Thiophene oligomers

The synthesis and study of thiophene oligomers have attracted a great deal of attention for a number of years because of their activity against microorganisms, and recently because of their use as model compounds or 

Two main factors have motivated the use of these compounds as starting monomers in the polymerization of thienyl oligomers. One is the lower polymerization potential of bithiophene and a-terthienyl. The second is that poly(bithiophene) and poly(a-terthienyl) are ex- 

VoL 2 Conductive Polymers Synthesis and Electrical Properties Intensity [a. u.] 

The elctrochemical polymerization of bisthienyl coronene represents anoAer route to polymers containing thiophene units [45]. The syndiesis scheme for bisthienyl coronene shown below illustrates the polymerization to the new polymer type. 

The 2-ethynyl thiophene used to synthesize the thiophene-substituted coronene offers another interesting possibility for synthesizing new polymers, i.e. via tetraarylcyclopenta-dienones or thienyl-substituted cy-clopentadienones to yield substituted terphenylenes [12,46,47]. 

---

Two years later, the same research group published an alternative to the solvent-free method, since this method cannot be applied to the synthesis of insoluble thiophene oligomers such as sexithiophene [43]. Obviously, their insolubility prevents separation of the AI2O3 solid support. The effectiveness of PdCl2(dppf)/KF in solution phase was tested in the synthesis of soluble quinquethiophene from diiodoterthiophene and 2-thi-... 

Redox molecules are particularly interesting for an electrochemical approach, because they offer addressable (functional) energy states in an electrochemically accessible potential window, which can be tuned upon polarization between oxidized and reduced states. The difference in the junction conductance of the oxidized and the reduced forms of redox molecules may span several orders of magnitude. Examples of functional molecules used in these studies include porphyrins [31,153], viologens [33, 34,110,114,154,155], aniline and thiophene oligomers [113, 146, 156, 157], metal-organic terpyridine complexes [46, 158-163], carotenes [164], nitro derivatives of OPE (OPV) [165, 166], ferrocene [150, 167, 168], perylene tetracarboxylic bisimide [141, 169, 170], tetrathia-fulvalenes [155], fullerene derivatives [171], redox-active proteins [109, 172-174], and hydroxyquinones [175]. 

The thiophene ring is a common building block for novel oligomeric and polymeric materials. The synthesis of monodisperse thiophene oligomers continues to be widely studied and the preparation of one class of oligomers, oligothienylenevinylenes <00CEJ1698,... 

Nagarajan R, Bruno FF, Samuelson LA, Kumar J (2004) Thiophene oligomer as a redox mediator for the biocatalytic synthesis of poly(3,4-ethylenedioxythiophene) [PEDOT]. Polym Prepr 45 195-196... 

Heteroacene derivatives, (I), were prepared by Park et al. (1), which were effective as organic thin-film transistors. Low-molecular-weight thiophene oligomers were also prepared by Hahn et aL (2) and used as organic semiconductors. 

Much work has been directed towards the synthesis of thiophene oligomers and polymers. This is due to the current interest in research on conducting polymers and molecular electronics (92CRV711). Two main approaches have been used for making such polymers (i) chemical (e.g. FeCl3) or electrochemical oxidation of monomeric thiophenes and (ii) transition metal-catalyzed cross-coupling reactions. 

Recently, microwave-assisted Suzuki couplings under solvent-free conditions resulting in thiophene oligomers have also been reported41. 

Melucci, M., Barbarella, G. and Sotgiu, G., Solvent-free, microwave-assisted ynthesis of thiophene oligomers via Suzuki coupling, /. Org. Chem., 2002,67, 8877-8884. 

The optical nonlinearity is strongly dependent on the extent of ir-electron delocalization from one repeat unit to another in the polymer (or oligomer) structure. This effective delocalization is not always equally manifested but depends on the details of repeat unit electronic structure and order. For example, in a sequentially built structure, the ir-delocalization effect on Y is found to be more effective for the thiophene oligomers than it is for the benzene oligomers (5). 

The (l)1Bu state is the lowest excited singlet state in all a-oligothio-phenes from 2T to an including 7T (96JPC18683) while the (2)1Ag is 6570 cm 1 above the lowest state (92JCP(96)2492 93SM(60)23). The photophysical properties of these thiophene oligomers are collected in Table 4. 

The fast rising transient absorption bands which appeared at all thiophene oligomers nT could be identified by their kinetics as transitions from Si to higher singlet states. 

II Conjugation alone cannot be relied upon to significantly enhance the optical nonlinearities. The conformational effects and the role of the substituents so that understanding of the molecular structure-property relation can be improved have been studied (89JPC7916). In the case of the thiophene oligomers, a rapid increase in the y value (large microscopic nonlinearity) as a function of N is found. 

Yanagi, H., Araki, Y., Ohara, T., Hotta, S., Ichikawa, M. andTaniguchi, Y. (2003) Comparative carrier transport characteristics on organic field-effect transistors with vapor deposited thin films and epitaxially grown crystals of biphenyl-capped thiophene oligomers. Advanced Functional Materials, 13, 767-73. 

Yoon, M.-H., Facchetfi, A., Stem, C.E. and Marks, T.J. (2006) Fluorocarbon-modified organic semiconductors Molecular architecture, electronic, and crystal tuning of arene- versus fluoroarene-thiophene oligomer thin-film properties. Journal of the American Chemical Society, 128, 5792-801. 

A. Facchetti, J. Letizia, M. H. Yoon, M. Mushrush, H. E. Katz, and T. J. Marks, Synthesis and characterization of diperfluorooctyl-substituted phenylene-thiophene oligomers... 

The solvent-free, microwave-assisted coupling of thienyl boronic acids and esters with thienyl bromides, using aluminum oxide as the solid support, served to rapidly check the reaction trends on changing times, temperature, catalyst, and base and easily optimize the experimental conditions to obtain the desired product in fair amounts. This procedure offers a novel, general, and very rapid route to the preparation of soluble thiophene oligomers. Quaterthiophene 265 was obtained in 6 min by reaction of 2-bromo-2,2 -bithiophene with bis(pinacolato)diborane(4) in 65% yield, whereas dithiophene 266 was obtained with 70% yield. The synthesis of new chiral 2,2 -bithiophenes also was reported. The detailed... 

---