on our understanding of the origins of the gate bias dependence of mobility in pentacene IGFETs, [8] we have designed and fabricated high-performance devices, comprising pentacene and a solution-processed, relatively high e BST gate insulator, that require operating voltages up to only 5 V.

We report investigations of the photoluminescence (PL) and electroluminescence (EL) of a thiophene oligomer for which we have devised a variety of substitutions aimed at enhancing the solid-state efficiency. We find that the absolute PL quantum efficiency in the solid state is up 37% for both powders or spin-coated thin films of the compound. The material thus becomes competitive for applications in organic light-emitting diodes (LEDs). EL efficiencies up to 1.2 cd/A are demonstrated in LEDs prepared with indium–tin–oxide and Ca–Al electrodes.

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Oligothiophene S,S-dioxides from dimers to pentamers were obtained in good yields by reaction of mono- and dibrominated thiophene S,S-dioxides with the appropriate thienyl stannanes in the presence of Pd(AsPh3)4 generated in situ. The reaction rate with brominated thiophene S,S-dioxides is greatly accelerated compared to that employing thienyl bromides to obtain the parent oligothiophenes. HF/6-31G*ab initio calculations on 2,2‘-bithiophene and the corresponding mono- and bis-S,S-dioxides show that the functionalization of the thienyl sulfur to the S,S-dioxide does not affect the π,π* nature of the frontier orbitals, decreases the energy of the LUMO much more than that of the HOMO, increases the degree of planarity of the molecular skeleton, and leads to higher syn anti rotation barriers about the carbon−carbon bond.