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...

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.

The regioselective synthesis of fluorescent oligothiophene isothiocyanates is described. The isothiocyanates were reacted with bovine serum albumin (BSA) following standard procedures and the optical properties of the oligothiophene-BSA conjugates were analyzed as a function of oligomer concentration, time, and irradiation power. The oligothiophene-BSA conjugates were chemically very stable and their photoluminescence characteristics persisted unaltered for several months. Photoluminescence data relative to the conjugate of an oligothiophene-S,S-dioxide isothiocyanate with monoclonal anti-CD8 antibody are reported. No fluorescence quenching was observed following the binding of the isothiocyanate to the antibody and the conjugate displayed high chemical stability and photostability.

Currently, thiophene-based oligomers and polymers are a matter of intense research activity in view of their technological applications in electrical and optical devices. [1][2][3][4] The efficiency of such devices is strictly related to the HOMO-LUMO energy gap of these materials, and a great amount of work has been done to functionalize oligo-and polythiophenes with a variety of Rand -substituents as well as to incorporate nonaromatic moieties, with the aim of reducing the energy gap. 1-5 S0022-3263(97)02108-7 CCC: $15.00